Unbalanced line. The return-path conductor of the unbalanced line is at ground potential. The return-path conductor of the balanced line carries a signal.
Stands for Universal Asynchronous Receiver/Transmitter, hardware used to translate between serial and parallel transmission.
See Also Universal Asynchronous Receiver/Transmitter (UART)
Stands for Universal Data Access, a Microsoft Corporation technology for enabling access to any kind of data source.
See Also Universal Data Access (UDA)
Stands for Universal Description, Discovery, and Integration, a platform-independent framework for businesses to advertise and discover each other.
See Also Universal Description, Discovery, and Integration (UDDI)
Stands for Uniqueness Database File, a text file used to supplement answer files during unattended installations of Microsoft Windows NT, Windows 2000, Windows XP, and Windows .NET Server.
See Also Uniqueness Database File (UDF)
Stands for Universal Disk Format, the successor to the CD-ROM File System (CDFS).
See Also Universal Disk Format (UDF)
Stands for Universal Disk File System, an older name for Universal Disk Format, the successor to the CD- ROM File System (CDFS).
See Also Universal Disk Format (UDF)
Stands for User Datagram Protocol, a transport layer protocol of the Transmission Control Protocol/Internet Protocol (TCP/IP) suite.
See Also User Datagram Protocol (UDP)
Stands for User Locator Service, an older name for Internet Locator Service, a Lightweight Directory Access Protocol (LDAP) directory service that enables Microsoft NetMeeting users to locate and contact other users for conferencing and collaboration over the Internet.
See Also Internet Locator Service (ILS)
Stands for Unified Messaging, technology that allows users to access their e-mail, telephone, and fax messages from a single mailbox.
See Also Unified Messaging (UM)
Stands for Universal Mobile Telecommunications System, another name for Wideband Code Division Multiple Access (W-CDMA), a third-generation (3G) cellular communications system deployed in Europe.
See Also Wideband Code Division Multiple Access (W-CDMA)
Deploying software without administrative intervention.
Overview
Many Microsoft Corporation products, such as Microsoft Windows 2000, Windows XP, Windows .NET Server, BackOffice products, and .NET Server products, support various forms of automatic deployment methods in which direct involvement of the administrator at the target machine is not required. Unattended installation also enables administrators to perform mass deployments of operating system and application software on networked computers. For example, an administrator can install Windows 2000 Professional on multiple desktop client computers without having to remain at each desktop during the installation process and answer the prompts displayed during a normal CD-based installation.
One way of performing an unattended installation is to make use of a distribution server, a network file server that stores the shared source files for the software you want to install. To perform an installation, the Setup program is started on the target computer, which then automatically connects to the share point on the distribution server, downloads the necessary installation files, and installs the software. With Windows 2000 and Windows XP, for example, unattended installation makes use of a special file called an answer file (Unattend.txt) that provides a fixed set of responses to the prompts for information usually generated by the Setup program, thus freeing the administrator from having to remain at the target machine during installation answering prompts. These prompts include information about the computer name, the network protocol to employ, what optional components to install, and so on. You use the answer file by appending the filename to the /u switch when you run Setup from the command prompt. The answer file is a plaintext file that can be customized using either a simple text editor such as Notepad or using the wizard-based tool Setup Manager provided by Windows 2000 and Windows XP.
Another way of performing unattended installations is to use disk-image duplication (or disk cloning) software such as Symantec Corporation's Ghost or PowerQuest Corporation's DriveImage software. In order to use disk cloning as an installation method, however, the master and target systems must have identical hardware configurations. To create a clone, you install and configure the operating system and applications on the master machine and then use the disk imaging software to capture an exact bit-image of the master machine's hard drive, which you then duplicate to the hard drives on the other computers, producing exact clones of the master machine. When you use disk-image duplication software, you must be careful that incompatibilities do not result. For example, if you clone a computer running Windows 2000, the cloned version will have the same security identifier (SID) as the original, which means that the two computers cannot coexist on the same network. Makers of disk-image duplication software can provide utilities for modifying SIDs to work around this problem. Note also that the master and target computers must be stand-alone member servers and not part of a Windows 2000 domain.
Another tool used for unattended installations included with Windows 2000 Server is Remote Installation Services (RIS), which you can use to install Windows 2000 Professional or Windows XP on client computers from a distribution server share point. An image of the fully configured operating system with any locally installed applications that will be required is created and stored on the network share, and the RIS boot disk is used to boot the client computers, connect to the RIS server, download the image, and install the operating system on the client computers. If the client computers have ROM (read-only memory) that supports Pre-Boot Execution Environment (PXE) architecture and have remote boot-enabled network adapter cards, a remote boot disk is not required for each client.
Finally, if you need to perform mass installations of software on large numbers of networked computers, you might consider using desktop systems management software such as Microsoft Systems Management Server (SMS).
See Also answer file ,disk imaging ,Systems Management Server (SMS)
Filename for an answer file, a text file that can be used to perform an unattended installation of Microsoft Windows 2000, Windows XP, and Windows .NET Server.
See Also answer file
An electrical cable in which the potential of the signal-carrying conductor is above ground while the return-path conductor is at ground potential.
Overview
An unbalanced line can be contrasted with a balanced line, in which both conductors carry a signal and have potentials that are equal in magnitude but 180 degrees out of phase. In computer networking, an example of an unbalanced line is coaxial cabling, in which the electrical signal is carried solely by the central conductor while the ground path (the internal wire-braid or mesh shielding inside the cable jacket, which is connected to the earth at one termination point of the cable) provides the unbalanced signal return path.
Unbalanced line. The return-path conductor of the unbalanced line is at ground potential. The return-path conductor of the balanced line carries a signal.
In contrast, an example of a balanced line is twisted-pair cabling, which comes in the shielded twisted-pair (STP) and the unshielded twisted-pair (UTP) varieties. Because of its electrical characteristics, unbalanced coaxial cabling is more susceptible to electromagnetic interference (EMI) than balanced STP cabling, but coaxial cabling is capable of higher transmission rates over longer distances.
See Also balanced line , coaxial cabling ,electromagnetic interference (EMI) ,shielded twisted-pair (STP) cabling ,
Stands for Universal Naming Convention, a convention for naming shared network resources.
See Also Universal Naming Convention (UNC)
One-to-one communication between stations on a network.
Overview
Unicasting represents full-duplex, point-to-point communication using directed traffic sent between two hosts on a network. Unicasting is contrasted with broadcasting, which is point-to-multipoint in nature and directs information to all accessible hosts on the network, and multicasting, another form of multipoint communication in which information is directed only to hosts that request it. Most user-initiated network traffic is unicast or directed traffic, but broadcast traffic is mostly for clients and services to announce themselves on the network and multicast traffic is for streaming multimedia communication. In the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite, the connection-oriented TCP supports unicasting of IP packets, and the connectionless User Datagram Protocol (UDP) provides broadcasting and multicasting of IP packets.
For a unicast packet to reach its destination host on the network, the packet header must contain the address of the destination host. In TCP/IP networking, this destination address is in the form of a logical address called an IP address. On an IP internetwork, routers (network devices that can forward packets to other networks) are usually configured to forward unicast packets (packets directed to a specific host on a destination network) and broadcast packets (packets directed to all users on a subnet, network, or internetwork).
Unicasting. Comparison of unicasting, multicasting, and broadcasting.
See Also broadcasting , connectionless protocol ,connection-oriented protocol ,Internet Protocol (IP) ,multicasting ,point-to-multipoint ,point-to-point ,routing ,Transmission Control Protocol (TCP) ,Transmission Control Protocol/Internet Protocol (TCP/IP) ,
An international character set standard that is capable of representing characters in all written languages.
Overview
The American National Standards Institute (ANSI) character set, which employs a single byte (8 bits) to represent each character, can represent up to 28 = 256 characters only. This is sufficient for most Western languages, but it is insufficient for languages such as Chinese or Arabic. In contrast, the Unicode character set employs two bytes (16 bits) per character and can thus represent a total of 216 = 65,536 characters-enough for every language in the world, plus punctuation and standard mathematical symbols.
Unicode is the native character encoding for file and object names in Microsoft Windows 2000, Windows XP, and Windows .NET Server. Filenames are also represented in the NTFS file system (NTFS) using Unicode characters. Before any operations are performed on strings, the Win32 subsystem converts all ANSI strings to Unicode. Many Win32 applications have entry points for both 8-bit ANSI and 16-bit Unicode string functions to ensure backward compatibility for running on older platforms such as Windows 95 and Windows 98.
For More Information
Visit the Unicode Consortium at www.unicode.org
See Also ASCII
Technology that allows users to access their e-mail, telephone, fax, and other forms of messages all from a single mailbox.
Overview
Unified Messaging (UM) has been a Holy Grail pursued by system and application vendors for years and which may finally be becoming a reality. The goal of UM is to make it easier for users to manage the various forms of personal communication they use in today's enterprise. For example, the typical knowledge worker might receive dozens of telephone calls, several faxes, a hundred or so e-mails, and numerous pager alerts and instant messaging (IM) communications in a single day. UM aims to simplify things by allowing users to access all these various forms of communication from a single interface, usually an Inbox of an e-mail program but also a telephone or even a Web browser.
Implementation
There are two basic approaches to implementing UM: unified and integrated. The unified solution stores all of a user's messages, regardless of their format, in a single message repository, usually the message store of a mail server. This approach best benefits companies that have not yet heavily invested in alternate messaging architectures such as voice mail and Private Branch Exchange (PBX) systems or network fax servers. Unified UM is typically implemented using third-party vendor add-on products for popular messaging and collaboration servers such as Microsoft Exchange Server or Lotus Notes. A new approach gaining some popularity is to replace traditional PBXs with PC-based PBXs-standard PCs configured with phone-line cards and specialized software to enable them to provide the functions of a traditional PBX together with advanced UM capabilities. Many of these PC-PBX systems are built on top of an Exchange architecture.
For companies that have already acquired voice mail systems and other legacy messaging systems, a better approach might be integrated UM, which makes use of existing messaging systems and ties them together using UM middleware. Integrated UM generally employs a peer-to-peer architecture that connects and synchronizes together various messaging systems such as e-mail, network fax, and voice mail. Integrated UM keeps each type of message stored in its own kind of repository and leverages the use of existing messaging hardware and software. The negative side is that the integrated UM solution generally requires some software customization to be performed to ensure that these various repositories replicate their message header information to one another to enable users to reliably access any form of message from any message interface, such as telephone, cell phone, or e-mail inbox.
Besides these above two solutions, which involve deploying hardware or software at the customer premises, a third approach to UM is gaining popularity, especially among small and mid-sized businesses. This third solution involves outsourcing UM needs to a third party or service provider. These subscription services are typically offered for a low monthly fee from telcos, cellular communication providers, Internet service providers (ISPs), or even portals such as Yahoo!. The user is typically assigned a telephone number, and others can telephone or send faxes to them using this number. Voice messages received are converted to .wav files and forwarded to the user's e-mail inbox, and faxes are converted to .tif files and similarly forwarded. Users then access their voice, fax, and e-mail messages either using their e-mail client or a standard Web browser.
Marketplace
Customer premises equipment (CPE) for implementing unified UM solutions are available from a number of infrastructure vendors including Avaya, Cisco Systems, Lucent Technologies, Mitel Networks, NEC Corporation, Nortel Networks, Siemens, and several others. In addition, many niche vendors specialize in unified and integrated UM solutions, including Active Voice and Captaris. Examples of some popular in-house UM systems include Unified Messenger from Avaya, OnePoint Messenger from Mitel, InternetPBX from COM2001.com (now Alexis), Telephony e-LinX from Esnatech, VoiXX from Intersis, MESSAGEmanager from System Solutions, and TOPCALL Communication Server ONE from TOPCALL International.
Examples of UM service providers include large carriers such as Bell Canada, Verizon Communications, MCI Worldcom, and many others. A number of companies also offer various forms of free UM services in exchange for advertising, including eFax.com, Hello Direct, JFAX.com, Verso Technologies, OneBox, ThinkLink, and uReach Technologies.
Prospects
The driving force behind the current wave of UM solutions is the proliferation of mobile workers using cell phones. Whether traditional UM infrastructure vendors will thrive in the face of the growing number of companies offering subscription-based UM solutions will be the big question to watch over the next few years.
For More Information
Visit the Unified Communications Consortium at www.unified-msg.com
See Also e-mail ,Internet service provider (ISP) ,portal ,Private Branch Exchange (PBX) ,telco
An addressing scheme for locating resources on the Internet.
Overview
A Uniform Resource Locator (URL) is essentially an "address" of a file stored somewhere on the Internet. URLs enable Internet client software such as Hypertext Transfer Protocol (HTTP) clients (most commonly Web browsers) and File Transfer Protocol (FTP) clients to locate and access resources stored on Web servers, FTP servers, and other kinds of Internet hosts. URLs are to the Internet what absolute paths are to network file systems and what Universal Naming Convention (UNC) paths are to shared file and print resources on a Microsoft Windows network.
Architecture
A URL is a hierarchical, structured address based on the Domain Name System (DNS), although a URL can also use an Internet Protocol (IP) address to locate its targets. A URL adheres to the following standard form:
[protocol:]//[domain or IP address][:port]/[path to target file]
The components of this syntax are:
Protocol: This is an optional address element that describes the application layer Internet protocol on the server that must be used in order for the client to access the resource. Examples include HTTP:, FTP:, and Telnet:.
Domain name or IP address: This represents the logical address of the server on which the resource resides and is a required element.
Port number: This is required only if the port on the server to which the client must connect to access the resource differs from the default port for that type of protocol. In other words, if no port number is specified, the request is assumed to use the well- known port number for the protocol being used. For example, the well-known port number for HTTP is port 80.
Path: This is required if the resource is not in the root of the server's directory that is mapped to the specified domain name or IP address. This path is usually a virtual path that is mapped to a hierarchical series of directories on the server or on the server's network.
Examples
The following are a few examples of URLs:
http://www.microsoft.com/train_cert/default.asp
ftp://172.16.8.99:3355/usr/bin/frag.Z
http://b12.sales.contoso.com/Sales%20Department/Wigs/Colors/red.html
The string %20 in the third URL is an example of character encoding. Because certain ASCII characters (such as spaces) are not allowed in URLs, they must be encoded using the form %xx , in which xx is the hexadecimal code for the character using the International Organization for Standardization (ISO) Latin-1 character set. The string %20 represents a space character.
See Also absolute path , Domain Name System (DNS) ,File Transfer Protocol (FTP) ,Hypertext Transfer Protocol (HTTP) ,IP address ,port number , Web browser
A device that can temporarily provide power to network components.
Overview
Downtime of computer networks and systems are the bane of today's 24x7 e-businesses, so guarding against such a condition is essential to the enterprise network architect. One of the key weapons in an administrator's arsenal is the uninterruptible power supply (UPS), which provides:
Temporary power against brief power outages to prevent mission-critical data from being lost and against extended power failures to enable servers and other key networking components to be shut down in an orderly manner
Power conditioning to eliminate harmful surges and brownouts that can corrupt data and damage equipment
The standard UPS is typically a device with rechargeable batteries and software drivers that can perform operations such as:
Notifying users that a shutdown is imminent and that they should save their work
Pausing services on servers to prevent new connections from being established
Providing enough power to perform a soft shutdown of servers and other key network components
Performing other actions as specified in a batch file that runs automatically
Providing power line conditioning and smoothing out spikes and dips in the power flow
UPS devices range from small stand-alone boxes to rack-mountable units to room-sized devices having integrated diesel generators for extended power generation. UPS devices are generally rated according to two values:
Volt-amp (VA) or kilovolt-amp (kVA) rating: This determines how much connected computer and networking equipment the UPS is capable of supporting. The larger the VA or kVA value, the greater the number and higher the power requirements of connected devices it can support. UPS devices range from small 1000 VA (1 kVA) units costing $100 or so and designed for Small Office/Home Office (SOHO) computer networks and servers to huge high-end 500-kVA units costing thousands of dollars and designed for mission-critical needs such as those of banks and hospitals. Note that UPS devices can also be rated in watts (W) since 1 VA is equal to 1 W and 1 kVA equals 1 kilowatt (kW), but for some reason general usage is to rate them in VA and kVA instead.
Time@100%(min): This is the amount of time that the unit can sustain maximum power generation. Small and mid-sized UPS devices can typically provide 100 percent power to network components and computers for between 5 and 15 minutes after a power failure. You can use additional battery packs with some UPS devices to extend uptime further.
Marketplace
Some of the larger vendors of UPS devices include American Power Conversion Corporation, Best Power, Clary Corporation, Exide Technologies, Falcon Electric, Liebert Corporation, ONEAC Corporation, OPTI- UPS, Powerware, Toshiba Corporation, Tripp Lite, and Tsi Power Corporation. An example of a high-end UPS is the Silcon DP300E from American Power Conversion, which comes in various models rated up to 1.6 megawatts (MW). Such high-end UPS devices are designed for the largest data centers.
Notes
Always test your UPS after installing it. Otherwise, you might be sorry when a power failure happens in your neighborhood! A good way of testing a UPS is simply to unplug a computer on your system and watch what happens. Some UPS units support Simple Network Management Protocol (SNMP), which lets you test the condition of the UPS at regular intervals and report the results to an SNMP management console.
Note also that the larger the load attached to a UPS unit, the shorter the time interval the unit can continue powering attached devices during a power outage. Be sure that your UPS unit can support your power needs for the time needed to properly shut down your system.
To determine what size UPS you need for your network, add the wattages of all the devices you want to connect and ensure that you buy a UPS unit whose VA rating exceeds this value by a good margin.
See Also disaster recovery
A text file used to supplement answer files during unattended installations of Microsoft Windows NT, Windows 2000, Windows XP, and Windows .NET Server.
Overview
Uniqueness Database Files (UDFs) are used to override the answer to prompts provided by answer files during unattended setup. These UDFs replace the general answers in the answer file with information that is specific to a given computer or small group of computers. UDFs typically supply information customized for individual computers such as computer name or username. By contrast, answer files specify more general setup options such as setup mode or time zone, which are the same for all computers on the network. Thus, although you might need only one answer file to deploy a number of Windows 2000 computers on your network, you would need a separate UDF for each computer to be deployed.
The way it works is that the UDF is merged into the answer file at the start of the graphical user interface (GUI) portion of setup. As a result, you can only use UDFs to define setup parameters for the GUI portion of setup, not the text portion. During the text portion of setup, only the answer file can specify parameters to use in response to setup prompts.
Notes
Windows XP and Windows .NET Server use Setup Manager to prepare unattended installation files for system deployment.
See Also answer file ,
Hardware used to translate between serial and parallel transmission.
Overview
Universal Asynchronous Receiver/Transmitter (UART) chips are employed in the PC architecture to translate signals between a computer's internal bus and its COM ports. This is because although COM ports use serial transmission, which sends bits one at a time, a computer's internal bus is parallel and carries multiple bits of data simultaneously. Another function of the UART is to add start and stop bits when translating from parallel to serial transmission in order to ensure synchronization between the COM ports and connected remote serial devices and to strip off these bits when translating from serial back to parallel for incoming serial transmission.
UARTs employ their own internal clocking system that runs separately from the computer's internal clock, so the UART must be fast enough to handle data transferred to it from the internal bus or bits may be overwritten and communications may become garbled. Internal modems have their own UART chips because they bypass the computer's COM ports for their transmission.
See Also bus ,modem ,serial transmission
A Microsoft technology for enabling access to any kind of data source.
Overview
Universal Data Access (UDA) is part of Microsoft Corporation's Windows Distributed Network Architecture (Windows DNA) framework and enables applications to access data from all kinds of sources, including Structured Query Language (SQL) databases, Lightweight Directory Access Protocol (LDAP) directory services, e-mail repositories, spreadsheets, text files, and mainframe and other legacy data sources. UDA relies on three components to connect applications and data browsers with back-end data sources:
Open Database Connectivity (ODBC): Used primarily to access SQL-compliant relational databases such as Microsoft SQL Server databases, Microsoft Jet databases, and Oracle databases.
OLE DB: Specifies a series of extensible Component Object Model (COM) interfaces for accessing various kinds of data sources, including SQL databases (through ODBC), non-SQL structured data sources, and unstructured data such as text files.
ActiveX Data Objects (ADO): Language- independent framework that enables applications to use OLE DB directly.
See Also ActiveX Data Objects (ADO) ,Component Object Model (COM) ,Joint Engine Technology (Jet) ,Lightweight Directory Access Protocol (LDAP) ,OLE DB ,open database connectivity (ODBC) ,SQL Server ,Structured Query Language (SQL) ,Windows Distributed interNet Applications Architecture (Windows DNA)
A platform-independent framework for businesses to advertise and to discover one another.
Overview
Universal Description, Discovery, and Integration (UDDI) is an emerging platform used to simplify the process of companies locating and doing business-to- business (B2B) transactions with one another over the Internet. UDDI provides a standard way for businesses to code what they do and store this information in a directory that is publicly accessible to other businesses. The UDDI project is spearheaded by Ariba, IBM, and Microsoft Corporation and is in beta testing by more than 130 companies who have joined on to the initiative.
UDDI is built on a framework of other open standards, including Extensible Markup Language (XML), Hypertext Transfer Protocol (HTTP), and Simple Object Access Protocol (SOAP).
Implementation
UDDI uses a series of specialized servers called registries. Ariba, IBM, and Microsoft maintain beta versions of these registries, but the intention is to eventually turn UDDI over to an open standards body once testing is completed. UDDI registries store directory information about businesses in a centralized location in three formats:
White pages: General information about businesses such as name, address, and description of business services.
Yellow pages: Businesses organized by categories according to tags defined by government standards.
Green pages: Information on the technical aspect of services which individual business systems expose for data exchange purposes using XML.
For More Information
Find out more about UDDI at www.uddi.org
See Also B2B ,directory ,e-business ,Hypertext Transfer Protocol (HTTP) ,Simple Object Access Protocol (SOAP) ,XML
The successor to the CD-ROM File System (CDFS).
Overview
CDFS was a file system used in Microsoft Windows NT 4 and earlier versions for accessing information stored on CD-ROM disks. Universal Disk Format (UDF) is the successor to CDFS and is capable of both reading from and writing to removable media such as CD, CD-R, CD-RW, DVD, write-once read-many (WORM), and magneto-optical (MO) media. UDFS is an open industry standard based on the following standards:
ISO 13346 from the International Organization for Standardization (ISO)
ECMA 167 from the European Computer Manufacturers Organization (ECMA)
OSTA UDF from the Optical Storage Technology Association (OSTA)
UDF supports a number of advanced features, including
Access control lists (ACLs)
Deep directory trees
Large file sizes up to 64 gigabytes (GB) in size
Long file names up to 255 characters long with a maximum path length of 1023 characters
Named streams
Sparse files
Unicode file names
UDF is implemented on the Windows 2000, Windows XP, and Windows .NET Server platforms. Some versions of these platforms require special drivers in order to write to certain media.
See Also CD-ROM File System (CDFS) ,file system
Another name for G.Lite, a splitterless version of Asymmetric Digital Subscriber Line (ADSL).
See Also G.Lite
A type of group in Microsoft Windows 2000 and Windows .NET Server.
Overview
Universal groups are one of three types of groups in Windows 2000 and Windows .NET Server, the other types being domain local groups and global groups. Universal groups differ from these other types in that they can include members from any domain in the forest and can be granted permissions for resources in any domain in the forest. Universal groups can contain user accounts, global groups, and even other universal groups, but they cannot contain domain local groups. Like global groups, all trusted domains have access to universal groups to grant them permission to access resources on the network.
Some typical uses for universal groups include
You can use global groups nested inside universal groups to dramatically reduce network traffic due to global catalog replication in a Windows 2000-based network. Use universal groups only when their membership changes infrequently, because excessive replication traffic can occur in a domain tree if their membership changes frequently.
You can also use universal groups to grant users access to resources that are located in multiple domains. Simply add global groups from each domain to a universal group and assign permissions for the resource to the universal group. This use of universal groups is similar to that of domain local groups, except that you can use domain local groups only to assign permissions for resources in a single domain.
Notes
You can create universal groups only when your Windows 2000 or Windows .NET Server domain is running in native mode, not when it is in mixed mode. In other words, you cannot use universal groups in a network that still has downlevel Microsoft Windows NT domain controllers. You can use them only in a network whose domain controllers are all running Windows 2000 or Windows .NET Server.
See Also domain local group ,forest ,global group ,group ,mixed mode ,native mode
Another name for Wideband Code Division Multiple Access (W-CDMA), a third-generation (3G) cellular communications system being deployed in Europe.
See Also Wideband Code Division Multiple Access (W-CDMA)
A convention for naming shared network resources.
Overview
The syntax for a Universal Naming Convention (UNC) path to a network resource is as follows:
\\server_name\share_name[\path]
where "share_name" is a shared folder, and "path" is the (optional) path to the file within the shared folder.
The UNC path \\max\sales\1998\report.doc indicates that the document report.doc is located within the shared folder called sales on the file server called max. The document is in the 1998 subfolder of the sales shared folder.
See Also shared folder
A high-speed serial interface for connecting peripherals to computers.
Overview
Connecting peripherals to computer systems has traditionally suffered from two problems:
Installing internal peripherals such as Peripheral Component Interconnect (PCI) modem cards involved opening the case, plugging the card into an available slot, screwing the card tight, and closing the case-a time-consuming and complicated process for the average user.
Transmission speeds supported by standard RS-232 serial and Centronics parallel interfaces have been too slow for some external peripherals such as scanners and digital cameras to operate at an acceptable speed.
Universal serial bus (USB) was created to address these issues by providing a low-cost industry-standard high- speed serial interface for external peripherals and supporting Plug and Play (PnP). Installing a USB peripheral is as simple as connecting it to a USB port on the computer using a standard USB cable. Provided the computer is running an operating system that supports PnP and USB, such as Microsoft Windows 2000 or Windows XP, simply plugging in the peripheral causes it to be automatically recognized and configured for use. Since USB provides "hot plug-able" support, no rebooting of the computer is necessary when connecting or disconnecting USB devices. USB makes it easy to connect peripherals to computers and eliminates the need to configure interrupt request (IRQ) settings or dual inline package (DIP) switches since configuration of devices is automatic.
USB ports are included on most computer systems sold today, and USB peripherals are becoming popular, especially in the consumer market where USB-capable monitors, mice, keyboards, joysticks, scanners, digital cameras, printers, and other devices are now proliferating. Besides ease of use, another advantage of USB over legacy serial and parallel port communications is that USB carries up to 500 milliamps (mA) of power to connected peripherals, eliminating the need for bulky power supplies for many types of peripherals.
Universal serial bus (USB). A typical example of using USB to connect peripherals to a computer system.
Implementation
USB uses a master/slave architecture in which the computer (master) controls all communications between itself and attached peripherals (slave devices). There are several types of USB peripherals:
Input/output (I/O) device: This is simply any peripheral that supports USB connectivity. Examples include USB keyboards, mice, scanners, and speakers.
Hub: This is a device with multiple USB ports that allows several peripherals to share a single USB port on the computer. USB hubs can be cascaded in a tiered star topology up to five levels deep to support greater numbers of peripherals.
Compound device: This is a USB peripheral that acts as both an I/O device and a hub. An example is a USB monitor, which allows a keyboard or mouse to be connected through it to the computer.
The USB controller on the host system (a computer having a built-in USB port on the motherboard or an installed USB serial card) manages communication between the host and the connected USB devices using a packet-based token-passing mechanism called transfer queuing. Two kinds of communication take place:
Initialization: The host controller continually polls the network to detect new USB devices and to ensure that connected devices are still attached and functioning.
Run time: USB devices make various kinds of requests to the host controller, which queues the requests and respond to them according to a first-in, first-out (FIFO) manner. Requests made by USB devices include requests for or reports of status information, and requests for reading data to or writing data from another device on the bus. All USB requests are handled by the host controller, which obtains and sends the necessary control information or data to the device.
USB theoretically supports up to 126 peripherals connected together, but in practice it is best to keep the number to about a dozen for best performance and to simplify troubleshooting should incompatibilities arise. USB devices can be no more than 16.4 feet (5 meters) apart or the same distance from a hub or computer. The entire cabling arrangement of a computer and multiple USB peripherals is collectively referred to as the USB bus, even if the actual topology is star-like instead of bus-like in appearance.
USB cables are terminated differently at each end, depending on whether they connect to a computer (upstream connection) or I/O device (downstream connection). This is mainly done to prevent loops in the USB bus. The Type A (flat) connector plugs into the USB port on the computer or into the device ports on the hub, and the Type B (square) connector generally connects to the device.
USB supports two speeds, which depend basically on the type of media used to connect peripherals:
1.5-megabits per second (Mbps) transmission over unshielded USB cabling
12-Mbps transmission over shielded USB cabling
It is usually possible to combine these two types of media into a single arrangement, because USB hubs and host controllers are generally autosensing and can determine which transmission speed to use depending upon the attached cabling. USB cabling is typically 4-wire cabling and uses a 28-gauge twisted pair for data transmission and a 20-gauge untwisted pair for carrying power to the devices. The table shows the pinning types for USB cabling.
Pin | Color | Function |
1 | Red | +5 V power |
2 | White | Data - |
3 | Green | Data + |
4 | Black | Ground |
Marketplace
For Small Office/Home Office (SOHO) users who want to network two computers, USB offers a simple way of doing this using USB direct cable networking. In this scenario, a USB port on one computer is connected directly to a USB port on another to provide Ethernet-like networking capability. Alternatively, more than two computers can be networked using a USB hub. Several companies offer the special USB cabling and software needed to implement USB direct cable networking, including ActionLink from ActionTec Electronics, EZLink USB from Anchor Chips, and USB Direct Connect from Belkin Components.
Prospects
USB is rapidly replacing serial and parallel interfaces as the most popular way of connecting peripherals to computer systems. A new specification called USB 2, or "high-speed USB," has been developed that is 40 times faster than USB 1.1, and USB 2-capable systems and software started becoming available toward the end of 2001 with support included in hardware from Intel Corporation, systems from Dell Computer Corporation, and in the Microsoft Windows XP operating system platform.
Notes
When you use shielded USB cabling, connect the shield to ground only at the USB host (computer), not at the USB devices (peripherals).
You can connect an Institute of Electrical and Electronics Engineers (IEEE) 1284-compliant printer that has a standard Centronics interface to a USB system by using a Centronics/USB adapter or patch cable.
See Also IEEE 1284 ,IEEE 1394 ,parallel transmission ,Plug and Play (PnP) ,RS-232 ,serial transmission
A standard for third-generation (3G) wireless cellular systems.
Overview
Universal Wireless Communications (UWC-136) is a standard developed by the Universal Wireless Communication Consortium (UWCC), a consortium of more than 85 telecommunications carriers and wireless product vendors.UWC-136 is part of the International Mobile Telecommunications-2000 (IMT-2000) initiative from the International Telecommunication Union (ITU), which promotes UWC-136 as a competing IMT-2000 standard.
UWC-136 is a pure Time Division Multiple Access (TDMA) cellular communication technology that is designed to be backward compatible with the existing IS-136 TDMA digital cellular phone system defined by the ANSI-136 and IS-41 standards and still widely used throughout North America. Implementation of UWC-136 is planned in phases by first upgrading existing IS-136 TDMA systems to IS-136+ to provide data rates of 43.2 kilobits per second (Kbps) over standard 30-kilohertz (kHz) channels. Then IS-136+ will be upgraded to IS-136HS, which is the high-speed component of UWC-136. Eventually, UWC-136 will provide packet- data services at speeds of up to 384 Kbps for wide-area coverage and up to 2 megabits per second (Mbps) for in-building coverage. Voice communication on this system will be high-fidelity wireline quality, comparable to that of Integrated Services Digital Network (ISDN).
For More Information
Visit the UWCC at www.uwcc.org
See Also 3G ,cellular communications ,IMT-2000 ,Integrated Services Digital Network (ISDN) ,International Telecommunication Union (ITU) ,Time Division Multiple Access (TDMA)
A popular operating system widely deployed in today's enterprise networking environments.
Overview
UNIX is a multiprocessing, multitasking, multiuser operating system developed at AT&T Bell Laboratories in 1969. UNIX was the first operating system written entirely in the C programming language and consequently has close ties to C. Writing UNIX in C made UNIX a portable operating system-that is, by changing the source code slightly, UNIX could be "ported" (made to work on) any computer hardware platform including PowerPC, MIPS, Intel, Sparc, and various mainframe and midframe computing architectures.
UNIX has the stability and robustness of an operating system that has evolved over 25 years, but it has a reputation for being difficult to learn and administer, mainly due to its high degree of dependence on the command line. On the other hand, this emphasis makes UNIX easier to administer for those familiar with the platform, because it allows administrative tasks to be scripted, scheduled, and executed to make complex and repetitive tasks simple to perform. In fact, much of the original drive toward developing scripting languages came from the UNIX administration environment.
UNIX was designed as a time-sharing environment, in contrast to the batch environment of most computing systems of the time. Using UNIX, multiple users could simultaneously access the system and run their programs, communicating interactively with the system using remote terminals. UNIX users could thus collaborate in real time on computing projects by sharing files and resources.
UNIX has established strong roots for itself in academic computing environments, where it is still frequently used and taught today. UNIX also has its roots in the Internet environment where UNIX and Transmission Control Protocol/Internet Protocol (TCP/IP) evolved hand-in- hand in the 1980s. TCP/IP is the underlying network transport used by all UNIX systems.
Types
There are actually two main "branches" in the UNIX "family tree":
BSD: Stands for Berkeley Software Distribution, an early variant of UNIX developed in the 1970s by students at the University of California at Berkeley, who licensed AT&T's version and added such new features as the C shell and vi editor to enhance the AT&T platform. BSD UNIX forms the basis of SunOS, a lightweight version of UNIX developed by Sun Microsystems that supports only a single processor. Another popular flavor of BSD is iXsystems (formerly BSDi) from Wind River Systems. There are also several free versions of BSD UNIX, including FreeBSD, OpenBSD, and NetBSD.
System V: Developed by AT&T Bell Laboratories with UNIX Systems Laboratories, this version incorporates some features of BSD UNIX. The source code for System V UNIX was owned by Santa Cruz Operation (SCO), which is now called Tarantella and is part of Caldera International. System V forms the basis of Solaris, a popular version of UNIX from Sun Microsystems (Solaris also includes some features of BSD), and several other versions.
Based on evolution from these two branches, UNIX has evolved into a number of different "flavors" as vendors have adapted it to their own proprietary hardware platforms. These flavors include the following:
Solaris: This version was developed by Sun Microsystems for its proprietary SPARC computing architecture and is the most popular version of UNIX in use today in the enterprise. Sun Enterprise Servers and Ultra Workstations running Solaris provide a powerful and reliable platform for high- performance computing. Solaris is based on SunOS 5.x, which itself was based on BSD with some features from System V R4 included. The current release of this product is Solaris 8, and it is compliant with the UNIX 98 standard.
AIX: This version by IBM is for its midrange RS/6000 and PowerPC computing platforms. AIX is currently in distant second place in the commercial UNIX marketplace behind Sun's Solaris operating system. Efforts are underway to port AIX to Intel Corporation's new 64-bit Itanium processor architecture, which, when complete, should make AIX a viable competitor to Solaris in the high-end server market. The current release of this product is AIX 5L, and it is compliant with the UNIX 98 standard.
HP-UX: This version was developed by Hewlett- Packard Company for its HP 9000 server line and is a close third-place finisher in the commercial UNIX market, behind Sun's Solaris and IBM's AIX platforms. HP-UX is based on System V but has some BSD features included. The current release of this product is HP-UX 11i, and it is compliant with the UNIX 95 standard.
Tru64 Unix: This version was originally called Digital Unix and was developed by Digital Equipment Corporation (DEC) for its Alpha processor platform. It was renamed Tru64 when DEC was acquired by Compaq Computer Corporation. Tru64 runs only on the Alpha platform, which is now discontinued but still widely used in high-end computing environments by many companies. Tru64 Unix is compliant with System V R3.2 and R4. The current release of this product is Tru64 Unix 5.1, and it is compliant with the UNIX 95 standard.
UnixWare: This version was developed by SCO (now Caldera) to port UNIX to the Intel PC platform. The current release is UnixWare 7, and it is compliant with the UNIX 95 standard.
IRIX: This version was developed by Silicon Graphics, Inc. (SGI) for its high-end SGI MIPS servers and graphic workstations. IRIX is based on System V R3.2 and R4. The current release is IRIX 6.5, and it is compliant with the UNIX 95 standard.
Linux: A UNIX-like operating system developed by Linus Torvalds which is rapidly growing in popularity for niche deployments in the enterprise.
Several attempts have been made to standardize the UNIX operating system. AT&T created a written standard called the System V Interface Definition (SVID) for its UNIX System V Release 4 (SVR4). In addition, an industry consortium has created a standard called Portable Operating System Interface for UNIX (POSIX). The actual trademark "UNIX" is now owned by The Open Group, a consortium of software and hardware manufacturers.
Notes
Microsoft Windows 2000 and Windows .NET Server support interoperability with UNIX to support real-world heterogeneous enterprise networking environments. This interoperability is provided through built-in protocols, services, and tools in the Windows 2000 operating system and also through Microsoft Services for UNIX 2, an add-on product from Microsoft Corporation. Built-in Windows/UNIX interoperability on the Windows 2000 platform includes such features as:
Common support for industry-standard protocols such as TCP/IP, Dynamic Host Configuration Protocol (DHCP), Domain Name System (DNS), and remote procedure call (RPC) functionality
Support for File Transfer Protocol (FTP) and Hypertext Transfer Protocol (HTTP) for cross- platform file sharing
Support for UNIX printing through Line Printer Daemon (LPD), Line Printer Queue (LPQ), and Line Printer Remote (LPR) utilities
Support for remote terminal emulation using Telnet
Support for cross-platform database access through open database connectivity (ODBC)
Support for industry-standard network management through Simple Network Management Protocol (SNMP) and Remote Network Monitoring (RMON)
Services for UNIX 2 provides additional interoperability features with the following UNIX platforms: Solaris, HP-UX, Digital UNIX, IRIX, and Linux. Several third-party companies also offer Windows- compatible interoperability solutions such as:
X Windows clients specifically designed for Windows-based PCs to support connectivity with UNIX servers
X Windows servers for Windows such as that from Hummingbird International
Server Message Block (SMB) to Network File System (NFS) protocol conversion for file sharing between platforms using products such as Samba
See Also AIX , Alpha platform ,BSD ,C++ ,HP-UX ,Intel-based platform ,Linux ,POSIX ,shell ,Transmission Control Protocol/Internet Protocol (TCP/IP) ,
Commands for administering various aspects of the UNIX operating system.
Overview
UNIX commands are text-based commands executed from the command line to perform a wide variety of administrative tasks. Most of these commands can also be used in batch files to perform complex administrative tasks from a single command. You can create batch files with a text editor such as ed or vi and schedule their execution by using the Cron command.
The table shows some commonly used UNIX commands, listed in alphabetical order with brief descriptions. Some commands have a simple syntax, such as the cd command for changing the current directory. Others are more complex and include scripting engines and even programming compilers, such as cc, the UNIX C compiler. Most versions of UNIX support most of these commands, but there are sometimes subtle differences from system to system. To see the exact syntax for any UNIX command, use the man command, which provides access to the UNIX online command reference. For example, to see the syntax for the cd command, type man cd at the UNIX command prompt. Note that that there are many UNIX commands not listed in this table-it would take many pages to list all the commands available on the various UNIX platforms.
Some of the commands below are asterisked (*), which indicates that they are specifically available on the Microsoft Windows 2000 platform when Microsoft Services for UNIX 2 is installed.
Command | Description |
alias | Displays or sets aliases for long command strings |
awk | Searches a file for a pattern and performs an action on the lines containing that pattern |
basename* | Removes the path leaving only the file name |
bg | Moves a stopped process into the background and restarts it |
cal | Displays a calendar |
cat* | Concatenates or displays files |
cc | C language compiler |
cd | Changes the current directory |
chgrp | Changes group ownership of a file |
chmod* | Changes access permissions for a file |
chown* | Changes individual ownership of a file |
chsh | Changes the default shell in a password file |
clear | Clears the screen |
cmp | Compares two files and returns the line numbers that differ |
cp* | Copies a file |
csh | C shell command interpreter |
date | Displays the date and time |
df | Displays the amount of free disk space in a file system |
diff | Displays differences between two files |
dirname* | Delivers all but the last level of the path in string |
du | Displays file system usage |
echo | Echoes the text typed following the command to the screen |
ed | Text editor |
elm | Text-based e-mail |
emacs | Text editor |
f77 | FORTRAN77 compiler |
fg | Moves a stopped process to the foreground and restarts it |
find* | Finds a file with specified characteristics |
ftp | File Transfer Protocol (FTP) client |
grep* | Searches a file for a text pattern |
head* | Displays the beginning of a file (by default, displays the first 10 lines) |
help | Displays help |
hostname | Displays the name of the current host system |
kill | Ends a running process |
ksh | Korn shell command interpreter |
ln* | Links files |
lpq | Displays the print queue |
lpr | Sends a print job to the spooler |
ls* | Lists files in a directory and displays the file statistics |
Sends e-mail | |
man | Accesses the online manual |
mkdir* | Creates a new directory |
more* | Displays a file one screen at a time |
mv* | Moves or renames a file |
passwd | Changes your password |
perl* | Interpretive scripting language |
pico | Text editor |
pine | Text-based e-mail program |
ps | Displays the status of a process |
pwd | Displays the name of the current directory |
rm* | Deletes a file |
rmdir* | Deletes a directory |
sed* | Stream line editor |
sh* | Invokes the Korn shell command interpreter |
sleep | Pauses a process |
sort* | Sorts or merges files |
split | Splits a file into multiple files |
tail* | Copies name file to standard output starting at designated place |
talk | Text-based chat |
tee* | Transcribes standard input to standard output |
telnet | Terminal emulator |
touch* | Creates a new file of designated name |
uniq* | Reports repeated lines in file |
uucp | UNIX-to-UNIX system file copies |
uudecode | Decodes a uuencoded file |
uuencode | Encodes a binary file |
vi* | Text editor |
wc* | Displays line, word, or character count in file |
who | Displays who else is logged on |
whoami | Displays the name of the user currently logged on |
whois | Finds remote users and sites |
write | Sends a message to a user |
See Also command , command interpreter ,command line ,
A command for transferring files between two UNIX hosts on a network.
Overview
UNIX-to-UNIX Copy (UUCP) is an early form of electronic messaging developed for UNIX platforms in the 1970s. UUCP was originally designed for transferring files over an asynchronous serial transmission interface such as RS-232. With UUCP, you can transfer files through a direct cable connection between serial ports on two computers by using a null modem cable or over telephone lines by using modems connected to the serial ports of each computer. A version of UUCP has been developed for transferring files over Ethernet as well. UUCP was also used for transferring electronic mail messages (e-mail) between hosts in the early days of the Internet. UNIX hosts used UUCP to transfer mail to a remote host simply by executing the MAIL program on the remote UNIX host and having it run on the local message file to be delivered.
UUCP uses the well-known port number 117 for its path service and number 540 for the UUCP daemon called UUCPD. UUCP has been eclipsed by File Transfer Protocol (FTP) as a general protocol for moving files between hosts and by Simple Mail Transfer Protocol (SMTP) for transferring e-mail messages.
Notes
Third-party vendors have created UUCP gateway connectors for Microsoft Exchange Server to enable Exchange to send and receive UUCP mail over the Internet.
See Also Ethernet , RS-232 ,serial transmission ,Simple Mail Transfer Protocol (SMTP) ,
Twisted-pair cabling with no internal shielding.
Overview
Unshielded twisted-pair (UTP) cabling is the most common form of network cabling for workgroups and departmental local area networks (LANs) because of its low cost, flexibility, and good performance. In UTP cabling the outer insulating jacket protects the cable from physical stress or damage but does not shield the cable from electromagnetic interference (EMI).
UTP cabling has an impedance of approximately 100 ohms and is available in various grades or categories based on data transmission capabilities, as shown in the table. Category 5 (Cat5) cabling and enhanced Category 5 (Cat5e) cabling are the most popular forms of UTP cabling in Ethernet networking. No agreed standards exist above Cat5e, but there is a proposed standard for Category 6 (Cat6) cabling and some manufacturers even offer UTP cabling they call Category 7 (Cat7) cabling.
Category | Use |
Category 1 (Cat1) | Analog voice grade |
Category 2 (Cat2) | Digital voice transmissions up to 4 megabits per second (Mbps) |
Category 3 (Cat3) | Digital transmissions up to 10 Mbps |
Category 4 (Cat4) | Digital transmissions up to 16 Mbps |
Category 5 (Cat5) | Digital transmissions up to 100 Mbps |
enhanced Category 5 (Cat5e) | Digital transmission up to 250 Mbps |
Notes
To ensure that Cat5e UTP cabling performs effectively, you should also use certified Cat5e wall plates, connectors, and patch panels. The patch panel or wall plate connections should be untwisted no more than 0.5 inch (1.3 cm); also try to keep cabling away from fluorescent lights, motors, and other sources of EMI. Do not bend the cable sharply-the radius of a bend should not be less than 10 times the diameter of the cable. Finally, avoid pulling cable wraps too tightly or crushing the cables in any way.
See Also Category 1 (Cat1) cabling ,Category 2 (Cat2) cabling ,Category 3 (Cat3) cabling ,Category 4 (Cat4) cabling ,Category 5 (Cat5) cabling ,Category 6 (Cat6) cabling ,Category 7 (Cat7) cabling ,electromagnetic interference (EMI) ,enhanced Category 5 (Cat5e) cabling ,Ethernet ,shielded twisted-pair (STP) cabling ,structured wiring
A type of identifier used in directory replication in Microsoft Windows 2000 and Windows .NET Server.
Overview
Update sequence numbers (USNs) are used for controlling directory replication between domain controllers. When an object is changed in Active Directory directory service, that change must be replicated to all other domain controllers. To do this, Active Directory assigns a USN to each changed object, incrementing the values with time. Each domain controller uses a table to keep track of its own current USN and the highest USN it has received from each of the other domain controllers on the network.
Update sequence number (USN). How Active Directory uses USNs during directory replication between domain controllers.
For example, if a domain controller called ABLE notifies a domain controller called BAKER of the updates it has for Active Directory, ABLE sends its own current USN number to BAKER, which compares the USN with the USN it recorded for ABLE in its internal table after the last directory update it received from ABLE. If the received USN is higher than the recorded USN, BAKER requests that ABLE send its updates for the directory. If the received USN is equal to or lower than the recorded one, BAKER knows that it has an up-to-date copy of ABLE's directory and does not request any updates.
USNs are more accurate than time stamps in controlling replication between domain controllers because time stamps depend on the clocks of domain controllers being exactly synchronized. However, time stamps are also employed if a tie-breaking mechanism is required for replication purposes. USNs also speed the recovery of Active Directory if a domain controller experiences a failure.
See Also Active Directory ,domain controller
A port on an Ethernet hub used to connect the hub to another hub.
Overview
Uplink ports allow Ethernet hubs to be cascaded to form larger networks using standard unshielded twisted-pair (UTP) patch cables. This method of connecting hubs does not use up any of the hub's station ports, which are used to connect computers to the hub. An arrangement of hubs connected together is typically known as a cascaded star topology.
When cascading hubs together, be sure that the root or main hub of your cascaded star arrangement of hubs is a high-quality hub. Do not create collision domains larger than about 150 computers-network performance can degrade seriously due to too many collisions on the network. If hubs cannot meet your bandwidth requirements, consider replacing the main hub with an Ethernet switch.
Notes
If you do not have an uplink port on your hub, you can still connect a station port on the hub to a station port on another hub by using a crossover cable.
See Also crossover cable , Ethernet ,hub ,topology ,
Stands for User Principal Name, the logon name of a user on a Microsoft Windows 2000 or Windows .NET Server network.
See Also user principal name (UPN)
Stands for uninterruptible power supply, a device that can temporarily provide power to network components.
See Also uninterruptible power supply (UPS)
Stands for Uniform Resource Locator, an addressing scheme for locating resources on the Internet.
See Also Uniform Resource Locator (URL)
A technology for load-balancing Hypertext Transfer Protocol (HTTP) traffic.
Overview
URL switching enables you to flexibly manage access to content stored on Web servers by load-balancing incoming HTTP requests. URL switching does this by directing these incoming requests to Web servers and caching servers based on the value of various substrings within the HTTP headers. For example, requests for static Web pages might be switched to a caching server, while requests for dynamic content are switched to a Web application server. Other criteria that can be used to switch requests include the size of the file requested and whether the content is multimedia in form. Once the switch determines the best server to service the request, it binds the request to that server for the duration of the session. Cookies can also be used with Uniform Resource Locators (URLs) to switch requests to specific Web servers in a server farm.
Other names sometimes used to describe URL switching include Web content switching and Layer-7 switching.
Marketplace
A leading vendor in URL switching devices arena is Alteon WebSystems (acquired by Nortel Networks). Other vendors of Web content switches include Cisco Systems, F5 Networks, Foundry Networks, and Intel Corporation.
See Also caching , Hypertext Transfer Protocol (HTTP) ,Layer 2 switch , Web server
Stands for universal serial bus, a high-speed serial interface for connecting peripherals to computers.
See Also universal serial bus (USB)
A global network of Internet hosts that supports tens of thousands of discussion groups covering every imaginable topic.
Usenet. How the Usenet system works on the Internet.
Overview
Usenet is part of the Internet's family of protocols and services and is typically used by individuals for such purposes as
Asking questions or requesting assistance on a particular issue
Distributing free information, including binary files such as images, sound clips, and multimedia files
Advertising products and services
Discussing issues of general, regional, local, or topical interest
Usenet currently consists of more than 60,000 newsgroups worldwide, and new groups are constantly being created as old ones disband. Most news servers do not carry the complete range of Usenet newsgroups because of the multigigabyte size of a full newsfeed. Other news servers exclude certain categories of groups, such as adult newsgroups. Some newsgroups are moderated, which means that new postings to the group must be approved by a moderator before they can appear in the newsgroup. This minimizes the "noise" or extraneous messages (mostly advertising).
Users typically gain access to the Usenet system by accessing a Usenet server running at their Internet service provider (ISP). Users typically must have an account and a password to connect to their ISP's news server to prevent unauthorized use, although free Usenet servers are also available across the Internet. Users employ Network News Transfer Protocol (NNTP) client software such as Microsoft Outlook Express to access newsgroups on the ISP's server and post new messages to them. Periodically, the ISP's news server replicates its newsgroup messages by pushing or pulling newsfeeds together with nearby Usenet servers on the Internet. As a result, within about 24 hours the messages posted by users are available on virtually every Usenet server in the world. A message typically remains on a news server for a few weeks or until enough new messages have been posted to the newsgroup to bump the old messages off the list.
Implementation
The Usenet system is based on NNTP, a standard Internet protocol defined in RFC 977. The architecture that supports Usenet has two components:
Client/server: This component allows user desktop machines running NNTP client software such as Outlook Express to interact with Usenet servers (NNTP hosts) to perform actions such as downloading a list of available Usenet newsgroups on the server, reading existing messages in the newsgroups, replying to existing messages, or posting new messages.
Server/server: This component allows NNTP hosts to communicate with each other, primarily for the purpose of replicating newsgroup messages between each other to ensure that each NNTP host has up-to-date messages in the newsgroups they host. An example of software that can function as an NNTP host is Microsoft Exchange Server, which fully supports NNTP and can be used to host newsgroup discussions.
The Usenet system of newsgroups is hierarchical in nature, similar to the Domain Name System (DNS). At the top level, Usenet consists of a number of top-level news hierarchies that identify either a range of topics (such as .rec for recreational topics) or a geographic location (such as .mn for Minnesota). Newsgroups are created hierarchically under these top-level categories, as in these examples:
forsale.mn
computers.forsale.mn
macintosh.computers.forsale.mn
The following table shows some of the popular top- level categories.
Category | Description |
alt | Alternative, which is a collection of various topics |
comp | Information on computer hardware, software, algorithms, and other topics |
misc | Miscellaneous information |
news | Information about Usenet itself |
rec | Recreational topics such as hobbies and travel |
sci | Scientific issues |
soc | Social issues |
talk | General discussion topics such as politics and religion |
Notes
If you do not have an NNTP client program or if your company firewall will not allow you to access Usenet services using the well-known port number for NNTP (port 119), you can still access Usenet (if you can browse the Internet using a standard Web browser such as Microsoft Internet Explorer). Web sites such as Google (www.google.com) offer a Web interface for reading and posting to Usenet newsgroups. Before doing this at work, however, make sure that your company does not have any policies forbidding employees to access Usenet.
See Also Exchange Server ,Internet ,Internet service provider (ISP) ,Network News Transfer Protocol (NNTP)
Credentials assigned to a user to enable secure access to network resources.
Overview
User accounts allow users to log on to desktop computers to access network resources such as shared folders, shared printers, e-mail services, and databases. An administrator generally grants a user access to these resources by assigning the user a set of credentials (user name and password) called a user account. In Microsoft Windows 2000 and Windows .NET Server networks, there are two basic types of user accounts:
Domain user accounts: These are created in Active Directory directory service and stored on domain controllers. You can log on to any computer on the network if you have a domain user account. Domain user accounts are usually referred to simply as user accounts.
Local user accounts: These are stored within the local directory database of a stand-alone server or desktop workstation that belongs to a workgroup instead a domain. A user who has a local user account can log on only to the computer on which the account was actually created.
Notes
A group account (or simply group) essentially acts as a container for holding user accounts. Group accounts simplify network administration by allowing multiple users to be granted or denied access to network resources in a single step-by granting or denying access for the group account to the resource.
See Also account ,domain user account ,group ,local user account
A transport layer protocol of the Transmission Control Protocol/Internet Protocol (TCP/IP) suite.
Overview
User Datagram Protocol (UDP) is one of two transport layer protocols within the TCP/IP protocol suite, the other being Transmission Control Protocol (TCP). Unlike TCP, which is intended for reliable, connection- oriented sessions between two hosts, UDP is a connectionless protocol in which packets are sent without first negotiating a connection between the sending and receiving hosts. UDP also does not include the segmentation, sequencing, flow control, acknowledgement, and retransmission features of TCP and is therefore an "unreliable" or "best-effort delivery" protocol in the same sense that ordinary mail ("snail mail") is unreliable-that is, there is no guarantee that a packet (or letter) sent will reach its intended recipient (think of TCP as the equivalent of "registered mail" in this analogy). Reliable delivery of UDP packets is the responsibility of application layer protocols above it that use UDP as their underlying network transport. UDP packets do include a checksum header, however, to ensure the integrity of packets that reach their destination.
Because of these characteristics UDP is considered the more "lightweight" of the two TCP/IP transport layer protocols. Although TCP is used for one-to-one communication between two hosts-in other words, for unicasting-UDP is used for one-to-many communication, which includes both broadcasting and multicasting. The following table summarizes the various differences between UDP and TCP-as you can see, UDP is a much simpler protocol than TCP, mainly because it does not need to include mechanisms relating to establishing, maintaining, and tearing down sessions between hosts. Note that neither UDP nor TCP includes fields for source and destination address in their headers-this is because the underlying network layer protocol called Internet Protocol (IP) is responsible for such addressing.
Feature | UDP | TCP |
Connection method | Connectionless | Connection-oriented |
Communication model | One-to-one | One-to-many |
Reliability | Best effort | Guaranteed |
Header length | 8 bytes | 20 bytes |
Header fields | Source and destination port, length, checksum | Source and destination port, sequence number, acknowledgement number, data offset, reserved, flags, window, checksum, urgent pointer, options, and padding |
Uses
Some examples of where UDP is employed include applications and services where name queries are performed, such as
Domain Name System (DNS)
NetBIOS name resolution (but if Windows Internet Name Service [WINS] is employed, then TCP is used instead)
Other examples include protocols and services where reliability is provided by
The application layer protocol itself-for example, Network File System (NFS) and Trivial File Transfer Protocol (TFTP)
Periodic advertisements-for example, Routing Information Protocol (RIP)
Notes
UDP is defined by RFC 768.
See Also connectionless protocol ,connection-oriented protocol ,Domain Name System (DNS) ,Internet Protocol (IP) ,NetBIOS name resolution ,Network File System (NFS) ,Routing Information Protocol (RIP) ,Transmission Control Protocol (TCP) ,Transmission Control Protocol/Internet Protocol (TCP/IP) ,Trivial File Transfer Protocol (TFTP) ,Windows Internet Name Service (WINS)
A mode of security on computers running Microsoft Windows 98 and Windows Me that protects shared folders and printers by requiring that users be authenticated by the network's security provider.
Overview
User-level security is more flexible and granular than share-level security, which protects shared resources using a password only. The security provider for enabling user-level security to be used on a Windows network can be a Windows 2000 or Windows .NET domain controller or a Novell NetWare server, depending on which network client the computer running Windows 98 or Windows Me has installed.
Windows 98 and Windows Me employ pass-through authentication for granting client requests for shared resources on the computer:
A remote client attempts to access a share on the computer running Windows 95 or Windows 98, passing it the user's credentials.
The computer forwards the user's credentials to the network security provider.
The security provider verifies the credentials and informs the computer.
The computer grants the user the specified level of access to the share, depending on the user's account and group memberships.
Notes
Use the Network utility in Control Panel to enable user-level security on a computer running Windows 98 or Windows Me.
See Also domain controller ,share-level security
Former name for Internet Locator Service, a Lightweight Directory Access Protocol (LDAP) directory service that enables Microsoft NetMeeting users to locate and contact other users for conferencing and collaboration over the Internet.
See Also Internet Locator Service (ILS)
A restricted mode of operation for processes in Microsoft Windows NT, Windows 2000, Windows XP, and Windows .NET Server.
Overview
User mode processes include components of the operating system that directly support user applications and the applications that use those subsystems. User mode processes have lower priority and fewer privileges than kernel mode processes, and they cannot access hardware directly.
User mode. Architecture of Windows 2000 User Mode.
User mode subsystems include
Win32 subsystem, which supports Windows applications
OS/2 subsystem, which supports OS/2 applications
POSIX subsystem, which supports POSIX- compliant applications
Security subsystem, which handles logon validation
See Also kernel mode
A unique identifier that enables a user to log on to a computer or network.
Overview
Usernames are part of a user's credentials, which include the username, password, and domain name. Each user on the network must have a unique username so that his or her personal folders can be protected against unauthorized access and for administrative purposes such as auditing. A typical example of a username might be "jsmith" or "jeffs" for a user named Jeff Smith. On a network running Microsoft Windows 2000 or Windows .NET Server, an administrator or an account operator can employ the Active Directory Users and Computers console to create and manage user accounts, and account restrictions can be created and managed using group policies.
See Also naming convention ,
The logon name of a user on a Microsoft Windows 2000 or Windows .NET Server network.
Overview
User principal names (UPNs) are one of three types of names by which objects in Active Directory directory service can be known, the others being distinguished names (DNs) and relative distinguished names (RDNs). The UPN for a user consists of the name of the User object in Active Directory that is associated with that user's account, followed by the @ sign and then typically by the Domain Name System (DNS) name of the container in Active Directory in which the User object resides. The UPN must be unique for every User object stored in Active Directory.
Examples
If the User object in Active Directory for user Jeff Smith has the name jsmith and if this User object resides in the container with a DNS name of sales.microsoft.com, the default form of Jeff Smith's user principal name is as follows:
jsmith@sales.microsoft.com
An alternative form of the user principal name for Jeff Smith is
jsmith@microsoft.com
In other words, the suffix for the user principal name of a user can be either the name of the tree or the name of the domain that resides above the object named in the tree. Note the similarity between the user principal name for Jeff Smith and what you might expect his Simple Mail Transfer Protocol (SMTP) e-mail address to be. The difference is that the user principal name is merely a property that is assigned to the user and can be set to any value.
See Also Active Directory ,distinguished name (DN) ,Domain Name System (DNS) ,Simple Mail Transfer Protocol (SMTP)
On Microsoft Windows platforms, user-definable information about a user's work environment, including desktop settings, network connections, printer connections, and Start menu items.
Overview
A user's profile is stored in a folder named after the username and consists of a registry hive and a series of subfolders whose functions are listed in the following table. If additional software is installed on the computer (such as Microsoft Internet Explorer), additional subfolders might exist within the user's profile folder.
Subfolder or Hive | Contents |
Application Data folder | Vendors' application-specific data |
Desktop folder | Files and shortcuts on the Windows desktop |
Favorites folder | Favorite locations |
NetHood folder | Shortcuts to Network Neighborhood items |
Personal folder | Users' personal documents |
PrintHood folder | Shortcuts to Printer Folder items |
Recent folder | Shortcuts to recently opened documents |
SendTo folder | Shortcuts for the SendTo menu |
Start Menu folder | Start menu shortcuts |
Templates folder | Shortcuts to templates |
ntuser.dat | Copy of the HKEY_CURRENT_USER hive |
ntuser.dat.log | Transaction log for error recovery |
A built-in group in Microsoft Windows 2000 and Windows .NET Server.
Overview
The Users group is a domain local group on domain controllers and a local group on stand-alone servers and computers running Windows 2000 or Windows XP. The initial membership of the Users group is the domain users global group, which consists of all ordinary users on the network. The Users group has no preassigned rights or permissions and should be assigned permissions to network resources that ordinary users will normally require.
See Also built-in group
Stands for update sequence number, a type of identifier used in directory replication in Microsoft Windows 2000 and Windows .NET Server.
See Also update sequence number (USN)
Stands for unshielded twisted-pair cabling, twisted-pair cabling with no internal shielding.
See Also unshielded twisted-pair (UTP) cabling
Stands for UNIX-to-UNIX Copy, a command for transferring files between two UNIX hosts on a network.
See Also UNIX-to-UNIX Copy (UUCP)
A method of encoding binary files into 7-bit ASCII text.
Overview
Uuencoding was originally developed for UNIX platforms to enable binary files to be attached to e-mail messages and transmitted over the Internet using Simple Mail Transfer Protocol (SMTP). The sending program first "uuencodes" the attachments and sends them with the e-mail message, which the receiving program "uudecodes" into its original binary format.
Although still popular on some platforms, uuencoding has been largely superseded by Multipurpose Internet Mail Extensions (MIME), a more sophisticated scheme for encoding multipart e-mail messages. Uuencoding is still used in one context in Microsoft Windows, namely for Basic Authentication on Internet Information Services (IIS). Basic Authentication transmits the user's credentials between the Web browser client and the IIS computer in uuencoded form. This method is commonly referred to as cleartext transmission, although if you were to capture authentication traffic by using a network sniffer such as Microsoft Network Monitor, you would be unable to read the user's credentials without uudecoding the information. Uuencoding is not considered a cryptographic algorithm because it is extremely simple to crack.
An encoding method similar to uuencoding that is implemented on Apple Macintosh systems is called BinHex.
Notes
You can tell that a binary file has been uuencoded into an ASCII text file if the text of the file starts with "begin 644..."
See Also Basic authentication , Internet Information Services (IIS) ,Multipurpose Internet Mail Extensions (MIME) ,Simple Mail Transfer Protocol (SMTP) ,
A standard for third-generation (3G) wireless cellular communication systems.
See Also Universal Wireless Communications (UWC-136)
