Page #38 (W)

X.25

A legacy packet-switching protocol.

Overview

X.25 is the earliest example of a packet-switching protocol designed to provide wide area network (WAN) connectivity. It was developed in the early 1970s and standardized in 1976 by the Comit Consultatif International T l graphique et T l phonique (CCITT), the precursor of the International Telecommunication Union (ITU). X.25 is a synchronous, connection-oriented, bidirectional, full-duplex packet-switching protocol originally designed for connecting "dumb terminals" (character-based terminals) to remote mainframe hosts.

X.25 was designed to be a reliable WAN service to compensate for the fact that dumb terminals lacked the processing power to include an X.25 protocol stack. To ensure this reliability of communication, X.25 includes extra protocol overhead in packet headers. In addition, because X.25 was designed when analog transmission over noisy copper telephone wire was the norm, X.25 packets have additional overhead for error correction. The result of these two factors is the comparatively low overall bandwidth of X.25, which originally operated at only 19.2 kilobits per second (Kbps) (although this was generally sufficient for character-based communication between mainframes and terminals). The current X.25 standard supports speeds up to 2 megabits per second (Mbps) over two pairs of wires, but most implementations are 64-Kbps connections through a standard DS-0 link. The X.25 standard is updated every four years, and versions after 1984 are backward compatible with the 1984 version.

Architecture

The X.25 standard roughly corresponds in functionality to the first three layers of the Open Systems Interconnection (OSI) reference model. The reason this correspondence is only rough is that X.25 was actually developed before the OSI model was created. Specifically, X.25 defines the following:

Also called the physical layer in X.25 terminology, this layer defines the physical interface for connecting routers or terminals at the customer premises with X.25 switches at the carrier's facilities. The physical layer interface of X.25 is defined by the X.21 standard, which was originally derived from the RS-232 interface for serial transmission. The original X.21 connector was a 15-pin connector, but a newer standard called X.21bis uses a more common 25-pin connector. In general, RS-232 and V.35 serial interfaces have largely replaced X.21 interfaces on X.25 equipment.

  • Data-link layer: Called the link access layer in X.25 terminology, this layer defines the encapsulation (framing) and error-correction methods used by X.25 and also enables X.25 equipment to initiate or terminate a communication session or initiate data transfer using the Link Access Procedure, Balanced (LAPB), which was derived from the earlier High-level Data Link Control (HDLC) protocol.

  • Network layer: Called the packet layer in X.25 terminology, this layer defines the way to deliver X.25 packets reliably between end nodes on an X.25 network using the Packet Layer Protocol (PLP) and is also responsible for call setup and termination and for X.25 addressing using the X.121 standard.

  • Implementation

    An X.25 network consists of four elements:

    X.25 supports two different types of connection- oriented communications:

    In typical SVC communications, a DTE initiates a communication session with another DTE by dialing its X.121 address and establishing a virtual circuit. Packets are then forwarded through the PDN by using the ID number of the virtual circuit established for that particular communication session. This ID number is called the logical channel identifier (LCI) and is a 12-bit address that identifies the virtual circuit. X.25 packets are generally 128, 256, or 512 bytes in size, although actual size can range from 64 to 4096 bytes depending on the implementation.

    X.25. Anatomy of a simple X.25 network.

    Issues

    X.25 is efficient for batch file transfer but not for interactive communication such as Telnet sessions, in which Transmission Control Protocol/Internet Protocol (TCP/IP) is run over X.25. If you often use Telnet from your X.25 terminal, you can improve efficiency by employing VanJacobsen TCP/IP Header Compression to reduce the overhead of the TCP/IP packet header from 40 bytes to 5 bytes (if your TCP/IP stack supports this feature). Another cause of X.25's inefficiency for interactive communication is the typical half-second latency in communication due to the store-and-forward nature of the packet-switching network. Frame relay does not use store-and-forward packet switching and hence has much less latency. In general, X.25 is not a good communication medium for applications that use TCP/IP because the high latency and low speed can sometimes cause TCP/IP applications to time out.

    Prospects

    Newer WAN technologies such as frame relay, Integrated Services Digital Network (ISDN), and T-carrier services are now generally preferred to X.25. However, X.25 networks still have applications in areas such as credit card verification, automatic teller machine transactions, and other dedicated business and financial uses. A striking example of how X.25 is still used is offered by the U.S. federal government's Automated Clearing House (ACH) network, a 30-year-old nationwide X.25 network used for collections payments between U.S. banks. Businesses use ACH to perform direct deposit of employee payroll, to credit and debit bank and credit institution funds transfers, and many other tasks. The U.S. Department of the Treasury has created a Web- based front end called www.pay.gov to this legacy X.25 system to allow ordinary citizens to pay taxes and other government fees such as those for National Parks permits.

    See Also Channel Service Unit/Data Service Unit (CSU/DSU) , connection-oriented protocol ,data communications equipment (DCE) ,data terminal equipment (DTE) ,DS-0 ,frame relay ,High-level Data Link Control (HDLC) ,Integrated Services Digital Network (ISDN) ,International Telecommunication Union (ITU) ,Open Systems Interconnection (OSI) reference model ,packet assembler/ disassembler (PAD) ,packet-switching services ,permanent virtual circuit (PVC) ,Public Switched Telephone Network (PSTN) ,router ,RS-232 ,switched virtual circuit (SVC) ,T-carrier ,Telnet ,terminal ,Transmission Control Protocol/Internet Protocol (TCP/IP) ,V.35 ,wide area network (WAN) ,

    x86 platform

    A PC hardware platform whose processor is based on the Intel 386 architecture microprocessor.

    Overview

    The x86, or Intel, platform is one of the two processor platforms supported by Microsoft Windows NT (the other being the Alpha platform) and the only processor platform supported by Microsoft Windows 2000. Intel-based systems have essentially caught up with Alpha in terms of speed and functionality and are used for everything from mobile laptop computers to desktop workstations to high-performance symmetric multiprocessing (SMP) servers.

    The x86 family is based on the 386 processor and includes the 486, Pentium, Pentium Pro, Pentium II, Pentium III, and Pentium IV processors. Intel processors are based on the complex instruction set computing (CISC) architecture, which uses a large set of basic processor instructions to simplify code compilation. The CISC architecture differs from the reduced instruction set computing (RISC) architecture of the Alpha platform, which uses fewer processor instructions and offers better performance.

    Notes

    The Windows .NET Server platform also supports the new Itanium 64-bit architecture from Intel Corporation.

    See Also 64-bit architecture ,Alpha platform

    X.121 address

    An address of an end node connected to an X.25 public data network.

    Overview

    Also called international data numbers (IDNs), X.121 addresses are similar to long-distance telephone numbers and are used by X.25 end nodes to call each other to set up communication sessions. X.121 addresses are used during the call setup phase of X.25 communication and are used to establish a switched virtual circuit (SVC) between the source node and destination node on the network.

    X.121 addresses are typically 14 decimal digits in length, unless fewer can suffice to uniquely determine the address of the destination node being called. The first four digits form the data network identification code (DNIC), with the first three digits indicating the country or region and the fourth digit indicating the carrier that owns the common packet-switching network being used to make the call. The remaining digits form the national terminal number (NTN) and identify the end node being called. An additional 1-byte header indicates the number of digits of both the source and destination nodes.

    Once an X.25 communication session is established, a 12-bit logical channel identifier (LCI) is assigned to the two hosts as the identification number of the virtual circuit that is established between them. The X.25 network uses the LCI in the headers of the X.25 packets for routing data between the nodes. The X.121 address is used only at call setup to establish the virtual circuit.

    See Also X.25 ,X-series

    X.400

    A set of electronic messaging standards defined in 1984 and 1988 by the International Telecommunication Union (ITU).

    Overview

    The X.400 standards are based on the Open Systems Interconnection (OSI) reference model developed by the International Organization for Standardization (ISO). X.400 defines global standards to enable users to send e-mail between X.400-compliant messaging systems.

    X.400 was originally intended to be the uniform, worldwide standard for global messaging, but the Internet's Simple Mail Transfer Protocol (SMTP) has far eclipsed it in popularity. X.400 is still used, however, in some parts of Europe by post, telephone, and telegraph (PTT) authorities.

    Implementation

    X.400 defines a global Message Handling System (MHS) that consists of a number of messaging components. From an administrative point of view, the primary building blocks of the MHS are management domains (MDs). Note that these management domains are not the same as DNS domains-the Domain Name System (DNS) is used by SMTP messaging services, not X.400. A management domain is a collection of X.400 messaging systems having at least one Message Transfer Agent (MTA) managed by a specific organization. X.400 management domains come in two varieties:

    An X.400 MHS consists of the following five kinds of messaging components:

    Each UA in an X.400 MTS is identified by a special X.400 address called an Originator/Recipient (O/R) address. The O/R address is the e-mail address of the X.400 user and can be quite complex compared to an SMTP e-mail address, which is one reason that SMTP has overtaken X.400 in popularity as a global messaging standard. An O/R address consists of a series of VALUE=ATTRIBUTE pairs separated by semicolons. Not all fields need to be complete-only those that uniquely identify the recipient are required. Here is an example of an X.400 address:

    C=US;A=MCI;P=MICROSOFT;O=SALES;S=SMITH;G=JEFF;

    The individual address fields are as follows:

    An X.400 message consists of a P1 envelope and its P2/22 message contents. The envelope contains the e-mail address information needed for routing the message to its destination. The X.400 protocol for a message envelope includes support for message tracking and delivery priority features. The X.400 protocol for the message content includes a header and body part for the message.

    What typically happens in the message transfer process is that a UA sends a message addressed to another UA in the MHS. The message is forwarded to an MTA in the local MTS, which either delivers the message locally or forwards it to a remote MTA for handling, depending on where the destination UA is located. The message is passed from MTA to MTA until it reaches the MTS of the destination UA, whereupon it is either delivered if the destination UA is connected or stored in an MS until the UA can retrieve it.

    See Also e-mail , P-series protocols ,Simple Mail Transfer Protocol (SMTP) ,

    X.500

    An International Telecommunication Union (ITU) recommendation for a global directory.

    Overview

    A directory is a tool designed to provide a single source for locating, organizing, and managing a network's resources within a business or enterprise. The X.500 recommendations define a global, hierarchical directory that includes the following features:

    Implementation

    From an administrative point of view, the building blocks of the X.500 directory service are Directory Management Domains (DMDs). An X.500 DMD is a collection of X.500 components that includes at least one Directory System Agent (DSA) and is managed by a Domain Management Organization (DMO). There are two types of DMDs:

    The three main components of an X.500 directory are

    To access information in the directory, a DUA connects to a local DSA and queries the directory by using the Directory Access Protocol (DAP), the standard X.500 protocol for locating, accessing, and modifying information in an X.500 directory. Various attribute-based search methods are possible using X.500-based directory services, including the following:

    When a DUA issues a query, the query travels through a chain of DSAs and a result set travels back along the same chain. These queries use DAP, while DSAs communicate with each other using the Directory System Protocol (DSP).

    Marketplace

    X.500 forms the architectural basis of Active Directory in Windows 2000 and Windows .NET Server, Novell Networks' Novell Directory Services (NDS) eDirectory, Oracle Corporation's Internet Directory (OID), and other popular directory services. Neither Active Directory nor NDS are full X.500 directories, however, although full X.500 directories are offered by a few vendors, including Global Directory Server from Critical Path, eTrust from Computer Associates, and DirX from Siemens.

    Prospects

    Despite the compelling features of X.500, it is not widely used for several reasons:

    The feature-heavy Directory Access Protocol (DAP) used by X.500 is particularly complex and has been widely replaced by the Lightweight Directory Access Protocol (LDAP) developed by the University of Michigan and standardized by the Internet Engineering Task Force (IETF). LDAP directories such as iPlanet's Directory Server and InnoSoft's IDDS are generally more popular than full X.500 ones and much easier to manage, though offering fewer features.

    See Also Active Directory , directory ,Directory Access Protocol (DAP) ,Lightweight Directory Access Protocol (LDAP) ,Novell Directory Services (NDS) ,

    xDSL

    Refers to different "flavors" of Digital Subscriber Line (DSL), a group of broadband telecommunications technologies supported over copper local loop connections.

    See Also Digital Subscriber Line (DSL)

    Xerox Network Systems (XNS)

    A suite of networking protocols developed by Xerox Corporation's Palo Alto Research Center (PARC) in the early 1980s.

    Overview

    Xerox Network Systems (XNS) is based on a five-layer model, in contrast to the seven-layer Open Systems Interconnection (OSI) reference model for networking. The layers of the XNS protocol stack are as follows:

    XNS is little used today, but it was important in the evolution of other networking protocols, such as IPX/SPX and TCP/IP.

    See Also IPX/SPX-Compatible Protocol ,protocol ,Transmission Control Protocol/Internet Protocol (TCP/IP)

    XHTML

    An enhanced version of Hypertext Markup Language (HTML) developed by the World Wide Web Consortium (W3C).

    Overview

    XHTML is basically a reformulation of HTML 4.01 in Extensible Markup Language (XML) using an XML document type definition (DTD). In other words, XHTML is HTML defined in terms of XML. XHTML thus includes all the functionality of HTML while being fully compliant with XML and including the extensibility and portability of XML.

    XHTML is expected to smooth the migration from HTML to XML by allowing developers to create HTML documents that contain XML functionality and are compliant with XML applications. Some of the advantages of using XHTML instead of HTML for Web content development include

    Web sites can be developed in or migrated to XHTML without worry of browser incompatibility because XHTML conforms to the operation of existing HTTP user agents. Migrating HTML sites to XHTML ensures that site content is XML-conforming, which is advantageous because it is likely that XML will eventually become the paradigm for developing all Web content.

    The current standard is XHTML 1, which was released in January 2000.

    For More Information

    Find out more about XHTML at www.w3.org/TR/xhtml1.

    See Also Hypertext Markup Language (HTML) , World Wide Web Consortium (W3C) ,

    XML

    Stands for Extensible Markup Language, a meta- language used as a universal standard for electronic data exchange.

    Overview

    XML is a derivative of Standardized Generalized Markup Language (SGML), an International Organization for Standardization (ISO) standard developed in 1986. SGML allows you to programmatically describe the structure and content of an electronic document. XML is basically a subset of SGML and is a language that can be used for creating other languages. Numerous XML "dialects" have been developed in the last few years for different sectors of industry. These dialects allow companies within each sector to exchange business information electronically and perform such business transactions as ordering, invoicing, and payment.

    The goals of the designers of XML were to create a meta-language for developing business dialects that

    History

    Electronic business communication emerged in the 1970s when companies such as Kmart and Sears developed proprietary electronic processes for simplifying communication between stores and suppliers. Using these processes, large companies were able to reduce the amount of paperwork involved in their business transactions and save costs because of the reduced amount of labor involved in processing electronic communications over paper ones. Toward the end of the 1970s the U.S. government and companies from the transportation and manufacturing sectors worked with the American National Standards Institute (ANSI) to develop uniform standards for the electronic exchange of business information. The result of this effort was the ANSI X12 standard, which formed the basis of electronic data interchange (EDI). EDI basically defines the format and protocols for electronic exchange of invoices, purchase orders, receipts, and other documents.

    EDI has been widely used by business, industry, and government, but its complexity and high cost have tended to limit its implementation in large enterprises. The high cost of EDI is mainly because it uses dedicated leased lines for exchange of business information between companies. To overcome the limitations of EDI, the World Wide Web Consortium (W3C) began developing XML in 1996 as an alternative to EDI, leveraging the popularity of Hypertext Markup Language (HTML) and the ubiquity of the Internet as a communication medium. Although EDI is intrinsically secure because it uses dedicated leased lines, XML requires an additional mechanism for ensuring secure transmission of electronic documents over the unsecure public Internet-for example, by employing a virtual private network (VPN).

    The initial XML standard called XML 1 was published in 1998, and the XML Working Group of the W3C steers further development of the language. XML standards continue to evolve, and the most important ones are outlined in the "Architecture" section later in this article.

    Uses

    One of the earliest implementations of XML was by Microsoft Corporation in their Channel Definition Format (CDF) push technology for the Web. Since then XML has been embraced by all sectors of industry and is supported by software from major vendors such as Microsoft, Oracle Corporation, SAP, and PeopleSoft and by application service providers such as Ariba and Commerce One. Hundreds of different dialects (schemas) have been developed for different industry sectors-for example, to name a few:

    Comparison

    XML resembles HTML in many ways-for example, both languages use plain text files that are marked up using tags. But there are significant differences between XML and HTML, namely the following:

    Architecture

    XML actually embraces a whole series of standards for different language and protocols, and it is still evolving as new pieces are developed and come into play. Some of the important parts of the XML specification include

    Implementation

    XML requires much stricter adherence to formatting rules than HTML does. Specifically, XML documents must be

    Examples

    A simple example of an XML document is the following, which contains the name and phone number for a sales contact person:

    <?xml version "1.0"?> <contact_book>     <contact type="sales">        <name>           <first_name>Jeff</first_name>           <last_name>Smith</last_name>        </name>        <phone>555-1212</phone>     </contact> </contact_book>

    The first line of this document (required) is called the prolog, and it informs the XML parser within an application that it is parsing an XML document and other relevant information. The rest of the document is called the document element and contains the data to be used by the application. Note how you can use XML to define tags that are used to describe what the data is about. The "extensible" nature of XML lies in the fact that you can create custom tags to describe different kinds of data as needed. For example, you could create an <im> tag to include the Instant Messaging (IM) identifier for the contact above.

    Prospects

    Although XML has captured the minds of industry and software vendors alike as a way to simplify and reduce the cost of doing business electronically, few businesses have actually implemented full XML-based business-to- business (B2B) solutions. The reasons for this include

    Despite these issues, it seems almost inevitable that XML will eventually succeed in dominating the electronic business marketplace due to its flexibility, power, and ease of use. A number of vendors are developing workaround approaches to help companies migrate from EDI to XML. For example, Vitria Technology, PaperFree Corporation, and several other companies have created Web-based applications that allow businesses to access EDI documents using a standard Web browser interface and to translate EDI messages into XML.

    For More Information

    Visit the W3C's XML site at www.w3.org/xml. Other useful sites to visit include www.xml.org and www.xml.com

    See Also B2B ,Electronic Business Extensible Markup Language (ebXML) ,electronic data interchange (EDI) ,Hypertext Markup Language (HTML) ,Simple Object Access Protocol (SOAP) ,Universal Description,Discovery,and Integration (UDDI),virtual private network (VPN) ,World Wide Web Consortium (W3C)

    XNS

    Stands for Xerox Network Systems, a suite of networking protocols developed by Xerox Corporation's Palo Alto Research Center (PARC) in the early 1980s.

    See Also Xerox Network Systems (XNS)

    X-series

    A series of standards and recommendations from the International Telecommunication Union (ITU) dealing with data communication over computer networks and telecommunication services.

    Overview

    Some of the more important X-series standards and recommendations include the following:

    See Also X.25 ,X.121 address ,X.400

    xSP

    Represents the various "flavors" of service providers that emerged in the late 1990s.

    Overview

    The earliest of the new breed of service providers that emerged in the 1990s was the Internet service provider (ISP), which originally meant a company that provided connectivity with the Internet, usually dial-up for home users and T1 or fractional T1 lines for businesses. As the number of ISPs on the market exploded in the late 1990s, many of the larger ones began to offer additional services to customers including Web hosting, custom Web application development, and e-commerce storefronts. Soon many players in the Internet marketplace began to refer to themselves as different types of service providers in an effort to differentiate themselves from one another according to the services they specialized in offering. Today a growing number of service providers have emerged, often overlapping in the services they offer and changing their focus to adjust to the market. Currently the list includes

    Together, all these different types of service providers are generally grouped under the acronym xSP , where x represents a variable that can be replaced by other letters, such as I for Internet or A for Application.

    See Also application service provider (ASP) ,caching service provider (CSP) ,commercial service provider (CSP) ,Management Service Provider (MSP) ,outsourcing ,storage service provider (SSP)

    X Window System

    A multiuser client/server graphical user interface (GUI) for UNIX environments.

    Overview

    The X Window System, also known simply as X , provides a multitasking GUI windowing environment for network-attached UNIX workstations and terminals. The Massachusetts Institute of Technology (MIT), Stanford, and IBM jointly developed X starting in 1984. The first popular release of the platform was X version 11, which came out in 1987. In 1988 the X Consortium was formed to steer development of the system, and Release Six of X, usually called X11R6, appeared in 1996 and remains the most popular version of X. In 1997 the X Consortium turned over responsibility for overseeing further development of X to The Open Group.

    Elements of X include the following:

    X is available on all UNIX platforms and on Linux distributions, and there are even versions for Microsoft Windows, OS/2, and the Macintosh platform.

    For More Information

    Visit X.Org at www.x.org

    See Also K Desktop Environment (KDE) ,UNIX



    Microsoft Encyclopedia of Networking
    Microsoft Encyclopedia of Networking
    ISBN: 0735613788
    EAN: 2147483647
    Year: 2002
    Pages: 36
    Authors: Mitch Tulloch, Ingrid Tulloch
    BUY ON AMAZON

    flylib.com © 2008-2017.
    If you may any questions please contact us: flylib@qtcs.net