Cabinet. A typical cabinet for mounting networking and telecommunications equipment.
The most widely used object-oriented programming language.
Overview
C++ is the standard programming language used for developing tools and applications for high-speed networking. This is because it is a compiled language whose object code is essentially native assembly language and hence runs extremely fast. For example, a form handler written for a Hypertext Markup Language (HTML) form that runs on a UNIX Apache Web server will run much faster if it is written in a compiled language such as C or C++ than if it is written in an interpretive language such as Perl or Microsoft Visual Basic, Scripting Edition (VBScript).
High-performance Internet Server API (ISAPI) applications and dynamic-link libraries (DLLs) written for Microsoft Internet Information Services (IIS) generally perform best when written in C++. Powerful distributed Web-based applications for the Internet can be developed using a combination of Active Server Pages (ASP) scripts in VBScript or JScript, with Microsoft ActiveX components and ISAPI dynamic-link libraries (DLLs) written in C++ using Microsoft Visual C++.
C++ programs for Windows network operating system platforms can be developed using a variety of tools:
Traditional C++ development tools using editors, compilers, and debuggers support low-level application programming interface (API) access, Microsoft Foundation Classes (MFC), software development kits (SDKs), and other supporting elements.
Rapid application development (RAD) tools provide integrated development environments (IDEs) that focus on providing developers with ease-of-use features.
Component deployment tools take a component- based approach to generating and reusing C++ code for building distributed and multitier applications.
C++ is based on the earlier C language that is used for developing applications, including Win32 applications for Microsoft Windows platforms. The standard form of C++ is called ANSI C++ and is defined by the American National Standards Institute (ANSI).
Notes
Microsoft Visual C++ combines the best features of these various classes of tools. It is an excellent choice for developing large-scale distributed networking applications based on the C++ language and Microsoft Corporation's own specific extensions to ANSI C++ that are designed for the Win32 platform.
See Also C#
Pronounced "C sharp," a programming language developed by Microsoft Corporation for its new .NET platform.
Overview
C# is an offshoot of C and C++ that is designed for rapid development of Web services for Microsoft's new .NET platform. C# is derived from C++ and has much of the power of C++ while improving on it in many respects from the point of view of the developer who needs to rapidly create and deploy applications in the e-economy.
The main weaknesses of C++ as a development environment are its complicated code-compile-debug development cycle, its potential of creating memory leaks that are difficult to troubleshoot, and the requirement of having applications explicitly manage memory resources. C# is similar to the Java platform developed by Sun Microsystems in that memory resources and garbage collection (object lifetime management) are automatically handled by the language itself, freeing developers to work on creating applications instead of being bothered by handling these intricate details. C# also simplifies access to external objects and simplifies the object creation process compared with C++. And although C++ applications require frequent use of include files to allow access to system services, in C# these services are transparently wrapped in objects. Like Java, C# thus relieves programmers of much of the chore of object and memory management, speeding up the development cycle in the process.
On the other hand, C# is unlike Java in that it maintains powerful features such as pointers, passing arguments through reference, overloading operators, and manually allocating memory. These features, basic to C++, are included in C# to provide programmers with the functionality should they need it, but they are deemphasized in C# compared to their common use in C++. For example, when pointers are used in C#, they need to be tagged in a section of code marked as "unsafe," making it easier to troubleshoot difficulties that may arise from their misapplication. Furthermore, C# has the advantage over Java of providing direct access to native Microsoft Windows services, and C# is easier to learn than Java for C++ programmers because its syntax is derived from and is similar to C++, whereas the syntax for Java has many differences from C++ that make it difficult for C++ programmers to use easily.
It is relatively simple to port existing C++ programs to C# (by contrast, it is much more difficult to port such applications to Java). The main disadvantage of C# as a development environment is a weakness shared with Java: unlike C++, which compiles programs into native machine code, C# compiles programs into an intermediate bytecode called intermediate language (IL) that is then interpreted by the .NET runtime. This means C# shares a performance hit similar to Java, which is also an interpreted platform. C++ will still be used for writing code modules where the best performance possible is required (such as device drivers), but C# performs sufficiently well on today's hardware platforms to be used as a primarily development platform. And because C# executes on the .NET runtime, it shares the interoperability features of this runtime, which allows modules written in C# to communicate with code written using other programming languages.
Microsoft plans to submit the C# language to the standards board of the European Computer Manufacturers Association (ECMA). The ECMA will then manage the language as a standard, which means that third parties wanting to use C# to develop applications will not be required to pay licensing fees to Microsoft. This will keep the cost of development tools and applications development low.
Notes
The Object Management Group (OMG), the creators of Common Object Request Broker Architecture (CORBA), is developing a language mapping for C# to enable it to communicate through CORBA with applications written in other languages such as C, C++, Cobol, Java, Python, and Ada.
See Also C++ ,.NET platform
A security standard for computer systems established by the National Computer Security Center (NCSC).
Overview
The NCSC is a U.S. government agency responsible for evaluating the security of software products. The C2 security standard is defined in the Trusted Computer Systems Evaluation Criteria (TCSEC) manual (or Orange Book) published by the NCSC.
The NCSC rated Microsoft's Windows NT 3.5 (with Service Pack 3) C2-compliant. The C2 designation assures that the base operating system satisfies a number of important security criteria. This designation also represents an independent, unbiased evaluation of the system architecture's security with regard to the government's operating and implementation standards. Windows 2000 also supports C2, but is still undergoing the NCSC evaluation process.
A C2 rating does not indicate that a system is free of security bugs; instead, this rating certifies that the computer system's underlying architecture is suitable for high-security environments in specific networking configurations. It is incorrect to say that Windows NT is C2-certified or runs in C2 mode. Only a complete computer system (including hardware) can be rated C2. A rating of C2 means that in a particular implementation, in a particular networking environment and configuration, using specific hardware and software, a computer network using a Windows NT operating system can apply for, and might receive, C2 certification.
According to the Orange Book, in a C-level system, the security policy must be based on what is known as Discretionary Access Control (DAC), which essentially means that users of the system can own objects (such as files and directories) and can control access to these objects by other users. A user who establishes control over an object is responsible for granting or denying all access rights to that object. In other words, the owner of an object grants or denies users access to the object at his or her discretion. This is in contrast to a B-level system, in which Mandatory Access Control (MAC) specifies that all objects have security levels that are defined independent of the object's owner.
For More Information
Visit the NCSC at www.radium.ncsc.mil.
See Also security
Stands for certificate authority, any entity (individual, department, company, or organization) that issues digital certificates to verify the identity of users, applications, or organizations.
See Also certificate authority (CA)
An enclosure with a built-in rack for holding and organizing patch panels, switches, hubs, routers, servers, and any other networking equipment within a wiring closet.
Overview
Cabinets can be wall-mounted or freestanding, come in various heights, and are usually standardized for 19-inch-wide shelving and paneling. They generally come in 83-inch and 48-inch heights, although many vendors offer custom-designed cabinets. The reason for standardizing the width to 19 inches (18.31 inches to be precise) is that hubs, switches, routers, and other networking devices are produced in this width so that they can be organized in racks and cabinets designed for this purpose. Cabinets come with a variety of accessories for organizing cables, power strips, and other equipment. Because heat can accumulate in cabinets, they usually include vented walls and have an exhaust fan on top. A cabinet will often have a locking front panel made of clear plastic so that status lights on equipment are easily visible. Shelves can be fixed, mounted, or sliding to enable easy access to the sides and backs of equipment.
Cabinet. A typical cabinet for mounting networking and telecommunications equipment.
Notes
When should you choose a cabinet instead of a rack? Choose a cabinet for equipment that is exposed to user traffic, and then you can lock equipment away when the room itself is not locked. Cabinets are also best for expensive networking equipment that you do not want anyone but authorized administrators to touch. Cabinets with filter fans installed can protect equipment in environments where dust is a problem. Use filler panels to enclose areas of the cabinet that are not occupied by equipment.
See Also premise cabling ,rack
A file with the extension .cab that stores compressed files, usually for distributing software on Microsoft platforms.
Overview
Cabinet files can contain multiple files in a compressed state, or a single compressed file can be spread over several cabinet files. During installation of software, the setup program decompresses the cabinet files and copies the resulting files to the user's system.
Cabinet files can be digitally signed using a Microsoft technology called Authenticode. This allows setup files to be downloaded safely over distrusted networks such as the Internet. Cabinet files are compressed using a compression algorithm called MSZIP, which is based on the Lempel-Ziv algorithm.
Notes
Cabinet files in Microsoft Windows 95 were located in the Win95 directory on the source CD, and most were represented as a series of large files with names such as Win95_1.cab and Win95_2.cab. Windows 98 uses a different naming convention and names many of its smaller cabinet files by function rather than by the order in which they are used during setup. Naming by function makes the extraction of files easier, which in turn makes the setup process smoother.
In Windows 95, if you want to extract specific operating system files from a cabinet file (for example, to replace a missing or corrupt file), you have to use the command- line utility called extract. Using Windows 98 and later versions, you can simply double-click on a cabinet file using Windows Explorer to view its contents in a new window, double-click on the specific file you want to extract, and specify the destination folder to send it to. You can also use the System File Checker tool to extract files without knowing which specific cabinet file they are located in.
See Also Authenticode
A device that allows your computer to access the Internet through dedicated broadband transmission networking services by means of your home cable TV (CATV) connection.
Overview
Cable modems modulate and demodulate analog signals like regular modems, but for transmission over broadband video services instead of telephone voice services. A cable modem can be internal or external and can interface with the coaxial cable connection at the user's end and the Cable Modem Termination System (CMTS) at the cable provider's head office.
Cable modem and Asymmetric Digital Subscriber Line (ADSL) are competing technologies for bringing high- speed broadband Internet services to homes and businesses. Cable modems offer downstream speeds of 10 megabits per second (Mbps) and higher, but competing technologies, lack of standards, and implementation costs have slowed widespread deployment and use of this technology.
There are two basic types of cable modem services:
One-way cable modems: These are used by unidirectional cable service providers. Most cable TV services are designed to carry information in one direction only-from the broadcaster to the customer premises. With one-way cable modems, the customer uses a regular telephone modem to send information to the cable company but uses the cable TV system with cable mode to receive signals from the company. The telephone modem handles all upstream communication, and the cable modem handles all downstream communication. One-way cable modems are typically cards installed inside a subscriber's computer.
Two-way cable modems: These require that the broadcasting cable company has converted its cabling and repeater infrastructure for bidirectional communication. Two-way cable modems are typically external devices connected to a network interface card (NIC) that is installed in the subscriber's computer. The cable modem is used for both upstream and downstream communication in this configuration. Most cable companies currently have initiatives under way to make such a conversion, but it requires a large capital investment. Therefore, it will be several years before these systems become widely available.
Implementation
In a typical one-way cable modem implementation, the CMTS uses separate subsystems for upstream and downstream connections that terminate at a router. The downstream subsystem is designed for converting incoming Internet Protocol (IP) traffic into radio frequency broadband signals that are broadcast using a broadband network hub (BNH) over cable TV wiring to local groups of connected subscribers. The upstream subsystem usually consists of banks of ordinary telephone modems to allow for easy expansion of services to additional subscribers. The router is used to route network traffic between clients and local content servers hosted by the cable provider, and to the Internet.
Downstream traffic is typically modulated using 64 or 256 Quadrature Amplitude Modulation (QAM) and can achieve speeds of 27 Mbps or higher. Upstream traffic is encoded using 16 QAM or Quadrature Phase Shift Keying (QPSK) and typically ranges from 320 kilobits per second (Kbps) to several Mbps.
Marketplace
Excite@Home was an early player in the cable modem marketplace, and many local cable companies followed offering high-speed Internet access to their cable TV subscribers. Consolidation in the industry has led to the emergence of several major players, including AT&T Broadband and AOL/Time Warner.
Cable modem. A one-way cable modem service.
Cable modem Internet access costs around $40 a month in most locations but, because their operation depends on the wiring infrastructure of cable TV systems, cable modems are almost exclusively offered for residential customers and home-based businesses. Some cable modem providers such as Cox Communications also compete with telcos by offering voice as well as data services.
The cable modem market is growing more rapidly than the Digital Subscriber Line (DSL) market, its main competitor. This is happening despite the security and bandwidth issues associated with cable modems (see below), and is driven largely by the complexity of DSL compared to the simplicity of cable modems and by the fact that cable modems are not limited by the 3-mile (4.8-kilometer) distance restriction that governs how far a DSL customer can be from the DSL provider's central office (CO). A Federal Communications Commission (FCC) survey found that cable modem subscribers in the United States grew from just over 100,000 in 1998 to almost 1 million in 1999. In 2000 worldwide cable modem usage was estimated at over 6 million subscribers, almost twice that of DSL.
Issues
Most cable television companies have jumped on the cable modem bandwagon to offer Internet access for their customers, but this has not been without problems. Existing tree-and-branch coaxial systems were built for one-way transmission and often require expensive upgrades to make them suitable for two-way data transmission. Some of the steps typically involved in the upgrade are upgrading core distribution networks to fiber (creating a hybrid fiber-coax network) and troubleshooting ingress noise due to poor shielding and loose connectors.
Many cable operators, although they support multimegabit upstream data speeds, have instead restricted upstream speeds to 128 Kbps or lower to help prevent customers from running rogue Web servers on their network in violation of their customer agreements. Also, all subscribers in a one-way cable modem local service area are essentially on a local area network (LAN) and, if they have a packet sniffer, they can see one another. If you are using a one-way cable modem with Microsoft Windows on your computer, you should disable file and print sharing so that other users in your local service area cannot see your system or access resources on it. The cable modem industry itself has proposed a solution to the security issue through an initiative called Data Over Cable Service Interface Specification (DOCSIS) that specifies encryption of all cable modem traffic using 56-bit Data Encryption Standard (DES), although DES is no longer secure.
Furthermore, although in theory a cable modem might support downstream transmission speeds of 10 Mbps or higher, in practice downstreams may be significantly less than 1 Mbps. This is because all cable modems serviced by a given neighborhood Cable Modem Termination System (CMTS) are essentially on a LAN and share the available bandwidth. Thus, the more modems deployed in a given neighborhood, the slower the system performs for Internet access, especially when some users are downloading large files or streaming media (a common occurrence among cable modem users). In this respect DSL has an advantage because it is a secure, private connection directly to the Internet.
Notes
If you have a one-way cable modem installed on a computer running Microsoft Windows 2000, Windows XP, or Windows .NET Server and it is not working properly, you might have IP Auto-Configuration Addressing enabled, causing an addressing problem that prevents packets from being routed successfully to your machine. Also, try checking with your cable service provider to determine whether you have correctly configured the line-in frequency, line-out phone number, and proxy server address.
For More Information
Visit Cablemodems.com at www.cablemodems.com
See Also Asymmetric Digital Subscriber Line (ADSL) ,broadband Internet access ,Data Over Cable Interface Specification (DOCSIS) ,modem
A length of installed cable connecting two network components that are not in immediate proximity to one another.
Overview
Laying cable runs is the main work of installing premise cabling in a customer premises. Types of cable runs include
Horizontal cable: Runs through building plenums (the space between the floor and the ceiling) and false ceilings, connecting wiring closets together and connecting patch panels to wall plates
Vertical cable: Runs through vertical building rises, connecting wiring closets on each floor with the building's main equipment room
Different grades of cabling must be used for different runs to ensure compliance with building codes and safety standards. Examples include polyvinyl chloride (PVC) cabling and plenum cabling. The Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) wiring standards specify guidelines for using cable types and grades.
Notes
Cables connecting computers to patch panels (drop cables) and connecting patch panels with hubs and switches (patch cables) are not generally referred to as cable runs because they are not permanently installed and are usually quite short. The term cable run generally applies to cables that run from the patch panels in a wiring closet to other parts of the building.
When installing horizontal or vertical cable runs, use the highest grade that your budget will allow in order to accommodate future upgrades of your network's speed and bandwidth. Use the enhanced Category 5 (Cat5e) cabling-which is a variety of unshielded twisted-pair (UTP) cabling-for all copper cabling installations. If you can, install parallel vertical runs of fiber-optic cabling with copper cabling in vertical rises to allow for future expansion of your network backbone. Installing two cables at once saves costs later, even if you need only the copper cabling now.
See Also cabling ,premise cabling
Any device for measuring the integrity and transmission characteristics of cabling.
Overview
Cable testers perform various functions to test network cabling for compliance with cabling standards developed by bodies such as the Telecommunications Industry Association (TIA), the International Organization for Standardization (ISO), and the International Electrotechnical Commission (IEC). Cable testers are useful to local area network (LAN) administrators, cable installers, and field service providers for testing and certifying cabling installations as compliant with these standards.
Cable tester. A handheld cable tester.
Cable testers come in a variety of forms, ranging from handheld to briefcase size. They are generally divided between those used for testing fiber-optic cabling and those used for testing copper cabling. Different testers have different capabilities, but their general function is to measure various electrical characteristics across different ranges of frequencies. Testers will typically measure some or all of the following parameters at various frequencies from 100 megahertz (MHz) to 350 MHz and beyond:
Attenuation, which is the decibel decrease in signal strength as a signal propagates through a physical medium
Impedance, which is the resistance to the flow of alternating current
Noise, indicated by the floor values for randomly generated electrical signals
Near-end crosstalk (NEXT), which is a decibel measurement of crosstalk taken at the end where a signal is injected
Attenuation to crosstalk ratio (ACR), which is the decibel difference between NEXT and attenuation values
PowerSum NEXT, which measures the crosstalk between a single pair of wires and all other pairs in the cable
The distance to a short or unterminated cable end, used for link-testing the continuity of circuits
Uses
Cable testers are particularly important for testing Category 5 (Cat5) and enhanced Category 5 (Cat5e) structured wiring deployments to see if they will properly support Gigabit Ethernet (GbE) networking. GbE pushes unshielded twisted-pair (UTP) cabling to its limits and requires top-quality properly installed cabling to operate as expected. Particularly in older buildings with existing Cat5 wiring, use a cable tester to measure such parameters as cross-talk and NEXT to determine if packet loss will occur over portions of the network. Particularly vulnerable portions include cable termination at RJ-45 wall jacks and at patch panels inside wiring closets. Poor quality patch cables between workstations and wall jacks are a common problem, as are patch panel connections that have been untwisted beyond the specified limits.
Marketplace
A number of vendors produce different kinds of test equipment for cable testing. Two of the top vendors of such equipment include Fluke Corporation, which offers a wide range of cable testers, and Hewlett- Packard.
Today's cable testers can perform comprehensive and programmable sets of autotests for a variety of cable types. A good cable tester can tell you at the push of a button whether installed wiring can support different kinds of networking architectures-such as coax, 10BaseT, 100BaseT, 100BaseVG, and Token Ring. Cable testers can also store measurements taken so that they can be analyzed separately afterward.
The most accurate types of cable testers for UTP cabling are those that can test and certify Category 5 (Cat5) cabling to Level II TSB-67 compliance and support the Institute of Electrical and Electronics Engineers (IEEE) TSB95 field testing specifications. An all-in-one cable tester is an invaluable tool and a good investment for the network administrator. It can make up for its cost in higher network availability.
Notes
Use a fiber-optic tester and an optoelectronic light source to test both ends of a new spool of fiber-optic cabling before beginning an installation with this cable. A good fiber-optic cable test should give you not only a pass/fail analysis of an installed cabling setup, but also quantitative values of the optical link capabilities of your wiring configuration.
See Also cabling ,network troubleshooting ,test equipment
Any wires used for connecting computers and networking devices together to enable them to communicate.
Overview
Cabling constitutes the passive portion of any computer network-the active portion consists of the servers, workstations, switches, routers, and other components. Good cabling provides the foundation for creating reliable local area networks (LANs). Cabling is also used for connecting LANs into wide area networks (WANs). Network administrators are usually involved in the deployment of LAN cabling, but WAN cabling is usually the responsibility of the telecommunications carriers whose services are being leased.
Two basic types of cabling are used in LAN networking environments:
Copper cabling: This type consists of insulated copper conductors that transmit signals using electrical voltages and currents. Copper cabling can be either coaxial cabling (such as thinnet or thicknet) that is used mainly in industrial environments, or the more commonly employed twisted-pair cabling. Twisted-pair cabling comes as either unshielded twisted-pair (UTP) cabling (commonly used in Ethernet or Fast Ethernet environments) or the less common shielded twisted-pair (STP) cabling (employed for token ring networks and sometimes for Gigabit Ethernet [GbE] installations). Copper cabling is mainly used for shorter cable runs such as horizontal cable runs between wiring closets and wall plates in work areas, for patch cables, and for equipment interconnects.
Fiber-optic cabling: This type is made of glass strands that transmit signals as light waves or pulses. Fiber-optic cabling can be either single- mode, which is used for the longest cable runs, or multimode, which has a much higher carrying capacity. Fiber-optic cabling is generally used for backbone cable runs such as vertical rises in buildings and building-to-building interconnects on a campus, for high-speed interconnects between networking devices in a wiring closet, and for connections to high-speed servers and workstations.
Implementation
The process of installing cabling in a building for purposes of computer networking is called establishing premise wiring. Unfortunately, cabling is only as good as the way it is deployed and only as good as the connectors and other elements that help make a computer network. Poor quality cabling, improperly installed cabling, or cheap wall plates and patch panels can make a cabling system perform more poorly than expected, particularly at GbE speeds.
The Telecommunications Industry Association (TIA) and the Electronic Industries Alliance (EIA) have defined a series of standards on the required electrical characteristics of commercial cabling for computer networking and also standards for the proper layout and organization of premise cabling. These standards are called the EIA/TIA wiring standards. The EIA/TIA-568A Commercial Building Telecommunication Cabling Standard specifies standards for each of the following:
Establishing wiring closets on each floor of the building to contain rack-mounted equipment such as hubs, switches, and patch panels
Running vertical backbone plenum cabling through building risers and building plenums, for connecting wiring closets to the main equipment room
Running horizontal polyvinyl chloride (PVC) cabling for each floor through false ceilings
Connecting the patch panels in the wiring closet to wall plates in computer work areas
Cabling installed in a building must also meet all legal requirements, including federal and local building regulations for fire safety. Do not attempt to wire a building unless you are fully familiar with the regulations.
Choosing the right kind of cabling at the beginning of an installation can save considerable expense when you later upgrade networking equipment for higher transmission speeds. Some tips for successfully outsourcing cabling installations for computer networks in buildings include
Hiring a qualified cabling consultant to review the scope and details of your plans and procedures. Hire cabling consultants who either have vendor certification (if you are using "channel cabling," that is, all the cabling components are being purchased from a single vendor) or an independent consultant affiliated with BICSI.
Ensure that if you outsource your cabling installation that the company you hire is properly licensed and insured for such work.
If you plan to lay cable yourself for your building, here are a few more guidelines:
Use no more than 30 to 40 pounds of pull when pulling cabling through a conduit or a plenum.
Do not excessively bend wiring (especially fiber- optic cabling), and be sure not to untwist UTP wires more than specifications allow.
Make sure data wiring and power cables are at least 6 inches (15 centimeters) apart, and, if they need to cross each other, they do this at a 90 degree angle.
Notes
Specialized cables-such as serial, parallel, or Small Computer System Interface (SCSI) cables-are used to connect peripherals and therefore do not serve the same purpose as the cables just discussed. Serial cables and other special purpose cables are generally very short and are not permanently installed.
Note that not all networks use physical cabling. Wireless networks can use infrared, microwave, radio, or some other form of electromagnetic radiation to allow networking components to communicate with each other. A cabled network is sometimes referred to as a wireline network, as opposed to a wireless network.
For More Information
Look at www.cablingstandards.com. You can also visit the BICSI Web site at www.bicsi.org.
See Also copper cabling ,fiber-optic cabling ,premise cabling
Also called a root certification, a digital certificate that can be used to verify the identity of the certificate authority (CA) itself.
Overview
The CA certificate contains the identification information and public key for the certificate authority it identifies. A certificate authority that is part of a hierarchical public key infrastructure (PKI) receives its CA certificate from the CA directly above it in the hierarchy. A root CA at the top of a PKI hierarchy must self-sign its own certificate, in effect certifying itself.
The CA certificate plays an important part in the workings of the Secure Sockets Layer (SSL) protocol. The CA's public key, contained in the CA certificate, is used to validate all other digital certificates that have been issued by that CA for entities (individuals, systems, companies, and organizations). When an entity such as a Web browser (perhaps Microsoft Internet Explorer) or a Web server (perhaps Microsoft Internet Information Services) requests a digital certificate from a CA, the CA certificate identifies the CA that issues the certificate.
This CA certificate is downloaded from a shared storage location at the certificate authority and installed onto the Web server or browser. Later, when the Web browser tries to access the Web server using the SSL protocol, the Web browser uses the CA certificate to validate the Web server's certificate. Similarly, the server can use the CA certificate to validate the browser client's certificate, if it has one.
Notes
The digital CA certificate for a CA must be kept in a location that is readily available for all servers and clients that will access it and install it on their Web browser or Web server. From this location, Web servers and Web clients that need to use the SSL protocol must obtain and install the CA certificate in their certificate stores. On Microsoft Certificate Server, this location is the default Web location http://Server_Name/certsrv, where Server Name is the name of the Microsoft Windows NT server on which Microsoft Certificate Server is installed.
Internet Explorer comes with the CA certificates of a number of certificate authorities preinstalled. These root certificates enable Internet Explorer to be used for functions such as SSL authentication and sending secure e-mail. If you want to use the services of a CA that does not have its CA root certificate installed in Internet Explorer, you can visit that CA's Web site to find instructions on how to obtain its root certificate. Administrators can also use the Internet Explorer Administration Kit (IEAK) for importing and installing root certificates on Web browsers prior to installation on client machines.
See Also certificate authority (CA) ,client certificate digital certificate
Generally, any mechanism for storing frequently needed information in accessible memory so that it can be quickly retrieved. This article focuses on caching as the temporary storage of Web content to enable faster access by users.
Overview
While caching in general has been around a long time and has been implemented in various ways in computer systems and networks (see Notes at the end of this article), caching of Web content has exploded over the last few years into a big industry. In the context of the Internet, caching means the copying of Web content to storage locations near the client so that the client's Web browser can more quickly access the information. Caching speeds up accessing Web content and makes better use of available bandwidth (especially important when the Internet is accessed over slow and costly wide area network [WAN] connections).
While the emergence of broadband Internet access may seem to sidestep the need for caching, this is not really so. Most analysts agree that with the Internet, providing faster access simply drives up demand further and raises expectations, leading to even greater traffic congestion and frustration for users. The solution to bogged-down Internet access is caching.
Implementation
Caching can be implemented in a variety of ways using different systems. The simplest arrangement is deploying cache servers on the network. A cache server is used to speed corporate access to Web content on the Internet by caching the Web pages that users most frequently request. Cache servers reduce network traffic and speed up access to frequently requested content by caching such content. If a user requests a page that has recently been cached, the page will be retrieved from the cache server instead of from the Internet. Pages are held in the cache until they expire.
The cache server locates the content closer to the users who need it than the Web servers that contain the original version of this content. In general, the closer the cache to the user, the faster the response time the user will experience when trying to access this content in a browser. In the enterprise, cache servers are typically deployed at the edge of the corporate network, thus reducing overall WAN link traffic and congestion and saving money. Many firewalls and proxy servers include some form of content caching. Although firewalls and proxy servers are mainly concerned with securing access between a private corporate network and an distrusted public network such as the Internet, including cache server functionality in these products enhances their overall performance.
Caching. How a simple cache server can improve access time for clients.
Caching servers are also useful in situations where a sudden increase in Web traffic is anticipated, as when a major sports or fashion event will be covered on the Web. In this case, simply throwing bandwidth at the problem of Internet traffic congestions is not a viable solution because increased bandwidth simply leads to increased demand from users. Instead, by preloading content from Web servers to caching servers, users will not overwhelm the Web servers on the day of the event and be disappointed. This use for caching servers is sometimes called "dealing with flash crowd control" or the "instant popularity problem." By using a network of cache servers in such situations and configuring replication appropriately between them, supply can be scaled for demand at virtually any level.
Dedicated cache servers are also used in high-traffic situations within the Internet backbone itself to reduce congestion on the backbone. Caching servers are often located at Internet service providers (ISPs) and Network Access Points (NAPs) for improving the overall performance and responsiveness of the Internet. This can be used to reduce the effect of bottlenecks on ISP networks and to reduce the cost of local ISPs buying bandwidth from regional ISPs.
Cache servers can even be deployed at the customer premises and configured with preloaded content to speed access to corporate intranets. Such a scenario is similar to the built-in content caching capability of Web browsers, but in this case the content is cached locally on a machine separate from the client machine to improve performance even more.
Types
As far as administrators are concerned, caching servers come in two basic types:
Caching appliances: These are preconfigured servers with preloaded caching software that can easily be dropped into the network by simply connecting them to a backbone switch and turning them on.
Caching software: This includes software from different vendors that can be loaded onto servers and deployed as needed within the network by the administrator. Caching software is especially targeted toward the needs of the corporate enterprise for improving remote access to corporate intranets.
In addition, from the user's perspective, cache servers can be implemented in different ways:
Transparent caching: Here cache servers filter Internet Protocol (IP) traffic to find out which Web pages are most requested in order to determine which pages should be cached and for how long. Transparent cache servers require no special modification or configuration of the client in order to operate, but the cache server itself must be placed at the edge of the WAN, typically by connecting it to a Layer 4 switch or WAN access router so that every request sent by a client is examined by the cache server first before routing it over the Internet to its intended destination.
Proxy caching: Here the administrator configures the client browsers to send their requests for Web content directly to the cache servers., If these cannot respond to the request (for example, if the page has not been cached or has expired from the cache), the cache server refers the client to the actual Web server hosting the content. This option requires more configuration but gives administrators greater control over what content users can access and provides an extra layer of security.
Reverse proxy caching: Here a group of cache servers typically serves as a front-end to the Web server on the Internet, which hides behind them. User requests go directly to the cache servers, which appear as Web servers to the client. Reverse proxy caching is typically used to deal with flash crowd control discussed earlier.
Cache servers also determine whether to flush content from their cache in different ways:
Active caching: Here the cache server uses intelligent heuristics to determine how long to keep a page in the cache before marking it expired. Many factors can be taken into account, including the requesting IP address, the hit statistics for the page, and how long the page has been cached.
Passive caching: When a client requests a page from the cache, the cache server issues an HTTP GET IF MODIFIED command to the originating Web server to determine if the cache contains the latest version of the page. If not, the cache requests the current version and then forwards this to the client. If the cache does hold the current version, this is returned immediately to the client. The main disadvantage of passive caching is that it introduces the extra step of having the cache server contact the Web server each time a client requests a page, which, unless the cache server is close (in a network sense) to the Web server, will have the opposite effect from what caching was intended to do-it will slow down the response time instead of speeding it up.
Forced caching: In this case the cache server is preloaded with content that users are expected to frequently access. Forced caching is typically used for improving access to corporate intranets for remote users.
Marketplace
The caching industry has exploded over the last few years, driven by the tremendous growth of the Internet and the demand of users for fast performance in accessing Web sites. In the arena of general caching software, the top players include Microsoft Corporation's Internet Acceleration and Security Server (IAS Server), the successor to Microsoft Proxy Server; Novell's Internet Caching System; and several UNIX-based solutions.
In the arena of prepackaged caching appliances, offerings are available from Dell Computer Corporation's (PowerApp.cache), Compaq (TaskSmart), Cobalt Networks (Qube), Network Appliance (NetCache), and many others. Caching appliances range from those targeted for corporate intranets to powerful caching solutions for ISPs and telecoms.
A number of pure-play caching vendors have had a large impact on the caching market. Some of the big players here include Inktomi Corporation, CacheFlow, and Network Appliance. Inktomi offers routers and load balancers from Alteon WebSystems and Foundry Networks bundled together with Inktomi's Traffic Server caching software. Cisco Systems and Lucent Technologies also offer high-end caching appliances with proprietary operating system kernels tuned for top performance in this application.
A relatively new approach is called predictive client- side caching and involves configuring a plug-in for client browsers that determines what kinds of content the client accesses and then attempts to preload such content during idle times so that when the client requests it the content is already in the browser cache. Blueflame, a product from Fireclick, is one example of predictive client-side caching technology. Fireclick also has a hosted version of this application called Netflame.
Packeteer has a product called AppCelera Internet Content Accelerator (ICX) that boosts performance by adapting requested content to the type of browser requesting it and by compressing content to make better utilization of available bandwidth.
Issues
Caching of dynamic Web content presents an ever-greater challenge as more and more Web sites move away from static informational content to deploying database-driven e-commerce Web applications. Dynamic Web pages are generated from databases in response to user queries and often include some form of personalization as well. Dynamic content cannot be cached the way static content can, but a number of vendors have come up with various methods for improving the performance of dynamic Web applications by using caching. For example, the components of a Web application can be cached for reuse across multiple user sessions. Another approach is to dynamically monitor the configuration of Web applications to better manage stale cache content. Vendors of dynamic caching systems include Xcache Technologies, Chutney Technologies, and SpiderSoftware. NetScaler offers a caching device that routes requests for static content to traditional cache servers while multiplexing requests for dynamic content over persistent connections with Web servers to improve performance.
Notes
Caching in a general sense is used in various ways by operating systems, applications, and network devices to improve performance by providing temporary storage of information that needs to be quickly accessed. Some common examples include
The file system cache for the Microsoft Windows 2000 operating system, which speeds up file access from hard disk drives
Offline Files in Windows 2000, which allows users to browse network file system content when disconnected from the network
Domain Name System (DNS) cache for caching recently resolved host names on a name server, which speeds up the resolution of host names for the DNS on the Internet
Address Resolution Protocol (ARP) cache on a host connected to an IP internetwork, which is used for caching IP addresses that have recently been resolved into MAC addresses, thus speeding up network communications between hosts
Microsoft Internet Security and Acceleration (ISA) Server, which allows Web content obtained from the Internet to be cached locally for faster access and reduction of WAN link congestion
Caching of open database connectivity (ODBC) connections for improved access to Microsoft SQL Server databases for Active Server Pages (ASP) applications written for Microsoft Internet Information Services (IIS)
For More Information
Find out more about caching technologies at these sites: www.caching.com and www.web-caching.com
See Also Content Delivery Provider (CDP) ,Web server
A protocol developed by Microsoft Corporation that allows multiple proxy servers to be arrayed as a single logical cache for distributed content caching.
Overview
Caching Array Routing Protocol (CARP) is implemented as a series of algorithms that are applied on top of Hypertext Transfer Protocol (HTTP). CARP allows a Web browser or downstream proxy server to determine exactly where in the proxy array the information for a requested Uniform Resource Locator (URL) is stored.
CARP enables proxy servers to be tracked through an array membership list that is automatically updated using a Time to Live (TTL) countdown function. This function regularly checks for active proxy servers in the array. CARP uses hash functions and combines the hash value of each requested URL with each proxy server. The URL/proxy server hash with the highest value becomes the owner of the information cached. This results in a deterministic location for all cached information in the array, which enables a Web browser or downstream proxy server to know exactly where a requested URL is locally stored or where it will be located after it has been cached. The hash functions result in cached information being statistically distributed (load balanced) across the array. Using hashing means that massive location tables for cached information need not be maintained-the Web browser simply runs the same hashing function on the object to locate where it is cached.
Advantages and Disadvantages
CARP provides two main benefits:
It saves network bandwidth by avoiding the query messaging between proxy servers that occurs with conventional Internet Cache Protocol (ICP) networks.
It eliminates the duplication of content that occurs when proxy servers are grouped in arrays, resulting in faster response times and more efficient use of server resources.
See Also caching ,proxy server
A name server in the Domain Name System (DNS) that can resolve name lookup requests but does not maintain its own local DNS database or zone file of resource records.
Overview
Caching-only name servers do not have their own DNS databases. Instead, they resolve name lookup requests from resolvers by making iterative queries to other name servers. When the responses to these queries are received, the caching-only name server caches them, in case another resolver issues the same request within a short period of time.
A caching-only name server is not authoritative for any particular DNS domain. It can look up names that are inside or outside its own zone.
Notes
Caching-only name servers are not the only kind of name server that performs caching of resolved queries. For example, primary name servers also cache name lookups that they perform. This caching generally improves the primary name server's response to name lookup requests from resolvers. Caching-only name servers are distinguished by the fact that they perform only one function: issuing iterative queries to other name servers and then caching the results.
Caching-only name servers provide support for primary and secondary name servers in environments where name lookup traffic is heavy. Using caching-only name servers where possible also reduces the overhead of zone transfers between name servers on a network.
See Also Domain Name System (DNS) ,name server
A company that maintains caching servers that speed the transfer of information across the Internet's infrastructure and offers managed access to these servers for a fee.
Overview
Many companies think that if they host their Web site at an Internet service provider (ISP) or at a major Web hosting company, the site will always be accessible from anywhere on the Internet, but this is not necessarily true. Traffic congestion can cause access to a Web server to be slow and unreliable from various parts of the Internet at various times of the day, and equipment failures (such as routes going down at Internet peering points) can make a server completely inaccessible to certain portions of the Internet until the problem is fixed. ("Peering" means two ISPs or other providers passing traffic between each other's customers.)
One solution is to mirror (replicate) your Web server at various points around the globe so that a copy of your site is relatively close to any location on the Internet. For companies with a global presence, this is a fairly simple solution to implement, but most companies do not have the presence or the resources to implement global mirroring. A better solution might be to use the services of a caching service provider (CSP), which maintains cached copies of your Web site at various points in the Internet's infrastructure and provides tools for managing and load balancing the content to handle traffic spikes that occur at certain times of the day or year.
CSPs maintain data centers around the world with caching server farms that have high-speed connections to the Internet's backbone. These caching servers are usually designed to cache Web content and often support features such as content management and proxying. Caching servers can also be used within the corporate network to speed access to large, distributed corporate intranets.
See Also caching
Stands for client access license, a license that grants a client machine access to a Microsoft BackOffice product running on a network of computers.
See Also client access license (CAL)
A security feature for remote access servers.
Overview
Callback provides an extra layer of security for users dialing in to a remote access server. When callback is configured, the client software dials in to a remote access server and has the user's credentials authenticated. The remote access server then disconnects the client and calls the client back at a previously specified phone number. Callback might be configured in the following ways:
To ensure that the user matches his or her credentials by verifying the telephone location
For accounting reasons-for example, to charge the phone bill to the remote access server instead of to the client
Callback. How callback is used in remote access.
Callback is an optional feature supported by the Routing and Remote Access service of Microsoft Windows 2000 and Windows .NET Server. Note that in Windows 2000 and Windows .NET Server, the phone number specified for callback is called the Caller ID number.
See Also remote access
A protocol that enables callback for establishing Point- to-Point Protocol (PPP) connections.
Overview
The Callback Control Protocol (CBCP) is part of Microsoft Corporation's implementation of the Point- to-Point Protocol (PPP). CBCP makes it possible for a PPP server to call back the remote dial-in client to complete initiation of a PPP dial-up session. Callback is a useful security feature for ensuring that dial-in clients are authentic.
Implementation
The CBCP is used during the third phase of establishing a PPP connection. After the PPP link is established using the Link Control Protocol (LCP) and the user's credentials are authenticated using Microsoft Challenge Handshake Authentication Protocol (MS-CHAP) or some other authentication protocol supported by the client, the network access server (NAS) at the PPP service provider can optionally initiate a PPP callback control phase, provided that callback is configured on the server. The NAS and the PPP clients both disconnect from the PPP link, and the NAS calls the client back using the specified callback phone number. If the client responds, the link is reestablished; no further authentication is needed, and compatible network protocols are negotiated so that data transmission can begin.
See Also Point-to-Point Protocol (PPP)
A group of connected local area networks (LANs) on a campus.
Overview
A campus area network (CAN) is intermediate in size between a LAN, which typically resides in a single room or building, and a metropolitan area network (MAN), which spans a city or metropolitan area. CANs exist on university campuses, in industrial parks, and in similar collections of buildings under a single administrative authority.
Implementation
CANs are typically built in hierarchical fashion starting from the top or inside with a core network (usually a collapsed backbone), a distribution network (routers or switches in different buildings), an access network (a collection of hubs and switches within wiring closets on each floor of the buildings), and finally end-user stations.
See Also local area network (LAN) ,metropolitan area network (MAN) ,Personal Area Network (PAN) ,wide area network (WAN)
Stands for campus area network, a group of connected local area networks (LANs) on a campus.
See Also campus area network (CAN)
Stands for carrierless amplitude and phase modulation, a line coding scheme in which data is modulated using a single carrier frequency.
See Also carrierless amplitude and phase modulation (CAP)
A collection of frames gathered from network traffic by a sniffer.
Overview
You can use packet-sniffing software such as Microsoft Network Monitor (which is included in Microsoft Systems Management Server and in a simplified form in Microsoft Windows 2000, Windows XP, and Windows .NET Server) for capturing all kinds of traffic on the network. This captured traffic is displayed in the capture window as a variety of statistics and details about the nature of the traffic. Additional windows can display details about individual packets that have been captured.
Capturing network traffic is a common troubleshooting technique on enterprise-level networks. Captures can show whether services such as Dynamic Host Configuration Protocol (DHCP), Windows Internet Name Service (WINS), Domain Name System (DNS), and other common network services are performing properly. Captures can also isolate servers that are generating excessive network traffic because of failed hardware. Captures can even be used to detect unauthorized traffic initiated by hackers and disgruntled employees and to profile network traffic for planning purposes.
See Also capture window ,sniffing
In Microsoft Network Monitor, the window that displays the statistics about the frames being captured on a network.
Overview
The capture window displays four kinds of real-time statistics concerning the traffic that an administrator captures using Network Monitor:
Graph statistics: A graphical representation of current network activity that shows the percent of network utilization, frames captured per second, bytes captured per second, and broadcasts or multicasts captured per second
Session statistics: Information about current sessions between computers on the network, showing which hosts have sent packets to each other
Station statistics: Information about various stations involved in sending or receiving packets, showing how many of each type of packet they have sent or received
Total statistics: Summary statistics about network activity since the capture began, including the number of frames, bytes, and frames dropped during the capture
Capture window. The capture window in Network Monitor, which is included with Microsoft Systems Management Server.
You can toggle these various windowpanes on and off during a capture to focus on statistics of interest.
See Also capture ,sniffing
An Internet surveillance system developed by the FBI (Federal Bureau of Investigation).
Overview
Carnivore was designed to help the FBI collect evidence to convict terrorists, drug smugglers, and hackers. Carnivore is a self-contained "black-box" Internet Protocol (IP) traffic filtering system that sniffs network traffic to find e-mail and other traffic that contains evidence of criminal activity.
Implementation
To use Carnivore, the FBI first obtains a court order to capture specific types of Internet traffic that may contain evidence of criminal wrongdoing by parties under consideration. The FBI then brings the Carnivore system to an Internet service provider (ISP) and connects the system to the backbone switches of the ISP's network. Carnivore then monitors all traffic through the ISP, capturing only such traffic as matches the filter conditions established in the court order (this is called minimization-Carnivore does not snoop all traffic indiscriminately). For example, Carnivore might filter all messages coming from or going to a specific IP address block or having a certain keyword in the Subject line.
See Also sniffing ,security
Stands for Caching Array Routing Protocol, a protocol developed by Microsoft Corporation that allows multiple proxy servers to be arrayed as a single logical cache for distributed content caching.
See Also Caching Array Routing Protocol (CARP)
A company that provides various kinds of telecommunication services to its customers.
Overview
A carrier provisions telecommunication services to the customer premises. These services include voice transmission, data transmission over analog modems, Integrated Services Digital Network (ISDN), digital subscriber line (DSL), frame relay, and anything else the carrier's equipment supports. There are two basic kinds of carriers: LECs and IXCs.
A local exchange carrier (LEC) is a company that provides access to both local and long-distance telephone services through the local loop connecting the telco's central office (CO) with the customer premises. In the United States, LECs are typically one of two kinds:
Local phone companies, now usually called Incumbent Local Exchange Carriers (ILECs) but sometimes called Regional Bell Operating Companies (RBOCs).
Competitive Local Exchange Carriers (CLECs), a new breed of LEC that has arisen in response to the Telecommunications Act of 1996. CLECs that offer primarily DSL voice/data services are often called DSL carriers.
An inter-exchange carrier (IXC), on the other hand, is a company that provides long-distance services only. IXCs own their own telecommunication facilities and provide long-distance services between LECs located in different toll-free areas. The big three in the IXC arena in order of size are AT&T, MCI WorldCom, and Sprint Corporation.
All long-haul carriers in the United States use Synchronous Optical Network (SONET) or ATM (Asynchronous Transfer Mode)/SONET as their backbone transport, and most local telcos use ATM in their own backbone networks also. Some LECs are also deploying Gigabit Ethernet (GbE) as an alternative to ATM for metropolitan area networks (MANs) that can bring GbE ports directly to the edge of corporate networks, providing end-to-end Ethernet connectivity for corporate wide area networks (WANs).
Outside the United States, most carriers use Synchronous Digital Hierarchy (SDH), essentially a variant of SONET.
Implementation
Almost all carriers rely on ATM as their underlying backbone transport and use this backbone to provision local and long-distance voice services, Internet access over DSL, leased-line WAN links, and other services. A large portion of the backbone of most long-haul carriers is currently at or near OC-48, providing maximum throughput of 2.488 gigabits per second (Gbps), but these backbone fiber connections are constantly being upgraded to support higher speeds such as OC-192 and OC-768.
Carriers usually house their high-speed switching equipment in large buildings called colocation centers. These centers have this name because under the Telecommunications Act of 1996, competitive local exchange carriers (CLECs) are allowed to lease space within carrier facilities for locating their own switching equipment. Another name for these centers is "data centers," and they are usually huge nondescript buildings that typically occupy hundreds of thousands of square feet. Sometimes several floors of existing buildings are used for data centers, but the trend is to build new centers to ensure that the highest standards of fire safety and security are used.
Marketplace
The global carrier market has become complex over the last decade, with traditional incumbent telcos in the United States and Post, Telegraph, and Telephone (PTT) companies in Europe being challenged on all sides by newcomers to the game. International carriers can be classified in different types, including
Incumbents: These are traditional or established carriers such as public telephone companies, long distance companies, and PTTs in Europe.
Cross-border infrastructure owners: These are companies that own the international lines carrying voice and data between countries, regions, and continents.
Resellers: This includes companies reselling both local loop and long haul services purchased wholesale from other carriers.
To complicate things further, many large national carriers are constantly making efforts to expand into the international market by acquiring, or merging with, other carriers, upgrading their core equipment to provision new high-speed data services, while relegating some existing services to the legacy domain and trying to cope with the ever-exploding amounts of Internet bandwidth demanded by their customers. Because of the state of flux of the global telecommunications carrier market, the enterprise architect's dream of simple, seamless, global WAN connectivity remains a complicated reality of tariffs, interoperability issues, and politics.
See Also Competitive Local Exchange Carrier (CLEC) ,Incumbent Local Exchange Carrier (ILEC) ,inter-exchange carrier (IXC) ,local exchange carrier (LEC) ,telecommunications services
A line coding scheme in which data is modulated using a single carrier frequency.
Overview
Carrierless amplitude and phase modulation (CAP) is used for transmission of voice information over a phone line. The transmission is considered "carrierless" because the carrier is suppressed before transmission and is reconstructed at the receiver. CAP is algorithmically similar to the quadrature amplitude modulation (QAM) line coding scheme, which encodes bits as discrete phase and amplitude changes, but it has different transmission characteristics.
Notes
Some competitive local exchange carriers (CLECs) deploy Asymmetric Digital Subscriber Line (ADSL) using CAP as the encoding method, but results of some independent tests suggest that CAP-encoded ADSL lines might cause spectral interference with proximate T1 lines and Integrated Services Digital Network (ISDN) circuits, resulting in bit errors that can reduce throughput. However, these tests might be misleading because of the limited number of ADSL circuits currently deployed by CLECs. Check with your carrier before signing up for ADSL services to get the latest information about this issue.
Such interference is not a problem with symmetric digital subscriber line (SDSL) technologies, which use the 2B1Q encoding scheme. Furthermore, ADSL deployed by incumbent local exchange carriers (ILECs) uses discrete multitone (DMT) technology, which does not produce the same degree of spectral interference as CAP. Competitive local exchange carriers generally do not use DMT for ADSL because they must deal with the copper local loop, which effectively supports DMT only about half the time but can support CAP about 85 percent of the time. Furthermore, DMT has been adopted as the standard for ADSL by both the American National Standards Institute (ANSI) and the International Telecommunication Union (ITU).
See Also line coding
The media access control method used by AppleTalk.
Overview
Carrier Sense Multiple Access with Collision Avoidance (CDMA/CA) is a type of media access control method for placing signals on baseband transmission networks. Because baseband networks can carry only one data signal at a time, there must be some way of controlling which station has access to the media at any given time. CSMA/CA is one such control method.
Uses
CSMA/CA is the standard access method for AppleTalk networks based on LocalTalk. LocalTalk is a legacy network media technology that specifies the proprietary cabling components of the original AppleTalk networking architecture. LocalTalk uses a bus topology or tree topology that supports up to 32 stations on a network.
Implementation
In networking technologies that use CSMA/CA as their access method, stations announce their intention to transmit before they actually transmit their data onto the network media. Each station "listens" constantly to the wire for these announcements, and if it hears one, it avoids transmitting its own data. In other words, on a CSMA/CA network, stations try to avoid collisions with signals generated from other stations. The extra signaling generated by CSMA/CA makes it a slower access method than the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method used in Ethernet networking.
See Also AppleTalk, Carrier Service Multiple Access with Collision Detection (CSMA/CD), Ethernet, media access control method
The media access control method used by Ethernet.
Overview
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a type of media access control method developed by Xerox Corporation in the 1970s for placing signals on baseband transmission networks. Because baseband networks can carry only one data signal at a time, there must be some way of controlling which station has access to the media at any given time. CSMA/CD is one such control method.
Implementation
In networking technologies that use CSMA/CD as their access method, a station first "listens" to the network media to make sure there is no signal already present from another station before it tries to place its own signal on the media. If a carrier signal is detected on the media, which indicates that a station is currently transmitting a signal, no other station can initiate a transmission until the carrier stops. If no carrier is detected, any station can transmit a signal.
If two stations listen to the wire and detect no carrier signal, they may both decide to send signals simultaneously. If this happens, a collision occurs between the two signals generated. Next, both stations detect the collision and stop transmitting their signals immediately, sending out a jamming signal that informs all other stations on the network that a collision has occurred and that they should not transmit. Meanwhile, the two stations whose signals created the collision cease transmitting and wait random intervals of time (usually a few milliseconds) before attempting to retransmit.
Issues
CSMA/CD is known as a contention method because computers contend for the chance to transmit data onto the network media. CSMA/CD is the standard access method for Ethernet networks. This method has two main drawbacks:
Only a relatively small number of computers can exist within any one collision domain. More computers will produce more collisions and slow overall network traffic.
CSMA/CD is not reliable beyond a distance of 1.5 miles (2.5 kilometers) because of signal attenuation.
Notes
The designation CSMA/CD derives from the following:
CS means that stations first s ense a c arrier present on the media before transmitting their own signals.
MA means that m ultiple stations can a ccess the network media.
CD means that if a c ollision is d etected because of multiple simultaneous transmission of signals, the stations that are transmitting signals stop, and then retransmit a short time later.
See Also Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) ,Ethernet ,media access control method
An alternating electromagnetic signal with a steady frequency upon which information is superimposed by some form of modulation.
Overview
The specific frequency at which a carrier signal runs is called the carrier frequency and is measured in hertz (Hz). The modulation of the carrier signal enables information such as voice or data traffic to be integrated into the carrier signal. The carrier signal thus "carries" the voice or data information using modulation technologies.
The type of modulation used in digital communication systems depends on whether the underlying carrier signal is analog or digital. For example, in digital radio or microwave communication, some form of digital-analog modulation, such as frequency-shift keying (FSK), is used to impose the digital (binary) information on the analog carrier wave. On the other hand, in Ethernet networking, a digital-digital encoding scheme called Manchester coding is used to enable the digital signal to carry binary 1s and 0s.
In Ethernet networks, the carrier signal plays an important role in the media access control method that Ethernet uses, namely the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method.
See Also Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
A layered form of star topology.
See Also star topology
A method for giving Web developers more control over how the pages of a Web site will look when displayed on a Web browser.
Overview
Cascading style sheets (CSS) is a standard from the World Wide Web Consortium (W3C) that gives Web developers control over design elements such as fonts and font sizes and allows two-dimensional overlapping and exact positioning of page elements. The CSS standard also makes it easier to globally change the style and appearance of a Web site without having to change elements on every page.
Implementation
Hypertext Markup Language (HTML) was designed for logical communication of linked information without much regard for its style or format, and it was not designed to provide a high degree of control over how that information is laid out on a page. Using CSS, a Web developer can control the appearance of an entire Web site, or a portion of it, using a single HTML page called a style sheet. The genius of CSS is that it separates the content of the page (formatted in HTML) from the page layout (defined in the style sheet using CSS).
Style sheets define the functions of different HTML tags on your site's Web pages and allow you to make global changes to your site's style by changing a single entry on a style sheet. Web pages then link to style sheets using a <LINK> tag.
For example, you can use a style sheet to define the <H1> tag as representing green, 18-point, Arial font text, and you can then apply this style to the entire site or a portion of it. Cascading style sheets involve the operation of several levels of style sheets that provide control over how an element on an HTML document is placed. CSS applies these settings in the following order:
The STYLE attribute in the object's tag
The STYLE element between the <TITLE> and <BODY> tags that specifies the style sheet to be used
The settings of the browser accessing the page and its default style sheet
Issues
The main factor that has prevented CSS from being widely implemented on most Web sites is that neither Microsoft Internet Explorer nor Netscape Navigator fully supports all aspects of the CSS standard. Thus, a CSS-enabled site may look one way in Internet Explorer and another way in Netscape, and neither may be what the site designer intended. One Web browser that fully implements the CSS standard is Opera, developed by Hakon Lie, who has been involved in the W3C.
For More Information
You can find the W3C standard for CSS version 2 at www.w3c.org/Style.
See Also Hypertext Markup Language (HTML)
Stands for Category 5 cabling, the most common grade of unshielded twisted-pair (UTP) cabling used for structured wiring in commercial buildings.
See Also Category 5 (Cat5) cabling
Stands for enhanced Category 5, a form of Category 5 (Cat5) cabling that supports higher speeds.
See Also enhanced Category 5 (Cat5e) cabling
Stands for Category 6 cabling, the proposed next step up from enhanced Category 5 (Cat5e) cabling.
See Also Category 6 (Cat6) cabling
Stands for Category 7 cabling, a still-hypothetical next step up from the proposed Category 6 (Cat6) cabling.
See Also Category 7 (Cat7) cabling
The top-level organizational structure for the Indexing service in Microsoft Windows 2000, Windows XP, and Windows .NET Server.
Overview
The catalog contains the master index and other persistent indexes. The Index Server catalog is located by default in the directory %systemdrive%\inetpub\catalog.wci, but this can be overridden during installation. The maximum size of the catalog for Index Server is 40 percent of the size of the documents being indexed (the corpus), so it is important when installing Index Server to locate the catalog on a drive with sufficient free space. For example, if you will be indexing 10 gigabytes (GB) of documents, you will need about 4 GB of space for locating your catalog.
Notes
An Indexing service query can span only one catalog at a time, so do not create multiple catalogs unless you want to completely separate the indexing of their documents-for example, if you are hosting Web sites for multiple companies on your server.
If you are hosting more than one virtual server on a Microsoft Internet Information Services (IIS) machine, and these virtual servers represent different companies, you might want to create multiple catalogs for the Indexing service, one for each virtual server. Each catalog will then be used for indexing a specific virtual server, and queries based on a catalog will return only results for content on the associated virtual server.
The lowest grade of unshielded twisted-pair (UTP) cabling.
Overview
Category 1 (Cat1) cabling was designed to support analog voice communication only. Cat1 cabling was used prior to 1983 for wiring installations of analog telephone systems, otherwise known as the Plain Old Telephone Service (POTS). The electrical characteristics of Cat1 cabling make it unsuitable for computer networking purposes, and it is never installed as premise wiring for structured cabling installations. Instead, all premise wiring should use either Category 3 (Cat3) cabling, Category 4 (Cat4) cabling, or Category 5 (Cat5) cabling, with Cat5 or enhanced Category 5 (Cat5e) cabling preferred for all new installations.
See Also cabling ,premise cabling ,structured wiring
The second-lowest grade of unshielded twisted-pair (UTP) cabling.
Overview
Category 2 (Cat2) cabling was designed to support digital voice and data communication. Cat2 cabling was capable of data transmissions up to 4 megabits per second (Mbps). It was used primarily in the installation of premise wiring for legacy Token Ring networks from IBM. The electrical characteristics of Cat2 cabling make it unsuitable for most networking purposes today, thus it is no longer installed as premise wiring. Instead, all premise wiring today should use only Category 3 (Cat3) cabling, Category 4 (Cat4) cabling, or Category 5 (Cat5) cabling, with Cat5 or enhanced Category 5 (Cat5e) cabling preferred for all new installations.
See Also cabling ,premise cabling ,structured wiring
The third-lowest grade of unshielded twisted-pair (UTP) cabling.
Overview
Category 3 (Cat3) cabling was designed to support digital voice and data communication at speeds up to 10 megabits per second (Mbps). It uses 24-gauge copper wires in a configuration of four twisted-pairs enclosed in a protective insulating sheath. Cat3 cabling is the lowest grade of UTP cabling that can support standard 10BaseT types of Ethernet networks and was often used for legacy 4-Mbps Token Ring installations.
Cat3 cabling still has a significant installed base in older buildings, and for basic 10BaseT Ethernet purposes, it is often cheaper to use existing Cat3 cabling than to upgrade to Category 5 (Cat5). Installing higher-grade cabling for backbone cabling in vertical rises and elevator shafts can extend the life of work areas that still use Cat3 cabling. However, if greater speeds are required at users' workstations, the best solution is to rewire the work areas using Cat5 cabling or enhanced Category 5 (Cat5e) cabling.
The following table summarizes the electrical characteristics of Cat3 cabling at different frequencies, which correspond to different data transmission speeds. Note that attenuation increases with frequency, while near-end crosstalk (NEXT) decreases.
Characteristic | Value at 10 MHz | Value at 16 MHz |
Attenuation | 27 decibels (dB)/1000 feet | 36 dB/1000 feet |
NEXT | 26 dB/1000 feet | 23 dB/1000 feet |
Resistance | 28.6 ohms/1000 feet | 28.6 ohms/1000 feet |
Impedance | 100 ohms ( 15%) | 100 ohms ( 15%) |
Capacitance | 18 pF/feet | 18 pF/feet |
See Also cabling ,Category 5 (Cat5) cabling enhanced Category 5 (Cat5e) cabling, premise cabling, structured wiring
The grade of unshielded twisted-pair (UTP) cabling just below standard Category 5 (Cat5) cabling.
Overview
Category 4 (Cat4) cabling was designed to support digital voice and data communication at speeds up to 16 megabits per second (Mbps). It uses 22-gauge or 24-gauge copper wires in a configuration of four twisted-pairs enclosed in a protective insulating sheath. Cat4 cabling can support standard 10BaseT types of Ethernet networks. It was also commonly used in older 16-Mbps Token Ring installations.
Cat4 cabling still has some installed base in older buildings where it is often cheaper to use the existing cabling than to upgrade to newer grades. Installing higher-grade cabling for backbone cabling in vertical rises and elevator shafts can extend the life of work areas that still use Cat4 cabling. However, if greater speeds are required at users' workstations, the best solution is to rewire the work areas using Cat5 cabling or enhanced Category 5 (Cat5e) cabling.
The following table summarizes the electrical characteristics of Cat4 cabling at different frequencies, which correspond to different data transmission speeds. Note that attenuation increases with frequency, while near-end crosstalk (NEXT) decreases.
Characteristic | Value at 10 MHz | Value at 20 MHz |
Attenuation | 20 decibels (dB)/1000 feet | 31 dB/1000 feet |
NEXT | 41 dB/1000 feet | 36 dB/1000 feet |
Resistance | 28.6 ohms/1000 feet | 28.6 ohms/1000 feet |
Impedance | 100 ohms ( 15%) | 100 ohms ( 15%) |
Capacitance | 18 pF/feet | 18 pF/feet |
See Also cabling ,Category 5 (Cat5) cabling enhanced Category 5 (Cat5e) cabling, premise cabling, structured wiring
The most common grade of unshielded twisted-pair (UTP) cabling used for structured wiring in commercial buildings.
Overview
Category 5 (Cat5) cabling was designed to support digital voice and data communication at speeds up to 100 megabits per second (Mbps). It uses 22-gauge or 24-gauge copper wires in a configuration of four twisted-pairs enclosed in a protective insulating sheath. It is still the highest official grade of UTP cabling currently recognized by the Electronic Industries Alliance (EIA) and Telecommunications Industry Association (TIA), although proposals have been made for higher Category 6 (Cat6) and Category 7 (Cat7) grades.
Cat5 cabling is the standard grade of UTP cabling for common networking architectures including 10BaseT Ethernet, Fast Ethernet, and 1000BaseT Gigabit Ethernet (GbE) over copper. Cat5 cabling typically makes up over 80 percent of the wiring in today's corporate network. Its continuing popularity in high-speed networks is because fiber-optic cabling is still about twice as expensive to deploy as Cat5 cabling. Because of its superior electrical characteristics, Cat5 cabling is recommended for all new structured wiring installations as well as for upgrading existing premise wiring to support higher-speed networks.
The following table summarizes the electrical characteristics of Cat5 cabling at different frequencies, which correspond to different data transmission speeds. Note that attenuation increases with frequency, while near-end crosstalk (NEXT) decreases.
Characteristic | Value at 10 MHz | Value at 100 MHz |
Attenuation | 20 decibels (dB)/1000 feet | 22 dB/1000 feet |
NEXT | 47 dB/1000 feet | 32.3 dB/1000 feet |
Resistance | 28.6 ohms/1000 feet | 28.6 ohms/1000 feet |
Impedance | 100 ohms ( 15%) | 100 ohms ( 15%) |
Capacitance | 18 pF/feet | 18 pF/feet |
Structural return loss | 16 dB | 16 dB |
Delay skew | 45 nanoseconds (ns)/100 meters | 45 ns/100 meters |
Marketplace
There are a vast number of different brands of Cat5 cabling on the market today. One estimate is that there are over 150 different vendors of such cabling. Not all Cat5 cabling is alike, and before undertaking a costly large-scale deployment of structured wiring, the enterprise architect should carefully investigate and compare the different brands.
Notes
For typical installations of Ethernet and Fast Ethernet, Cat5 cables in work areas should be no more than 300 feet (90 meters) long, and Cat5 patch cords should be no longer than 33 feet (10 meters). Check the Ethernet specifications for exact lengths permitted.
Many vendors offer an enhanced Cat5 (Cat5e) cabling grade with electrical characteristics exceeding those of standard Cat5. Cat5e cabling typically supports data transmission up to frequencies of 350 MHz, and new standards are under development to allow even higher data transmission frequencies. Cat5e networking is recommended for Gigabit Ethernet (GbE) over copper, but properly installed Cat5 cabling should work in most GbE setups. For best performance of GbE networks, however, use Cat5e or Cat 6 cabling whenever possible.
There is widespread agreement that Cat5 cabling will not be able to support networks beyond GbE, such as the 10 GbE architecture currently under development. Such ultra-high speed networks will likely work only with fiber and will not support copper.
See Also cabling ,enhanced Category 5 (Cat5e) cabling ,premise cabling ,structured wiring
The proposed next step up from enhanced Category 5 cabling (Cat5e).
Overview
Category 6 (Cat6) is a proposed Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) cabling standard that represents the next step up from Cat5 and Cat5e cabling. The following table shows the current draft standard for Cat6 cabling characteristics.
Characteristic | Value at 250 MHz |
Attenuation | 19.9 decibels (dB)/1000 feet |
NEXT | 44.3 dB/1000 feet |
Impedance | 100 ohms ( 15%) |
Return loss | 20.1 dB |
Delay skew | 45 nanoseconds (ns)/100 meters |
PS-NEXT | 42.3 dB |
Implementation
Great care must be taken when installing Cat6 cabling in order to have it achieve its design goals because poorly installed Cat6 cabling may offer only Cat5 performance. To get best performance out of Cat6 cabling for high-speed networking, be sure to follow the following guidelines:
Buy cabling that has a definite spline separating the cables to reduce crosstalk.
Make sure twists in wires are kept flush with connections at all termination blocks to avoid return-loss issues.
Liberally use a hook and loop fastener, such as Velcro, to prevent cable from becoming excessively crimped.
Issues
Despite the formation in 1999 of a TIA task force to develop an official standard for a proposed Cat6 cabling, this unshielded twisted-pair (UTP) cabling standard remains to be ratified. The inherent engineering difficulties of standardizing cabling transmission characteristics for transmissions at 250 MHz and above, coupled with the fact that many cabling vendors have released their own "Category 6 or better" cabling, has led to some confusion in the marketplace. A factor that has slowed the development of Cat6 standards was the successful implementation of 1000BaseT Gigabit Ethernet (GbE) over Cat5 cabling, which relaxed the immediate need for a higher Cat6 class of cable and slowed the momentum of the Cat6 standards process. Cat6 thus currently appears as a solution in search of a problem, and the emergence of 10 Gigabit Ethernet (10 GbE) may have little impact on this situation because 10 GbE is envisioned as a fiber-only architecture.
At present Cat6 cabling solutions are essentially "channel solutions," meaning that their successful implementation depends on purchasing cabling components (such as cabling, patch panels, jacks, and wall plates) from a single vendor. Until an official Cat6 standard emerges, networking architects are recommended to use such channel solutions from a singling cabling vendor. In fact, some believe that "mix and match" cabling solutions for Cat6 will never be achieved due to the careful tuning of components that must be performed to make such systems work.
Notes
Existing cable testing equipment will not be able to evaluate Cat6 cabling because Cat6 not only includes additional parameters that must be measured but also specifies at least 10 dB greater precision in measuring existing cabling transmission characteristics over that achieved by Cat5e test equipment.
Some experts believe that it may be possible in the future to run 10 gigabits per second (Gbps) over Cat6 cabling at distances up to 330 feet (100 meters). If this is the case, then Cat6 cabling may well serve as an alternate infrastructure solution to fiber for the emerging 10 GbE standard. Such uses of Cat6 cabling are likely to be restricted to niche applications such as switch-switch and switch-server interconnects and within some types of storage area networks (SANs).
Another name for the proposed Cat6 cabling standard is Class E.
See Also 10GbE, cabling, enhanced Category 5 (Cat5e) cabling, premise cabling, structured wiring
A still-hypothetical next step up from the proposed Category 6 (Cat6) cabling.
Overview
There is as yet no official Telecommunications Industry Association (TIA) standard for unshielded twisted-pair (UTP) cabling above Cat6, and Cat6 itself is still under consideration and has not been ratified as a standard. Nevertheless, many cabling vendors are offering "Category 7 cabling solutions" that exceed the specifications of Cat6. Such marketing should be taken with a grain of salt, as they do not represent true Cat7 standards, but such "channel solutions" involving cabling system elements purchased from a single vendor can provide significantly better performance than standard enhanced Category 5 (Cat5e) cabling. It is likely that when a true Cat7 standard emerges, it may require the replacement of the ubiquitous RJ-45 connector, which is a weak point in the transmission architecture for UTP cabling. Unfortunately, this means that existing networking equipment such as switches and routers may not be able to support Cat7.
While the Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) has currently abandoned efforts to standardize Cat7, standards efforts are underway on the international scene through the efforts of the International Standards Organization (ISO) and International Electrotechnical Commission (IEC).
Notes
Another name for proposed Cat7 cabling is Class F cabling.
See Also cabling ,Category 6 (Cat6) cabling premise cabling, structured wiring
Stands for Callback Control Protocol, a protocol that enables callback for establishing Point-to-Point Protocol (PPP) connections.
See Also Callback Control Protocol (CBCP)
Stands for class-based queuing, an emerging technology for WAN traffic management.
See Also class-based queuing (CBQ)
Stands for Core-Based Trees, a multicast routing protocol.
See Also Core-Based Trees (CBT)
Stands for Copper Distributed Data Interface, a form of Fiber Distributed Data Interface (FDDI) deployed over copper cabling instead of fiber.
See Also Copper Distributed Data Interface (CDDI)
Stands for Common Desktop Environment, a graphical user interface (GUI) or desktop environment developed for UNIX systems.
See Also Common Desktop Environment (CDE)
Stands for Channel Definition Format, an open standard created by Microsoft Corporation for Microsoft Internet Explorer version 4 (and proposed as a standard to the World Wide Web Consortium) that defines a "smart pull" technology for webcasting information to users' desktops.
See Also Channel Definition Format (CDF)
Text files used for creating Active Channels, Active Desktop items, and channel screen savers for managed webcasting of content to users' desktops.
Overview
CDF files are based on the Channel Definition Format (CDF) standard. CDF files provide a mechanism for allowing users to select the content they want to download from a Web site, and they let administrators schedule content for delivery to users' desktops.
CDF files are used to convert existing Web sites into Active Channels without the need to change the existing site in any way. You simply create a CDF file using a text editor such as Microsoft Notepad and include it in your site. This will allow the content of the site to be webcast to users' browsers. The CDF file must be saved with the extension .cdf, and a link on your site should point to this file so that users can subscribe to the channel.
A typical CDF file defines a channel hierarchy for the different Web sites making up the Active Channel. This channel hierarchy contains a table of contents for webcasting the content and consists of a top-level channel, subchannels, and actual content items (Web pages). The simplest format for a CDF file is a list of Uniform Resource Locators (URLs) that point to specific Web pages in the site. More advanced CDF files can contain information such as the following:
A map of the hierarchical structure of the URLs in the Web site
Logical groupings of different content items within a site that can differ from the observable link structure of the site itself
The title of each referenced Web page and a brief abstract of its contents
Information controlling the scheduling of content updates
The syntax of advanced CDF file items is based on the Extensible Markup Language (XML), an open specification that provides extensibility to standard Hypertext Markup Language (HTML) files. More than one CDF file can be created for a site, allowing users to subscribe to information in different fashions. For example, a news site can have separate CDF files for news, sports, and weather subscriptions.
Notes
For specific information on the syntax of CDF files and how to create them, refer to the Microsoft Internet Client software development kit (SDK).
Channels in Active Channel enable personalized delivery of Web content using Web applications designed for Internet Information Service (IIS) for Windows 2000. Active Server Pages (ASP) can be used for dynamically generating personalized CDF files for users. Cookies can also be used for dynamically generating customized CDF files for users. These CDF files can be customized on the basis of preferences that a user specifies on an HTML form prior to subscribing to the channel.
See Also Channel Definition Format (CDF)
Stands for CD-ROM File System, a file system designed for read-only CD-ROM media.
See Also CD-ROM File System (CDFS)
Stands for Code Division Multiple Access, a second- generation (2G) digital cellular phone technology that uses spread-spectrum techniques, popular in the United States and some other parts of the world.
See Also Code Division Multiple Access (CDMA)
A proposed third-generation (3G) upgrade for existing Code Division Multiple Access (CDMA) cellular telephone systems.
Overview
CDMA2000 was developed by the Telecommunications Industry Association (TIA) and is part of the International Mobile Telecommunications 2000 (IMT-2000) initiative of the International Telecommunication Union (ITU). CDMA2000 will boost the bandwidth of existing cdmaOne cellular systems to 2 megabits per second (Mbps), making global broadband wireless communications a reality.
Because the proposed CDMA2000 upgrade includes only a small portion of the overall wireless communication market, the IMT-2000 initiative also includes proposed upgrades to Time Division Multiple Access (TDMA) systems such as Global System for Mobile Communications (GSM). A competing upgrade for TDMA systems is General Packet Radio Service (GPRS), which is closer to implementation than CDMA2000 and might therefore win more initial support than CDMA2000.
The term CDMA2000 1x is commonly used to describe various interim CDMA2000 systems that use existing IS-95a base stations to provide 2.5G cellular services. CDMA2000 1x systems expect to achieve data transmission speeds of between 144 kilobits per second (Kbps) and 307 Kbps and are therefore classed as 2.5G cellular services instead of the much faster 3G services envisioned by IMT-2000. The proposed 3G system Wideband CDMA (W-CDMA) is comparable to CDMA2000 2x , meaning the 3G version of CDMA2000.
The CDMA Development Group is also promoting a different CDMA2000 upgrade called High Data Rate (HDR) CDMA2000 1x . This platform is based on the American National Standards Institute (ANSI) IS-95c standard and is viewed as an alternative upgrade path from CDMAone toward 3G. It involves a hardware upgrade using chipsets produced by QUALCOMM (the originators of CDMA) and may provide data speeds up to 2.4 Mbps, even better than the 2 Mbps speeds anticipated by W-CDMA. In some respects HDR may be superior to W-CDMA, the 3G version of CDMA being proposed by the ITU in its IMT-2000 initiative. Specifically, while W-CDMA is expected to provide 2 Mbps service only for stationary users, and much slower speeds for mobile and roaming users, HDR is intended to provide the same 2.4 Mbps speed for all users whether mobile or stationary. However, W-CDMA has the advantage of being a proposed standard for implementing compatible systems throughout the world. Only time will tell which system achieves market dominance.
For More Information
Visit the CDMA Development Group at www.cdg.org
See Also 3G, cellular communications, cdmaOne, Code Division Multiple Access (CDMA)
The commercial name for the Code Division Multiple Access (CDMA) cellular communications system used in North America and parts of Asia.
See Also Code Division Multiple Access (CDMA)
Stands for content delivery network, a method for efficiently pushing out content over the Internet to users.
See Also content delivery network (CDN)
Stands for Collaboration Data Objects, a collection of Component Object Model (COM) objects that allow developers to create Microsoft Internet Information Services (IIS) Web applications that send and receive electronic mail.
See Also Collaboration Data Objects (CDO)
Stands for Content Delivery Provider, a company that builds and operates a content delivery network (CDN).
See Also content delivery network (CDN)
Stands for Cellular Digital Packet Data, a type of packet-switched data transmission network operating as an overlay for a cellular communications system.
See Also Cellular Digital Packet Data (CDPD)
A file system designed for read-only CD-ROM media.
Overview
CD-ROM File System (CDFS) is an International Organization for Standardization (ISO) standard (ISO 9660) for a read-only formatting standard for CD-ROM media. CDFS provides the same kind of file and directory management for CD-ROM devices that the file allocation table (FAT) and NTFS file systems (NTFS) do for hard disks.
CDFS is implemented on the Microsoft Windows 95, Windows 98, Windows Millennium Edition (Me), Windows NT, Windows 2000, Windows XP, and Windows .NET Server platforms. On 32-bit Windows systems, CDFS uses a 32-bit protected-mode driver that replaces the 16-bit real-mode Microsoft CD-ROM Extension (MSCDEX) driver that was used in the legacy 16-bit Windows and MS-DOS operating system platforms. In Windows 95 and Windows 98, the file system driver that supports CDFS is called Cdfs.vxd, and in Windows NT, Windows 2000, Windows XP, and Windows .NET Server, it is called Cdfs.sys.
CDFS is optimized for reading compact discs that have a standard data block size of 2048 bytes (2 KB). The Microsoft implementation of CDFS includes a dynamic, protected-mode cache pool for caching CD-ROM data to improve read performance. This allows CDFS to read ahead to ensure that playback of multimedia content from CDs is smooth and seamless. On Windows 95 OEM Service Release 2 (OSR2), Windows 98, Windows Me, Windows NT, Windows 2000, Windows XP, and Windows .NET Server platforms, CDFS includes a number of enhancements over the original version of CDFS for Windows 95, such as
CD-XA support for optimized reading of Moving Picture Experts Group (MPEG) video CDs having larger block sizes of 2352 bytes.
Auto-Run, which allows applications on CDs to start immediately when the CD is inserted into the drive. To do this, the operating system reads the Autorun.inf file that is stored in the root of the directory structure on the CD.
CDFS does have several limitations compared to disk file systems such as FAT and NTFS, namely:
Names for files and directories can be no longer than 32 characters
Directories can be nested only eight levels deep
CDFS is now considered a legacy format and is being replaced by the industry-standard Universal Disk Format (UDF), the new standard for read-only disk media.
Notes
Using Services for Macintosh, you can create an Apple Macintosh-accessible volume on a CDFS volume by following the same steps you would use to make an NTFS volume accessible to Macintosh clients. Of course, the CDFS volume has one difference: it is read-only.
See Also file system ,Universal Disk Format (UDF)
Stands for Consumer DSL, a broadband transmission technology based on Digital Subscriber Line (DSL) technology.
See Also Consumer DSL (CDSL)
A 53-byte packet of data, the standard packet size used by Asynchronous Transfer Mode (ATM) communication technologies.
Overview
Cells are to ATM technologies what frames are to Ethernet networking. In other words, they form the smallest element of data for transmission over the network.
Cell in ATM. Details of an ATM cell.
ATM cells are standardized at a fixed-length size of 53 bytes to enable faster switching than is possible on networks using variable-packet sizes (such as Ethernet). It is much easier to design a device to quickly switch a fixed-length packet than to design a device to switch a variable-length packet. (Switching a fixed- length packet is easier because the device knows in advance the packet's exact length and can anticipate the exact moment at which the last portion of the packet will be received. With variable-length packets, the device must examine each packet for length information.) Using fixed-length cells also makes it possible to control and allocate ATM bandwidth more effectively, making support for different quality of service (QoS) levels for ATM possible.
The functions of information stored in the 5-byte header of an ATM cell include the following:
Providing information about the physical layer transmission method being used
Providing flow control to enable a steady flow of cell traffic and to reduce cell jitter
Specifying virtual path or channel identification numbers so that multiplexed cells belonging to the same ATM connection can be distinguished from cells belonging to other ATM connections and cells can be switched to their intended destination
Specifying the nature of the payload contained in the cell-that is, whether it contains actual user data or ATM cell-management information
Specifying the priority of the cell to determine whether the cell can be dropped in congested traffic conditions
Providing error checking by means of an 8-bit field containing cyclical redundancy check (CRC) information for the header itself
Two kinds of header formats are used in ATM cells:
User-Network Interface (UNI) format: Used for communication between end nodes and an ATM network
Network-Node Interface (NNI) format: Used within the ATM network itself after the cell has been multiplexed for transmission over its virtual path
Notes
Why a 48-byte data payload for ATM cells? This is the result of a trade-off between larger 64-byte payloads that contain more data but take longer to package and unpackage- and are therefore not suitable for real-time transmissions such as voice or multimedia-and shorter 32-byte payloads that provide better real-time transmission but are inefficient for larger amounts of data. By compromising at a 48-byte payload size, ATM has good transmission capabilities for both voice and data communication, providing efficient packet transfer with low latency.
See Also Asynchronous Transfer Mode (ATM)
In wireless communication technologies, the geographical region that is covered by a transmission facility.
Overview
The term cell is most often used in reference to cellular phone technology, but it can also be used in reference to the coverage areas for transmission of cordless telephones, satellite transmissions, wireless local area networks (LANs), packet radio, and paging technologies.
Cells range in size from a few dozen feet to thousands of miles in diameter, depending on the technology being used, the power of the transmission station, and the terrain topography. The following table summarizes typical cell size ranges for different wireless communication technologies. These figures are only approximate because wireless technologies are constantly evolving.
Satellite-based systems have by far the largest cell sizes and are rapidly increasing in popularity. Cellular phone technologies in rural areas typically use cells with a radius of 6 to 30 miles (10 to 50 kilometers), while cells in urban areas range in size from 0.6 to 6 miles (1 to 10 kilometers). For highly dense urban areas, cell sizes as small as 330 feet (100 meters) can be used, especially in high-tier Personal Communications Devices (PCD) cellular technologies.
Wireless Technology | Cell Radius |
Wireless LANs | 10 to 100 meters |
Cellular telephone | 0.1 to 50 kilometers |
PCD | 0.1 to 1 kilometer |
Satellite-based | 1000 kilometers or more |
Notes
When a mobile caller using a cell phone passes from one cell to another, the cellular phone system transfers the call to the system servicing the adjacent cell, a process called roaming.
See Also cellular communications ,wireless networking
A type of packet-switched data transmission network operating as an overlay for a cellular communications system.
Overview
Cellular Digital Packet Data (CDPD) is a specification for overlaying digital data transmissions on the existing circuit-switched Advanced Mobile Phone Service (AMPS) analog cellular phone service. CDPD was developed by IBM together with a consortium of Regional Bell Operating Companies and other organizations to leverage the existing installed base of AMPS cellular equipment in the United States to provide low-cost, packet-switched data services. CDPD was first offered in 1994 by Bell Atlantic Mobile.
Uses
CDPD is typically used to provide wireless access to public packet-switched networks such as the Internet so that mobile users can access their e-mail and other services. Multiple users can share the same channel; the user's modem determines which packets are destined for the user's machine. CDPD also supports IP multicasting and is an open standard based on the Open Systems Interconnection (OSI) reference model for networking.
Architecture
CDPD makes use of idle times between calls in cellular phone network channels for interleaving packets of digital data. In other words, CDPD makes use of the "bursty" nature of typical voice transmission on the AMPS cellular system. Voice communication has gaps or pauses where packet data can be inserted and transmitted without interfering with the communication taking place between customers.
CDPD uses the Reed-Solomon forward-error-correcting code to encode each block or burst of data sent, and includes built-in RC4 encryption to ensure security and privacy of the transmitted data. CDPD is also based on the industry standard Internet Protocol (IP), allowing data to be transmitted to and from the Internet.
Although CDPD supports data transmission rates of 19.2 kilobits per second (Kbps) and higher, actual data throughput is usually around 9.6 Kbps. This is because of the large overhead added by CDPD to each data block transmitted. This overhead is designed to ensure that communications are reliable and to maintain synchronization between the modems at each end of the transmission. In addition, a color code is added to every data block to detect interference resulting from transmissions on the same channel from neighboring cell sites.
Implementation
A typical implementation of CDPD consists of three components:
Mobile-End System (M-ES): A user device such as a laptop equipped with a cellular modem. This system communicates in full-duplex mode with a Mobile Data Base Station (MDBS) using the Digital Sense Multiple Access protocol, which prevents collisions of data streams from multiple Mobile-End Systems.
Mobile Data Base Station (MDBS): A telco device for receiving and transmitting CDPD data.
Mobile Data Intermediate System (MDIS): Provides the central control for a CDPD network.
Marketplace
In the United States, the main providers of CDPD services are AT&T Wireless and Verizon Wireless. CDPD is also supported by several carriers in Canada and by AirData in New Zealand. CDPD is not widely deployed as a cellular data transmission technology, and coverage in the United States is spotty even in urban areas.
For More Information
You can find the CDPD Forum at www.cdpd.org
See Also cellular communications ,wireless networking
A group of technologies that support roaming cell-based wireless communications.
Overview
Common to all forms of cellular communications is the concept of the cell. Instead of using one extremely powerful transmission to provide coverage for a geographical area (for example, a city, state, region, or country), the area is divided into a series of overlapping smaller areas called cells. Each cell has a relatively low-power base station that provides coverage for users within that cell, and when a user moves (roams) from one cell to another, the first base station seamlessly hands off servicing of the user to the second base station and the user experiences no interruption in communication.
Cellular communications. How a cellular phone system works.
Dividing the coverage area into many cells makes cellular communications systems more complicated than systems that use a single high-powered base station. Complex functions such as call setup and tear down, call authorization, call handoff, routing of call traffic, and call billing must operate seamlessly as users move between cells, and they require complex technology to implement. These functions are provided by connecting groups of base stations to a mobile switching center (MSC), which coordinates the activities of the base stations and connects them to the Public Switched Telephone Network (PSTN) so wireless users can call wireline users and vice versa.
Communications channels also need to be allocated to cells in such a way that no two adjacent cells use the same set of channels. This is necessary to ensure that signals from one base station do not interfere with those from base stations in adjacent cells. Repeating patterns of cells are used to optimize coverage of an area while maintaining the maximum number of available channels per cell. For example, in a typical cellular system, if you travel in a straight line every third cell utilizes the same set of frequencies.
Cells can also vary considerably in size. Because each cell can only support a certain number of users, cells in dense urban areas are much smaller than those in sparsely inhabited rural areas, and when population increases cells may need to be split to continue to provide adequate coverage. All this makes cellular communications complicated to implement, which explains why the technology took so many years to be widely deployed after the it was first conceived. The main advantage of such systems, however, is that, because cells are relatively small, the mobile client (cell phone or other device) does not need to be very powerful, which saves considerably on the size and the cost of the phone and has helped propel the widespread use and popularity of these systems.
History
The concept of a cellular phone system originated in 1947 at AT&T Bell Laboratories. AT&T first proposed a commercial cellular system to the Federal Communications Commission (FCC) in the late 1960s, and the FCC allocated the necessary frequency spectrum in 1974. A demonstration system was tested in Chicago in 1978, but the first actual cellular system was deployed in Japan in 1979 by the Nippon Telephone and Telegraph Company. This was followed by Europe in 1981, and finally in 1983 AT&T began deploying Advanced Mobile Phone Service (AMPS), the first nationwide cellular system in the United States.
AMPS was a first-generation (1G) analog cellular phone system that used frequency modulation for voice transmission and frequency-shift keying (FSK) for transmission of signaling information. AMPS uses channels within the 800 megahertz (MHz) frequency band of the electromagnetic spectrum, and channel access is provided by using Frequency Division Multiple Access (FDMA) as the media access method. AMPS achieved widespread implementation across the United States in the 1980s and is still widely used.
Another wave of cellular communications systems was developed in the 1990s and is still widely deployed. These second-generation (2G) systems differ from AMPS in being digital rather than analog in nature, and they can support not just voice but also data transmission, typically at 9.6 to 19.2 kilobits per second (Kbps). Several competing digital cellular systems have evolved and have become widely deployed:
Time Division Multiple Access (TDMA): The main TDMA system deployed in the United States is AT&T Wireless Services, a subsidiary of AT&T. This TDMA system operates in the same 800 MHz frequency band as AMPS and is sometimes referred to as either Digital AMPS (D-AMPS) or North American TDMA (NA-TDMA).
Code Division Multiple Access (CDMA): CDMA was developed by QUALCOMM, and its cdmaOne system is the most popular digital cellular system in North America. Other CDMA carriers include Bell Mobility, Sprint Corporation, and Verizon Communications. CDMA is more complex than TDMA, but it is more efficient in its utilization of the frequency spectrum.
Global System for Mobile Communications (GSM): This system is based on TDMA and is the most popular digital cellular phone technology in Europe and much of Asia. GSM can operate in the 900, 1800, or 1900 MHz frequency bands. There are only a few GSM carriers in the United States, and they cover only a small portion of the market. An example is VoiceStream Wireless Corporation.
Personal Communications Services (PCS): These systems can be TDMA-based or CDMA- based and operate in the higher 1900 MHz frequency band using smaller cell sizes than 800 MHz cellular systems. Sprint PCS, for example, is based on CDMA technology.
Issues
A major concern that has been receiving media attention lately is the safety of using cellular phones. While most agree that using a cellular phone while driving can increase the chances of an accident (and some civic and municipal governing bodies have passed laws regulating use of cellular phones while driving), scientists continue to debate whether other health hazards might be associated with the long-term use of cell phones. A particular concern is the possible link between cell phone use and brain cancer, particularly among children whose brains are still forming. Cell phones transmit microwave energy through their antennas, and microwaves have a known heating effect (think of a microwave oven, for example). When a cell phone is held next to the ear, some of the microwave energy penetrates into the brain, and while the FCC has mandated emission standards for cellular phones to keep these emissions below dangerous levels, some researchers believe that long-term exposure to even low-level microwave radiation may be harmful to the brain and other organs, particularly in children. On the other hand, many researchers believe that there is no conclusive link between brain cancer and cell-phone use, and in general such associations between cancer and environment factors are difficult to prove (consider how long it took to resolve the controversy over whether cigarettes caused lung cancer). The American Medical Association and other bodies continue to call for further study of these issues, but it will likely take years for studies to produce any significant results. Some governing bodies are already taking steps, however, such as the United Kingdom, which has mandated that mobile telephone handsets must now come with leaflets warning of potential health risks to children who use these devices over a prolonged period.
Impact
Cellular communications have revolutionized business and personal communications worldwide. About one-third of Americans now own cell phones, and this is growing at an annual rate of 25 percent. Use of cell phones in Europe and Asia is even higher on a percentage basis, and some analysts predict that by 2005 there will be more than 1.25 billion cell phones and other cellular communications devices used around the world.
Cell phones have helped catalyze the e-business revolution by providing employees with 24x7 connectivity with the office-something desirable from a management point of view but debatable in terms of the quality of life for workers. Using data-enabled cell phones and wireless Personal Digital Assistants (PDAs), workers can perform tasks such as sending and receiving e-mail, checking inventory and ordering products, reboot servers, and perform other essential tasks to keep an e-business operating. Nonbusiness uses of data-enabled cell phones including browsing the Web, checking weather reports, downloading news headlines, confirming airline reservations, checking stock quotations, and many other uses. Although many of these uses are exciting, most data operations are still tedious over slow 2G cellular systems, and the real promise of cellular Internet access will not be realized until 3G comes into full operation in the latter part of this decade.
Prospects
The current 2G cellular systems will be replaced over the next few years by broadband cellular systems that support much higher rates of data transmission than can now be achieved. Proposed 3G systems will support data transmission as fast as 2 megabits per second (Mbps) and include
Wideband CDMA (W-CDMA): This system has the support of the International Telecommunication Union (ITU) as the upgrade for TDMA-based systems such as GSM, offers data rates of up to 2 Mbps, and is expected to become widely deployed in Europe and Asia.
CDMA2000: This system has been proposed by QUALCOMM as the upgrade for CDMA systems, has the support of the ITU as an alternative to W-CDMA, offers data rates of up to 2 Mbps, and is expected to become widely deployed in North America.
Enhanced Data rates for Global Evolution (EDGE): This system is a proposed upgrade by AT&T for its North American TDMA system but offers data rates of only 384 Kbps.
Because 3G technologies are not expected to be deployed until 2003 (or more likely 2005), some carriers are implementing interim 2.5G systems to provide increased data rates over 2G until 3G becomes a reality. Some 2.5G systems starting to be deployed include
CDMA2000 1X: This system developed by QUALCOMM supports data transmission at 307 Kbps, and is an interim step toward the 3G version CDMA2000 (sometimes called CDMA2000 2X).
General Packet Radio Service (GPRS): This system upgrades AT&T's existing TDMA system to support data rates of 115 Kbps and is an interim step toward EDGE.
For More Information
For the latest news about the cellular industry, visit the Cellular Telecommunications & Internet Association's (CTIA) World of Wireless at www.wow-com.com
See Also 2G, 2.5G, 3G, broadband wireless communications, CDMA2000, Code Division Multiple Access (CDMA), Enhanced Data Rates for Global Evolution (EDGE), General Packet Radio Service (GPRS), Global System for Mobile Communications (GSM), Personal Communications Services (PCS), Time Division Multiple Access (TDMA), wireless networking
A telco switching facility.
Overview
The central office (CO) provides access to the Plain Old Telephone Service (POTS), leased lines, and other services that a telco offers its customers in a given geographical area (usually a dozen square miles or so). The CO contains the Class 5 switching equipment that connects telephone subscribers to both local and long-distance phone services. A telco typically has one CO servicing several dozen square miles in urban areas, so in large cities there may be many COs for each telco.
A typical CO may look like a fortress built to withstand an earthquake or any other natural disaster. Building standards for COs are high because of the importance of the communications infrastructure to a nation's economic health and safety. Banks of batteries and diesel generators provide backup power in case of blackouts so that phone communication will not be disrupted.
Multitudes of twisted-pair copper telephone lines from customer premises usually enter the building through the underground cable vault. These twisted-pair lines are grouped into bundles of thousands of lines, forming large cables 3 to 4 inches (7.5 to 10 centimeters) in diameter. The cables have grounding mesh to provide a drain for unwanted electrical surges and tough polyvinyl chloride (PVC) insulating jackets that are pressurized to prevent water from seeping in at cable junctions. Steel racks organize these cables as they enter the cable vault.
From the vault, the cables snake their way to the main cross-connect grid. It is in these steel-frame grids that all the individual twisted-pairs fan out and connect through feeders to the main switching equipment. The importance of the feeders is that they allow any incoming twisted-pair line to connect to virtually any switching bank. This makes it possible for customers to move to a different part of the city and maintain their old telephone number at their new location. It also allows for redundancy: if a switch fails, a CO technician can rewire the feeder blocks quickly and easily. The main switches are then used to route calls to other local subscribers or to a long-distance telecommunications carrier such as AT&T or MCI WorldCom. In the United States, these Class 5 telephone switches are usually Basic-5ESS switches, also called AT&T basic rate switches.
Subscribers are connected to their local CO through a segment of wiring called the local loop. This wiring is typically copper, but telcos lay fiber to the building for customers requiring high-speed services not supported by copper lines.
Finally, a telco may have dozens of COs in a dense urban area, all interconnected using cross-links and redundant switches. Each CO services subscribers within a specific geographical area and manages voice and data traffic in that area. When a call is made from a customer over the local loop to the CO, the call is either switched to another circuit within the same exchange (for local calls) or switched to a trunk line joining the CO to a CO belonging to a different company, typically a long-haul carrier such as Sprint or AT&T. For business customers needing multiple telephone lines at their location, the carrier typically deploys a private branch exchange (PBX) at the customer premises.
Notes
Not all of a telco's switching and telecommunications equipment is located at a central office. There are recent trends where telcos have pushed out their ATM switching gear to within 500 feet (150 meters) of residential neighborhoods in pursuit of better-quality Digital Subscriber Line (DSL) connections. These remote stations shorten the copper DSL connections between customer premises and the provider's switching gear, and the remote Asynchronous Transfer Mode (ATM) concentrators are then connected to the COs using fiber.
In countries and regions other than the United States, a CO is often referred to as a public exchange.
See Also telco
An acronym for Centr al Office Ex change Service, a business telephone service provided by telcos.
Overview
By using a Centrex instead of a Private Branch Exchange (PBX), a business can eliminate the necessity of having its own dedicated switching facilities at its customer premises. Centrex also eliminates the need for customers to upgrade to expensive new telephones because existing telephone lines and touch tone phones can be used with it. This frees the customer from the need to invest in the cost and management of customer premises equipment (CPE).
Centrex services partition the switching capabilities of the telco's central office (CO) equipment and allow a portion of these capabilities to be dedicated to a particular customer. In essence, the business customer is leasing dedicated switching facilities at the CO to enable a large number of employee telephones to be routed through a few telephone lines. All routing of calls to individual employee telephones takes place using the Centrex. Configuration changes can be performed at the CO instead of requiring technicians to visit the customer premises. This can save the cost of installing a local PBX at the customer premises.
Centrex can handle advanced communication features such as internal call handling, inbound and outbound call handling, and multiparty calling. Each individual connected can have customized calling features just as they can with a PBX. Maintenance is entirely the responsibility of the telco central office, which provides around-the-clock support.
Notes
Some carriers such as Pacific Bell Telephone Company also offer Integrated Services Digital Network (ISDN) as a Centrex service in addition to standard business ISDN lines.
See Also central office (CO) ,Private Branch Exchange (PBX) ,telco
Also called digital certificate, a technology for verifying the identity of the user or service you are communicating with.
See Also digital certificate
Any entity (individual, department, company, or organization) that issues digital certificates to verify the identity of users, applications, or organizations.
Overview
Before issuing a digital certificate to someone, the certificate authority (CA) must verify the user's identity according to a strictly established policy, which can involve face-to-face communication, examination of a driver's license with photograph, or another method of establishing a user's identity. When the user's identity has been verified, the certificate is issued to the user. This certificate can then be presented by the user as a "digital driver's license" to identify himself or herself during network transactions.
CAs can be trusted third parties such as the private companies VeriSign, CyberTrust, and Nortel Networks; or they can be established within your own organization using Microsoft Certificate Server. CAs can be stand- alone authorities with their own self-signed certificates (that is, they validate their own identity as a root CA), or they can be part of a hierarchy in which each CA is certified by the trusted CA above it (up to a root CA, which must always be self-certified).
For digital certificates to work as an identification scheme, both client and server programs must trust the CA. In other words, when a client program presents a certificate to a server program, the server program must be able to validate that the certificate was issued by a valid and trusted CA. Certificate authorities also maintain a certificate revocation list (CRL) of revoked certificates. Certificates issued by CAs expire after a specified period of time.
CAs are necessary for the functioning of a public key infrastructure (PKI), which is essential to the widespread acceptance and success of any public key cryptography system. Microsoft Windows 2000 and Windows .NET Server can use standard X.509 digital certificates to authenticate connections across unsecured networks such as the Internet and to provide single sign-on using smart card authentication systems.
For More Information
Visit VeriSign, Inc. at www.verisign.com
See Also digital certificate ,public key infrastructure (PKI)
A feature of Microsoft Internet Information Server (IIS) version 4 and Internet Information Services (IIS) version 5 (for Windows 2000) and later that allows mapping between user accounts and digital certificates.
Overview
Certificate mapping is useful when an organization issues client certificates to users. Client certificates are digital certificates that verify the identity of client software (Web browsers) belonging to users. Client certificates are often used in situations in which mobile clients using laptops require secure access to a corporate intranet site.
Before users can be granted remote access to the corporate intranet, they must be authenticated by the Web server they are connecting to. IIS supports four kinds of Web authentication mechanisms:
Anonymous access: Allows anonymous users access to Web sites-such as public sites on the Internet.
Basic authentication: Passes a user's credentials over the network as clear text. Although this mechanism is not very secure, it has the advantage of being able to work through a firewall or a proxy server.
Microsoft Windows NT Challenge/Response Authentication (called Integrated Windows Authentication in Windows 2000): A secure authentication method that does not actually pass the user's credentials over the network but uses a cryptographic exchange instead. The only Web browser that supports this authentication method is Microsoft Internet Explorer. This method cannot work through a firewall or a proxy server.
Certificate mapping: Uses the Secure Sockets Layer (SSL) protocol to authenticate users by examining the contents of their client certificate in order to log them on to the network without requiring them to enter their credentials.
Client certificates provide verification of identity, but certificate mapping associates a user's account with the user's client certificate and permits the user to log on to the network. The user typically utilizes a Web browser with SSL protocol to connect to a secure company Web site. The company Web server checks the Web browser's client certificate. If the certificate is valid, the user is automatically logged on using his or her user account without ever having to enter credentials and can access whatever intranet resources for which the account has permissions.
IIS allows two kinds of client certificate mappings:
One-to-one mappings between user accounts and client certificates on the user's browser. This type of mapping is typically used to allow users secure access to corporate intranet resources-for example, to view or modify their employee information.
Many-to-one mappings can map several client certificates to a single user account. Many-to-one mappings have the advantage of permitting administrators to allow multiple certificates to be used to grant users access to the corporate intranet utilizing a single Windows account. For example, you can set a rule that maps all certificates issued by the CA of an agency that provides your company with temps to a single Windows account.
Notes
Certificate mapping is also supported by Active Directory directory service in Microsoft Windows 2000, Windows XP, and Windows .NET Server operating systems. You can use the Active Directory Users and Computers administrative tool for this purpose.
See Also digital certificate
A file containing an entity's identification information and public key that is submitted to a certificate authority (CA) in order to obtain a digital certificate.
Overview
A certificate request is a text file encoded using Base64 encoding. This text file is generated by an application in response to the entity's request for a key pair and digital certificate. The entity here refers to the individual, system, company, or organization requesting the certificate. The certificate request is then submitted to a CA to obtain a digital certificate for the entity.
The Key Manager utility in Internet Services Manager (the utility used to manage Microsoft Internet Information Services) can be used to generate a key pair and a standard public key cryptography standards (PKCS) #10 format certificate request file. The certificate request file is a simple text file that can be viewed with Microsoft Notepad. If this file is submitted to a public certificate authority, such as VeriSign, or to the company's own certificate authority, such as Microsoft Certificate Server, a standard X.509 format digital certificate will be granted in return.
See Also digital certificate ,public key infrastructure (PKI)
A list, maintained by a certificate authority (CA), of digital certificates that have been issued and later revoked.
Overview
A certificate revocation list (CRL) is similar to lists of revoked credit card numbers that credit card companies used to give to vendors. The certificate authority makes the CRL publicly available so that users can determine the validity of any digital certificate presented to them.
Creating and maintaining a CRL is an essential ingredient in running a public key infrastructure (PKI) to support public key cryptography systems. Microsoft Certificate Server includes a Web-based utility called the Certificate Administration Log Utility that can be used to revoke certificates and maintain a CRL.
See Also digital certificate ,public key infrastructure (PKI)
An education-delivery company, such as a school or training center, that is qualified by Microsoft Corporation for the delivery of Microsoft Official Curriculum (MOC) courseware.
Overview
Certified Technical Education Centers (CTECs) are one of Microsoft's main channels for training on Microsoft products and technologies. CTECs can deliver training in a variety of forms, including instructor-led, self- paced, customized, and online training. CTECs are one of the ways that Microsoft contributes to solutions addressing the shortage of skilled IT professionals worldwide.
Network professionals who want to become familiar with Microsoft operating systems, applications, and development platforms can take MOC courses offered by CTECs at various locations around the world. Microsoft CTECs are also a source for the best in self-paced training materials on Microsoft products and services.
For More Information
Visit the Microsoft CTEC site at www.microsoft.com/ctec.
See Also Authorized Academic Training Provider (AATP)
Stands for Common Gateway Interface, a mechanism by which a Web browser can request a Web server to execute an application.
See Also Common Gateway Interface (CGI)
A standard form of challenge/response authentication protocol.
Overview
Challenge Handshake Authentication Protocol (CHAP) is a standard authentication protocol defined in RFC 1994. CHAP is one of several authentication schemes used by the Point-to-Point Protocol (PPP), a serial transmission protocol for wide area network (WAN) connections. Other authentication schemes supported by PPP include Password Authentication Protocol (PAP), Shiva Password Authentication Protocol (SPAP), and Microsoft Challenge Handshake Authentication Protocol (MS-CHAP). PAP is a widely implemented authentication protocol, but CHAP is more secure than PAP because CHAP encrypts the transmitted password and PAP does not. SPAP and MS-CHAP are vendor-specific implementations.
CHAP is an encrypted authentication scheme in which the unencrypted password is not transmitted over the network. A typical CHAP session during the PPP authentication process works something like this:
The client connects to a network access server (NAS) and requests authentication.
The server challenges the client by sending a session ID and an arbitrary string.
The client uses the MD5 one-way hashing algorithm and sends the server the username, along with an encrypted form of the server's challenge, session ID, and client password.
A session is established between the client and the server.
To guard against replay attacks, the challenge string is chosen arbitrarily for each authentication attempt. To protect against remote client impersonation, CHAP sends repeated, random interval challenges to the client to maintain the session.
Notes
CHAP is supported by the Routing and Remote Access (RRAS) service of Windows 2000 and Windows .NET Server as a way to allow non-Microsoft clients to dial in and receive authentication for a Remote Access Server (RAS) session, and to allow Microsoft RAS clients to connect to any industry-standard PPP server.
With Cisco routers, CHAP repeatedly challenges the connecting host every two minutes after the connection is established. This helps to prevent session hijacking by hackers on the network.
See Also authentication provider, challenge/response, Microsoft Challenge Handshake Authentication Protocol (MS-CHAP), Password Authentication Protocol (PAP), Point-to-Point Protocol (PPP)
A mechanism for securely authenticating users over a network.
Overview
Challenge/response provides a way of employing user credentials to negotiate a connection over a network without actually passing these credentials over the network. Challenge/response forms the basis of the Integrated Windows authentication method supported by Internet Information Services (IIS) on Microsoft Windows 2000, Windows XP, and Windows .NET Server (Integrated Windows authentication was previously called Windows NT Challenge/Response authentication in Internet Information Server [IIS] on Windows NT).
Implementation
The basic steps of challenge/response are straightforward and independent of the actual platform they are implemented on. Here are the steps in a typical challenge/response authentication session:
The client contacts the server to access a resource on the server.
The server requests the client to authenticate itself and sends the "challenge," a randomly generated string, to the client.
The client hashes the challenge string together with the client's password using a predetermined hashing algorithm. The result of this hash is called the "response."
The client sends the response to the server, together with the client's name.
The server meanwhile has performed the identical hashing operation as the client has, using the client's credentials, which are securely stored ahead of time in the server's security database.
When the server receives the client's hash, it compares this with its own hash. If the hashes are identical, the client's identity is authenticated and the client is allowed to access the desired resource.
See Also authentication protocol ,hashing algorithm ,Integrated Windows Authentication
A set of features in Microsoft Windows 2000, Windows XP, and Windows .NET Server for managing user settings and installing applications.
Overview
Change and Configuration Management is included in Windows 2000, Windows XP, and Windows .NET Server to provide support for tasks such as
Installing initial operating systems on new computers
Managing deployment of software on computers
Managing user desktop configuration settings and user personal folders
Change and Configuration Management is an umbrella term for two underlying Microsoft technologies that make these things possible. These two technologies are
Intellimirror: Enables management of user preferences, user documents, and software installation and maintenance.
Remote Operating System (OS) Installation: Uses Remote Installation Services (RIS) to install Windows 2000, Windows XP, and Windows .NET Server on remote machines.
See Also IntelliMirror
The delivery method in Active Channel, a technology for Microsoft Internet Explorer that allows Web content to be "webcast" to users.
Overview
Channels deliver content to users' Web browsers. The content is displayed as ordinary Web pages and can be viewed off line. Channels are delivered to users by way of Microsoft Corporation's Channel Definition Format (CDF) technology. Channels can contain Hypertext Markup Language (HTML); Microsoft ActiveX controls; Microsoft Visual Basic, Scripting Edition (VBScript); Java applets; and other dynamic Web elements. The channels that a user has subscribed to are listed as part of his or her channel bar.
See Also Channel Definition Format (CDF)
A telecommunications device that consolidates multiple Digital Signal Zero (DS0) channels into a larger single digital transmission.
Overview
Channel banks usually combine 24 different voice and data 64 kilobits per second (Kbps) DS0 channels into a single 1.544 megabits per second (Mbps) DS1 channel, though some channel banks are capable of combining up to 96 DS0 channels into a 6.312 Mbps DS2 channel. The channel bank combines the individual DS0 signals using a technique called multiplexing, which allows multiple signals to be combined for transmission over a single line. The resulting multiplexed digital signal can then be sent over a T1 line to the telco.
A channel bank interfaces with the wires of the local loop connection that carry the phone signals from the customer premises to the telco's central office (CO). In a typical business scenario, the channel bank would be connected to the front end of an analog Private Branch Exchange (PBX) to support multiple telephones within the company. The channel bank also includes circuits for converting the analog voice signals into digital data signals, usually using pulse code modulation (PCM). The resulting modulated digital signal conforms to the standard 64-Kbps DS0 format. The digital signals can then be routed through the digital switching backbone of the Public Switched Telephone Network (PSTN) as necessary.
Channel banks are typically located at the telco's CO and support the digital switching functions of the PSTN. However, channel bank equipment can also be installed at customer premises for larger enterprises.
Notes
Channel banks are becoming obsolete as older analog PBXs are being replaced by digital PBXs. When a digital PBX is used, the PBX can be connected directly to the T1 line using a Channel Service Unit (CSU), making a channel bank unnecessary.
See Also Channel Service Unit (CSU) ,DS-0 ,multiplexer (MUX) ,Private Branch Exchange (PBX) ,pulse code modulation (PCM) ,T1
A component of Microsoft Internet Explorer version 4 and later that displays the available user-subscribed Active Channels.
Overview
Active Channels provide a way of webcasting information to subscribers using Microsoft Corporation's Channel Definition Format (CDF) technology. When Internet Explorer is first installed on a user's machine, a selection of preloaded channels is stored in the channel bar according to the country or region preference the user specified during installation. These channels are stored by default in C:\Windows\Favorites\Channels on a computer running Microsoft Windows 95 or Windows 98 and in C:\Winnt\Profiles\<user>\Favorites\Channels on a computer running Windows NT, Windows 2000, Windows XP, or Windows .NET Server. When a user subscribes to a channel, a small red gleam appears next to the icon representing the channel. This gleam indicates that new content has been downloaded and is available for browsing.
Notes
Administrators who plan to deliver and install Internet Explorer on users' machines can use the Internet Explorer Administration Kit (IEAK) to customize the users' set of startup channels. Administrators can thus use the channel bar to deliver important information to users through the company's intranet site.
See Also Channel Definition Format (CDF)
An open standard created by Microsoft Corporation for Microsoft Internet Explorer version 4 (and proposed as a standard to the World Wide Web Consortium [W3C]) that defines a "smart pull" technology for webcasting information to users' desktops.
Overview
Based on the Extensible Markup Language (XML), Channel Definition Format (CDF) lets administrators create Active Channels for delivery of content through the user's Web browser and Active Desktop elements and channel screen savers for delivery directly to the user's desktop. Channel content can be personalized, and delivery can be scheduled according to users' needs and preferences. Using CDF also reduces server load and allows delivery of just the needed content, instead of requiring users to download large quantities of unnecessary content.
Consider the delivery of Web content to the user's browser using Active Channels. A Web site can be made into an Active Channel through the addition of a CDF file, a simple text file formatted using XML. It forms a kind of table of contents of the logical subset of the Web site that comprises the Active Channel. A link is then created to the CDF file on the Web site. The user clicks the link to subscribe to the Active Channel and download the CDF file. The Active Channel then appears on the channel bar on the user's desktop. The content for the channel is downloaded to a cache on the user's system. Channel updates are accomplished by scheduled Web crawls, using either the publisher's predefined schedule or a user's customized one. Users can also receive updates to channels by e-mail.
Some of the advantages of using CDF for the distribution of Web information to users include
Simplicity: Turning an existing Web site into a channel merely involves creating a CDF file with a text editor and creating a hyperlink to this file.
Structure: CDF describes how to logically group information in a hierarchical structure, independent of the content format.
Personalization: Standard Hypertext Transfer Protocol (HTTP) cookies can be used to deliver personalized information to users.
Administrator control: The administrator can control how much of the site can be downloaded by users.
User control: The user can use CDF to specify which portions of a site to download to his or her browser, instead of pulling a lot of content off the site and hoping that it contains the needed information.
Notes
CDF is not true webcasting in the sense of Internet Protocol (IP) multicasting because it is a "pull" technology. True webcasting is supported by Microsoft Windows Media Player for delivery of content using IP multi- casting.
In Microsoft Windows Media Player, a mechanism that supplies clients with information needed to receive and render Advanced Streaming Format (ASF) streams.
Overview
A Windows Media Player channel specifies the multicast address and port number the clients should listen to in order to receive the stream. The channel also specifies the data types and formats in the stream, enabling the client to correctly render the stream. Windows Media Player saves channel information as files with the extension .nsc. A Windows Media Player channel is analogous to a television channel or a radio frequency: if a Windows Media Player client is tuned to a channel at the right time, the client receives streaming information sent by the Windows Media Player server.
Windows Media Player channels also support additional features, such as
Roll over to unicast: If clients cannot receive a multicast, you can configure the channel so that they automatically receive a unicast of the same program.
Stream distribution: On corporate Transmission Control Protocol/Internet Protocol (TCP/IP) networks whose routers do not support multicasting, one Windows Media Player server can distribute a stream to other Windows Media Player servers that are each located on separate local area network (LAN) segments. These secondary servers can then multicast to clients on their own LAN segment.
A device that is used to connect a synchronous digital telecommunications line to a computer network.
Overview
Channel Service Units (CSUs) are used to link local area networks (LANs) into a wide area network (WAN) using telecommunications carrier services such as Digital Data Services (DDS), T-carrier services such as a T1 line, and frame relay links.
The function of a CSU is to terminate the carrier's digital line at the customer premises. It also provides signal amplification and allows the carrier to perform remote loopback testing to monitor and troubleshoot the integrity of the line. Some CSUs also support Simple Network Management Protocol (SNMP) features that allow the unit to be monitored by the service provider.
Channel Service Unit (CSU). Using a CSU in a WAN link.
CSUs are always used together with Data Service Units (DSUs), which convert signals from data terminal equipment (DTE) such as a router, switch, multiplexer (MUX), or dedicated server on the LAN to a signaling format suitable for transmission over the line.
Implementation
The service provider interface of the CSU terminates at the telco's digital line where it enters the customer premises. The other interface of the CSU then connects to a DSU, and then the DSU connects with data terminal equipment (DTE) on the LAN (a router, switch, or other LAN device). The DTE is typically an RS-232 or a V.35 serial transmission interface.
Typically, the telecommunications service provider will lease the CSU to the customer, having preconfigured it for the type of digital line to be supported. CSUs usually come in a dual Channel Service Unit/Data Service Unit (CSU/DSU) package that drops into the edge of the network to provide WAN link connectivity. Dedicated stand-alone CSUs are typically used only for interfacing with installed customer premises telecommunications equipment that contains integrated DSUs. This installed equipment could be a channel bank, Private Branch Exchange (PBX), T1 multiplexer, or some other device. Some access routers have built-in CSU/DSUs as well.
See Also Channel Service Unit/Data Service Unit (CSU/DSU) ,Data Service Unit (DSU)
A device that combines the functions of both a Channel Service Unit (CSU) and a Data Service Unit (DSU).
Overview
Channel Service Unit/Data Service Units (CSU/DSUs) are placed between the telephone company network and the customer network at the demarcation point and are the local interfaces between the data terminal equipment (DTE) at the customer premises and the telco's digital communications line (such as a T1 line).
CSU/DSUs package digital data into a format suitable for the particular digital transmission line they are servicing and buffer and rate-adapt digital signals going to and from the telephone company network. CSU/DSUs ensure that data frames are properly formed and timed for the telephone company network and provide a protective barrier to electrical disturbances that can harm customer premises equipment (CPE).
Channel Service Unit/Data Service Unit (CSU/DSU). Using a CSU/DSU to connect two local area networks (LANs) over a wide area network (WAN) link
Implementation
CSU/DSUs essentially function as the digital counterpart to analog modems. They are typically external units that look similar to an external modem, but they can also come in sizes that can be mounted in a rack. Unlike analog modems, CSU/DSUs do not perform signal conversion because the signal at both ends is already digital.
Digital lines usually terminate at customer premises with four-wire connections having various connector types, including RJ-45, four-screw terminal blocks, and M-block connectors (used for V.35 interfaces). The four-wire connection is joined to the appropriate connector on the CSU/DSU. The CSU/DSU typically adjusts itself to the line speed of the digital data service (DDS) line using an autosensing feature. The customer's CSU/DSU then connects directly to the customer's router and from there connects to the customer's network.
At the other end of the DDS line at the central office (CO), the telco has a similar CSU that interfaces with a multiplexer to feed into the carrier's backbone network.
Notes
When purchasing CSU/DSUs, consider first the traffic requirements of your wide area network (WAN) link and make sure they support the full range of data rates for the DDS lines you plan to use (56 K, 64 K, or T1 speed).
See Also Channel Service Unit (CSU) ,Data Service Unit (DSU)
Stands for Challenge Handshake Authentication Protocol, a standard form of challenge/response authentication protocol.
See Also Challenge Handshake Authentication Protocol (CHAP)
A domain in a Microsoft Windows 2000 or Windows .NET Server domain tree whose Domain Name System (DNS) name is a subdomain of the parent domain.
Overview
As an example, if the name of the parent or company domain is microsoft.com, some typical names of child domains might include dev.microsoft.com, marketing.microsoft.com, and support.microsoft.com.
You can create new child domains using the Active Directory Installation Wizard. You must create a child domain in an existing domain tree, because creating a new tree automatically creates a new parent domain. A two-way transitive trust exists between a parent domain and its child domains.
See Also Active Directory ,domain tree
Stands for Customer Information Control System, the multipurpose transaction monitor for IBM mainframe computing environments.
See Also Customer Information Control System (CICS)
Stands for classless interdomain routing, an alternative way of classifying Internet Protocol (IP) addresses from the traditional class A-E system. Also called supernetting.
See Also classless interdomain routing (CIDR)
Stands for Common Internet File System, a public version of the Server Message Block (SMB) file- sharing protocol that has been tuned for use over the Internet.
See Also Common Internet File System (CIFS)
Stands for Common Information Model, a schema for defining manageable network objects.
See Also Common Information Model (CIM)
Part of the Microsoft Windows Management Information (WMI) architecture.
Overview
CIM Object Manager (CIMOM) functions as a broker for object requests within the WMI architecture. CIMOM supplies the required interfaces between management applications (WMI consumers) and a managed physical or logical network object (WMI providers).
CIMOM abstracts management information from a variety of different providers, including the Win32 programming interfaces for Windows applications and the Simple Network Management Protocol (SNMP) for managed network devices. CIMOM then employs the WMI application programming interface (API) to present the gathered information in a consistent fashion to the network management application.
A network management application can register with CIMOM in order to receive notifications when specific network events occur, such as a full disk or a saturated network path. If such a condition arises, the WMI provider for the managed device or application sends a notification to CIMOM, which notifies the registered management application of the condition.
See Also Common Information Model (CIM) ,Windows Management Instrumentation (WMI)
Stands for CIM Object Manager, part of the Microsoft Windows Management Information (WMI) architecture.
See Also CIM Object Manager (CIMOM)
Stands for Classical IP, a method for running Internet Protocol (IP) packets over Asynchronous Transfer Mode (ATM) networks.
See Also Classical IP (CIP)
Stands for Committed Information Rate, a way of guaranteeing bandwidth in frame relay services.
See Also Committed Information Rate (CIR)
A path between two points over which an electrical signal can pass.
Overview
In telecommunications, a circuit is a path over which voice, data, or other analog or digital signals can pass. A physical circuit is a collection of wires or cables that are connected with switches or other devices; it can be thought of as a straight line between the two endpoints.
Circuit. Two basic types of circuits.
For an electrical signal to actually flow between two points in a circuit, the circuit must be closed-that is, there must be a return path for the current. Two basic types of circuits are used in serial transmission for telecommunications technologies:
Balanced circuits, such as those based on the RS-422 interface, use a separate signal path and return path, with two separate wires.
Unbalanced circuits, such as those based on the RS-232 interface, use a single signal path, adding ground to complete the circuit.
Balanced circuits typically support higher data transmission rates because the are less susceptible to noise caused by electromagnetic interference (EMI) than unbalanced circuits.
Notes
A typical serial interface such as RS-232 includes specifications for a number of different types of circuits, including data circuits, control circuits, timing circuits, secondary circuits, and ground connections.
See Also circuit-switched services ,virtual circuit
Any service or server that provides proxy services using a specially installed component on the client computer to form a circuit between the proxy server and the client computer.
Overview
Microsoft Proxy Server is a product that combines firewall and proxy server functions and has two services for providing circuit-level proxy functions:
Winsock Proxy Service: This enables Microsoft Windows Sockets clients such as Microsoft Windows Media Player, RealAudio, and Internet Relay Chat (IRC) to function as if they are directly connected to the Internet. The Winsock Proxy Service provides Windows NT Challenge/Response Authentication with clients, regardless of whether the clients support it, and supports Windows Sockets version 1.1-compatible applications on computers running Windows. The Winsock Proxy Service can control access by port number, protocol, and user or group. Ports can be enabled or disabled for specific users or groups, and the list of users that can initiate outbound connections on a given port can differ from the list of users that can listen for inbound connections on that port.
SOCKS Proxy Service: This includes support for the SOCKS 4.3a protocol. The SOCKS Proxy Service provides support for Macintosh-based and UNIX-based client computers, while the Winsock Proxy Service supports only Windows-based computers. SOCKS uses Transmission Control Protocol (TCP) and can be used to control access to the Telnet, File Transfer Protocol (FTP), Gopher, and Hypertext Transfer Protocol (HTTP) protocols. The SOCKS Proxy Service does not support RealAudio, streaming video, or Windows Media Player clients.
Notes
Circuit layer proxies support a wider variety of protocols than application layer proxies.
See Also application layer proxy
A type of firewall that provides session-level control over network traffic.
Overview
Similar in operation to packet filtering routers, circuit-level gateways operate at a higher layer of the Open Systems Interconnection (OSI) reference model protocol stack. Circuit-level gateways are host-based and reside on individual clients and servers inside the network, rather than on a dedicated machine as they do with other types of firewalls. Circuit-level gateways examine incoming Internet Protocol (IP) packets at the session level-Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)-and act as relays by handing off incoming packets to other hosts. Circuit-level gateways are rarely used as a stand-alone firewall solution; instead, they are typically used in combination with application layer proxy services and packet filtering features in dedicated firewall applications.
Microsoft Proxy Server combines the features of packet filtering, circuit-level gateways, and application layer proxy to provide a full firewall solution for protecting your corporate network. Proxy Server supports both the SOCKS protocol, which provides nontransparent circuit-level gateway security, and the Winsock Proxy, which provides transparent circuit-level gateway security.
See Also firewall ,proxy server
A term describing any telecommunications service that provides switched connections between a telco and their customers.
Overview
Circuit switching is the oldest form of digital communications used by telecommunications carriers. Circuit- switched telecommunications services can be provided to businesses by both local telcos and long-distance carriers. The Public Switched Telephone Network (PSTN) is the classic example of a circuit-switched service (even its earlier analog form as the Plain Old Telephone System [POTS], it was also circuit-switched in operation). Another common example, often used for wide area network (WAN) connectivity between companies and remote branch offices, is Integrated Services Digital Network (ISDN).
Circuit-switched services. How typical circuit-switched services work.
Circuit-switched services are generally more suitable than packet-switched services for real-time transport of delay-sensitive traffic such as voice and video. This is because, after the circuit has been set up for a given connection, traffic is routed with minimal delay to its destination. Circuit-switched services offer guaranteed delivery and differentiated services that are only beginning to be offered by packet-switched services such as Ethernet that carry best-effort Internet Protocol (IP) traffic.
Uses
In the enterprise, circuit-switched services are often used as backup lines for more expensive leased lines. For example, if your more expensive T1 line fails, you can switch to a dial-up ISDN line (if it comes from a different provider). When using circuit-switched services, it is a good idea to monitor their usage because when a certain usage level is reached, leased lines may become economically preferable. Leased lines such as T1 lines use dedicated switches that are set up in a permanent configuration for as long as the customer leases the services.
Implementation
With circuit-switched services, a new switched circuit must be established each time one local area network (LAN) attempts to connect to a remote LAN. Circuit-switched services are temporary circuits only, and when the connection is terminated the circuits are torn down. Different switches can be used for each connection established, depending on availability and traffic, so the quality of services can vary between connections.
Typically, your corporate LAN is connected to these services through bridges, routers, modems, terminal adapters, or other equipment, depending on the type of service being offered. At the other end of the connection is the telco central office (CO), which sets up switches on demand to connect you to your remote branch office LAN. When you disconnect the WAN link, the switches are freed up for other purposes.
Advantages and Disadvantages
One advantage of circuit-switched services is that they are generally less expensive than leased lines. This is because switches are not dedicated to your network as leased lines are, and can therefore be used for other purposes when you are not using them. The cost for circuit-switched services is usually based on usage.
Another advantage of circuit-switched services is that you are not restricted to a single destination as you are with leased lines; you can dial up any destination that supports services similar to yours.
A disadvantage with circuit-switched services is that they are usually dial-up in nature, and a dial-up connection takes time to be established. This connection time varies with the technology used. For example, analog modems might take 10 to 20 seconds to establish a connection, and an ISDN terminal adapter might take only 1 to 2 seconds. This latency interval tends to make circuit-switched lines unsuitable for dedicated services, such as those used for connecting company Web servers to the Internet.
Another disadvantage of circuit-switched services is that the quality can differ substantially between connections, because each circuit is a temporary connection that can exist along different paths, switches, and communication devices.
See Also Integrated Services Digital Network (ISDN) ,leased line ,Multiprotocol Label Switching (MPLS) ,packet-switching services ,T1
A feature of Microsoft Exchange Server whereby transaction logs can be overwritten when full. Circular logging lowers disk space usage but reduces the chances of successfully recovering from a system crash.
Overview
Exchange Server databases, such as the directory database and information store, maintain special log files called transaction log files. These log files improve the performance and fault tolerance of the databases, and help track and maintain changes made to them. Transactions are immediately written both to the log files and to memory, and only afterward to the database files. Transaction logs are normally kept on a different drive from the database files to ensure fault tolerance in case of a disaster that causes data loss, such as a crashed disk or a power failure.
When circular logging is enabled, only a few transaction log files are maintained, and these are overwritten when they become full. This prevents log files from continually building up, which saves disk space. However, circular logging has the disadvantage of allowing you to perform only full backups, rather than incremental or differential ones, because you can restore information only up to the last full backup.
Notes
Do not use circular logging if data recoverability is of high importance to your mail system, which is almost always the case with e-mail. Circular logging is enabled by default. You should always disable it and ensure that you have enough free disk space to hold the transaction files. The only reasons you might want to enable circular logging would be if you run low on disk space or if your server is being used for noncritical data only, such as a public news server.
More precisely called "object class," a logical grouping of objects within Active Directory directory service in Microsoft Windows 2000 and Windows .NET Server.
Overview
Objects are organized within Active Directory by their classes. Examples of object classes can include users, groups, computers, domains, and organizational units (OUs). Each class of objects has its own defining properties or attributes, as laid out in the Active Directory schema. Grouping objects logically into classes makes it easier to find and access these resources on the network.
Active Directory comes with predefined object classes. You can create additional classes or modify existing ones using the Active Directory schema.
See Also Active Directory ,object (Active Directory)
A type of Internet Protocol (IP) network where the first octet of IP addresses ranges between 0 and 126 inclusive.
Overview
Class A networks were originally intended for very large internetworks. Using the default class A subnet mask of 255.0.0.0, each class A network can support a maximum of 16,777,214 individual hosts with unique IP addresses-large enough for the largest of enterprise networks. However, there are no longer any Class A networks available since all 125 of them were assigned in the 1980s to large corporate customers, organizations, and the military. None of these organizations actually has networks large enough to require the full 16 million host addresses provided by their Class A addresses, so classless interdomain routing (CIDR) was developed by the Internet Engineering Task Force (IETF) to allow unused Class A addresses to be reassigned to other users.
Notes
For a huge private network not directly connected to the Internet or hidden behind a firewall using Network Address Translation (NAT), the Internet Assigned Numbers Authority (IANA) recommends using Class A addresses whose first octet is 10. This provides millions of possible host addresses ranging from 10.0.0.1 to 10.255.255.254.
The Class A address 127.0.0.1 is reserved for loopback and represents the local host being used.
See Also Class B ,Class C ,Class D ,Class E ,classful domain IP address
A type of Internet Protocol (IP) network where the first octet of IP addresses ranges between 128 and 191 inclusive.
Overview
Class B networks were originally intended for large internetworks. Using the default Class B subnet mask of 255.255.0.0, each Class B network can support up to 65,534 individual hosts with unique IP addresses-large enough for the largest of enterprise networks. There are 65,536 possible Class B networks that can be assigned, and in the early days of internetworking a number of these network IDs were assigned to large corporations and to the military by the Internet Assigned Numbers Authority (IANA). Today it is virtually impossible to obtain a class B network ID.
Notes
For a large private network not directly connected to the Internet or hidden behind a firewall using Network Address Translation (NAT), IANA recommends using Class B addresses whose first two octets range from 172.16 to 172.31. This provides many thousands of host addresses ranging from 172.16.0.1 through 172.31.255.254 and up to 16 different subnets if required.
See Also Class A ,Class C ,Class D ,Class E IP address
An emerging technology for wide area network (WAN) traffic management.
Overview
Traditionally, WAN traffic for different classes of service (CoS) have been managed using router-based schemes that provide best-effort control of bandwidth allocation. For example, a router might be configured to allow no more than 20 percent of available bandwidth for streaming video. Such a bandwidth allocation is relative (percentage) rather than absolute (bps) in nature, which does not give much granularity for control of traffic, particularly mission-critical network traffic.
Class-based queuing (CBQ) is an emerging technology for WAN access routers that allows network managers to classify traffic types into a hierarchy of classes and then assign absolute bandwidth allocations to each type. For example, CBQ could first be used to divide traffic types according to different kinds of business applications and then to create a second level classifying traffic according to the department using the application. The customer relationship management (CRM) software used by the marketing department could then be assured a given minimum bandwidth.
Implementation
CBQ operates at Level 2 (the network layer) for IP traffic and works with any IP protocol including Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or Internet Control Message Protocol (ICMP). To implement CBQ in a corporate WAN setting, a CBQ-capable access router would be inserted at the WAN edge of each corporate local area network (LAN) and configured with suitable classes to control allocation of bandwidth for WAN traffic. CBQ classes are implemented on routers by using policies.
Class-based queuing (CBQ). Implementing CBQ on a WAN.
See Also wide area network (WAN)
A type of Internet Protocol (IP) network where the first octet of IP addresses ranges between 192 and 223 inclusive.
Overview
Class C networks were originally intended for small internetworks. Using the default Class C subnet mask of 255.255.255.0, each Class C network can support up to 254 individual hosts with unique IP addresses. There are 16,777,216 possible Class C networks that can be assigned, and it is relatively easy to obtain a Class C network ID from your Internet Service Provider (ISP) should your company require it.
Notes
For a small private network not directly connected to the Internet or hidden behind a firewall using Network Address Translation (NAT), the Internet Assigned Numbers Authority (IANA) recommends using Class C addresses whose first three octets range from 192.168.0 through 192.168.255. This provides thousands of host addresses ranging from 192.168.0.1 through 192.168.255.254 and up to 256 different subnets if required.
See Also Class A ,Class C ,Class D ,Class E IP address
A type of Internet Protocol (IP) network where the first octet of IP addresses ranges between 224 and 239 inclusive.
Overview
Class D addresses are used exclusively for multicasting purposes. Most Class D addresses whose first octet is 224 are reserved for special purposes, as shown in the table. For multicasting within a private network not directly connected to the Internet or hidden behind a firewall using Network Address Translation (NAT), the Internet Assigned Numbers Authority (IANA) recommends using Class D addresses whose first octet is 239. For multicasting over the Internet you must first obtain a multicast address from IANA.
Address | Reserved for |
224.0.0.1 | Multicasting to all hosts on the local subnet |
224.0.0.2 | Multicasting to all routers on the local subnet |
224.0.0.4 | Multicasting to all Distance Vector Multicast Routing Protocol (DVMRP) routers |
224.0.0.5 | Multicasting to all Multicast Open Shortest Path First (MOSPF) routers |
224.0.0.9 | Multicasting to all Routing Internet Protocol version 2 (RIPv2) routers |
224.0.0.10 | Multicasting to all Interior Gateway Routing Protocol (IGRP) routers |
224.0.18.255 | Dow Jones multicasting service |
See Also Class A ,Class B ,Class C ,Class E IP address
A type of Internet Protocol (IP) network where the first octet of IP addresses ranges between 240 and 255 inclusive.
Overview
Class E addresses are reserved for research and experimental purposes, and are not used in ordinary computer networking. Class E addresses have a first octet that ranges from 240 to 255. The only Class E address commonly used is 255.255.255.255, which represents a local area network (LAN) broadcast.
See Also Class A ,Class B ,Class C ,Class D IP address
An Internet Protocol (IP) network that uses a default subnet mask.
Overview
A classful domain is an IP network that contains only the single default subnet. All hosts on the network are therefore in the same broadcast domain. For example, a network that is using a Class A address such as 10.0.0.0 and the default subnet mask 255.0.0.0 is a classful domain. The same would be true of a network using a Class B address such as 172.11.0.0 with subnet mask 255.255.0.0 and a network using a Class C address such as 192.16.33.0 with subnet mask 255.255.255.0-all three of these are examples of classful domains and are networks with only one subnet and therefore one broadcast domain.
Generally speaking, classful domains are not the way to go as they are very busy places with respect to traffic and may be susceptible to broadcast storms. Large networks are thus subnetted into a group of subnets forming what is called a classless domain.
See Also classless domain ,IP address ,subnetting
A routing protocol for classful networks.
Overview
Classful routing protocols require that all Internet Protocol (IP) addresses on a network have the same subnet mask. For example, if a Class B network ID of 172.24.0.0 is subnetted into several subnets such as 172.24.1.0, 172.24.2.0, and 172.24.3.0, then classful routing requires that all these subnets have the same subnet mask. The reason for this limitation is that when routers using classful routing protocols exchange routing table updates with one another, the subnet mask is not included in the updates.
Examples of common classful routing protocols include Routing Information Protocol (RIP) versions 1 and 2, and Interior Gateway Routing Protocol (IGRP).
See Also Interior Gateway Routing Protocol (IGRP) ,routing ,Routing Information Protocol (RIP) ,routing protocol
A method for running Internet Protocol (IP) over Asynchronous Transfer Mode (ATM).
Overview
Classical IP (CIP) is an alternative to LAN Emulation (LANE) as a way of transporting IP packets over Asynchronous Transfer Mode (ATM) networks. CIP is based on RFC 1577 and supports only IP and no other network protocols (LANE can also be used to transport multiprotocol traffic such as IP and Internetwork Packet Exchange [IPX] over ATM). CIP has a number of benefits that make it an attractive solution in many situations:
CIP is fast-speeds of 25 megabits per second (Mbps), 155 Mbps, 625Mbps, and 2.4 gigabits per second (Gbps) are supported, making it competitive with Fast Ethernet and Gigabit Ethernet (GbE) as a network transport.
CIP is a circuit-switched technology that provides full bandwidth to each station on the network.
CIP is simpler to implement, manage, and troubleshoot than LANE and utilizes fewer ATM resources (virtual circuits).
CIP utilizes bandwidth effectively by eliminating broadcast traffic (as described later).
Implementation
CIP groups IP hosts together into groupings called logical IP subnets (LISs). Each LIS has an Address Resolution Protocol (ARP) server to support IP broadcasts. Because broadcasts are essential to IP for the operation of the ARP, CIP implements a device called an ARP server to eliminate the need for these broadcasts. This is necessary because ATM in itself does not support broadcasts. When an IP host appears on a CIP network, it first registers itself with the ARP server so that IP communications can take place.
When an IP host needs to communicate with a target host on the network, it passes the IP address of the target host to the ARP server in the LIS, which returns the ATM address of the target host. To make this possible, the ARP server must be preconfigured with mappings of the IP and ATM addresses of each IP host in the LIS. When the host knows the target host's ATM address, a switched virtual circuit (SVC) can be established between the two hosts and IP packets can be transmitted to the target host.
An alternative implementation of CIP is called CIP over PVC (permanent virtual circuit). This is used primarily for wide area network (WAN) connections that are always on.
See Also Asynchronous Transfer Mode (ATM) ,LAN Emulation (LANE)
A way of displaying the desktop and its contents that was first used in Microsoft Windows 95.
Overview
The Classic desktop presents users with a graphical user interface (GUI) that allows icons, shortcuts, files, and folders to be placed on it. These desktop items provide a simple way for users to launch and access frequently used programs and network resources. The Start menu provides another tool for launching programs and accessing resources. The taskbar displays the programs currently running and the Windows-to- network resources that are open.
The choice of GUI for users' client computers can make a big difference in employee productivity. The following are two factors involved in determining whether network administrators should maintain the Classic desktop or upgrade to the newer Active Desktop included with Microsoft Internet Explorer beginning with version 4:
The cost of introducing users to the new desktop paradigm
The necessity for tight integration among the desktop, the corporate network, and the Internet
Classic desktop. The Classic desktop, first used in Windows 95.
An Internet Protocol (IP) network that uses a variable-bit subnet mask.
Overview
A classful domain is an IP network that uses a default subnet mask such as 255.0.0.0 for Class A networks, 255.255.0.0 for Class B, or 255.255.255.0 for Class C. Classful domains thus have only one subnet and one broadcast domain. By contrast, a classless domain is an IP network that uses a variable-bit subnet mask (VBSM), also called a classless subnet mask, to divide the network into two or more subnets, each of which represents a different broadcast domain.
Classless domains are generally built using routers. Each subnet within a classless domain constitutes a unique broadcast domain and collision domain, which generally improves performance over using a single broadcast and collision domain. Switches operate at the data-link layer and can partition a network into different collision domains, but they do not create different broadcast domains.
See Also classful domain ,IP address ,subnetting
Also called supernetting, an alternative way of classifying Internet Protocol (IP) addresses from the traditional Class A-E system.
Overview
Classless interdomain routing (CIDR) is a more efficient routing mechanism than the original method of segregating network IP addresses into classes named Class A, B, and C. The trouble with the old system is that it leaves too many unused IP addresses. For example, while Class A networks support large numbers of network nodes, there are not enough Class A networks to go around and none of the owners of these networks make anywhere near full use of the large number of IP addresses available to them. As a result, large numbers of Class A (and Class B) IP addresses go unused, and CIDR was developed as a way of reclaiming those IP addresses for allocation elsewhere. Similarly, while many Class C network IDs are available, many companies require more than the 254 IP addresses available on a Class C network, but not nearly as many as the 65,534 IP addresses available on any Class B network.
History
In the late 1980s, prescient architects of the Internet foresaw that the standard Class A-E method for assigning IP addresses would eventually fail and that the routing tables used by the core routers of the Internet's backbones would eventually grow unmanageably large. The class system provides for a huge number of IP addresses, but for only about 2 million different IP networks. As the number of networks attached to the Internet grew exponentially, a time was anticipated when there would be no more network numbers left to assign for new Class B and C networks (all Class A networks were assigned early on).
In the early 1990s the Internet Engineering Task Force (IETF) produced a group of Requests for Comments (RFCs), namely RFCs 1517 to 1520, that brought a way out of the dilemma. These RFCs formed the basis of CIDR and provided a way of not only reducing the growing load on the Internet's core routing tables (another result of the Internet's rapid growth) but also of reusing unused IP addresses to make about 8 million additional IP networks of Class C size available for assignment to other companies and organizations.
Uses
CIDR is used primarily by routers and gateways on the backbone of the Internet for routing packets across the Internet. CIDR is not used much in private networks because most networks are hidden behind firewalls and can use any arbitrary block of IP addresses, such as the 10.x.y.z block allocated by Internet Network Information Center (InterNIC) for general, private use. Instead, CIDR comes into its own on the Internet backbone to facilitate routing and ensure the continued functioning of the Internet. However, CIDR is viewed only as a workaround to the issues of insufficient numbers of IP networks available for allocation and maintaining the routing tables of backbone routers at workable sizes. Most Internet architects see IPv6 as the real solution to these issues and expect the need to change over to this system of addressing in the next few years.
Implementation
CIDR replaces the old class method of allocating 8, 16, or 24 bits to the network ID, and instead allows any number of contiguous bits in the IP address to be allocated as the network ID. For example, if a company needs a few thousand IP addresses for its network, it can allocate 11 or 12 bits of the address for the network ID instead of 8 bits for a Class C (which would not work because you would need to use several Class C networks) or 16 bits for Class B (which is wasteful).
CIDR assigns a numerical prefix to each IP address. For example, a typical destination IP address using CIDR might be 177.67.5.44/13 (the last part being pronounced "slash thirteen"). The suffix /13 indicates that the first 13 bits of the IP address identify the network, while the remaining 32-13 = 19 bits identify the host. In subnetting notation, the CIDR address 177.67.5.44/13 would be equivalent to the combination of IP address 177.67.5.44 and subnet mask 255.255.128.0 (see the following table). As another example, in CIDR notation an old style class B network 132.16.0.0 with default subnet mask 255.255.0.0 would be represented simply as 132.16/16.
The prefix helps to identify the Internet destination gateway or group of gateways to which the packet will be forwarded. Prefixes vary in size, with longer prefixes indicating more specific destinations. Routers use the longest possible prefix in their routing tables when determining how to forward each packet. CIDR enables packets to be sent to groups of networks instead of to individual networks, which considerably simplifies the complex routing tables of the Internet's backbone routers. The table shows the different CIDR values and their subnet mask equivalents.
Class | CIDR Value | Subnet Mask |
Class A | /8 | 255.0.0.0 |
/9 | 255.128.0.0 | |
/10 | 255.192.0.0 | |
/11 | 255.224.0.0 | |
/12 | 255.240.0.0 | |
/13 | 255.248.0.0 | |
/14 | 255.252.0.0 | |
/15 | 255.254.0.0 | |
/16 | 255.255.0.0 | |
Class B | /17 | 255.255.128.0 |
/18 | 255.255.192.0 | |
/19 | 255.255.224.0 | |
/20 | 255.255.240.0 | |
/21 | 255.255.248.0 | |
/22 | 255.255.252.0 | |
/23 | 255.255.254.0 | |
/24 | 255.255.255.0 | |
Class C | /25 | 255.255.255.128 |
/26 | 255.255.255.192 | |
/27 | 255.255.255.224 | |
/28 | 255.255.255.240 | |
/29 | 255.255.255.248 | |
/30 | 255.255.255.252 | |
/31 | 255.255.255.254 | |
/32 | 255.255.255.255 |
Notes
There might seem to be an alternate solution for avoiding the waste of IP addresses from Class A and Class B networks. Consider, for example, a company with a network of 5,000 nodes. Assigning a single Class B network ID such as 166.33.0.0 to the network would provide 65,534 possible IP addresses for hosts, which is far too many-60,534 addresses would remain unused. A solution to this might seem to be assigning the company a contiguous set of 20 Class C network IDs. Because each Class C address provides 254 possible host addresses, 20 contiguous Class C network Ids, such as 198.15.1.0 through 198.15.20.0, would provide 20 x 254 = 5080 possible IP addresses, which is just right, plus a few to spare.
Using this technique does eliminate the wasting of IP addresses, but it creates a new problem: to handle routing between your corporate network and the Internet, you need to add 20 new entries to the routing tables on Internet routers, one entry for each network ID you are using. Following this method quickly overwhelms the routers that form the Internet's backbone because as their routing tables grow their performance slows down. CIDR was devised to address this very issue by decreasing the number of entries required in the Internet's routing tables. CIDR does this by supernetting the 20 Class C networks above into a single supernet, which needs only one entry in the routing tables.
See Also Internet ,IP address ,IPv6 ,routing
A routing protocol for classless networks.
Overview
Classless routing protocols allow different portions of an Internet Protocol (IP) network to use different subnet masks for the same network ID. Then, when routers using classless routing protocols exchange routing table updates with one another, the subnet mask is included in each update. Thus, given a single IP network ID, variable length subnet masking (VLSM) may be used to create different subnets having different subnet masks. Classless routing protocols also help conserve network resources by using route summarization.
Examples of common classless routing protocols include Border Gateway Protocol (BGP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF).
See Also Border Gateway Protocol (BGP) ,Enhanced Interior Gateway Routing Protocol (EIGRP) ,Open Shortest Path First (OSPF) ,routing ,routing protocol
Stands for Component Load Balancing, a Microsoft clustering technology supported by Microsoft Windows 2000 Server and Windows .NET Server and provided by Microsoft Application Center 2000.
See Also Component Load Balancing (CLB)
The process of sending data over a network in an unencrypted form.
Overview
By using a packet sniffer or software such as Network Monitor, anyone who can capture cleartext packets can read the information within them. Cleartext authentication methods are sometimes the best choice in a heterogeneous network environment where users running different operating system platforms need to access resources on network servers. For example, UNIX clients that need to access a Microsoft Internet Information Services (IIS) machine using a Web browser will need to be authenticated using a cleartext method called Basic Authentication.
Notes
Basic Authentication is usually described as cleartext authentication, but in actuality Basic Authentication weakly encrypts data using the well-known Uuencoding algorithm. This algorithm is in the public domain and can easily be decrypted by knowledgeable users.
The standard AppleTalk protocol uses clear-text authentication for allowing Apple Macintosh clients to access shared folders on Macintosh file servers using AppleShare.
See Also Basic authentication
Also called Basic authentication, an authentication scheme that passes a user's credentials over a network in encrypted form.
See Also Basic authentication
A new display technology from Microsoft Corporation designed for liquid crystal display (LCD) displays.
Overview
Microsoft ClearType is intended to provide more readable displays on LCD screens for laptops, tablet PCs, and Pocket PCs. ClearType does this by a process known as sub-pixel rendering, which triples the perceived number of pixels for vertical screen resolution (horizontal resolution remains unchanged). The results are easier-to-read text that appears more like a page out of a book than text displayed on a screen. ClearType was designed with Microsoft Reader in mind, a technology developed by Microsoft for eBooks.
ClearType technology was developed by Microsoft Research, but a similar subpixel rendering concept was developed in the 1970s by Steve Gibson for the Apple II platform. ClearType support will be included in all future releases of Microsoft Windows and Microsoft Office.
For More Information
You can find out more about ClearType at research.microsoft.com
Stands for Competitive Local Exchange Carrier, a telco that competes with existing incumbent telcos under the terms of the Telecommunications Act of 1996.
See Also Competitive Local Exchange Carrier (CLEC)
A workstation or computer, usually belonging to a single user, as opposed to a server, which is shared by many users.
Overview
Planning the hardware, software, configuration, deployment, and maintenance of clients is as important to the network administrator as the other server-related activities.
Choice of a client operating system depends on various considerations. For example, in determining whether to install Microsoft Windows Millennium Edition (Me), Windows 2000 Professional, or Windows XP Professional on client computers, users should consider the following:
Both client operating systems, in conjunction with Microsoft Internet Explorer versions 5 and later, offer the same desktop configuration options, similar utilities, and similar support for features such as user profiles, hardware profiles, and system policies.
Windows 2000 Professional or Windows XP Professional will provide client machines with better performance, greater reliability, and more robust security, but they have higher hardware requirements than Windows Millennium Edition (Me).
Windows Me supports a broader range of devices and legacy software applications, and includes power-management support-making it a better solution for mobile users.
Notes
In configuring clients to operate on a network, appropriate software must be installed on each client to allow it to access servers on the network. For example, to access Windows 2000 servers, client machines require Microsoft client software such as Client for Microsoft Networks. To access Novell NetWare servers, client machines require NetWare-compatible clients, such as Client for NetWare Networks.
See Also client/server ,server
A license that grants a client machine access to a Microsoft BackOffice product running on a network of computers.
Overview
Every client computer on a network, regardless of whether it is running a Microsoft or non-Microsoft operating system, requires a client access license (CAL) if it will be accessing any of the following Microsoft Windows NT, Windows 2000, Windows XP, or Windows .NET Server services:
File services, for accessing shared files and folders on a server
Print services, for accessing shared network printers
Remote Access Service (RAS) or Routing and Remote Access Service (RRAS)
File and Print Services for NetWare (FPNW)
File and Print Services for Macintosh (FSM)
Microsoft Transaction Server (MTS) and Microsoft Message Queue (MSMQ) Server access
Windows NT Terminal Server functionality
Client access licenses can operate in one of two modes:
Per Server licensing, which is based on concurrency of access to network resources
Per Seat licensing, which is the more commonly implemented solution and is supported by all BackOffice applications
See Also license
A digital certificate obtained for a client application (such as a Web browser) that can be used by the client to digitally sign data it transmits.
Overview
Client certificates can be used to enable client machine authentication for the purpose of secure communication over the Internet using the Secure Sockets Layer (SSL) protocol.
Client certificate. Importing a client certificate in the Internet Explorer Properties dialog box.
A client obtains a certificate from a certificate authority (CA) by submitting a certificate request file. The CA responds by issuing a client certificate, which contains the client's identification information in encrypted form, along with the client's public key. The client certificate must then be installed on the client's Web browser. Microsoft Internet Explorer can import client certificates into the browser's certificate store using the Personal button on the Content tab of the Internet Options dialog box. Administrators can also use the Internet Explorer Administration Kit (IEAK) for preconfiguring client certificates prior to installation on user computers.
In SSL communication, a Web server can validate the identity of a client using the certificate installed on the client. With Microsoft Internet Information Server (IIS) version 4, client certificates can be mapped to Microsoft Windows NT user accounts by way of a process called certificate mapping. (Windows 2000, Windows XP, and Windows .NET Server support a similar feature in their Internet Information Services.) Certificate mapping makes it easier for administrators to control access to content located on the Web server.
Notes
Use client certificates when it is important for servers to validate the identity of clients-for example, when your enterprise includes mobile users with laptops who need to remotely and securely access the company's intranet server using Internet Explorer.
See Also digital certificate ,public key cryptography
A networking component in Microsoft Windows 95, Windows 98, Windows Millennium Edition (Me), Windows 2000, Windows XP, and Windows .NET Server that makes it possible to access file and print services on all of the above versions of Windows, as well as Windows NT, Windows for Workgroups, and LAN Manager dedicated servers and peer servers.
Overview
Client for Microsoft Networks works with any combination of NetBEUI, IPX/SPX-Compatible Protocol, and Transmission Control Protocol/Internet Protocol (TCP/IP) protocols. Client for Microsoft Networks cannot be used for accessing non-Microsoft servers such as Novell NetWare servers. You must install Client for NetWare Networks to access these servers. Windows 95 and Windows 98 allow you to install more than one client at a time to access different kinds of servers on the network.
Use the Network utility in Control Panel to install Client for Microsoft Networks on a computer running Windows 95 or Windows 98. Then use the property sheet of Client for Microsoft Networks to configure the computer to either participate in a workgroup or log on to a Windows NT, Windows 2000, or Windows .NET Server domain.
Notes
In Windows NT, the equivalent component is called the Workstation service, but in Windows 2000, Windows XP, and Windows .NET Server, the component is Client for Microsoft Networks, as in Windows 95 and Windows 98.
In Microsoft Windows 95, Windows 98, Windows Millennium Edition (Me), Windows 2000, Windows XP, and Windows .NET Server, a networking component that makes it possible to access file and print services on Novell NetWare servers.
Overview
Client for NetWare Networks requires that the IPX/SPX-Compatible Protocol be installed. Client for NetWare Networks cannot be used for accessing Microsoft servers such as Windows NT, Windows 2000, and Windows .NET Server. You must install Client for Microsoft Networks to access these servers. Windows 95 and Windows 98 allow you to install more than one client at a time to access different kinds of servers on the network.
Use the Network utility in Control Panel to install Client for NetWare Networks on a computer running Windows 95 or Windows 98. Then use the property sheet of Client for NetWare Networks to configure the preferred NetWare server, to select the first drive letter to use for mapping network drives from NetWare command-line utilities, and to enable processing of logon scripts on the preferred server.
Notes
Client for NetWare Networks can connect to NetWare 3 and earlier servers, or NetWare 4 servers running in bindery emulation mode. If you want to use Client for NetWare Networks to connect to a NetWare 4 server running Novell Directory Services (NDS), you must also install Service for NetWare Directory Services on the Windows 95 or Windows 98 client. This service is available with Windows 95 OSR2 or Service Pack 1 for Windows 95, and is included with Windows 98.
Before installing Client for NetWare Networks on a computer running Windows 95 or Windows 98, make sure you remove any real-mode NetWare requestor software running on the machine, such as NETX (the NetWare 3.x client shell) or VLM (the NetWare 4.x client shell).
A shared directory on a network file server to which users on your network can connect to install client software locally on their client computers.
Overview
Creating a client installation is the first step in preparing to install software over the network. To create a client installation point, create a directory on a server and share the folder with full permissions for administrators and read-only permissions for ordinary users. Either copy the installation files for the software from the CD to the shared directory, or run the setup program using a special switch to copy the files so that they can be used for network installation-for example, to uncompress the cabinet files on the CD. Users can then connect to the shared directory, run the setup program, and complete the installation process.
A paradigm for deploying two-tiered distributed applications.
Overview
In the client/server model, an application is split into a front-end client component and a back-end server component. The front-end client part of the application runs on a workstation and receives data that is input by the user. The client component prepares the data for the server by preprocessing it in some fashion, and then sends the processed information to the server, usually in the form of a request for some service. The back-end server component receives the client's request, processes it, and returns information to the client. The client receives the information returned from the server and presents it to the user by way of its user interface. Usually most of the processing is done at the back end (server end) where database servers, messaging servers, file servers, and other resources are located.
An example of a simple client/server application is a Web application that is designed for Microsoft Internet Information Services (IIS) using a combination of server-side Active Server Pages (ASP) programming and client-side scripting. The ASP scripts run on the Web server, while the client-side scripts run on the client Web browser.
See Also distributed application
A Microsoft Windows 2000, Windows XP, and Windows .NET Server service that provides Windows clients with access to Novell NetWare file, print, and directory services.
Overview
Client Services for NetWare (CSNW) is an optional service that can be installed on Windows 2000, Windows XP, and Windows .NET Server machines to enable them to directly connect to file and print resources on Novell NetWare servers. In other words, CSNW is a Microsoft version of the NetWare redirector for Windows 2000, Windows XP, and Windows .NET Server.
Client Services for NetWare (CSNW). How CSNW works.
CSNW is a full-featured, 32-bit client for NetWare networks that can be installed on Windows 2000 Professional or Windows XP Professional by using the Network and Dial-Up Connections utility in Control Panel. If you are connecting to a NetWare 3.12 or earlier server, you must specify a preferred NetWare server for access to its bindery. If you are connecting to NetWare 4, specify the Novell Directory Services (NDS) tree and default context. CSNW supports browsing NDS trees, but it does not support administration of NDS trees.
CSNW supports connections to servers running version 2, 3, or 4 of NetWare, including both bindery emulation and NDS on NetWare 4. CSNW includes support for NetWare Core Protocol (NCP), Large Internet Protocol (LIP), and Long filenames (LFNs). Additional options are included for printing and login script support. CSNW requires installation of the NWLink protocol, but if it is not installed already, it will be added automatically when you install CSNW on a machine.
Notes
On a machine running Windows 2000 Server, NetWare connectivity is provided by Gateway Services for NetWare (GSNW). Use CSNW to provide your Windows 2000 Professional or Windows XP Professional clients with dedicated access to Novell NetWare servers; use GSNW only to provide occasional access to NetWare servers from Windows 2000 Professional or Windows XP Professional clients.
An alternative to using CSNW on Windows 2000 Professional machines is to use the Novell Client for Windows 2000.
See Also bindery ,Gateway Service for NetWare (GSNW) ,Novell Directory Services (NDS) ,NWLink
In networking, any part of the network whose data transmission paths are unpredictable and vary from session to session.
Overview
Clouds are often used in networking diagrams to represent packet-switching services. In these services, a packet sent from one node to another follows an unpredictable path because, at any given moment, different routers or other devices can be used to forward the packet toward its destination. The Internet is an example of a packet-switching cloud for Transmission Control Protocol/Internet Protocol (TCP/IP) networking because data sent between two points can travel over many possible paths. This is why the Internet is graphically represented as a cloud in drawings of wide area networks (WANs). Other examples of packet-switching services include frame relay and X.25 networks.
Cloud. The Internet depicted as a cloud of paths and connections.
Circuit-switched services are often represented as clouds as well. In circuit-switched services, communication switches at various telco and carrier central offices (COs) and switching facilities are temporarily used for establishing circuits between two communicating nodes. Each time communication is terminated and reestablished, different sets of switches can be used.
See Also telecommunications services
A group of two or more nodes within a system supporting clustering.
Overview
When a client on a network tries to access shared resources or applications on a cluster, the cluster appears to the client as a single node or server instead of the group of nodes or servers it really is.
In the Cluster service of Microsoft Windows 2000 and Windows .NET Server Enterprise Server and Datacenter Server, each node in a cluster is a completely independent computer system that must be running Windows 2000 Enterprise Server. Typically, such nodes are connected by a shared storage bus such as an external Small Computer System Interface (SCSI) disk subsystem or RAID array.
See Also clustering
An application that can run on a node of a cluster and can be managed as a cluster resource.
Overview
In the Cluster service on Microsoft Windows 2000 or Windows .NET Server Enterprise Server and Datacenter Server editions, a cluster-aware application is one that can run on a node of a cluster and can be managed as a cluster resource. Cluster-aware applications can be written to access the Windows 2000, or Windows .NET Server Cluster service by using its cluster application programming interface (API). Cluster-aware applications also implement the extension dynamic-link libraries (DLLs) of Cluster Administrator, which allow them to be managed using Cluster Administrator. These features allow developers to write high-scalability applications that can operate across the different nodes in a cluster.
A cluster-aware application is one that is aware of the fact that it is running on a cluster and can make use of the scalability, load balancing, and failover aspects of clustering to provide high availability for mission-critical business environments. Cluster-aware applications include database applications such as Microsoft SQL Server, messaging applications such as Microsoft Exchange Server, and Web applications for running on Web servers such as Microsoft Internet Information Services (IIS).
See Also clustering
Any technology that enables two or more servers to appear to clients as a single system.
Overview
A cluster consists of a group of computers functioning together as a unit, running a common set of applications, and presenting a single image to client systems. Clustering can be implemented in various ways, but its basic goals are to provide businesses with high availability, high reliability, and high scalability solutions for mission critical business operations.
Clustering solutions generally come in two basic types:
Stateful clustering: The goal of this type of clustering is to provide high availability and high reliability for fast, uninterrupted service in high- demand environments that can tolerate minimal downtime (stateful clustering is not designed to scale applications out to handle more users-this is the purpose of stateless clustering discussed next). Stateful clustering works by connecting independent computer systems into a single entity called a cluster, with each system within the cluster being called a node. Generally some multiple of two is used as the number of nodes within a cluster, for example, 2, 4, 8, 16, or 32 nodes. The different nodes within a cluster are usually connected using a shared disk subsystem which typically consists of a hard disk system or RAID-5 array connected to each node using a fast Small Computer System Interface (SCSI) bus or fiber channel connection. The result is a cluster of computer systems that acts and functions as if it were a single system. In stateful clustering, the nodes within a cluster generally share the workload, and when one node fails its workload fails over (transfers to) another node in the cluster with no interruption of services from the user's perspective. When the failed node comes back online, the workload fails back to this node and normal operation of the cluster resumes. Failover in stateful clustering systems can be implemented in different ways, and this is discussed below. An example of a platform supporting stateful clustering is the Cluster service of Microsoft Windows 2000 Enterprise Server and Datacenter Server. Another name for stateful clustering is shared storage or shared something clustering.
Stateless clustering: The goal of this type of clustering is to provide high availability and high reliability by enabling administrators to scale out applications to meet increased demand as the number of users and traffic generated increase. Stateless clustering uses a group of nodes (servers) that are not connected in any way apart from the underlying network connectivity. No failover occurs between nodes when a node in the cluster fails. Instead, some form of load balancing is used to share the workload between the different nodes, and if one node fails the other nodes pick up the extra workload with no interruption of services. Stateless clustering is supported by three Microsoft products: Network Load Balancing (NLB), Component Load Balancing (CLB), and Application Center 2000. Another name for stateless clustering is shared- nothing clustering, and SQL Server 2000 supports a form of shared-nothing clustering known as Federated Server Groups.
Stateful clustering solutions themselves generally fall into three different categories, depending on if and how failover occurs between different nodes:
Active/active clustering: This type of clustering makes the most efficient use of system resources because there are no redundant nodes: all nodes run active processes. If one node of a cluster fails, other nodes take on the failed cluster's workload. The latency for failover in this scenario is typically 15 to 150 seconds, depending on the hardware/software configuration. Active/active clustering is supported by the Cluster service of Microsoft Windows 2000 Enterprise Server and Datacenter Server, and by the Cluster service of Windows .NET Server (discussed later in this article).
Active/standby clustering: Nodes are paired within a cluster, with one node designated to take over should another node fail. If an active node fails, a standby node assumes its workload. Latency for failover is also 15 to 150 seconds. Active/standby clustering is a more expensive solution than active/active because the standby node is essentially doing nothing unless the active node fails.
Fault-tolerant clustering: Nodes are paired within a cluster, and all nodes perform all tasks simultaneously. This is an expensive solution from a hardware point of view, but latency for failover is reduced to a second or less.
Marketplace
Many different clustering solutions are in the marketplace, but this article focuses on four different clustering technologies delivered by Microsoft platforms and products, namely:
Windows clustering, previously known as Microsoft Cluster Services (MCSC)
Network Load Balancing (NLB)
Application Center 2000
Component Load Balancing (CLB)
You can find additional information in separate articles on each of these four solutions.
Windows clustering is a feature of Microsoft Windows 2000 Advanced Server and Datacenter Server and of Windows .NET Server. Windows clustering is probably Microsoft's best-known clustering platform and was originally developed for Microsoft Windows NT Server Enterprise Edition where it was code-named Wolfpack during its development. Windows clustering is a stateful clustering solution that enables system architects to create clusters from groups of independent computer systems and to run and manage cluster-aware applications. Using Windows clustering, you can build two-way clusters (that is, clusters with only two nodes) on Windows 2000 and Windows .NET Server Enterprise Server edition or four- way clusters on Windows 2000 and Windows .NET Server Datacenter Server edition (Windows NT Server Enterprise Edition supported only two-way clustering). In Windows clustering a cluster connects nodes together using a shared file system and clusters can utilize active/active clustering for maximum reliability and availability. Windows clustering makes an excellent choice for clustering database and messaging applications for enterprises.
Network Load Balancing (NLB) is a stateless clustering solution included with Windows 2000 and Windows .NET Server Enterprise Server and Datacenter Server editions, and it was formerly called Windows Load Balancing Services (WLBS) on the Windows NT Server 4 platform. NLB provides load balancing of Internet Protocol (IP) traffic to up to 32 independent network nodes (servers) and is typically used to build farms of Web servers or Exchange 2000 Outlook Web Access (OWA) servers for large enterprises. When one node in an NLB cluster goes down, the load is simply redistributed to the remaining nodes.
Application Center 2000 is a part of Microsoft Corporation's .NET Server family, and is a stateless clustering platform designed to provide a single point of management for farms of Web servers. Appcenter is typically used in conjunction with NLB and CLB to provide high availibity, high reliability clustering that can scale out to large numbers of users. Appcenter manages a collection of servers in a Web farm as a single entity and can be used to create new clusters, join servers to existing cluster, remove nodes from clusters, deploy applications and application components to different nodes within a cluster, move components between nodes of a cluster, monitor the performance of a cluster, and manage load balancing of network connections to cluster nodes and COM+ components within a cluster-aware application.
Component Load Balancing (CLB) is supported by all versions of Windows 2000 Server and is used to provide load balancing of COM+ objects across distributed applications deployed on up to 16 nodes (servers). CLB is a stateless clustering solution that requires no special hardware but needs Microsoft Application Center 2000 in order to operate.
See Also Application Center, cluster, cluster-aware application, Component Load Balancing (CLB)
Stands for Connection Manager Administration Kit, a wizard-based tool for creating custom connectivity solutions, and a component of Internet Connection Services for Microsoft Remote Access Service (RAS).
See Also Connection Manager Administration Kit (CMAK)
A grade of cabling that is resistant to combustion.
See Also plenum cabling
Stands for connected network; in Microsoft Message Queue (MSMQ) Server terminology, a name for a collection of computers in which any two computers can directly communicate.
See Also connected network (CN)
Stands for Canonical Name record, a Domain Name System (DNS) resource record for assigning an alias to a host.
Overview
A CNAME record is used to map an alias to the canonical name (true name) of a host on the Internet or a private Internet Protocol (IP) internetwork. The CNAME record thus lets you use more than one name to refer to a single host on the network. If a name server is queried by a resolver to look up a host and the queried name is an alias in a CNAME record, the name server replaces the alias name with the canonical name of the host being looked up and then looks up the address of the canonical name.
Examples
Here is an address record for the host named server12 in the microsoft.com Internet domain. This record has the IP address 172.16.8.5, followed by a CNAME record indicating that the name bobby (or the fully qualified domain name bobby.microsoft.com) is an alias for the same host:
server12.microsoft.com IN A 172.16.8.55 bobby IN CNAME server12
Uses
CNAME records are generally used to hide the true names of important servers on your network from the outside world.
Stands for Common Name Resolution Protocol, a proposed Internet Engineering Task Force (IETF) standard for a protocol to replace Uniform Resource Locators (URLs) with a simpler, more natural scheme for navigating the Web.
See Also Common Name Resolution Protocol (CNRP)
Stands for central office, a telco switching facility.
See Also central office (CO)
Short for coaxial cabling, a legacy form of network cabling.
See Also coaxial cabling
A legacy form of network cabling.
Overview
Coaxial cabling (or simply coax) is a form of network cabling that was used in legacy 10Base2 and 10Base5 Ethernet networks. The name coax comes from its two-conductor construction in which the conductors run concentrically with each other along the axis of the cable. Coaxial cabling in local area networks (LANs) has been largely replaced by twisted-pair cabling (structured wiring installations) within buildings, and by fiber-optic cabling for high-speed network backbones.
Coaxial cabling. Typical coax cabling.
Coaxial cabling generally consists of a solid copper core for carrying the signal, covered with successive layers of inner insulation, aluminum foil, a copper braided mesh, and outer protective insulation. A solid conductor provides better conductivity than a stranded one, but is less flexible and more difficult to install. The insulation is usually polyvinyl chloride (PVC) or a nonstick coating; the aluminum foil and copper mesh provide shielding for the inner copper core. The mesh also provides the point of grounding for the cable to complete the circuit.
Types
Coaxial cabling comes in various types and grades. The most common are the following:
Thicknet cabling, which is an earlier form of cabling used for legacy 10Base5 Ethernet backbone installations. This cabling is generally yellow and is referred to as RG-8 or N-series cabling. Strictly speaking, only cabling labeled as IEEE 802.3 cabling is true thicknet cabling.
Thinnet coaxial cabling, which is used in 10Base2 networks for small Ethernet installations. This grade of coaxial cabling is generally designated as RG- 58A/U cabling, which has a stranded conductor and a 53-ohm impedance. This kind of cabling uses bayonet Neill-Concelman (BNC) connectors for connecting to other networking components, and must have terminators at free ends to prevent signal bounce.
ARCNET cabling, which uses thin coaxial cabling called RG-62 cabling with an impedance of 93 ohms.
RG-59 cabling, which is used for cable television (CATV) connections.
In addition, a number of special types of coaxial cabling are sometimes used for certain networking purposes. An example is twinax cabling, which consists of two conductors first enclosed in their own insulation and then enclosed in a single copper mesh and insulating jacket. Twinax is used in legacy IBM networks for connecting AS/400 systems to 5250 terminals. Other more exotic varieties include triax, quadrax, and ribbon types of coaxial cables.
Notes
Coaxial cabling is often used in heavy industrial environments where motors and generators produce a lot of electromagnetic interference (EMI), and where more expensive fiber-optic cabling is unnecessary because of the slow data rates needed. Coaxial cabling is also used frequently in IBM mainframe and minicomputer environments. A device called a splitter can be used to fork one coaxial cable into two-for example, when connecting two 3270 terminals to one IBM mainframe system. A splitter is used at either end of the connection so that the signals for both terminals can be sent over a single coaxial cable. Coax multiplexers can be used to connect eight or more terminals to a single controller.
See Also cabling ,fiber-optic cabling ,twinax cabling ,twisted-pair cabling
Short for co mpressor/dec ompressor, an encoding algorithm used for recording digital audio or video.
Overview
A codec compresses transmitted data at the sending end and decompresses it at the receiving end. Microsoft Windows Media Player uses different codecs to provide streaming multimedia information over a Transmission Control Protocol/Internet Protocol (TCP/IP) network such as the Internet. Windows Media Player provides a number of different codecs for different purposes. You can select a codec to give you the audio or image quality and image size that you want for your transmission.
A second-generation (2G) digital cellular phone technology popular in the United States and parts of Asia.
Overview
Code Division Multiple Access (CDMA) can be used to refer both to a type of digital cellular phone system and to the specific media access method used by this kind of cellular system. CDMA was developed by QUALCOMM in 1993, and it was adopted and ratified by the Telecommunications Industry Association (TIA) as part of their Interim Standard 95, specifically as TIA standard IS-95a.
CDMA supports combined voice and data over a single channel and supports circuit-switched data transmission at a rate of 14.4 kilobits per second (Kbps), although in practice speeds are more typically around 13 Kbps. Speeds up to 19.2 Kbps are also possible by using special error detection and correction techniques.
Architecture
CDMA uses the spread spectrum wireless networking technology-first developed for military communication systems in the 1940s because it spreads its transmission over a large bandwidth, making it difficult to jam. Instead of dividing the available radio spectrum into a series of discrete channels using the older Time Division Multiple Access (TDMA) media access method, a CDMA channel occupies the entire available frequency band. In other words, all CDMA users on a given network utilize the same frequency band.
What enables users to share the same spectrum is that CDMA assigns a special digital code sequence to each user. Users thus share time and frequency resources on the available bandwidth, and their individual communications are channeled using these codes. The code tag identifies the conversation to the transmission station and enables multiple users to simultaneously access the network and divide its frequency resources between them-hence the name Code-Division Multiple Access.
Without knowledge of a conversation's code tag, eavesdropping on CDMA conversations is difficult, making CDMA a more secure cellular phone technology than the Advanced Mobile Phone Service (AMPS) still used widely in the United States. CDMA also has a much higher call capacity than AMPS and is comparable to the Global System for Mobile Communications (GSM) standard for cellular communication used in Europe. The disadvantage is that CDMA is more complex to implement than TDMA digital cellular technologies.
Marketplace
The main CDMA-based system in the market today is QUALCOMM's cdmaOne, the operation of which is now supervised by the CDMA Development Group and independent organization. The cdmaOne system uses 64 different codes called Walsh sequences, and in theory thus supports up to 64 concurrent users talking over a single 1.25-megahertz (MHz)-wide channel. In practice, however, this works out to more like 20 concurrent users, especially when data transmissions are included. The cdmaOne system has approximately 30 million users in the United States and Asia, with its competitors being Global System for Mobile Communications (GSM), which has 150 million users worldwide; Digital Advanced Mobile Phone Service (D-AMPS), with 15 million users in the United States; and Personal Digital Cellular (PDC), with 45 million users in Japan.
A special upgrade to cdmaOne called IS-95b provides improved data speeds of up to 115 Kbps, though real speeds are usually more like 64 Kbps (the version of IS-95b in Japan and Korea supports packet-switched data rates of only 64 Kbps). Further proposed 2.5G and third-generation (3G) improvements fall under the umbrella name of CDMA2000, and the CDMA technology itself forms the basis of much of the proposed IMT-2000 standard for 3G cellular communications from the International Telecommunication Union (ITU).
Another cellular system in the United States based on CDMA technologies is Sprint PCS from Sprint Corporation, which operates at a higher frequency band than cdmaOne but uses CDMA as its media access control method.
Notes
CDMA does not assign specific frequencies to each user as do other competing systems. Instead, CDMA uses a unique technique where every channel uses the entire available spectrum. CDMA was developed and first used by the military during World War II by the English to thwart German attempts at jamming transmissions. The Allies utilized CDMA to transmit over different frequencies, instead of one, which made it extremely difficult for the Germans to pick up entire signals.
For More Information
You can find the CDMA Development Group at www.cdg.org QUALCOMM is at www.qualcomm.com
See Also 2G, 2.5G, 3G, Advanced Mobile Phone Service (AMPS), CDMA2000, cellular communications, Global System for Mobile Communications (GSM), Time Division Multiple Access (TDMA)
A technique for enhancing the speed of wireless networking.
Overview
Coded Orthogonal Frequency Division Multiplexing (COFDM) is employed by the 802.11a wireless networking standard as a way to work around the difficulty of radio frequency (RF) interference caused by scattering and reflection off of buildings, walls, and similar objects. The problem with wireless networking, as opposed to wireline (wired) networking, is that although wireline networks usually have a single path between different stations, wireless communication is often multipath. As the figure shows, a signal can travel between a wireless client and a base station along different paths due to reflection of RF signals off walls and other objects. The result is that when the signal arrives it is spread out in time. If the time over which a signal can be spread out by interference (the delay spread) is less than the time between individual packet transmissions (the symbol rate), then the receiver can still be processing one packet while the next one arrives, creating a problem.
COFDM works around this problem not only by slowing down the symbol rate (the rate of packet transmission) but also by cramming as much information as possible into each packet. COFDM thus transmits data in a massively parallel fashion, ensuring that each symbol can be processed in a time less than the delay spread between divergent signal paths. Each symbol transmitted is proceeded and followed by a cyclic prefix called a guard interval, which helps give the baseband processor time to receive and process the information. Additional error-correcting information is encoded to help reduce the effects of interference.
Coded Orthogonal Frequency Division Multiplexing (COFDM). COFDM helps overcome the delay spread caused when signals are reflected off of interfering objects.
Using COFDM, wireless networks based on 802.11a can break through their current speeds of 10 to 20 megabits per second (Mbps) to achieve speeds of 155 Mbps or even higher.
See Also 802.11a ,wireless networking
A switch with a combination of input and output ports, the connections between which can be remotely reconfigured by commands entered into a computer.
Overview
Code-operated switches are useful in environments where remote switching is needed for file-sharing or monitoring purposes. For example, you could use an RS-232 serial code-operated switch to remotely switch between pieces of data terminal equipment (DTE), such as servers or routers for running diagnostics from a remote console.
Code-operated switch. 
Implementation
Internal dual inline package (DIP) switches are usually used to configure the code-operated switch so that a different arming character can be used to trigger each connected device. The code-operated switch then examines the incoming data stream for these special text-string codes in order to determine to which device it should route data. An example might be the remote switching between printers. A remote computer could send an embedded switching character to specify which printer connected to the code-operated switch should be used for printing the data.
A remote user can connect to the company network using a modem that interfaces with a code-operated switch, and then use the switch to control a variety of serial-controlled devices in an industrial environment, such as a group of laboratory instruments. Code-operated switches are available from different vendors in configurations supporting up to 64 different serial devices from one remote connection.
Besides embedding switching characters in the data stream, embedded control characters can also be used to directly control the code-operated switch.
See Also switch
Stands for Coded Orthogonal Frequency Division Multiplexing, a technique for enhancing the speed of wireless networking.
See Also Coded Orthogonal Frequency Division Multiplexing (COFDM)
Restarting the computer by turning the power switch off and then on, or by shutting down the computer, turning it off, and then turning it on again.
Overview
If you perform the first type of rebooting, open files will not be properly closed and data can be lost. However, this method ensures that memory is cleared and devices are properly reset. An example might be when you reconfigure the settings of a legacy modem using the modem utility in Control Panel. You might find that you must cold boot your machine in order for the new configuration settings to fully take effect.
See Also boot
A popular tool for developing dynamic Web applications.
Overview
Cold Fusion from Allaire Corporation lets developers rapidly create and deploy dynamic Web applications that can access information from databases and other data sources. Cold Fusion uses a set of proprietary tags that are similar to Hypertext Markup Language (HTML) tags. The resemblance to HTML tags makes Cold Fusion a comfortable platform for experienced HTML users to develop database-driven applications that can be accessed from Web browsers. Cold Fusion is available for both UNIX and Microsoft Windows platforms, and it is an alternative to Microsoft Corporation's Active Server Pages (ASP) technology for developing dynamic Web applications.
For More Information
Find out more about Cold Fusion at www.allaire.com
See Also Active Server Pages (ASP)
A collection of Component Object Model (COM) objects that allow developers to create Microsoft Internet Information Services (IIS) Web applications that send and receive electronic mail.
Overview
Collaboration Data Objects (CDO) is a Microsoft object library that provides messaging capability for applications written in Microsoft Visual Basic, Microsoft Visual C++, and Win32 Virtual Machine for Java. CDO provides distributed Web applications with a standard way to quickly and easily create, send, post, receive, read, and manipulate messages using interfaces based on standard Internet protocols such as Simple Mail Transfer Protocol (SMTP), Network News Transfer Protocol (NNTP), and Multipurpose Internet Mail Extensions (MIME).
CDO was formerly known as Active Messaging, and on the Microsoft Windows NT platform was called Collaboration Data Objects for Windows NT Server (CDONTS). CDONTS actually provides a subset of CDO functionality that includes messaging services but omits the calendaring and workflow functions that CDO on Microsoft Exchange 2000 provides.
CDO 2 is included with IIS in Windows 2000, Windows XP, and Windows .NET Server to support the built-in SMTP and NNTP services. CDO 2 does not support mailboxes, but it does support protocol events to enable programmers to write routines that respond to incoming messages and process outgoing messages. Developing mail-enabled applications using CDO is easier and faster than building custom Common Gateway Interface (CGI) mail programs, and CDO applications have more flexibility than typical CGI applications.
CDO is also a powerful development tool for Exchange Server 2000 and is the premier application programming interface (API) for building collaborative solutions using Exchange. CDO in Exchange 2000 consists of three components:
CDO for Exchange 2000: Used for building collaborative solutions that make use of e-mail, contact management, and scheduling.
CDO Workflow Objects for Exchange: Used to build workflow and routing applications.
CDO for Exchange Management: Used for creating and managing mailboxes and recipients.
Notes
For security reasons, scripts running on Microsoft Internet Explorer cannot access CDO.
See Also Exchange Server ,Internet Information Services (IIS)
An enterprise networking methodology in which the network backbone consists of a single device.
Overview
In a traditional network, local area networks (LANs) are multipoint connections connected using a backbone cable. For example, in a building, a fiber-optic backbone might run from floor to floor and connect with a hub in a wiring closet on each floor. In contrast, collapsed backbones make use of centralized switches, which provide virtual point-to-point connections for LAN connections. These switches are located in the same place as the network servers-in fact it was the move toward centralized location of network servers that helped drive the development of collapsed backbones.
In a typical collapsed backbone scenario, instead of having a hub for each floor located in that floor's wiring closet, a set of stackable Ethernet switches would be located in the equipment room in the basement, with individual cables running from this closet through vertical rises to wiring closets on each floor where hubs distribute connections to stations in work areas.
Advantages and Disadvantages
The advantages of using a collapsed backbone are that they eliminate the costs of backbone cabling installation, they require fewer devices, their equipment administration is more centralized, and they offer higher available bandwidth for each station. The disadvantages are that collapsed backbones generally are not feasible for use in more than one building, they require more cabling, they use more expensive devices, and they have a more limited distance capability.
See Also backbone ,network
A condition that occurs when two or more computers on a network try to transmit signals over the same wire at the same time.
Overview
Collisions are inevitable on a network as long as there is more than one computer on the network. Handling collisions is one of the main functions of a network access method. For example, in Ethernet networks, collisions often occur when two or more stations attempt to place frames on the wire at the same time. To handle this situation, Ethernet uses the access method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
When a station begins transmitting a signal and detects a collision, the station stops transmitting and issues a jam signal to tell the other station that a collision has occurred. Both stations then stop transmitting and wait a random length of time before retransmitting their signals. The amount of time the stations wait before retransmitting increases with the number of collisions occurring on the network.
See Also Carrier Sense Multiple Access with Collision Detection (CSMA/CD) ,collision domain Ethernet
An area of a network where signals transmitted by different stations with that area can collide.
Overview
In Carrier Sense Multiple Access with Collision Detection (CSMA/CD) networks such as Ethernet, a collision can occur if two computers on the network attempt to transmit signals at the same time. When a collision occurs, the network is momentarily offline and no computers can communicate on it.
The larger the collision domain of an Ethernet network, the more computers present and the higher the probability of collisions occurring and negatively affecting network performance. When collision domains become too large, so many collisions occur that network communications become possible. As a result, it is important to segment Ethernet networks to keep collision domains small enough that the effect of collisions is minimized on the network.
Segmenting a collision domain can be accomplished using bridges, switches, routers, and other devices. For example, if two Ethernet hubs are connected directly to a third hub, the resulting local area network (LAN) is still only a single collision domain because only hub connections are used between segments of the network. But if the two hubs are directly connected to an Ethernet switch, you have two collision domains because the switch enables the two networks to function independently. Routers also segment networks into broadcast domains to prevent the occurrence of broadcast storms.
See Also broadcast domain, broadcast storm, Carrier Sense Multiple Access with Collision Detection (CSMA/CD), collision, Ethernet
Stands for Component Object Model, an object-based software architecture developed by Microsoft Corporation that allows applications to be built from binary software components.
See Also Component Object Model (COM)
An extension of Microsoft Corporation's Component Object Model (COM).
Overview
COM was originally designed for building component-based applications to run on single systems. Distributed COM (DCOM) was an evolution of COM that enabled COM components to reside on different machines and communicate with each other over the network. COM+ takes DCOM a step further by providing services and features that enable large, distributed, multitier applications to be built for enterprise-wide systems and the Internet.
COM+ is part of the Microsoft Windows Distributed Network Architecture (DNA) programming paradigm. The COM+ that is supported by the Microsoft Windows 2000, Windows XP, and Windows .NET Server operating system platforms is basically COM plus Microsoft Transaction Server (MTS) and Microsoft Message Queue Server (MSMQ), minus some legacy COM functionality no longer needed. Some of the enhancements of COM found in COM+ include the following:
Dynamic load balancing, which distributes client requests across multiple equivalent COM components
Registration of COM+ objects that is cached in memory to speed up the process of locating and instantiating components
In-memory database (IMDB), which provides quicker data access to applications by lowering overhead
Publish and subscribe services, which provide an event mechanism enabling multiple clients to subscribe to published events and send notification to subscribers when events are fired
Integration of Microsoft Message Queue Server (MSMQ) to support queued asynchronous calls issued to COM components, which let clients invoke methods on COM components using an asynchronous model for increased reliability over poor network connections and in disconnected usage scenarios
Integration of Microsoft Transaction Server (MTS) into COM, which supports attribute-based programming, improvements in transactions, security and administration services, and improved interoperability with other transaction environments through support for the Transaction Internet Protocol (TIP)
Inclusion of basic COM services in the COM+ runtime to simplify and speed up application development.
See Also Component Object Model (COM) ,Distributed Component Object Model (DCOM)
A compiled software component based on Microsoft Corporation's Component Object Model (COM) technology.
Overview
COM components generally refer to the physical files that contain the classes that define COM objects. COM components also include additional code, such as code to register the component in the registry and code for loading the component.
COM components can be written in many languages using tools such as Microsoft Visual Basic, Microsoft Visual J++, Microsoft Visual C++, and Microsoft Visual FoxPro. COM components that support Automation can be called by scripting languages such as Microsoft Visual Basic, Scripting Edition (VBScript) or Microsoft JScript.
COM component object. In-process, local, and remote COM components.
COM components can be run on an application server, a Web server, a Microsoft Transaction Server (MTS), or a client. COM components can be stand-alone applications or reusable software components, and they make the development of Web applications comparable to the development of system applications.
COM components interact with each other and with user applications in a client/server fashion. The client therefore uses the functionality of the server component by creating instances of classes that the server component provides and calling their properties and methods.
COM components can be designed to run in three different modes:
In-process: The component executes in the calling application's process space.
Local: The component executes in its own process space.
Remote: The component executes in a process space on another machine.
An in-process COM component has the extension .ocx or .dll, while an out-of-process COM component (one running outside the calling application process) has the extension .exe. COM components can run on another machine in a manner transparent to the calling application by using the Distributed Component Object Model (DCOM).
When developing applications based on COM components, use in-process components to optimize the speed of object access but use out-of-process components to maximize thread safety.
See Also Component Object Model (COM)
A method by which actions can be performed on a computer running Microsoft Windows by typing text into a command prompt window. The commands that are available depend on which version of Windows is used.
Overview
Examples of commands common to most Windows platforms include the Attrib command, Cacls command, Copy command, Dir command, and Diskcopy command. In addition to these Windows commands, some special commands are available only when certain networking services or protocols are installed. For example, if Transmission Control Protocol/Internet Protocol (TCP/IP) is installed on a computer running Windows, a number of TCP/IP commands are available, including the Arp command, Ping, Tracert, and Nbtstat.
Commands are useful for administering different aspects of a system or network using a command-line interface, such as a telnet connection or a command prompt. Commands are also often used in writing batch files that can perform a group of operations on a system or network service. You can run such a batch file directly, or you can schedule its operation for a predetermined time.
Finally, many Windows programs can be started in different ways from the command prompt using optional switches. For example, Windows Explorer can be run by typing explorer.exe from the command prompt. These programs are normally run using a graphical user interface (GUI), are started by desktop shortcuts, and are not usually referred to as commands.
See Also UNIX commands ,Windows commands
The underlying service or daemon that supports a command-line interface.
Overview
The command interpreter is a system process that allows users to type text commands into the command line or command prompt and execute them to perform various routines and manage system and networking resources. Traditional UNIX systems make heavy use of the command line, while MS-DOS, the legacy Microsoft operating system, is basically a command shell for running text-based commands to control operating system processes.
On Microsoft Windows 2000, Windows XP, or Windows .NET Server systems, users can open a command prompt window to issue text-based commands to the underlying command interpreter Cmd.exe. These text-based Windows commands represent only a subset of the full functionality of the GUI-based administration tools available on the Windows 2000, Windows XP, and Windows .NET Server platforms.
The command interpreter is sometimes referred to as the operating system shell, especially on UNIX platforms. Entering a command into the command interpreter is referred to as "working at the command line."
See Also command line ,command prompt
A general name for any user interface that allows text-based commands to be entered and executed on a system. The term command line is popular in UNIX environments, but Microsoft Windows systems use command prompt to mean essentially the same thing.
See Also command interpreter ,command prompt
A Microsoft Windows application that allows text- based Windows commands to be entered and executed.
Overview
The Windows command prompt provides a command-line interface (CLI) similar to those provided by UNIX systems. The command prompt can be used for running operating system tasks, configuring networking services, and even accessing resources and applications over the network. Many common administrative tasks can be performed from the command prompt, although the command prompt itself provides a more complex and less intuitive interface than the usual desktop graphical user interface (GUI) of Windows operating systems. However, administrators who have spent significant time working in UNIX networking environments often find the command prompt a more familiar paradigm for administering a Windows-based network.
Command prompt. The command prompt in Windows 2000.
Notes
In Windows 2000, Windows XP, and Windows .NET Server, the command prompt application is Cmd.exe, located in the %SystemRoot%\system32 folder. In Windows Millennium Edition (Me), it is called the MS-DOS prompt, has the executable filename Command.com, and is in the \Windows folder. The Windows 2000, Windows XP, and Windows .NET Server versions can be configured using the Console utility in Control Panel.
For security reasons, should you wish as an administrator to disable the command prompt on a Windows 2000, Windows XP, or Windows .NET Server machine, you can accomplish this by either renaming Cmd.exe to something only you yourself are aware of, set NTFS file system (NTFS) permissions so that only Administrators can access it, or (not recommended) delete Cmd.exe entirely from the system.
See Also command interpreter
Microsoft Corporation's platform for building and managing e-commerce solutions.
Overview
Commerce Server 2000 reduces the time it takes to develop and deploy complex e-commerce solutions. Commerce Server is based on Microsoft Site Server version 3 Commerce Edition (SSCE), and it builds on the strength of this earlier product. Commerce Server is part of the Microsoft Windows .NET Server family.
Commerce Server provides core services for managing your e-commerce site, including
Profile system: Used to profile and manage customers and trading partners
Product catalog system: Lets you manage millions of different products and find them quickly
Targeting system: Lets you target customers with personalize one-on-one marketing
Business processing pipelines system: Lets you customize your products to meet your customers' needs
In addition to these services, Commerce Server includes administration and development tools for building and managing your site, sample e-commerce sites you can use as templates and models, a data- warehousing decision-making system, help-desk customer support functionality, and much more.
For More Information
Find out more at www.microsoft.com/commerceserver/.
See Also .NET platform
Internet service providers (ISPs), online service providers, telephone and cable network operators, and other companies.
Overview
CSPs provide software services such as community access to mail, news, chat, and conferencing services. By utilizing these services of a CSP, customers do not have to acquire licenses for the software the CSP provides.
See Also xSP
A way of guaranteeing bandwidth in frame relay services.
Overview
Committed Information Rate (CIR) provides a way of guaranteeing minimum bandwidth for frame relay customers. Because customers on a frame relay network share the network, it is possible that service providers might oversubscribe the service-with the result that some customers receive insufficient bandwidth. Another situation where this can be a problem is if many customers try to access the frame relay network at the same time.
CIR guarantees that data throughput on frame relay connections will not drop below a previously agreed- upon contractual lower limit. However, CIR does permit short bursts of traffic to occupy greater amounts of bandwidth.
See Also frame relay
A graphical user interface (GUI) or desktop environment developed for UNIX systems.
Overview
Common Desktop Environment (CDE) was developed by IBM, Sun Microsystems, and Hewlett-Packard under the Common Open Software Environment (COSE) initiative. CDE is a paradigm that is widely used in the UNIX industry. CDE is based on various industry standards including the X Window System (X11) release 5, X/OPEN, OSF/Motif 1.2, and others.
CDE is designed to provide UNIX users with a simple and consistent desktop interface that includes
Standard Windows-management features
File-system browsing tools supporting multiple views
Customizable user interface-management tools for changing backdrops, mouse and keyboard settings, and screen savers
Extensive and easily accessed online help features
Multiple workspaces for increasing available desktop area
See Also UNIX ,X Window System
A mechanism by which a Web browser can request a Web server to execute an external application.
Overview
Common Gateway Interface (CGI) was developed in the UNIX networking environment to allow Web browsers to execute "gateway" applications on Web servers. These gateway programs are typically written either in a compiled language such as C or in an interpreted language such as Perl. CGI allows Web servers to run scripts or programs on the server and send the output to the client Web browser, thus turning the Web into a platform for running dynamic applications instead of merely presenting static information to clients.
Architecture
CGI programs are called "gateway" programs because the Web server passes the CGI request to the external program, which then runs as a separate process to process the input data, generate the results, and pass these results back to the Web server, which then returns them properly formatted in Hypertext Markup Language (HTML) to the client.
The main disadvantage of CGI is that each request must spawn a new CGI process and that, after the request is satisfied, the process is killed. Thus a Web server experiencing multiple simultaneous requests from clients will spawn multiple copies of the gateway process, each of which consumes memory and processing overhead. The fact that processes are terminated at the end of each request limits CGI to single-step Web applications and requires much ingenuity to handle data across a multistep user session.
Examples
CGI applications are often used as form handlers for Web forms, and are executed using a <FORM> tag embedded in the form document. When a Web client such as Microsoft Internet Explorer submits a form or otherwise passes information to a Web server using CGI, the Web server receives the information from the client and passes it to the gateway program for processing. The gateway program then returns the result of the processing to the server, which returns it to the Web browser as an HTML page. Here's a simple example:
<FORM METHOD=POST ACTION= "http://www.northwind.microsoft.com/cgi-bin/results.pl">
In this example, the Perl script results.pl in the cgi-bin directory functions as the form handler for processing the information submitted using the form.
Common Gateway Interface (CGI). How CGI works.
Notes
Although CGI was developed for UNIX-based systems, it is supported by most Web servers, including Microsoft Internet Information Services (IIS). Microsoft Internet Server API (ISAPI) is a set of server extensions for IIS that functions similarly to those of CGI but uses fewer resources. The main difference is that with CGI the system creates a unique process for every CGI request, but ISAPI extensions do not require separate processes. This makes ISAPI applications generally more responsive than CGI applications.
See Also Internet Server API (ISAPI) ,UNIX ,Web server
A schema for defining manageable network objects.
Overview
The Common Information Model (CIM) defines a set of schema for describing information collected for network and systems management purposes. CIM was developed by the Distributed Management Task Force (DMTF), formerly named the Desktop Management Task Force, as an extensible, object-oriented schema for managing information collected from computers, networking devices, protocols, and applications.
CIM supports management of two types of objects:
Hardware objects: This includes router interfaces and disks.
Software objects: This includes applications, application components, and services.
Another way of defining the different types of information that can be described by CIM is as follows:
Static information: Examples include the capacity of a hard drive on a desktop computer or the specific applications installed on a server.
Dynamic information: Examples include the current bandwidth being used on a port on a switch or router.
Uses
CIM is similar to the Simple Network Management Protocol (SNMP) and Desktop Management Interface (DMI) standards. However, unlike SNMP and DMI, CIM is able to manage the widest possible range of hardware and software systems. CIM also shows the relationships between the different hardware and software components of an enterprise network more completely, making it easier to troubleshoot complex distributed systems and applications.
CIM information that is collected can be shared between systems on a peer-to-peer basis. This information sharing allows network devices to not only be managed from a centralized management console but also to talk to one another to resolve problems as they arise.
CIM was designed by the DMTF to operate together with their Web-Based Enterprise Management (WBEM) initiative to provide a broad WBEM/CIM framework for managing resources across a network.
Architecture
CIM is based on an object-oriented programming model that allows inheritance to be used to grant subclasses the characteristics of their parent classes. CIM classes have the properties, methods, and associations typical of object classes. CIM supports both physical and logical objects and models these objects for purposes of network management applications. CIM is also extensible and allows vendors to define the features of their products using inherited subclasses. The fact that these subclasses are inherited from standard parent classes ensures that data collected from different vendors' systems will be compatible with the CIM standard.
CIM consists of two parts: a language definition specifying the constructs and methods that can be used to model network and system resources, and a set of schema that describes how specific types of resources will be represented.
CIM supports three kinds of schema:
Core schema: These define general areas of network and system management, and core CIM classes are platform-independent.
Common schema: These define specific areas of management.
Extension schema: These define the management of vendor-specific technologies.
Notes
Microsoft Systems Management Server (SMS) 2 is capable of collecting CIM data from managed systems and exporting this data to other enterprise management applications, such as NetView from Tivoli Systems and Unicenter from Computer Associates.
See Also Distributed Management Task Force (DMTF) ,Web-Based Enterprise Management (WBEM)
A public version of the Server Message Block (SMB) file-sharing protocol that has been tuned for use over the Internet.
Overview
Common Internet File System (CIFS) is a remote file system access protocol that enables groups of users to collaborate and share documents over the Internet or within corporate intranets. CIFS is an open, cross-platform technology that is based on the native file-sharing protocols of Microsoft Windows platforms. It is supported by other platforms such as UNIX.
CIFS has been viewed as a possible replacement for both the File Transfer Protocol (FTP) and the Network File System (NFS) file system protocols. CIFS supports encrypted passwords and Unicode filenames, and it can be used to mount a remote file system as a directory or drive on the local machine. CIFS also includes features not supported by NFS, including write-ahead and native support for locks. Microsoft Corporation's Distributed file system (Dfs) is covered as part of the CIFS specification.
Microsoft has submitted CIFS to the Internet Engineering Task Force (IETF). CIFS client and server software is available for the Windows 2000 operating system platform.
See Also Server Message Block (SMB)
A proposed Internet Engineering Task Force (IETF) standard for a protocol to replace Uniform Resource Locators (URLs) with a simpler, more natural scheme for navigating the Web.
Overview
The existing Internet naming systems (domain names and URLs) are not particularly user friendly, as anyone knows who has ever picked up the phone and heard someone ask, "What is the URL for [name of Web page]?" After tediously repeating a long string of characters and slashes, you begin to wish the Internet community could come up with something different.
Enter the proposed Common Name Resolution Protocol (CNRP), an initiative of Network Solutions, AT&T, and other companies. Using CNRP, users could enter the name of a company into their browsers to reach the company home page, then enter a product name to reach the page for a particular product, and enter "2000 Sales Figures" to retrieve a document with these figures. Areas where CNRP might excel include government and public information portals and corporate intranets. Wireless Internet access may also benefit by eliminating the need to enter long, complex URLs on small keypads in order to access specific content on the Internet.
Architecture
CNRP basically runs on top of Hypertext Transfer Protocol (HTTP) as an Extensible Markup Language (XML)-encoded service. A user could enter "Go:2000 Sales Figures" into the browser's address bar, and the browser would encode this request in XML and forward it to a CNRP name server. The name server would then return the URL of the requested page to the browser, which would then request the actual content from where it is located on the Internet or corporate intranet.
Common Name Resolution Protocol (CNRP). How the CNRP works.
Current Web browsers do not support CNRP, and until they do, users who want to use this service will have to download a plug-in for their browser to provide this functionality. Network Solutions offers a free, downloadable CNRP plug-in, as do several other vendors. Whether CNRP will become widely used will depend largely on social inertia (most people are used to URLs despite being fed up with them) and on whether CNRP is natively supported by the next release of Microsoft Internet Explorer and other Web browsers.
See Also Uniform Resource Locator (URL)
A component architecture that specifies technologies for creating, distributing, and managing component programming objects over a network.
Overview
Common Object Request Broker Architecture (CORBA) was developed by the Object Management Group and its member companies and was designed to provide interoperability between applications in heterogeneous distributed environments.
In a CORBA environment, programs request services through an object request broker (ORB), which allows components of distributed applications to find each other and communicate without knowing where applications are located on the network or what kind of interface they use. ORBs are the middleware that enable client and server programs to establish sessions with each other, independent of their location on the network or their programming interface.
The process of a client invoking a call to an application programming interface (API) on a server object is transparent. The client issues the call, which is intercepted by the ORB. The ORB takes the call and is responsible for locating a server object that is able to implement the request. When it has located such an object, the ORB invokes the object's method and passes it any parameters submitted by the client. The results are then returned to the client. ORBs communicate among themselves using the General Inter-ORB Protocol (GIOP) or the Internet Inter-ORB Protocol (IIOP) so that any ORB can fulfill any client request on the network.
Uses
CORBA is primarily used in the UNIX world as an underlying architecture for developing distributed applications. CORBA is not natively supported by Microsoft Windows, which uses its own distributed object management architecture called Distributed Component Object Model (DCOM). The OMG has indicated, though, that it plans to include support for Microsoft's new C# programming language in CORBA, which should promote interoperability between UNIX applications using CORBA and Web services developed under Microsoft Corporation's new .NET platform.
For More Information
Find out more about CORBA from the Object Management Group at www.omg.org
See Also C# ,Distributed Component Object Model (DCOM) ,.NET platform ,UNIX
A group of hosts managed by Simple Network Management Protocol (SNMP) running SNMP agents.
Overview
Communities provide a simple way of partitioning and securing a network for SNMP management. SNMP agents and management systems use community names as the mechanism for authenticating SNMP messages. All SNMP agents belonging to the same community share the same community name, which functions as a kind of shared password for those agents so that they can be recognized by the SNMP management program and other agents. SNMP messages sent by SNMP management systems to a specific community are accepted only by hosts configured to belong to that community. If an SNMP agent program receives an SNMP message with a community name that it is not configured to recognize, it typically drops the message and sends a trap message to the SNMP management program indicating that a message was not authenticated on that machine.
Notes
An agent can be a member of one or more communities. By default, all agents belong to the public community. If all community names including public names are removed from an SNMP-managed host, the host will accept all SNMP messages sent to it.
See Also agent ,Simple Network Management Protocol (SNMP)
A telco that competes with incumbent telcos under the terms of the Telecommunications Act of 1996.
Overview
Before 1996, the U.S. telecommunications market was dominated by a small group of telcos called Incumbent Local Exchange Carriers (ILECs) or Regional Bell Operating Companies (RBOCs) and a group of long- distance carriers or inter-exchange carriers (IXCs). The Telecommunications Act of 1996 was designed to open things up by allowing new companies to compete with the established ones in both the local and long- distance markets.
Competitive Local Exchange Carriers (CLECs) generally compete with ILECs for provisioning the local loop market, but unlike ILECs, which focus more on residential and large enterprise customers, many CLECs have targeted the small to mid-sized business market that has traditionally been poorly served by ILECs. CLECs generally offer high-speed data services and often focus on specific niche services such as Digital Subscriber Line (DSL)-these are often known instead as DSL providers-but many CLECs also offer a much wider spectrum of services including voice (local and long-distance), high-speed Internet access (using DSL), virtual private networks (VPNs), and business-to-business data links.
CLECs come in many types and range from smaller start-ups that piggyback on ILECs and purchase services wholesale from them for reselling purposes, to larger start-ups building out new fiber from their own switching centers, to large IXCs such as AT&T (which acquired Teleport, a pioneering CLEC) and MCI WorldCom (which bought MFS, another pioneer CLEC) competing in the local telco market.
Like ILECs, CLECs are generally concerned about provisioning buildings, not individual users. However, some CLECs are also getting into the Building Local Exchange Carrier (BLEC) markets by offering to-the-desktop services for building-out wiring and deploying services not just to buildings but also within them.
Advantages and Disadvantages
One of the advantages that CLECs have over traditional ILECs is that, instead of competing directly with ILECs by building switching centers, building out wiring to customer premises, and purchasing expensive Class 5 telephone switches to handle all-important voice traffic, they can save implementation costs considerably by colocating their switching equipment at ILEC COs, sharing the existing local loop infrastructure owned by ILECs, and focusing on purchasing more modern, less expensive switching gear dedicated to specific uses such as DSL. Using less expensive and more modern equipment theoretically means the CLEC can offer its services at a discount compared to the ILEC, but CLECs also have to pay the ILECs for the right to use their services, including line provisioning and maintenance, switching interconnects, and colocation services.
Competitive Local Exchange Carrier (CLEC). Two ways CLECs can provision customers with voice and data services.
On the other hand, contracting CLECs to provision telecommunication services for your company instead of ILECs has an element of risk. This is evidenced by the changing nature of the CLEC market, in which some startups have failed and others have been acquired, and by litigation initiated by some CLECs against ILECs. An example is Pronto, a project of mega-bell SBC Communications, which is building out thousands of neighborhood DSL remote terminals to shorten customer DSL connections, thereby improving DSL reliability and data rates. CLECs that want to provide their own DSL services to the same customers have complained to the Federal Communications Commission (FCC) that Pronto cuts them out of the loop because they cannot service Pronto customers using DSLAMs colocated at SBC's COs. Other RBOCs are contemplating similar projects, which could undercut the operations of many CLECs.
Implementation
Because the ILECs own the infrastructure of the last-mile (local loop) wiring serving residential and business markets in the United States, the Telecommunications Act required ILECs to open up use of the local loop to CLECs, allow CLECs to colocate their equipment at the ILEC central offices (COs), and allow CLECs to lease use of the local loop from the ILECs that provision it. CLECs can architect to provision services in many ways, two of which are shown in the diagram.
The top part of the diagram shows a DSL modem at the customer premises that connects the customer's local area network (LAN) over the local loop to a DSL Access Multiplexer (DSLAM) colocated by the CLEC at the CO of the customer's incumbent telco. The CLEC's DSLAM is connected to the ILEC's switching backbone to provide the customer with voice and data services. Such an arrangement is typical of a CLEC that functions as a DSL provider.
The bottom part shows an Integrated Access Device (IAD) at the customer premises. The IAD converts the Internet Protocol (IP) packets of the customer's Ethernet LAN to Asynchronous Transfer Mode (ATM) cells for transmission over the T1 line to an ATM switch colocated by the CLEC at the ILEC's CO. In this case, the ILEC is responsible for provisioning the T1 for the CLEC and acts as a wholesaler of T1 services toward the ILEC. The CLEC's ATM switch then filters the voice and data traffic from the customer. The voice traffic is routed either to the ILEC's telephone switch or directly to an IXC, while the data traffic is routed to an ISP for Internet access or to an IXC for long-haul wide area network (WAN) connections to branch offices.
Marketplace
The CLEC landscape is constantly changing, but some of the bigger players include Covad Communications Company (www.covad.com), Intermedia Communications (www.intermedia.com), Cogent Communications (www.cogentco.com), and others. Despite these big players and the large number of smaller CLECs on the market, analysts estimate that CLECs currently have less than 10 percent of the local telecom market, with more than 90 percent still in the hands of the incumbent ILEC/RBOCs.
CLECs that provision DSL services typically pay RBOCs about $15 to $25 per month to use their local loop connection for deploying such services, the cost of which must be recovered when they resell such services to businesses and consumers, typically at $40 to $60 per month. A newer technology called line sharing may lower the cost for CLECs to lease lines from ILECs, and the savings might be passed on to consumers.
Prospects
Times have gotten tough in the telecommunications industry in general at the start of the new millennium. After the dot-com crash of 2000, sources of venture capital for new telecom startups has dried up, leaving some CLECs in financial difficulty (the same difficulties are faced by 3G wireless vendors and other segments of the telecom sector). Although a few CLECs have failed and others have been acquired, there were still about 200 different CLECs in the marketplace in 2001, with a market capitalization of about $6 billion.
One aim of the Telecommunications Act of 1996 was to open the doors for cable TV companies and utilities to begin competing with ILECs for residential and business voice and data services. When the act was passed, these companies were slow to build out these services, and a diverse host of CLECs appeared to compete in the residential, office, and metropolitan marketplaces. Now, however, cable companies and utilities are beginning to deploy high-speed data services in large rollouts, and so now many CLECs face competition on two fronts instead of from just the ILECs. Finally, FCC rulings have begun to come down on the side of large carriers, which may squeeze many smaller startups out of the marketplace or lead to their being acquired by big carriers such as AT&T and SBC.
The shakeout in the CLEC market and its uncertain future makes it advisable for businesses to use due diligence in investigating the financially viability of CLECs before deploying mission-critical WAN services just to save costs over similar services offered by RBOCs. Many enterprise network architects believe that leasing reliable services for WAN and Internet access from large RBOCs is more important than saving a few dollars by using CLECs. On the other hand, some e-commerce companies have chosen to go with CLECs because they can typically provision services much faster than traditional RBOCs. CLECs that are building out their own infrastructure (own their own fiber) are probably more likely to succeed in the long term. Enterprise network architects should also consider purchasing redundant services from different carriers to provide fault-tolerance for their WAN connections, but they should realize that as the CLEC market consolidates, their options may narrow.
For More Information
You can find industry news on CLECs at www.clec.com and www.clec-planet.com. A current list of CLECs can be found at www.dslreports.com/clecs.
See Also carrier ,Digital Subscriber Line (DSL) ,Incumbent Local Exchange Carrier (ILEC) ,inter-exchange carrier (IXC) ,line sharing ,local loop ,Regional Bell Operating Company (RBOC) ,telco
A domain model in Microsoft Windows NT in which every domain trusts every other domain with two-way trusts.
Complete trust model. Shown for Windows NT.
Overview
The complete trust model is rarely implemented in Windows NT-based networks unless the motivation for using Windows NT is being driven from the bottom up. For example, if a number of individual departments start implementing domains, the company might soon find itself implementing the complete trust model in order to make administration of these domains more efficient. This model also might be used in a situation in which two companies using Windows NT merge into a single company. Because of the large number of trusts in a complete trust model, there are additional security concerns about who is able to administer what. The following table outlines the pros and cons of using this domain model.
Advantages | Disadvantages |
Scalable to any number of user accounts. | Complex to set up and administer. |
Suitable for merging companies or organizations with no central MIS department. | Multiple local groups must be created in each resource domain. |
Notes
When you upgrade a Windows NT network based on the complete trust model to a Windows 2000 or Windows .NET Server network, you can maintain the relative independence of each domain by migrating each domain to be the root domain of a domain tree. Each domain tree would have a single domain, namely the root domain. Two-way transitive trusts can then be established between the trees to form a domain forest.
See Also multiple master domain model ,single domain model ,single master domain model
A Microsoft clustering technology supported by Microsoft Windows 2000 and Windows .NET Server Server and provided by Microsoft Application Center 2000.
Overview
Component Load Balancing (CLB) is a Microsoft clustering solution that is designed to increase the availability and reliability of distributed applications developed using Microsoft COM+ technologies. A CLB cluster consists of a group of up to 16 nodes (servers) running a distributed COM+ application whose components are distributed across the cluster. A CLB cluster is connected together by a network and is managed by Application Center 2000. When one node in a CLB cluster fails, its workload is distributed to the remaining nodes and the COM+ application keeps running.
Implementation
CLB is usually used in conjunction with other forms of clustering such as Network Load Balancing (NLB) and Microsoft Cluster Services (MCSC) for Microsoft Windows 2000 and Windows .NET Server Enterprise Server edition. This scenario is often used for farms of Web servers running mission-critical Web applications. Typically, NLB is used to handle load balancing of Web servers at the front-end, while CLB load balances COM+ application logic in the middle tier and MCSC provides clustering for back-end database servers. Application Center 2000 is then used to manage the CLB middle tier of the Web farm.
Component Load Balancing (CLB). Using CLB to provide high reliability and availability for distributed COM+ applications.
To distribute the load across the nodes of the CLB cluster, CLB uses round-robin techniques and polling algorithms based on server response time. Because CLB uses frequent polling, be sure to implement your CLB cluster on a fast network (100 megabits per second [Mbps] or faster).
See Also Application Center , clustering
An object-based programming architecture developed by Microsoft Corporation that allows applications to be built from binary software components.
Overview
Component Object Model (COM) is both a set of specifications for building application components and a group of underlying services for supporting these components. COM defines a standard method for building components and specifies what these components will look like at the binary level. Because it is a binary standard, COM is language-neutral and COM components can be written using a variety of programming languages including C++, Visual Basic, Java, COBOL, SmallTalk, and other languages. COM also has the following features:
Support for object-oriented programming features including encapsulation, inheritance, and polymorphism.
Location transparency, which allows COM components to be moved from one system to another.
Portability, which lets you port COM applications to other operating system platforms.
Code reuse, the result of encapsulation of COM functionality and a way to speed up application development.
Loose coupling, which lets you replace components with new ones that have similar interfaces for accessing them.
Stable version transitioning, which lets you upgrade some components within an application without breaking the application.
History
Microsoft released an earlier technology, Object Linking and Embedding (OLE), in 1991. OLE enabled functionality from one application to be embedded into another application. For example, a Microsoft Excel spreadsheet could be embedded into a Microsoft Word document and still maintain its spreadsheet functionality. OLE used an underlying technology called Dynamic Data Exchange (DDE) that was complicated in its operation.
In 1995, OLE 2 was released, which replaced DDE with COM as an underlying architecture for OLE. It was soon realized that COM had more uses than embedding one document within another, and COM became the fundamental technology while OLE withdrew to the background. COM has continued to evolve, and its present version is called COM+, which is discussed in a separate article in this chapter.
Implementation
The basic unit of COM is the COM component object, a binary programming object that complies with the COM standards. COM objects can be implemented as either executable (.exe) files or dynamic-link libraries (DLLs) and can function in one of two roles: COM servers (providers) and COM clients (consumers). COM servers and clients interact with each other by using COM interfaces.
A COM interface is a set of methods by which a COM object exposes its functionality to other COM objects. COM interfaces allow COM objects to invoke one another through the methods and properties residing in these interfaces. COM interfaces consist of groups of related functions implemented by the COM class. An interface is basically a table of pointers to functions that are implemented by the object. The table represents the interface, and the functions to which the table points represent the methods of that interface. COM objects can expose multiple interfaces. Each interface has its own unique interface ID (IID), and COM interfaces begin with the letter "I." For example, IUnknown is an important COM interface that must be included in every COM object. IUnknown provides reference counting and interface querying mechanisms and allows navigation to all other interfaces exposed by COM objects.
When COM-based applications are written, COM components are identified by globally unique identifiers (GUIDs), a 16-byte (128-bit) alphanumeric string that is uniquely generated using the current data and time and other information. COM component objects must be registered in order for location transparency to work. You can manually register a COM component object using Regsrv32.exe and other tools.
COM also includes a set of COM services implemented as Win32 library routines (DLLs). These DLLs are used to create new instances of COM components, find and keep track of the location of COM component objects, and perform remote procedure calls (RPCs) for communicating between component objects.
Notes
An alternate component object technology found primarily in the UNIX world is Common Object Request Broker Architecture (CORBA).
For More Information
Learn more about COM at the Microsoft COM site at www.microsoft.com/com
See Also ActiveX, COM+, COM component object, Distributed Component Object Model (DCOM), globally unique identifier (GUID), object linking and embedding (OLE)
Stands for Computing Technology Industry Association, a computer industry trade association formed in 1982.
See Also Computing Technology Industry Association (CompTIA)
An account in the Active Directory directory service of Microsoft Windows 2000 and Windows .NET Server that signifies that a particular computer is a part of a Windows 2000 or Windows .NET Server domain.
Overview
Windows 2000 and Windows .NET Server domain controllers can store three types of accounts: user accounts, group accounts, and computer accounts. Windows 2000 and Windows .NET Server use computer accounts to determine whether a particular system that a user is employing to attempt to log on to the network is part of the domain. When the NetLogon service running on a client computer connects to the NetLogon service on a domain controller in order to authenticate a user, the NetLogon services challenge each other to determine whether they both have valid computer accounts. This allows a secure communication channel to be established for logon purposes.
In order for a Windows 2000, Windows XP, or Windows .NET Server machine to join a domain, the machine must have a computer account created for it in Active Directory. There are two ways to create this account:
Use Active Directory Users and Computers in Windows 2000 and Windows .NET Server to create a computer account for the machine, and then have the machine join the domain.
Use an administrator account to create a computer account while installing Windows 2000, Windows XP, or Windows .NET Server on the machine.
Notes
Machines running Windows 95, Windows 98, and Windows Millennium Edition (Me) can participate in domain authentication, but they do not have computer accounts in Active Directory. This is why the logon box for a Windows 95 or Windows 98 machine has a hard-coded domain name and can log on to only one domain.
If you reinstall Windows 2000 on a machine, you must delete the old computer account and create a new computer account, even if the machine has the same name as before.
See Also account
In Microsoft Windows 2000, Windows XP, or Windows .NET Server, a service responsible for enabling the browsing of network resources using Network Neighborhood and Windows Explorer.
Overview
The Computer Browser service simplifies the user task of locating and accessing network resources by eliminating the need for users to remember Universal Naming Convention (UNC) paths or other network syntax and by eliminating the need for all computers on the network to maintain their own list of all available network resources.
The Computer Browser service maintains a distributed series of lists called browse lists that contain information about shared resources available on the network. Different computers on the network have different roles. These computers include the following:
Domain master browser: Collects and maintains the master browse list for the domain, and synchronizes this list with other domain master browsers in different domains. In a Windows NT network, the domain master browser must be the Primary Domain Controller (PDC).
Master browser: Collects and maintains the master browse list for the domain and distributes this list to backup browsers in the domain. This can be a computer running Windows 2000, Windows XP, Windows .NET Server, Windows NT, Windows 95, Windows 98, or Windows for Workgroups.
Backup browser: Maintains copies of the browse list received from the master browsers and distributes this list to any network client requesting a network resource. This can be a computer running Windows 2000, Windows XP, Windows .NET Server, Windows NT, Windows 95, Windows 98, or Windows for Workgroups.
Potential browser: Any computer on the network configured so that it can assume the role of a master browser or backup browser if required. This can be a computer running Windows 2000, Windows XP, Windows .NET Server, Windows NT, Windows 95, Windows 98, or Windows for Workgroups.
Nonbrowser: Any computer that cannot be a browser but can share resources with the network.
Computer Browser service. How the Computer Browser service works.
When a client tries to access a shared resource on the network, such as a shared folder on a file server, it first contacts the master browser for a list of backup browsers. Then it contacts a backup browser for a copy of the browse list. When the client has the browse list, it contacts the file server for a list of shares, and then connects to the desired share.
Notes
The Workstation service and Server service must be started for the Computer Browser service to function.
See Also browse list ,browsing
A Microsoft Windows 2000, Windows XP, and Windows .NET Server management console that provides a single integrated desktop tool for managing local and remote machines.
Computer Management. A typical Computer Management console.
Overview
Computer Management combines a number of administrative utilities from Windows NT with additional Windows 2000, Windows XP, and Windows .NET Server tools to provide an easy way of viewing and managing properties of any computer running Windows 2000, Windows XP, or Windows .NET Server on the network. Using Computer Management, an administrator can perform the following actions on local and remote machines:
Create and manage shares
Display a list of connected users
Manage services such as Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS)
Start and stop system services
Configure properties of storage devices
Monitor system events and application errors
Display device settings and add new device drivers
To use Computer Management for modifying administrative settings, you must be a member of the Administrators group.
See Also Microsoft Management Console (MMC)
For computers running Microsoft Windows, a name that identifies a computer on the network.
Overview
Computer names can be up to 15 characters in length. In Windows NT, Windows 95, and Windows 98, you specify a computer's name using the Network utility in Control Panel. (In Windows 2000, Windows XP, and Windows .NET Server, use the Network Identification tab of the System utility in Control Panel.) The computer must be restarted if its name is changed. This name is used by services that perform NetBIOS name resolution on the network, such as the Windows Internet Name Service (WINS). Computer names provide a friendly way of accessing network resources without having to remember complex numerical addresses such as IP addresses.
A hidden 16th character is appended to the computer name to form the NetBIOS name for NetBIOS- aware networking services on the machine. Each NetBIOS-aware service has a different NetBIOS name, some of which are based on the name of the computer and others of which are based on the name of the domain in which the computer resides.
Notes
Give friendly names, derived from some common source such as A Midsummer Night's Dream , to groups of computers offering related services. For example, you could call your servers Puck, Oberon, and Titania. This makes it easy to remember that these computers all belong to the same group.
A general term describing the integration of computer and telephone technologies.
Overview
By joining computer systems with switched telephone services, users can access advanced functions such as automatic incoming call routing, call display, and power dialing. For example, a computer can use computer-telephony integration (CTI) to issue commands to a telephone switch to control call routing of calls.
CTI applications generally fall into one of two categories:
Call-control applications: Allow computers to dial numbers, establish conference calls, and other functions. The computer essentially replaces the touch-tone telephone keypad.
Media-processing applications : Deal with more complex issues, such as voice recognition, speech synthesis, and converting fax messages to e-mail. These applications pursue the goal of completely integrated unified messaging in which voice, fax, e-mail, and video conferencing features are combined.
CTI is made possible on Microsoft Windows platforms by operating system application programming interfaces (APIs) such as Microsoft Corporation's Telephony Application Programming Interface (TAPI). The range of products and technologies that support CTI continues to evolve. Cross-platform, vendor-neutral standards have not yet been established.
A computer industry trade association formed in 1982.
Overview
The Computing Technology Industry Association (CompTIA) currently has more than 7500 members. CompTIA membership includes resellers, value-added resellers (VARs), distributors, manufacturers, and training companies in the United States and Canada. The goals of CompTIA are to foster professional competence and business ethics among its members and throughout the computer industry. CompTIA provides its members with educational opportunities, a professional network, and a forum for the development of ethical, professional, and business standards in the computing industry. A number of committees meet to consider issues such as software licensing and electronic warranty forms.
For More Information
Visit the CompTIA online at www.comptia.org
Stands for COM Transaction Integrator, a component of Microsoft SNA Server version 4 that provides client applications with access to two popular mainframe transaction processing (TP) environments, Customer Information Control System (CICS) and Information Management System (IMS).
See Also COM Transaction Integrator (COMTI)
A component of Microsoft SNA Server version 4 that provides client applications with access to two popular mainframe transaction processing (TP) environments, Customer Information Control System (CICS) and Information Management System (IMS).
Overview
COM Transaction Integrator (COMTI) works in conjunction with Microsoft Transaction Server (MTS), making CICS and IMS programs appear as MTS components that can be used with other MTS components to build distributed applications. COMTI includes both a Microsoft Windows NT Server run-time environment and a development tool called Component Builder, which can import mainframe COBOL code and automatically generate an object compatible with MTS. This lets developers program in the visual, object- oriented environments they are accustomed to, allowing them access to host transactions without needing to learn the intricacies of Systems Network Architecture (SNA).
See Also SNA Server ,Systems Network Architecture (SNA)
A term referring to the simultaneous access to a network resource by more than one client.
Overview
Concurrency is an important issue in the licensing of a server operating system or application. For example, the Per Server licensing mode for Microsoft Windows NT Server is based on concurrency. If you purchase 10 client access licenses (CALs) for your Windows NT Server, a maximum of 10 concurrent connections can legally be formed with that server for accessing network resources.
Notes
Some products, such as Microsoft Outlook 98, do not support concurrent access. In other words, you cannot install a central copy of Outlook 98 on a server and have thin clients run this program from the centralized location. Instead, you must install one copy of Outlook 98 on each client that needs to run it.
In Microsoft Message Queue (MSMQ) Server terminology, a name for a collection of computers in which any two computers can directly communicate.
Overview
Computers in the same connected network must be running the same network protocol. A connected network (CN) is essentially a label describing how MSMQ servers are related in an enterprise. CNs are logical groupings of computers that can communicate directly using MSMQ messages. When you install an MSMQ server, you specify a connected network for each network address on the server.
When you specify connected networks for your MSMQ enterprise, it is a good idea to use meaningful labels so that administrators can easily select a connected network from a list when they need to override the default connected network settings.
A link between two computers for the purpose of exchanging information.
Overview
An example would be a Microsoft Windows NT Workstation, Windows 95, or Windows 98 client computer accessing a shared folder or printer on a Windows 2000 server. The term connection is also used to describe the establishment of communication over a WAN link, as in using a dial-up connection over a modem.
When a client computer tries to connect to a server, the success or failure of the attempt can depend upon whether
The server has shared the resource that the client wants to connect to
The client has been properly authenticated or has permission to access the resource
The client is properly licensed to connect to the server, and free licenses are available
See Also client access license (CAL) ,license
Any transport layer protocol that relies on broadcast packets instead of directed packets.
Overview
Connectionless protocols can only offer "best-effort" delivery and cannot guarantee that packets will arrive in the correct order or even at all. Connectionless protocols cannot guarantee delivery of packets. Instead, reliability of packets is handled by the application itself or some higher layer of the protocol stack.
An example of a connectionless protocol is the User Datagram Protocol (UDP), which is part of the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite. UDP provides connectionless services for delivering small packets of information commonly called datagrams. Another connectionless transport layer protocol is the Appletalk Transaction Protocol (ATP), part of the AppleTalk suite of protocols.
Notes
The term connectionless is also used to describe any delivery mechanism where complete addressing information (the address of the source and the address of the destination) is included in every packet. Packets are then placed on the network and are delivered to their destination independently, sometimes taking different routes and arriving in a mixed-up order that needs to be sorted out using packet numbers.
In this more general context, we can also examine protocols at other layers such as the network layer and data link layer. Most local area network (LAN) protocols at these layers are connectionless. For example, at the network layer we have IP, Internetwork Packet Exchange (IPX), Datagram Delivery Protocol (DDP), which is part of the legacy AppleTalk protocol suite, and DECnet Routing Protocol (DRP), which is part of the legacy DECnet protocol suite, all connectionless. At the datalink layer we have Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), and others, again all connectionless. An example of a data-link layer protocol that is connection-oriented instead of connectionless is Asynchronous Transfer Mode (ATM).
See Also connection-oriented protocol ,protocol
A wizard-based tool for creating custom connectivity solutions, and a component of Internet Connection Services for Microsoft Remote Access Service (RAS).
Overview
The Connection Manager Administration Kit (CMAK) is used to customize the Microsoft Connection Manager (CM) client component. Internet service providers (ISPs) can use this tool to customize dial-up installation packages for their customers. Customization features include
Animated logon screen, which can include a custom logo
Desktop icons
The language the dialer displays to the customer
Support numbers and help files
Various connect actions that the dialer performs when dialing, such as shutting down applications or downloading files
Any transport layer protocol that establishes a connection first in order to reliably send packets over the network.
Overview
Connection-oriented protocols guarantee delivery of packets by making use of acknowledgments and retransmission of data. Connection-oriented protocols are used primarily for reliable delivery of large packets of data, as opposed to the unreliable connectionless protocols that are used to deliver small datagrams.
An example of a connection-oriented protocol is TCP, which is part of the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite. The TCP protocol uses a TCP three-way handshake to establish a connection between two hosts on a network. During session establishment, the hosts negotiate the TCP window size, segment size, and other information needed to ensure reliable and efficient communication. A TCP connection is terminated using a similar handshake procedure. Another example of a connection-oriented transport layer protocol is Sequenced Packet Exchange (SPX), part of the NetWare suite of protocols. The legacy network service provider (NSP) protocol of the DECnet suite of protocols is also a connection-oriented transport layer protocol.
Notes
At lower Open Systems Interconnection (OSI) levels such as the network layer and data link layer, most local area network (LAN) protocols are connectionless instead of connection-oriented. Asynchronous Transfer Mode (ATM) is an exception and is connection- oriented. In ATM a virtual circuit (data pathway) is first established prior to sending any data. Instead of addressing data packets (actually cells) with source and destination addresses as in a connectionless protocol, ATM assigns the circuit number to the cells to ensure they reach their destination. Because circuit numbers are much smaller than network addresses, connectionless protocols such as ATM have less overhead than connection-oriented protocols such as IP or Ethernet.
See Also connectionless protocol ,protocol
A technique for optimizing Active Server Pages (ASP) applications running on Microsoft Internet Information Server (IIS) version 4 and Internet Information Services (IIS).
Overview
Connection pooling allows more efficient implementation when connecting ASP front-end applications to a back-end database. Connection pooling involves the pooling of open database connectivity (ODBC) connections to reduce the frequency at which ODBC connections need to be opened and closed on heavily accessed servers. Connection pooling improves ASP performance for ODBC-enabled Web applications and provides a graceful way to manage connection timeouts.
To use ODBC connection pooling on IIS, perform the following steps:
Configure the database driver using ODBC in Control Panel.
Enable connection pooling in the Microsoft Windows NT registry.
Open individual connections in your Microsoft ActiveX Data Objects (ADO) code right before data access is needed for an ASP page and release connections as soon as the data has been accessed.
When connection pooling is enabled, the ODBC driver will check the connection pool for idle connections it can reuse before creating a new connection in response to an ODBC request. When connections are released, they are returned to the connection pool instead of being closed.
Notes
You can control the amount of time an idle connection remains in the pool using the CPTimeout registry setting, which has a default setting of 60 seconds.
See Also Active Server Pages (ASP)
A computer running Microsoft Exchange Server that is dedicated for routing messages to other sites and foreign mail systems using Exchange connectors.
Overview
Large companies often require servers that are dedicated to message routing because of the high volume of message traffic they experience. In a typical high- volume site configuration, one server might be optimized as a home server for users' mailboxes, another server for dedicated public-folder replica hosting, and a third for providing dedicated messaging connectivity with other sites and foreign messaging systems. A connectivity server can have one or more connectors installed on it to provide connectivity with Exchange sites, Simple Mail Transfer Protocol (SMTP) hosts, X.400 messaging systems, or Microsoft Mail postoffices. The more connectors you have on a server, the greater its hardware requirements become.
Notes
On Exchange 5.5 systems, run the Performance Optimizer wizard after you have installed your connectors on the Exchange server. This will allow your server to take maximum advantage of its particular hardware configuration.
A device that terminates a segment of cabling or provides a point of entry for networking devices such as computers, hubs, and routers.
Overview
Connectors can be distinguished according to their physical appearance and mating properties, such as jacks and plugs (male connectors) or sockets and ports (female connectors). They can also be distinguished by their different pinning configurations, such as DB9 and DB15 connectors, which have 9 and 15 pins, respectively. In addition, connectors are distinguished by the kind of electrical interfaces they support. Examples of different types of connectors include
Connectors for serial interfaces, such as RS-232 and V.35
Ethernet connectors, such as RJ-45 and bayonet Neill-Concelman (BNC) connectors
Fiber-optic cabling connectors, such as SC and ST connectors
Connector (device). Common networking and telecommunications connectors.
A single connector may be used for a variety of purposes and different interfaces. For example, the DB-60 connector supports any of the following interfaces: V.35, X.21, EIA-530, EIA/TIA-232, and EIA/TIA-449. So you cannot always tell from the appearance of a connector what its function is-it depends on the interface it implements.
There are literally dozens of types of connectors used in networking, and the networking professional needs to be familiar with many of them. The illustration shows some of the common connector types used in different aspects of networking and telecommunications. Many of these connectors are discussed in separate articles elsewhere in this book.
A component of Microsoft Exchange Server 5.5 that enables message transfer and directory synchronization between Exchange Server and Lotus cc:Mail systems.
Overview
Lotus cc:Mail uses a shared-file messaging architecture similar to that of Microsoft Mail. The Connector for Lotus cc:Mail is implemented as a Microsoft Windows NT service on Exchange Server and supports the following functions:
Message transfer between Exchange Server and cc:Mail messaging systems
Synchronization of directory information between Exchange Server and cc:Mail servers
Only one Connector for Lotus cc:Mail can be installed on a given computer running Exchange Server, and that connector can connect to only one cc:Mail postoffice. However, multiple computers running Exchange Server can each have a cc:Mail connector installed in order to connect to multiple postoffices throughout a cc:Mail messaging system. The Lotus cc:Mail programs export.exe and import.exe must be installed on the computer running Exchange Server for connectivity to be established.
The Connector for Lotus cc:Mail can be used to provide connectivity with database versions 6 or 8 cc:Mail postoffices.
A component of Microsoft Exchange Server 5.5 that enables message transfers and directory synchronization between Exchange Server and Lotus Notes systems.
Overview
The Connector for Lotus Notes allows either single or multiple Lotus Notes servers to be accessed from a single machine running Exchange Server. The Connector for Lotus Notes is implemented as a Microsoft Windows NT service on Exchange Server and supports
Message transfer between Exchange Server and Lotus Notes
Synchronization of directory information between Exchange Server and Lotus Notes
The Connector for Lotus Notes also converts message content to Rich Text Format (RTF) and converts Object Linking and Embedding (OLE) objects on Exchange Server to Lotus Doclinks objects.
The Connector for Lotus Notes supports Lotus Notes 3.x and Lotus Notes/Domino 4.x .
Notes
Be sure to install the Lotus Notes client on the computer running Exchange Server prior to attempting to install the Connector for Lotus Notes on the machine. The connector needs this client to log on to the Lotus Notes mail server. If you have trouble establishing connectivity, check that the connector has a valid Lotus Notes ID and that this ID has the appropriate permissions needed to access the databases on the machine running Lotus Notes.
A component of Microsoft Exchange Server 5.5 used to connect Exchange sites or to connect an Exchange organization to foreign mail systems.
Overview
Connectors are components of Exchange that can be used to route messages over a messaging system. Connectors are implemented on Exchange as Microsoft Windows NT services and can be stopped and started using the Services utility in Control Panel.
Various types of connectors can be installed on Exchange, including the following:
Site Connector: Used for establishing high-speed messaging links between different sites in an Exchange organization
X.400 Connector: Used for establishing connectivity with a foreign X.400 messaging system such as those found in different parts of Europe
Dynamic RAS Connector: Used for establishing dial-up connectivity between sites in an Exchange organization
Internet Mail Service: Used for establishing connectivity with the Internet's Simple Mail Transfer Protocol (SMTP) messaging system
Microsoft Mail Connector: Used for establishing messaging connectivity with legacy Microsoft Mail networks
Connector for Lotus cc:Mail: Used for establishing connectivity with a foreign cc:Mail system
Connector for Lotus Notes: Used for establishing connectivity with a Lotus Notes network
See Also Connector for Lotus cc:Mail ,Connector for Lotus Notes
A broadband transmission technology based on Digital Subscriber Line (DSL) technologies.
Overview
Consumer DSL (CDSL) was developed by Rockwell and is a slower technology than the more common DSL variant called Asymmetric Digital Subscriber Line (ADSL). CDSL provides data rates of about 1 megabit per second (Mbps) downstream (about 128 kilobits per second [Kbps] upstream) to the customer premises over standard Plain Old Telephone Service (POTS) local loop wiring.
However, CDSL has the advantage of not requiring installation of a splitter at the customer premises. CDSL can operate only at distances of up to 18,000 feet (5500 meters) from the telco's central office (CO).
See Also Digital Subscriber Line (DSL)
In Microsoft Windows 2000 and Windows .NET Server, an object in Active Directory directory service that can contain other objects.
Overview
Examples of containers include organizational units (OUs), domains, and local networks. Domains are the core containers for organizing the structure of Active Directory. The other kinds of objects in Active Directory are leaf objects, which cannot contain other objects.
Objects created in a container inherit the discretionary access control list (DACL) of the container itself. In other words, a child object obtains its permissions from its parent object by inheritance.
Notes
Groups are not containers; they are security principals.
See Also Active Directory
In Microsoft Management Console (MMC), any node in a console tree to which other nodes can be added.
Overview
The usual icon for a container in MMC is the folder icon. The highest-level container in a console is the console root node. Beneath this node in the hierarchy are the top-level nodes for individual snap-ins that have been installed. Administrators who are creating new MMC consoles can create additional containers (folders) for organizing their console trees as desired.
See Also Microsoft Management Console (MMC)
In NTFS file system (NTFS), a file system object (such as a directory) that can contain other objects (such as files).
Overview
Objects created in a container inherit the access control list (ACL) of the container itself. In other words, a child object obtains its permissions from its parent object by inheritance. For example, if a directory on an NTFS volume has read permission assigned to the Everyone group, any new file that you create or save in the directory will inherit the same permission. Using containers therefore simplifies the assignment of permissions to objects in the file system.
See Also NTFS file system (NTFS)
A feature of Microsoft Internet Explorer that allows you to control user access to Web sites based on the content ratings of the sites.
Overview
The Internet provides individuals with access to a wide variety of information, but some of this information might be unsuitable for certain viewers. For example, parents are often concerned about their children being exposed to violent or sexually explicit material on the Internet.
Content Advisor lets you control the kind of Internet content that can be accessed using Internet Explorer. This is a useful feature in corporate networks that have high-speed connectivity to the Internet because it can be used to discourage improper use of Web browsers on employee machines, thus helping to implement a company's acceptable use policy for the Internet. With Content Advisor, you can specify ratings settings to indicate acceptable levels of content to view with regard to sex, nudity, violence, and offensive language, and you can password-protect these settings.
Notes
Content Advisor functions properly only with Web sites that are rated.
A tool included with Microsoft Site Server and Microsoft Site Server Express that lets Web server administrators perform content analysis and link management of Web sites.
Overview
Content Analyzer can visually display the structure and integrity of a site in the form of a diagram called a Web map. Web maps allow administrators to visually examine a site's structure and quickly identify problems, such as loops and broken links. Web maps display various Web content items using different icons and can use a variety of colors to convey different kinds of information. You can also use Content Analyzer to search Web maps for various kinds of information using predefined Quick Searches. When you find an item of interest on a Web map, you can open your Web page editing tool directly from the Web map. You can also export Web map information into a database or spreadsheet file for further analysis.
Content Analyzer can also generate predefined site reports you can use to identify broken links and analyze the structure of Web sites. These site reports can be generated in Hypertext Markup Language (HTML) format for easy reading and evaluation and can identify changes to the content of a site, broken links, and other information.
A feature of a proxy server such as Microsoft Proxy Server.
Overview
Content caching allows a proxy server to cache the results of a client request. The next time a client requests the same content, it is retrieved from the cache to improve performance. Content remains in the cache for a predetermined period of time, or until the cache becomes full and old content is moved to allow new content to be cached.
Microsoft Proxy Server makes use of distributed caching, which lets content caching take place closer to users and allows caching activity to be load-balanced across several Proxy Servers for scalability and fault tolerance. For example, within corporate intranets, caching can be moved toward the branch office and workgroup levels of the organization. For Internet service providers (ISPs), caching can be moved toward regional points of presence (POPs). Distributed caching is particularly effective for solving network bandwidth problems associated with Internet push technologies.
Microsoft Proxy Server's distributed caching can be implemented in two ways:
Array-based caching: In this approach, an array or group of proxy servers works together and is administered as a single, logical entity. A cache array provides load balancing, fault tolerance, scalability, and ease of administration. Cache arrays can provide a higher cache hit rate than an individual proxy server because of the larger size of the virtual cache.
Hierarchical caching: In this approach, you arrange proxy servers in a hierarchy by branch office or department. Requests from clients are then forwarded up the hierarchy until the requested object is found in a proxy server's cache.
See Also proxy server
A method for efficiently pushing out content over the Internet to users.
Overview
The idea of content delivery networks originated as the next evolutionary step up from caching of Web content on the Internet. Like caching, the idea of CDN is to deliver content to users over the Internet as efficiently and quickly as possible. CDN takes caching a step further by actively pushing content out rather than passively caching frequently-requested content. A company that builds and operates a CDN is sometimes called a Content Delivery Provider (CDP).
Akamai Technologies developed the first CDN solution to efficiently deliver streaming media content over the Internet. Akamai accomplished this through alliances with regional Internet service providers (ISPs) for hosting their caching servers around the globe and setting up their own advanced Web hosting centers. Akamai then used forward-proxy caching servers at their hosting centers to push content out to caching servers at the edges of the Internet, allowing users around the globe to access streaming media presentations from nearby caching servers instead of from centralized streaming media servers many network hops away. The result was a system with better performance than existing implementations of streaming media.
CDNs are now used not just for streaming media but for supporting a wide variety of different kinds of content delivered over the Internet including static and dynamic Web content, video-on-demand, and other services.
Implementation
There are many ways to implement a CDN, and the technology continues to evolve rapidly. A simple example would be a CDP that hosts content for a company on Web and media servers in its data center and then uses a private network to push this content out to caching servers colocated at points of presence (POPs) of regional ISPs located near the company's customers. The private network could be a satellite link, leased lines, private backbone networks (such as those owned by AT&T Wireless), or a leased portion of Internet backbone bandwidth (usually an expensive solution). Private peering arrangements between ISPs and the CDP enable the CDN to work. Personalization servers keep track of user personalization data for customers in different regions.
The resulting CDN can be envisioned as a "content island" within the ocean representing the Internet-only subscribers on this island can make use of the CDN to improve access to hosted content. With many CDNs in existence around the globe and run by different CDPs, interoperability between them becomes an issue-what if a subscriber of one CDN wants to access content in a different CDN? To solve this problem, the Content Bridge is a vendor consortium of CDNs and CDPs whose aim is to move toward developing new protocols to enable interoperability between different CDNs so that a subscriber of one CDN can access content from the network of a different CDN. The Internet Engineering Task Force (IETF) is also working on a number of draft protocols to support interoperability between different CDNs.
Content delivery network (CDN). A simple example of how a CDN works.
Marketplace
Since Akamai blazed the path, a plethora of new CDNs have arisen and are vying for market dominance. These include
CDNs built by private companies such as Yahoo! that have put together their own custom CDNs using technologies from vendors such as Cisco Systems.
Companies such as Inktomi Corporation, CacheFlow, and Network Appliance that create hardware and software tools that can be used to build custom CDNs.
Pure-play CDPs who build virtual CDNs by leasing bandwidth needed to push out their content to cache servers from bandwidth providers (BPs) such as Exodus Communications and Globix Corporation.
Despite the proliferation of CDN vendors and solution providers, Akamai and Inktomi, two early comers to the market, have established their platforms as cornerstone solutions for many large ISPs and enterprise customers. Akamai has more than 4000 caching servers distributed at POPs in over 50 countries. Other prominent players include new companies such as Digital Island. Existing networking companies are also releasing CDN solutions to the marketplace, though, including Cisco and Lucent Technologies.
A number of vendors are offering turnkey CDN solutions that can be easily implemented to improve content hosting performance for corporate intranets, ISPs, carriers, and others who need it. A leading vendor of CDN-in-a-box solutions is EdgeStream.
See Also caching
A company that builds and operates a content delivery network (CDN).
See Also content delivery network (CDN)
A component of Microsoft Indexing service that can read a specific document format and turn it into a stream of text characters.
Overview
Content filters are an essential part of the indexing process on Indexing service because they determine which types of documents can be read and indexed. Indexing service includes content filters for popular file formats such as
ASCII text
Hypertext Markup Language (HTML) pages
Microsoft Word documents
Microsoft Excel spreadsheets
In addition, many third-party companies have produced content filters for their own document formats, allowing these documents to be indexed by Indexing service when their content filters have been installed. Content filters also handle the presence of embedded objects in documents and recognize when a language shift occurs in a multilingual document.
A condition that occurs when two or more stations on a network try to access the network medium simultaneously. In other words, the stations are contending for control of the medium.
Overview
There are different ways of resolving contention issues on a network. One way is to use a single station as the master or primary station that controls all communication on the network. Other devices on the network function as slave, or secondary, stations. The entire system is known as a master-slave system. The master station normally functions in transmit mode, while the slave stations operate in receive mode. The master station tells individual slave stations when they should switch to transmit mode in order to transmit information over the network. This kind of scenario is used in networks based on IBM's Systems Network Architecture (SNA).
In Ethernet networks, the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method is used to resolve contention on the network by allowing collisions to occur, and then resolving them successfully.
See Also Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
A mechanism for preventing users of Microsoft Internet Explorer from viewing Web sites that contain objectionable language, violence, nudity, or sexually explicit content.
Overview
These ratings are configured on the Web server on a site-by-site basis. Ratings for each category have been established at four levels of acceptable use. The Webmaster who creates the site can then include information about the levels of objectionable language, violence, nudity, or sexually explicit content present on their site. Content ratings are defined by the Recreational Software Advisory Council (RSAC). The user can configure her browser to a specified accessibility level for each type of content by using the Content Advisor feature of Internet Explorer.
For More Information
RSAC can be found at www.rsac.org
A command sent from one Usenet host to another.
Overview
Control messages are defined in the Network News Transfer Protocol (NNTP) specifications. Control messages can be issued by Usenet hosts to perform actions such as
Creating a new newsgroup on the host
Deleting a newsgroup
Canceling messages that have already been posted
Control messages are simple text commands. You can troubleshoot a Usenet host by using telnet to connect to port 119 and manually typing various control messages and examining their results.
See Also Network News Transfer Protocol (NNTP) ,Usenet
A Microsoft Windows feature consisting of a number of utilities for configuring hardware devices and operating system services.
Overview
The following table shows some of the more common Control Panel utilities in Windows 95, Windows 98, Windows NT, Windows 2000, Windows XP, and Windows .NET Server and briefly describes their function. Note that some utilities are named differently in the various Windows versions, such as 32-bit ODBC for Windows NT and ODBC (32 bit) for Windows 95 or 98; these utilities are listed separately here. Note also that some Control Panel utilities are present only when additional Windows components have been installed. For example, the GSNW utility is present only when Gateway Services for NetWare has been installed. Finally, installing additional third-party software can add new utilities to Control Panel associated with that software.
Control Panel. Windows 2000 Control Panel.
Control Panel Utility | Function | Windows 95 and 98 | Windows NT | Windows 2000 | Windows XP and .NET Server |
32-bit ODBC | Database connectivity | x | x | x | |
Accessibility Options | Help for visually or motor-impaired individuals | x | x | x | |
Add New Hardware | Hardware installation wizard | x | |||
Add/Remove Hardware | Hardware installation wizard | x | x | ||
Add/Remove Programs | Installs new software or Windows components | x | x | x | x |
Administrative Tools | Shortcut to Administrative Tools program group | x | x | ||
Console | Command prompt window | x | |||
Date/Time | Date, time, time zone | x | x | x | x |
Desktop Themes | Configures appearance of desktop | x | |||
Devices | Startup profiles for hardware devices | x | |||
Dial-Up Monitor | Monitors RAS connections | x | |||
Display | Screen and desktop settings | x | x | x | x |
Folder Options | Enables Active Desktop and determines how folders are displayed | x | x | ||
Fonts | Installs new fonts | x | x | x | x |
Game Controllers | Configures joysticks | x | x | x | |
GSNW | Gateway Services for NetWare | x | x | x | |
Internet | Internet Explorer options | x | x | ||
Internet Options | Internet Explorer options | x | x | ||
Keyboard | Style and response rate | x | x | x | x |
Licensing | Changes licensing mode and configures replication | x | x | x | |
MacFile | Services for Macintosh | x | |||
Messaging profiles | x | x | x | x | |
Message Queuing | Configuration options for Microsoft Message Queue Server | x | x | ||
Modems | Modem settings | x | x | ||
Mouse | Mouse settings | x | x | x | x |
Multimedia | Audio/video settings | x | x | ||
Network | Networking clients, services, protocols, and adapters | x | x | ||
Network and Dial-up Connections | Creates and configures network connections | x | x | ||
ODBC (32-bit) | Database connectivity | x | |||
Passwords | Configures passwords, enables remote administration, and enables user profiles | x | |||
PC Card | Settings for Personal Computer Memory Card International Association (PCMCIA) cards | x | x | x | x |
Phone and Modem | Modem and TAPI location settings | x | x | ||
Ports | COM port settings | x | |||
Power Management | Advanced power management settings | x | x | x | |
Power Options | Advanced power management settings | x | x | ||
Printers | Adds printer wizard and manages printers | x | x | x | x |
Regional Options | Currency and other settings for countries and regions | x | x | ||
Regional Settings | Currency and other settings for countries and regions | x | x | ||
Scanners and Cameras | Configures these devices | x | x | ||
Scheduled Tasks | Schedule system management tasks | x | x | ||
SCSI Adapters | SCSI device settings | x | |||
Server | Server role | x | |||
Services | Starting and stopping services | x | |||
Sounds | System sounds | x | x | ||
Sounds and Multimedia | Audio/visual hardware/software and system sounds | x | x | ||
System | Boot, file system, profiles, devices, environment, network identification, and other functions (depending on the version of Windows) | x | x | x | x |
Tape Devices | Tape drive settings | x | |||
Telephony | TAPI location settings | x | x | ||
UPS | Uninterruptible Power Supply settings | x | |||
Users | User profiles | x |
Refers to a set of registry keys in Microsoft Windows 2000, Windows XP, and Windows .NET Server that contain configuration information used for system startup.
Overview
Control sets define certain aspects of the Windows 2000, Windows XP, and Windows .NET Server boot process to allow Windows 2000, Windows XP, and Windows .NET Server to boot up successfully. Up to four control set subkeys are located under the HKEY_LOCAL_MACHINE\SYSTEM registry key, including the following:
ControlSet001, ControlSet002, and so on, which represent backup copies of control sets that successfully started the system.
CurrentControlSet, which refers to the control set that was used to successfully boot the system under its current configuration. The key here is a pointer to one of the ControlSet00x registry keys.
The Select registry key found under HKEY_LOCAL_ MACHINES\SYSTEM identifies which of the control set keys corresponds to the current, default, failed, and Last Known Good configurations. If the current control set cannot start the system, you can press the Spacebar when indicated during the boot process to select the last known good configuration, which is the last control set that worked for sure.
See Also registry
The process of updating routing tables after a change in the routing topology of an internetwork.
Overview
When a change occurs in the routing infrastructure of an internetwork, information concerning the change needs to be replicated to all routers that need to know about it. The process by which all routers gradually become aware of the change that occurred is called convergence.
Examples of occurrences that affect the routing infrastructure of an internetwork include adding a new router to the network, having an existing router fail on the network, and adding a new route to the routing table of a router on the network. When any of these situations arise, the routing protocol used to provide communications between the routers on your network is used to communicate these changes to all the routers that need to be aware of them. It typically takes time (from minutes to hours) for such changes to propagate completely through the internetwork's routing infrastructure, and as routers become updated with the new routing information, the network is said to "converge" toward its final state.
Convergence is important-if it does not occur fully, some routes may be unavailable on the network, making some parts of the network inaccessible. Furthermore, some packets may end up disappearing into "black holes" instead of arriving at their destination.
Notes
A more popular usage of the term convergence is to describe the merging of voice, data, and video services for transmission over a single network.
See Also black hole ,internetwork ,routing
A small text file that the Web server saves the Web browser during a Hypertext Transfer Protocol (HTTP) session.
Overview
Cookies were originally intended to enable session state information to be maintained for Web applications across multiple HTTP requests. However, most commercial Web sites also use cookies to record information about the client's usage patterns, including the date and time the client visited the site, which pages were accessed, and Web browser preferences. Other uses for cookies include providing custom or personalized pages for users visiting Web sites, targeting advertising to users according to the pattern of their previous visits to a site, and enabling online shopping carts to function.
Cookies use the storage system of the client for saving this information instead of storing it on the server. Because the vast number of clients might visit the site only once, it would be inefficient to dedicate a large portion of server storage to tracking anonymous clients who might never return. Furthermore, client preferences (such as IP address) might change between sessions, especially for dial-up clients, so servers would have no way of recognizing clients if cookie information were saved on the server. Cookies therefore provide a way for the server to recognize that the client previously visited the site and record what the client did during previous visits, allowing the server to customize the HTTP session to meet the needs of the client (or the needs of the site's advertisers!).
Cookies are harmless text files and cannot be used to transmit a virus to the client. Cookies are simply passive holders of information; they cannot be used by hackers and other unauthorized users to "get" information off your computer such as your e-mail address. Nevertheless, most Web browsers, such as Microsoft Internet Explorer, have an optional setting that allows users to reject cookies. However, rejecting cookies can result in poorer browsing experiences on sites that are cookie-dependent. You can also delete any cookies on a computer running Microsoft Windows by deleting the contents of the Cookies subdirectory within the user profile directory on your hard drive (do not delete the directory itself, however!)
Notes
Web applications written using Microsoft Active Server Pages (ASP) technology can use cookies for maintaining session state information.
Shareware sites offer a variety of third-party browser plug-ins for managing or disabling cookies.
See Also Hypertext Transfer Protocol (HTTP) ,Web browser ,Web server
One of the two basic types of physical cabling media (the other being glass, or fiber-optic, cabling).
Overview
Copper cabling is cheap and flexible, but it is susceptible to electromagnetic interference (EMI), has limited range because of attenuation, and generates electromagnetic radiation that can be intercepted by nearby equipment.
The types of copper cabling commonly used in networking include
Twisted-pair cabling, such as unshielded twisted-pair (UTP) cabling and shielded twisted-pair (STP) cabling
Coaxial cabling, such as thinnet and thicknet
For more information on these types of copper cabling, refer to their individual entries in this book.
Implementation
UTP cabling of Category 5 (Cat5) grade is the most commonly used copper cabling in networking environments today. Cat5 cabling comes in either solid core or stranded cabling. Solid core cabling is stiffer, but it has better conductivity and less attenuation, and it is simpler to terminate than stranded cabling. Stranded cabling is more flexible and easier to work with than solid cabling, and it is more resistant to breaking or fracturing. Use solid core UTP cabling for fixed horizontal cable runs, cross-connects, and backbone cabling; use stranded UTP cabling for locations where equipment is frequently moved, for short cable runs between computers and wall plates, or as patch cables in the wiring closet.
See Also cabling ,coaxial cabling fiber-optic cabling, unshielded twisted-pair (UTP) cabling
A form of Fiber Distributed Data Interface (FDDI) deployed over copper cabling instead of fiber.
Overview
Copper Distributed Data Interface (CDDI) can send data over unshielded twisted-pair (UTP) cabling at 100 megabits per second (Mbps), but cable lengths are limited to about 330 feet (100 meters). The architecture and operation are similar to FDDI, but CDDI is not as commonly implemented as FDDI (and because FDDI is usually considered a legacy networking architecture now, CDDI is also likely to fade away quickly.
If cost is an issue, CDDI offers an alternative to FDDI. CDDI still provides a 100-Mbps network with redundancy, but at reduced cost because copper cabling is cheaper than fiber-optic cabling. Note that CDDI does not provide the security that FDDI does: copper cabling can be tapped, but fiber-optic cabling cannot.
See Also Fiber Distributed Data Interface (FDDI)
A backup type in which all the selected files and folders are backed up, but the archive attribute is not marked for each file and folder.
Overview
Copy backups (or simply copies) do not interrupt the normal backup schedule because they do not change the state of the archive bit on files being backed up. Copy backups are typically used to produce additional copies of backup tapes. Copy backups might be used for
Archiving information in a different location
Generating tapes of month-end financials, which can then be given to the accounting department
Providing branch offices with copies of information on file servers
See Also backup ,backup type
Making a replica of files.
Overview
On Microsoft Windows platforms, files can be copied using a graphical user interface (GUI) tool such as Windows Explorer or from the command prompt using the copy command. Some inheritance issues are associated with copying files on Windows NT, Windows 2000, Windows XP, and Windows .NET Server platforms that use the NTFS file system (NTFS). Specifically, copying a file within or between different NTFS volumes causes the file to inherit the permissions of the folder into which it is copied. For example, if a file on the NTFS drive C has read permission for everyone and it is copied to a directory on the NTFS drive D, which has change permission for everyone, the copy of the file inherits the change permission from the directory it is moved to.
See Also moving files
Stands for Common Object Request Broker Architecture, a component architecture that specifies technologies for creating, distributing, and managing component programming objects over a network.
See Also Common Object Request Broker Architecture (CORBA)
A multicast routing protocol.
Overview
Internet Protocol (IP) multicasting relies on the spanning tree algorithm to ensure delivery of information to intended recipients. Spanning-tree technologies can be implemented in two basic ways:
Dense mode: Used for Webcasting and other large-scale multicast events.
Sparse mode: Used for delivering multicast information to specific, small pockets of users on a large internetwork.
Core-Based Trees (CBT) is one of two sparse-mode protocols commonly used, the other being Protocol Independent Multicast Sparse Mode (PIM-SM).
Architecture
CBT works by having a single core router create a single multicast routing tree, regardless of the multicast transmission's source. Then, when a multicast client wants to register to receive a multicast transmission, the client contacts the nearest upstream multicast router by sending an Internet Group Membership Protocol (IGMP) packet to the router. If the member router is already receiving the multicast transmission, it registers the client and forwards the transmission to the client. If the router is not receiving the transmission, it contacts the next router upstream, and this continues until, if necessary, the core router is reached. The core router then adds the member routers to the multicast group and forwards the transmission to the member router, which then passes it to the client.
See Also dense mode ,multicasting ,Protocol Independent Multicast-Sparse Mode (PIM-SM) ,routing ,sparse mode
An aspect of an object in Performance for which usage statistics can be collected.
Overview
Performance is a Microsoft Windows 2000, Windows XP, and Windows .NET Server administrative console for monitoring system resources. It can be used to collect status information about various objects. For example, if the object Processor is being studied, the Performance Monitor collects information on all counters that belong to this object. This includes counters such as
% User Time: The percentage of the time the processor is in user mode executing a nonidle thread
% Privileged Time: The percentage of the time the processor is in kernel mode executing a nonidle thread
Interrupts/sec: The number of device interrupts a processor receives per second
If the machine is a multiprocessor system, each instance of each counter can be monitored. Performance Monitor counters are usually one of two types:
Instantaneous counters, which display the most recent value of a measurement- for example, Processor: % Processor Time
Average counters, which display the average of the last two measured values- for example, LogicalDisk: Avg. Disk Bytes/Read
A two-letter code identifying top-level domains for countries and regions in the Domain Name System (DNS).
Overview
Country codes are a way of geographically identifying a domain name as belonging to a particular country or region. They are an alternative to the more commonly used organizational codes such as .com, .org, and .net. This table lists the various country codes in the DNS system.
Code | Country/Region | Code | Country/Region |
ad | Andorra | ca | Canada |
ae | United Arab Emirates | cc | Cocos (Keeling Islands) |
af | Afghanistan | cf | Central African Republic |
ag | Antigua and Barbuda | cg | Congo (Congo Republic) |
ai | Anguilla | ch | Switzerland |
al | Albania | ci | Cote d'Ivoire |
am | Armenia | ck | Cook Islands |
an | Netherlands Antilles | cl | Chile |
ao | Angola | cm | Cameroon |
aq | Antarctica | cn | China |
ar | Argentina | co | Colombia |
as | American Samoa | cr | Costa Rica |
at | Austria | cu | Cuba |
au | Australia | cv | Cape Verde |
aw | Aruba | cx | Christmas Island |
az | Azerbaijan | cy | Cyprus |
ba | Bosnia and Herzegovina | cz | Czech Republic |
bb | Barbados | de | Germany |
bd | Bangladesh | dj | Djibouti |
be | Belgium | dk | Denmark |
bf | Burkina Faso | dm | Dominica |
bg | Bulgaria | do | Dominican Republic |
bh | Bahrain | dz | Algeria |
bi | Burundi | ec | Ecuador |
bj | Benin | ee | Estonia |
bm | Bermuda | eg | Egypt |
bn | Brunei Darussalam | eh | Western Sahara |
bo | Bolivia | er | Eritrea |
br | Brazil | es | Spain |
bs | Bahamas | et | Ethiopia |
bt | Bhutan | fi | Finland |
bv | Bouvet Island | fj | Fiji Islands |
bw | Botswana | fk | Falkland Islands (Islas Malvinas) |
by | Belarus | fm | Micronesia |
bz | Belize | fo | Faroe Islands |
fr | France | jp | Japan |
fx | France (Metropolitan) | ke | Kenya |
ga | Gabon | kg | Kyrgyzstan |
gd | Grenada | kh | Cambodia |
ge | Georgia | ki | Kiribati |
gf | French Guiana | km | Comoros |
gh | Ghana | kn | Saint Kitts and Nevis |
gi | Gibraltar | kp | Korea (North) |
gl | Greenland | kr | Korea (South) |
gm | Gambia | kw | Kuwait |
gn | Guinea | ky | Cayman Islands |
gp | Guadeloupe | kz | Kazakhstan |
gq | Equatorial Guinea | la | Laos |
gr | Greece | lb | Lebanon |
gs | South Georgia and South Sandwich Islands | lc | Saint Lucia |
gt | Guatemala | li | Liechtenstein |
gu | Guam | lk | Sri Lanka |
gw | Guinea-Bissau | lr | Liberia |
gy | Guyana | ls | Lesotho |
hk | Hong Kong SAR | lt | Lithuania |
hm | Heard and McDonald Islands | lu | Luxembourg |
hn | Honduras | lv | Latvia |
hr | Croatia (Hrvatska) | ly | Libya |
ht | Haiti | ma | Morocco |
hu | Hungary | mc | Monaco |
id | Indonesia | md | Moldova |
ie | Ireland | mg | Madagascar |
il | Israel | mh | Marshall Islands |
in | India | mk | Macedonia, Former Yugoslav Republic of |
io | British Indian Ocean Territory | ml | Mali |
iq | Iraq | mm | Myanmar |
ir | Iran | mn | Mongolia |
is | Iceland | mo | Macau SAR |
it | Italy | mp | Northern Mariana Islands |
jm | Jamaica | mq | Martinique |
jo | Jordan | mr | Mauritania |
ms | Montserrat | ru | Russian Federation |
mt | Malta | rw | Rwanda |
mu | Mauritius | sa | Saudi Arabia |
mv | Maldives | sb | Solomon Islands |
mw | Malawi | sc | Seychelles |
mx | Mexico | sd | Sudan |
my | Malaysia | se | Sweden |
mz | Mozambique | sg | Singapore |
na | Namibia | sh | St. Helena |
nc | New Caledonia | si | Slovenia |
ne | Niger | sj | Svalbard and Jan Mayen Islands |
nf | Norfolk Island | sk | Slovak Republic |
ng | Nigeria | sl | Sierra Leone |
ni | Nicaragua | sm | San Marino |
nl | Netherlands | sn | Senegal |
no | Norway | so | Somalia |
np | Nepal | sr | Suriname |
nr | Nauru | st | Sao Tome and Principe |
nu | Niue | sv | El Salvador |
nz | New Zealand | sy | Syria |
om | Oman | sz | Swaziland |
pa | Panama | tc | Turks and Caicos Islands |
pe | Peru | td | Chad |
pf | French Polynesia | tf | French Southern Territories |
pg | Papua New Guinea | tg | Togo |
ph | Philippines | th | Thailand |
pk | Pakistan | tj | Tajikistan |
pl | Poland | tk | Tokelau |
pm | St. Pierre and Miquelon | tm | Turkmenistan |
pn | Pitcairn | tn | Tunisia |
pr | Puerto Rico | to | Tonga |
pt | Portugal | tp | East Timor |
pw | Palau | tr | Turkey |
py | Paraguay | tt | Trinidad and Tobago |
qa | Qatar | tv | Tuvalu |
re | Reunion | tw | Taiwan |
ro | Romania | tz | Tanzania |
ua | Ukraine | vi | Virgin Islands (US) |
ug | Uganda | vn | Viet Nam |
uk | United Kingdom | vu | Vanuatu |
um | US Minor Outlying Islands | wf | Wallis and Futuna Islands |
us | United States | ws | Samoa |
uy | Uruguay | ye | Yemen |
uz | Uzbekistan | yt | Mayotte |
va | Vatican City State | yu | Yugoslavia |
vc | Saint Vincent and The Grenadines | za | South Africa |
ve | Venezuela | zm | Zambia |
vg | Virgin Islands (British) | zr | Congo (Democratic Republic of Congo) |
zw | Zimbabwe |
A small device for connecting two cables to make a longer cable, sometimes called an inline coupler.
Overview
Inline couplers do not provide any amplification or signal boost, and can cause attenuation and signal degradation unless they are of high quality. One example would be a small box that accepts two Category 5 (Cat5) cables with RJ-45 connectors and links them to form a longer cable. Another example would be the BNC barrel connector for joining two lengths of thinnet cabling. A third example would be a coupler with two RJ-11 connectors for joining two phone lines.
Coupler. A Category 5 UTP (unshielded twisted-pair) coupler.
The term coupler is also used to refer to modular connectors that can snap into customizable patch panels to allow different kinds of cabling to be mixed in one patch panel.
Stands for customer premises equipment (CPE), telecommunications equipment that is installed at the customer's location.
See Also customer premises equipment (CPE)
Stands for cyclical redundancy check, an error- checking technique for ensuring packets are successfully delivered over a network
See Also cyclical redundancy check (CRC)
A Microsoft Windows 2000, Windows XP, or Windows .NET Server built-in identity that is used as a security context for running services and operating system functions.
Overview
The membership of the Creator Owner system group cannot be modified directly. The Creator Owner system group includes only the user who created or took ownership of a network resource and is functionally equivalent to that user's primary group. The Creator Owner system group has full permissions on the resource, but the rights of the Creator Owner system group cannot be modified. Whoever creates a file system object or print job becomes the Creator Owner of that object or job.
See Also built-in identities
Information required from users who want to log on to a network and access its resources.
Overview
Credentials, which are formed by combining a user's username and password, identify users so that they can be authenticated by the network security provider. Credentials for access to one network do not guarantee access to another network.
In networks that are based on Microsoft Windows NT, Windows 2000, and Windows .NET Server, computers called domain controllers are responsible for authentication of user's credentials. In addition, trust relationships can be established between Windows NT domains to allow user's credentials to be authenticated from anywhere in the enterprise. Windows NT, Windows 2000, Windows XP, and Windows .NET Server support single- user logon, which allows a user to use a single set of credentials for accessing resources anywhere on a network.
See Also authentication protocol ,password ,username
A cabling installation tool used for attaching connectors to cabling.
Overview
Crimpers are used to terminate cables by applying appropriate pressure to contacts within a connector so that it remains physically attached to the cable without soldering. A crimper is an essential component of a network administrator's toolkit. Crimpers can include built-in strippers for removing the outer insulation from a cable. They can include a set of dies for crimping different kinds of connectors, or they can be specialized for a single type of termination. Crimpers are most often used for terminating Category 5 (Cat5) unshielded twisted-pair (UTP) cabling with RJ-45 connectors. A good crimper should be made of heavy-duty metal and be able to cut, strip, and terminate a cable easily.
Crimper. A crimper with connector set.
See Also cabling ,connector (device)
Stands for certificate revocation list, a list, maintained by a certificate authority (CA), of digital certificates that have been issued and then later revoked.
See Also certificate revocation list (CRL)
Stands for Customer Relations Management, a type of business application used to manage business-to- consumer (B2C) connections
See Also Customer Relationship Management (CRM)
Twisted-pair cabling with the send and receive pairs of wires crossed.
Overview
Crossover cables are primarily used for connecting hubs to each other. In addition, a small, two-station local area network (LAN) can be established by connecting two computers together with 10BaseT network interface cards (NICs) and a crossover cable. This configuration is often utilized when one computer is used to test the networking functions of another because it allows the computer being tested to be isolated from the network. The illustration shows the pinning configuration of a crossover cable.
Crossover cable. Pinning for a crossover cable.
See Also cabling ,twisted-pair cabling
A form of interference in which signals in one cable induce electromagnetic interference (EMI) in an adjacent cable.
Overview
The ability of a cable to reject crosstalk in Ethernet networks is usually measured using a scale called near-end crosstalk (NEXT). NEXT is expressed in decibels (dB), and the higher the NEXT rating of a cable, the greater its ability to reject crosstalk. A more complex scale called Power Sum NEXT (PS NEXT) is used to quantify crosstalk in high-speed Asynchronous Transfer Mode (ATM) and Gigabit Ethernet (GbE) networks.
The twisting in twisted-pair cabling reduces the amount of crosstalk that occurs, and crosstalk can be further reduced by shielding cables or physically separating them. Crosstalk is a feature of copper cables only-fiber-optic cables do not experience crosstalk. Crosstalk can be a problem for unshielded twisted-pair (UTP) cabling. To minimize crosstalk, make sure that
You do not untwist or sharply bend the UTP cabling
The cable ends connected to a patch panel or wall plate are untwisted no more than 0.5 inch (1.3 centimeters)
See Also cabling ,near-end crosstalk (NEXT)
A core component of the latest versions of Microsoft Windows that provides application programming interfaces (APIs) for cryptographic security services that provide secure channels and code signing for communication between applications.
Overview
CryptoAPI provides a set of standard Win32 libraries for managing cryptographic functions using a single consistent interface independent of the underlying cryptographic algorithms and ciphers. CryptoAPI interfaces with modules called cryptographic service providers (CSPs), such as the Microsoft RSA Base Cryptographic Provider, to provide cryptography functions such as hashing, data encryption and decryption, key generation and exchange, digital signature issuance and verification, and so forth.
CryptoAPI is natively supported by the latest versions of Windows NT, Windows 98, Windows 2000, Windows XP, and Windows .NET Server. Microsoft Internet Explorer version 4 provides CryptoAPI support for Windows 95. The current version of CryptoAPI is version 2.
See Also cryptography
A standard that defines the general syntax for data that includes cryptographic features such as digital signatures, encryption, and certificate chains.
Overview
Cryptographic Message Syntax Standard, also known as PKCS #7, specifies the format in which the data is signed and encrypted, and the types of encryption algorithms used.
Data encrypted according to the PKCS #7 standard can have multiple digital certificates attached, including certificate revocation lists (CRLs). Certificates include information concerning the issuer and serial number of the public key of the signer so that the recipient can decrypt the message.
See Also cryptography ,digital certificate ,encryption
In networking and telecommunications, the process of securely transmitting data over a network in such a way that if the data is intercepted, it cannot be read by unauthorized users.
Overview
Cryptography involves two complementary processes:
Encryption: The process of taking data and modifying it so that it cannot be read by distrusted users.
Decryption: The process of taking encrypted data and rendering it readable for trusted users.
Encryption and decryption are performed using algorithms and keys. An algorithm, a series of mathematical steps that scrambles data, is the underlying mathematical process behind encryption. There are a variety of cryptographic algorithms that have been developed based on different mathematical processes.
Some algorithms result in stronger encryption than others-the stronger the algorithm, the more difficult the encrypted data is to crack. For example, Network and Dial-up Connections in Microsoft Windows 2000 supports standard 40-bit RAS RC4 encryption, but if you are located in the United States or Canada, you can get a stronger 128-bit version. Similar versions are offered for Windows NT.
Encryption algorithms involve mathematical values called keys. Earlier cryptography systems were secret key encryption systems in which only the hosts involved in transmitting and receiving the encrypted transmission knew the key. This key had to somehow be transported securely to anyone needing to decrypt a message. This was the main disadvantage with secret key cryptosystems.
Most cryptography today involves a process called public key encryption, which uses two different keys:
A public key that is distributed to any user (or to any client program) requesting it
A private key that is known only to the owner (or the owner's client program)
To send an encrypted message, the sender uses his or her private key to encrypt the data, and the recipient uses the sender's public key to decrypt it. Similarly, the recipient can return a response to the original sender by using the sender's public key to encrypt the response, and the original sender uses his or her private key to decrypt it.
See Also digital certificate ,public key cryptography
Stands for Carrier Sense Multiple Access with Collision Avoidance, the media access control method used by AppleTalk.
See Also Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Stands for Carrier Sense Multiple Access with Collision Detection, the media access control method used by half-duplex Ethernet networks.
See Also Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
Stands for Client Services for NetWare, a Windows 2000, Windows XP, and Windows .NET Server service that provides Microsoft Windows clients with access to Novell NetWare file, print, and directory services.
See Also Client Services for NetWare (CSNW)
Stands for caching service provider, a company that maintains caching servers that speed the transfer of information across the Internet's infrastructure and offers managed access to these servers for a fee.
See Also caching service provider (CSP)
Stands for commercial service provider, typically Internet service providers (ISPs), online service providers, telephone and cable network operators, and other companies.
See Also commercial service provider (CSP)
Stands for cascading style sheets, a method for giving Web developers more control over how the pages of a Web site will look when displayed on a Web browser.
See Also cascading style sheets (CSS)
Stands for Channel Service Unit, a device that is used to connect a synchronous digital telecommunications line to a computer network.
See Also Channel Service Unit (CSU)
Stands for Channel Service Unit/Data Service Unit, a device that combines the functions of both a Channel Service Unit (CSU) and a Data Service Unit (DSU).
See Also Channel Service Unit/Data Service Unit (CSU/DSU)
A text file having the extension .csv, which contains fields of data separated by commas and a carriage return/linefeed at the end of each record. The extension .csv stands for comma-separated values.
Overview
These files are often used as a standard format for importing and exporting information between applications. For example, in Microsoft Exchange Server you can modify the properties of a group of mailboxes by exporting the properties of the mailboxes to a .csv file, opening this file as a spreadsheet in Microsoft Excel, modifying the properties using string functions and search/replace, exporting the information back into another .csv file, and then importing the modified file back into Exchange. Many applications can export log files or other information as .csv files. These files can then be imported into a spreadsheet or database program where they can be subjected to further inspection and analysis. Graphics and charts can also be generated from the imported information.
Stands for Certified Technical Education Center, an education-delivery company such as a school or training center that is been qualified by Microsoft Corporation for the delivery of Microsoft Official Curriculum (MOC) courseware.
See Also Certified Technical Education Center (CTEC)
Stands for computer-telephony integration, a general term describing the integration of computer and telephone technologies.
See Also computer-telephony integration (CTI)
A control sequence that has different effects depending upon the operating system involved.
Overview
Holding down the Control, Alt, and Delete keys simultaneously produces the following results (depending upon the operating system being used):
MS-DOS: Restarts the computer.
16-bit Microsoft Windows, Windows 95, and Windows 98: Shows running tasks and allows you to terminate a task. A second Ctrl+Alt+Delete will restart the computer.
Windows NT: Brings up the Windows NT Security dialog box.
Windows 2000: Brings up the Windows Security dialog box.
Windows XP and Windows .NET Server: Brings up the Windows Security dialog box.
See Also secure attention sequence (SAS)
Any user-created method for authentication of clients on a network.
Overview
Microsoft Internet Information Services (IIS) lets you create custom authentication schemes to control access to Web content. These can be implemented using several different technologies, including Active Server Pages (ASP), Internet Server API (ISAPI) authentication filters, or Common Gateway Interface (CGI) applications. For example, using any of these technologies, you can create an authentication scheme to
Perform a search of a client's credentials in a custom user database
Examine a client's digital certificate to determine whether to allow access
Use cookies or some other mechanism to establish whether the client should be authenticated
See Also authentication protocol
Customer Information Control System (CICS) enables transaction-based applications to operate on IBM mainframe systems. More than half of all mainframes running today still use some CICS applications. Traditionally CICS-based applications are accessed using legacy IBM 3270 text-based terminals, but newer products allow CICS transactions to be wrapped in Hypertext Markup Language (HTML) for access by Web browsers. One such product is Shadow AutoHTML for CICS/TS from Neon Systems, which also supports Open Database Connectivity (ODBC) interfaces for CICS applications.
For More Information
Find Neon Systems at www.neonsys.com
See Also Systems Network Architecture (SNA)
A general term referring to your local company's networking environment.
Overview
The term customer premises is typically used by service providers who provide leased or contractual services to help you implement and support your network. For example, a cabling company would install cabling at your customer premises and call this installation "premise wiring." A telecommunications company might send a representative to a customer premises in order to install a Channel Service Unit/Data Service Unit (CSU/DSU) or other device in the wiring closet to enable wide area network (WAN) communication. Typically, your company is responsible for the physical security of such installed equipment, but the actual configuration and monitoring of the equipment often takes place at the telco's central office (CO).
See Also customer premises equipment (CPE) ,enterprise resource planning (ERP)
Telecommunications equipment that is installed at the customer's location.
Overview
Customer premises equipment (CPE) is installed to terminate wide area network (WAN) links and local loop connections between the customer and the carrier's central office (CO) and to route traffic between the customer premises and the carrier from which telecommunications services are leased. Common examples of customer premises equipment include telephones, modems, Channel Service Unit/Data Service Units (CSU/DSUs), Private Branch Exchanges (PBXs), and Integrated Access Devices (IADs) routers.
Generally, the telecommunications service provider is responsible for configuring and monitoring the equipment, which is purchased or leased by the customer from the carrier. For example, when installing a CSU/DSU as a termination for a T1 line, the configuration of the CSU/DSU is likely to have been done previously by the service provider. The carrier then uses Simple Network Management Protocol (SNMP) and loopback monitoring to determine, from its central office, whether the remotely installed equipment is functioning correctly.
Alternatively, customers may purchase or lease their own CPE from third-party vendors. In this case the customer is usually responsible for configuring and monitoring the equipment. In general, cost/benefit is usually on the side of leasing CPE from the provisioning carrier because the cost of replacing defective or failed equipment or upgrading equipment to support enhanced services is the burden of the carrier.
See Also carrier ,customer premises local loop, telecommunications services, wide area network (WAN)
A type of business application used to manage business-to-consumer (B2C) connections.
Overview
Customer Relationship Management (CRM) software is used to manage mission-critical business information concerning the direct relationship between a business and its clients. CRM enables business users to quickly and easily access up-to-date information concerning client accounts, and it provides the tools for managing and growing a company's relationship with its customers. CRM software enables a business to manage leads, distribute timely information to customers, and coordinate a multitude of other customer-centric activities.
Marketplace
Some of the bigger players among CRM vendors include Onyx Technology, Pivotal Software, and Siebel Systems. CRM software takes a variety of forms depending on whether it is being implemented in small, mid-sized, or enterprise-level businesses. Some CRM vendors build their products around customer databases, while newer players and CRM startups often use the application service provider (ASP) model for outsourcing CRM needs.
The worldwide market revenue for all forms of CRM software and services was estimated at over $10 billion in 2000 and may reach $25 billion by 2003, according to some analysts.
Notes
A recent offshoot of CRM is partner relationship management (PRM), which helps companies to manage the more indirect relationship they have with business channels and supply chain partners, among others. Besides the big CRM players, a number of emerging pure-play PRM vendors are attracting market share. These include Allegis, ChannelWave, and many others.
See Also B2B ,enterprise resource planning (ERP)
A recipient in Microsoft Exchange Server that does not reside in the Exchange organization.
Overview
When creating a custom recipient, you specify the e-mail address of the remote user first, and then configure the properties of the recipient. An example of a custom recipient is the Simple Mail Transfer Protocol (SMTP) address of a user on the Internet.
Custom recipients are often created on Exchange servers to place frequently used foreign addresses in the global address book so that users do not have to specify the recipient's e-mail address manually or maintain their own personal address books. Custom recipients can be used for various other purposes in Exchange, such as to enable a user's Internet mail to be forwarded to his or her Exchange mailbox.
An error-checking technique for ensuring that packets are successfully delivered over a network.
Overview
A cyclical redundancy check (CRC) is a number that is mathematically calculated for a packet by its source computer and then recalculated by the destination computer. If the original and recalculated versions at the destination computer differ, the packet is corrupt and needs to be resent or ignored.
The mathematical procedure for performing a CRC is specified by the International Telecommunication Union (ITU) and involves applying a 16-bit polynomial to the data being transmitted by the packet for packets of 4 KB of data or less, or a 32-bit polynomial for packets larger than 4 KB. The results of this calculation are appended to the packet as a trailer. The receiving station applies the same polynomial to the data and compares the results to the trailer appended to the packet. Implementations of Ethernet use 32-bit polynomials to calculate their CRC.
