A packet-switching protocol for wide area network (WAN) connectivity that uses a public data network (PDN) that parallels the voice network of the Public Switched Telephone Network (PSTN). The current X.25 standard supports synchronous, full-duplex communication at speeds up to 2 Mbps over two pairs of wires, but most implementations are 64-Kbps connections via a standard DS0 link.
X.25 was developed by common carriers in the early 1970s and approved in 1976 by the CCITT, the precursor of the International Telecommunication Union (ITU), and was designed as a global standard for a packet-switching network. X.25 was originally designed to connect remote character-based terminals to mainframe hosts. The original X.25 standard operated only at 19.2 Kbps, but this was generally sufficient for character-based communication between mainframes and terminals.
Because X.25 was designed when analog telephone transmission over copper wire was the norm, X.25 packets have a relatively large overhead of error-correction information, resulting in comparatively low overall bandwidth. Newer WAN technologies such as frame relay, Integrated Services Digital Network (ISDN), and T-carrier services are now generally preferred over 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.
Graphic X-1.X.25. An X.25 network.
How It Works
The X.25 standard corresponds in functionality to the first three layers of the Open Systems Interconnection (OSI) reference model for networking. Specifically, X.25 defines the following:
The physical layer interface for connecting data terminal equipment (DTE), such as computers and terminals at the customer premises, with the data communications equipment (DCE), such as X.25 packet switches at the X.25 carrier’s facilities. The physical layer interface of X.25 is called X.21bis and was derived from the RS-232 interface for serial transmission.
The data-link layer protocol called Link Access Procedure, Balanced (LAPB), which defines encapsulation (framing) and error-correction methods. LAPB also enables the DTE or the DCE to initiate or terminate a communication session or initiate data transfer. LAPB is derived from the High-level Data Link Control (HDLC) protocol.
The network layer protocol called the Packet Layer Protocol (PLP), which defines how to address and deliver X.25 packets between end nodes and switches on an X.25 network using permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). This layer is responsible for call setup and termination and for managing transfer of packets.
An X.25 network consists of a backbone of X.25 switches that are called packet switching exchanges (PSEs). These switches provide packet-switching services that connect DCEs at the local facilities of X.25 carriers. DTEs at customer premises connect to DCEs at X.25 carrier facilities by using a device called a packet assembler/disassembler (PAD). You can connect several DTEs to a single DCE by using the multiplexing methods inherent in the X.25 protocol. Similarly, a single X.25 end node can establish several virtual circuits simultaneously with remote nodes.
An end node (DTE) can initiate a communication session with another end node by dialing its X.121 address and establishing a virtual circuit that can be either permanent or switched, depending on the level of service required. Packets are routed through the X.25 backbone network by using the ID number of the virtual circuit established for the particular communication session. This ID number is called the logical channel identifier (LCI) and is a 12-bit address that identifies the virtual circuit. Packets are generally up to 128 bytes in size, although maximum packet sizes range from 64 to 4096 bytes, depending on the system.
NOTE
Related protocols include X.3, which defines the parameters for configuring a PAD; X.28, which defines the data transmission interface between the DTE and the PAD; and X.29, which defines a control protocol between the DTE and the PAD.
The Remote Access Service (RAS) for Microsoft Windows NT and the Routing and Remote Access feature of Microsoft Windows 2000 support X.25 PADs for dial-up clients and smart cards for direct connection to the X.25 network of remote access servers and clients.
TIP
X.25 is efficient for file transfer but not for interactive communication such as Telnet sessions, in which 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 reason that X.25 is inefficient 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.
TIP
The X.25 standard is updated every four years. Versions after 1984 are backward compatible with the 1984 version. However, if a network using the 1980 version of X.25 needs to communicate with a DTE on a network based on a later version, you must properly configure the routers connecting these networks to ensure effective communication.
x86 platform
A computer platform whose processor is based on the Intel 386 architecture microprocessor. 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, and Pentium III processors. Intel processors are based on the CISC (complex instruction set computing) architecture, which uses a large set of basic processor instructions to simplify code compilation. The CISC architecture differs from the RISC (reduced instruction set computing) architecture of the Alpha platform, which uses fewer processor instructions and offers better performance.
X.121 address
Also called an international data number (IDN), an address of an end node (computer or terminal) that is connected to an X.25 public data network.
How It Works
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 virtual circuit between the source node and destination node on the network.
X.121 addresses can be up to 14 decimal digits in length (if that many digits are required 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 and the fourth digit indicating the carrier that owns the common packet-switching network being used to make the call. The last 8 to10 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 a 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.
X.400
A set of standards defined in 1984 and 1988 by the International Telecommunication Union (ITU) for computer-based handling of e-mail. The X.400 standard is based on the Open Systems Interconnection (OSI) reference model and other protocols developed by the International Organization for Standardization (ISO). X.400 provides global standards that enable users to send e-mail between any X.400-compliant messaging systems. X.400 is widely considered to be the standard framework for global messaging, although the Simple Mail Transfer Protocol (SMTP) for Internet e-mail might have an even better claim to the title. X.400 is widely implemented in Europe by most post, telephone, and telegraph (PTT) authorities. Microsoft Exchange Server supports messaging connectivity with X.400 mail systems through the X.400 Connector, an optional component available with the Enterprise Edition of Exchange Server 5.5.
How It Works
X.400 defines a global Message Handling System (MHS) that consists of a number of messaging components. From an administrative point of view, the building blocks of the MHS are management domains (MDs). (MDs are not the same as DNS domains—the Domain Name System [DNS] is used for SMTP mail, not X.400 messaging services.) A management domain is a collection of messaging systems with at least one Message Transfer Agent (MTA) managed by a specific organization. X.400 management domains come in two varieties:
Administrative Management Domains (ADMDs): Messaging systems managed by an administrator or a registered private agency. These are the top-level management domains that handle third-party messaging traffic. An example is a telephone carrier service company such as AT&T.
Private Management Domains (PRMDs): Unique subscriptions to an ADMD, such as telephone numbers of users. PRMDs can send or receive messages from an ADMD, but PRMDs cannot communicate directly with each other.
An X.400 MHS consists of the following five kinds of messaging components:
Message Transfer Systems (MTS’s): Collections of one or more MTAs that function together to provide message forwarding services for a particular X.400 domain.
Message Transfer Agents (MTAs): Route and deliver transport messages to and from User Agents (UAs) and with other MTAs. An MTA corresponds to a mail server in a typical LAN–based messaging system. MTAs maintain a database of all UAs registered in their domain and routing tables that indicate how messages should be forwarded to other domains.
Messages Stores (MS’s): Temporarily store messages that an MTA has received until they can be processed and forwarded for delivery. X.400 thus uses a store-and-forward method of message delivery.
User Agents (UAs): Provide messaging functionality directly to users. From a practical point of view, a UA can be identified as the e-mail client software that a user is running; from an abstract point of view, a UA is a domain belonging to a user and consisting of additional subcomponents. The goal of an X.400 MHS is to facilitate exchange of messages between different UAs.
Access Units (AUs): Gateways between an X.400 MHS and another messaging system such as a telex or fax system.
Graphic X-2.X.400. The X.400 Message Handling System.
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. (This is one reason that SMTP is overtaking X.400 in popularity.) 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:
Country (C) is United States
ADMD (A) is MCI
PRMD (P) is Microsoft (company name)
Organization (O) is Sales Department of Microsoft
Surname (S) is Smith
Given name (G) is Jeff
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 P-series protocols
X.400 Connector
A type of connector that is available as an optional component of Microsoft Exchange Server. The X.400 Connector can be used in several ways:
To link two different sites in an Exchange organization for messaging connectivity and directory replication. One advantage of using the X.400 Connector for this purpose instead of the Site Connector is that you can use the X.400 Connector to control both the size of the messages and the schedule for routing them.
To link two different sites in an Exchange organization by using a public or private X.400 messaging network as the messaging backbone.
To provide messaging connectivity between an Exchange organization and a foreign X.400 messaging system.
The X.400 Connector works with several different network transports, including these:
TP0/X.25 for X.25 packet-switched networks
TP4/CLNP for connecting to host-based systems
TCP/IP
TIP
Use the X.400 Connector instead of the Site Connector to link sites that are connected by dedicated or leased lines with a bandwidth of 64 Kbps or less, because the X.400 Connector uses about 30 percent less bandwidth than the Site Connector for communication overhead.
X.500
A recommendation from the International Telecommunication Union (ITU) that specifies a global, hierarchical directory service. Features of X.500 include the following:
A standards-based directory service for those applications that require directory information
A single, global, hierarchical namespace of objects and their attributes
Data management functions for viewing, adding, modifying, and deleting directory objects
Search capabilities for customizing complete data queries
How It Works
X.500 defines a global directory service that consists of several components. 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:
Administrative Directory Management Domains (ADDMDs): Directory services managed by a registered private agency that provide public directory services. Examples of ADDMDs are Four11 and Bigfoot, which provide public X.500 directory services over the Internet.
Private Directory Management Domains (PRDMDs): Directory services that provide private directory access. An example is a domain controller hosting Active Directory on a network running Microsoft Windows 2000.
Three main components are involved in maintaining and accessing X.500 directory services:
Directory Information Base (DIB): The actual hierarchical database that contains all the information in the directory. X.500 uses a distributed directory hierarchy in which different subsets of the DIB are found on different servers at different locations. From the user’s point of view, however, the entire global X.500 directory appears to be accessible from the local directory server that the Directory User Agent (DUA) connects to. A schema is used to define the various classes of objects and their attributes, which can be stored in the directory. The Directory Information Tree (DIT) is the naming hierarchy that describes the hierarchical structure of the DIB.
Directory System Agent (DSA): A particular server that maintains a subset of the DIB and provides an access point to the directory for DUAs to connect. Each DSA is responsible for a subset of the DIB and includes a set of naming contexts that define objects that are near each other in the DIT. DSAs also communicate with each other for directory replication purposes to ensure that each DSA’s subset of the DIB is current and complete and to maintain the integrity of the whole X.500 directory system.
Directory User Agents (DUAs): The client software that accesses the X.500 directory on behalf of the user. DUAs can perform such actions as searching, reading, updating, and deleting information in the directory, depending on the level of functionality of the client and the level of access granted to the user. The functionality of a DUA can be built into any type of software, including e-mail clients, Web browsers, or even the operating system itself.
Graphic X-3.X.500. The X.500 directory service.
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:
White pages searches, for name-to-address lookups
Yellow pages searches, for looking up a category
Browsing, for listings related to a given attribute
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).
NOTE
X.500 forms the basis of Active Directory in Windows 2000, the directory service of Microsoft Exchange Server, and Novell Directory Services (NDS).
A simplified version of DAP called the Lightweight Directory Access Protocol (LDAP) is more widely implemented than the feature-heavy DAP. LDAP was developed by the University of Michigan for use on TCP/IP networks such as the Internet and is widely implemented in Simple Mail Transfer Protocol (SMTP) client software such as Microsoft Outlook Express for querying online directories about SMTP users.
X.509 digital certificate
See digital certificate
Xerox Network Systems (XNS)
A suite of networking protocols developed by Xerox Corporation’s Palo Alto Research Center (PARC) in the early 1980s. Xerox Network Systems (XNS) is little used today, but it was important in the evolution of other networking protocols, such as IPX/SPX and TCP/IP.
How It Works
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:
Level 0 (media access layer): Maps to the OSI physical layer and data-link layer and performs similar functions. XNS does not tie into any one media access protocol and supports the Ethernet, Token Ring, High-level Data Link Control (HDLC), and X.25 protocols, among others.
Level 1 (network layer): Maps to the OSI network layer and defines the Internet Datagram Protocol (IDP). IDP functions similarly to the Internet Protocol (IP) of TCP/IP and uses a logical addressing scheme that requires four-byte network numbers, four-byte host numbers, and two-byte socket numbers for both source and destination addresses. IDP is responsible for delivering datagrams by using unicast, multicast, and broadcast methods. Level 1 also defines the Routing Information Protocol (RIP), which handles dynamic routing and has evolved into later versions for use in IPX/SPX and TCP/IP networks.
Level 2 (transport layer): Maps to the OSI transport layer and defines the Sequenced Packet Protocol (SPP). SPP functions similarly to the Transmission Control Protocol (TCP) of TCP/IP and is responsible for providing reliable transmission of IDP packets, including sequence numbers and acknowledgments. Level 2 also defines the Packet Exchange Protocol (PEP), which functions similarly to the User Datagram Protocol (UDP) of TCP/IP. For troubleshooting purposes, the Echo Protocol (EP) functions similarly to the ping utility of TCP/IP.
Level 3: Maps to the OSI presentation layer and includes the Filing Protocol (FP), Clearinghouse Protocol (CP), Printing Protocol (PP), and others.
Level 4: Maps to the OSI application layer.
XNS has no level that maps to the OSI session layer.
XHTML
Stands for Extensible Hypertext Markup Language, a proposed version of Hypertext Markup Language (HTML) from the World Wide Web Consortium (W3C). XHTML 1 is basically a reformulation of HTML 4 in Extensible Markup Language (XML) and can smooth the migration from HTML to XML by allowing developers to create HTML documents that contain XML functions.
The advantages of using XHTML instead of HTML for Web content development include the following:
Easier portability to nonstandard user interfaces
The ability to create new document type definitions (DTDs)
Web sites can already be migrated to XHTML because XHTML conforms to existing Hypertext Transfer Protocol (HTTP) user agents (Web browsers). Migrating ensures that content is XML-conforming, which is advantageous because XML is the future Web content paradigm.
On the Web
•
W3C XHTML information : http://www.w3.org/TR/xhtml1/
XML
Stands for Extensible Markup Language, a family of standards for the exchange of structured information that was developed by the World Wide Web Consortium (W3C). XML is viewed as the successor to Hypertext Markup Language (HTML), which is still commonly used for creating Web sites on the Internet and for publishing corporate intranet content. XML and its various components allow richly formatted and structured information to be delivered over the Web, and XML promises to be widely used in electronic commerce and electronic business applications.
How It Works
Like HTML, XML uses embedded tags to mark up documents for formatting purposes and to create relationships between documents (that is, to create hypertext). In fact, XML is a restricted subset of Standard Generalized Markup Language (SGML), which has existed for years but is unsuitable for implementation on the Web.
Unlike HTML, with its fixed syntax of tags, XML allows users to declare and use their own tags by using document type definitions (DTDs), which define the syntax, structure, and meaning of their tags. In other words, XML does not specify the set of available tags or their syntax, but instead functions as a meta-language for creating and describing other markup languages.
Various DTDs have been created for different subject areas, such as science, commerce, and documentation. XML also extends the idea of a “document” to include not only text files but also e-commerce transactions, server application programming interfaces (APIs), vector graphics, and many other forms. As a result, XML is far more universal than HTML.
XML also uses Extensible Stylesheet Language (XSL), in which you can define classes of XML documents and how they are formatted. You can use the XML Linking Language (XLL) to create links in XML documents to external objects such as multimedia objects, and use the XML Pointer Language (XPointer) to define link addresses in an XML document. These two languages go beyond the simple anchor tag (<A>) of HTML and provide ways to create one-to-many links, bidirectional links, read-only links, and other complex structural interactions between XML documents. Other components of the XML system include namespaces, query languages, and schema languages, many of which are still under development.
This example illustrates two of the XML markup types:
Processing instructions: Supply necessary information to the application parsing the XML document, such as <?XML VERSION="1.0">, which tells the application that the document being parsed is written in XML.
Elements: Surround content with start and end tags, as in <QUOTE>…</QUOTE>. Elements of the form <…/>, such as <LAUGHTER/>, are called empty elements.
Other types of XML markup include the following:
Attributes, which are name-value pairs that extend the definition of a start tag.
Comments, which are represented by <!--…-->, as in HTML.
CDATA sections, such as <![CDATA[…]]>, which indicate to the parser in the application reading the document that the enclosed section is to be read unparsed. This might be used for computer code, for example.
Entity references, which specify reserved and special characters. For example, < represents the less than symbol (<) that indicates the beginning of an element’s start tag.
XML also includes declarations that enable the XML document to communicate various types of meta-information to the application parsing the document. These include declarations for new elements, lists of attributes, and new entities. In the preceding sample XML document, for example, the elements <HUMOR>, <BOB>, <SALLY>, <LAUGHTER/>, and <QUOTE> would all need to be declared using <!ELEMENT…> declaration statements.
NOTE
Microsoft’s Channel Definition Format (CDF) was one of the earliest uses for XML in Internet environments.
On the Web
•
W3C’s XML site : http://www.w3.org/XML
XNS
See Xerox Network Systems (XNS)
X-series protocols
A series of standards and recommendations developed by the International Telecommunication Union (ITU) relating to data communication over networking and telecommunication services. These standards and recommendations include the following:
X.25: Defines a protocol for a global packet-switching network for wide area network (WAN) connectivity. Related X-series protocols include X.3, X.28, X.29, X.92, X.96, X.110, and X.121.
X.400: Defines a standard for a global message handling system for e-mail. Related protocols include X.402, X.403, X.407 (ANS.1), X.408, X.411, X.413, X.420, and the P-series protocols.
X.500: Defines a recommendation for a global directory service. Related protocols include X.501, X.509, X.511, and X.518 through X.521.
See also X.25, X.400, X.500
X Window System
A client/server system for running a graphical user interface (GUI) that uses windows in a UNIX environment. The X Window System, also known as “X,” was developed jointly by the Massachusetts Institute of Technology (MIT), Stanford, and IBM starting in 1984, and its implementation is standardized by The Open Group. The X Window System is device-independent and provides a multitasking windowing environment for client computers.
The X Window System runs on network-attached workstations and terminals that access UNIX servers or mainframe computers. Elements of the X Window System include the following:
X servers, which form the server portion of the X Window System. These are typically UNIX servers but can also be mainframe computers running UNIX.
X clients, which are UNIX desktop workstations running the X Window System client software. The client software enables the workstations to display a windowed GUI in the X Window System environment. An alternative to using workstations is using X terminals, which are dumb terminals that have no operating system and use a ROM routine to implement X client software. X terminal machines must be connected to X servers by using local area network (LAN) connections, in contrast to character-based dumb terminals, which usually use serial connections such as RS-232 or X.21.
X protocol, the standard protocol for transmitting requests for windows functionality between X clients and X servers.
An X window manager, such as OSF/Motif, which implements windows features such as menus, toolbars, and gadgets that provide the look and feel of a windows GUI environment and allows X applications to run.
