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:
Physical layer:
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:
Data terminal equipment (DTE): These include routers and computers at the customer premises that need to communicate using X.25.
Data communications equipment (DCE): These include modems and Channel Service Unit/Data Service Units (CSU/DSUs) that connect the customer's DTE to the X.25 network. You can also connect multiple 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.
Packet assembler/disassembler (PAD) : This is a device that you can use to connect a dumb terminal to an X.25 network. The dumb terminal is DTE, and the PAD resides between the DTE and the DCE. Note that a PAD is not required for a router having built-in X.25 support-PADs are only required to connect dumb terminals that have no processing power to implement X.25 directly. The X.3 standard defines the various parameters used for configuring a PAD, the X.28 standard defines the data transmission interface between the DTE and the PAD, and the X.29 standard defines the control protocol between the DTE and the PAD.
Public data network (PDN): This is the X.25 "cloud" of circuits and packet-switching exchanges (PSEs) (switches) at the carrier facility that you can use to connect DTE at different locations together and carry the X.25 packets from one DTE to another. These PDNs typically run parallel to the carrier's Public Switched Telephone Network (PSTN) voice network, and each carrier typically operates its own PDN. Different carriers' PDNs can be interconnected to form larger X.25 networks using the X.75 standard.
X.25 supports two different types of connection- oriented communications:
Permanent virtual circuit (PVC): Here the carrier sets up the switches for the connection ahead of time, emulating a dedicated point-to-point communications link for the connected nodes. PVCs are used when large amounts of data regularly need to be transferred over PDNs, such as for connections between banks and credit agencies.
Switched virtual circuit (SVC): The switches are set up when the call is initiated and are torn down afterward. SVCs are used when only small amounts of information need to be occasionally sent over PDNs.
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) ,
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
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
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:
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 (MTSs): 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 local area network (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 (MSs): 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.
X.400. Architecture of an X.400 Message Handling System (MHS).
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.
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:
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 e-mail , P-series protocols ,Simple Mail Transfer Protocol (SMTP) ,
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:
A vendor-neutral standards-based directory based on ITU recommendations
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
X.500. Architecture of an X.500 directory.
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:
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 directory service on a network running Microsoft Windows 2000.
The three main components of an X.500 directory are
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. This ensures that each DSA's subset of the DIB is current and complete and helps 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.
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).
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:
It is a complex directory that was originally based on network protocols used by the Open Systems Interconnection (OSI) reference model and incurs considerable protocol overhead on Transmission Control Protocol/Internet Protocol (TCP/IP) networks.
It has no native global name registration facility such as the Internet's Domain Name System (DNS), which makes it difficult to implement it as a truly global standard.
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) ,
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)
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:
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 Transmission Control Protocol/Internet Protocol (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 Internetwork Packet Exchange/Sequenced Packet Exchange (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).
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)
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
Easier portability to nonstandard user interfaces
The ability to create new DTDs
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) ,
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
Supports a wide range of applications and processing schemes including client/server, distributed, and multi-tier
Is easy to implement and use over the Internet
Creates documents that are human-readable
Is formal, concise, easy to understand, and has few optional features
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:
ACORD XML: An XML schema developed by the insurance industry for electronic exchange of life insurance information
ADPr: Stands for Active Digital Profile; used for exchanging provisioning information for IT (information technology) products and services
AgXML: Schema for the agricultural products sector developed by Monsanto, Archer Daniels Midland, and other members of this sector and managed by an independent nonprofit organization called AgXML LLC
BizTalk: An XML framework from Microsoft for general e-commerce
BPML: Stands for Business Process Modeling Language, a schema developed by a consortium of 80 companies to standardize how business processes are modeled
CDA: Stands for Clinical Document Architecture, a schema for health care companies to share clinical and patient information electronically
CXML: An XML schema developed by Commerce One for e-commerce
MathML: Used by the academic sector for formatting documents containing mathematical expressions
RosettaNet: XML schema for the high-tech manufacturing industry that has been embraced by over 300 different vendors
XKMS: Stands for XML Key Management Specification, an XML specification developed by the financial and credit sector for secure electronic banking and credit transactions
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:
XML markup describes the data itself, but HTML markup determines how the data is displayed in a Web browser. In other words, although HTML is a markup language, XML is a language for creating new markup languages. Note that unlike HTML, XML itself does not control how data is displayed. Instead, this feature is supported in XML through an associated standard called Extensible Stylesheet Language (XSL).
HTML uses a fixed set of tags, but XML allows users to create their own tags to describe any kind of data.
XML documents are easier to search than HTML ones because each kind of data has its own unique type of tags associated with it.
HTML documents are basically designed for display in Web browsers, but XML can be used as a general language that allows all types of applications to communicate with one another.
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
Extensible Stylesheet Language (XSL): Specifies how XML documents are displayed in a Web browser or other standard interface and allows XML documents to be translated into other types of documents such as HTML Web pages. Although XML documents contain the content (data) used by an application, XSL style sheets format the content of these documents to make them visually understandable. For example, an XSL style sheet could specify how an electronic invoice is displayed in HTML. Microsoft Internet Explorer includes a default XSL style sheet that can be used to display the contents of XML documents in the browser window.
Extensible Stylesheet Language Transformations (XSLT): Allows XML documents to be translated into other types of documents regardless of whether they ever need to be human-readable. XSLT is essentially one-half of XSL, the "translation" part, without any consideration of the "display" part.
XPath: Provides a programmatic syntax for querying XML documents to locate and select specific types of data within them. For example, an XML- enabled application could use XPath to extract pricing information from invoice documents.
Xpointer: Complements XPath by allowing you to identify a portion within an XML document for further processing.
XML Query: Like XPath, XML Query also lets you search for and extract specific kinds of information from an XML document, but XML Query offers a more database-like approach to this process. XML Query is currently still under development by the W3C.
XLink: Provides a standard way of representing hyperlinks within XML documents.
Xinclude: Copies a portion of an XML document into the current document. Compared to Microsoft's Object Linking and Embedding (OLE) technology, XLink is like Linking and XInclude is like Embedding. XInclude is also currently under development by the W3C.
Xbase: Provides a method of coding relative hyperlinks within XML documents.
Document type definition (DTD): A language that allows you to define an XML dialect. Each dialect requires an associated DTD that specifies the structure and syntax of valid and well-formed documents for that dialect. DTD is a complex language that is different from XML and much harder to read and is expected to be superceded by XML Schema, which is described next.
XML Schema: A newer method for defining XML dialects than the older DTD specification. XML Schema uses XML itself to create special documents called schemas that describe the structure and syntax of a particular XML dialect. Hundreds of XML schemas have been developed (see the Implementation section below).
Resource Description Framework (RDF): Specifies how document metadata such as copyright and search keywords can be embedded in XML documents. RDF makes it easier to manage large numbers of XML documents in a collection or repository.
Document Object Model (DOM): Allows XML documents to be parsed into the native object format of a standard programming language. DOM makes it easier for developers to XML-enable their applications.
SAX: Similar to DOM, but instead of reading the entire XML document into memory to process it (as DOM does), SAX allows applications to read through XML documents in real time and trigger events when certain conditions arise. Unlike the other XML standards described above, which are standardized by the W3C, SAX is an open-source project.
Simple Object Access Protocol (SOAP): Enables applications to access remote objects using XML. SOAP is a specification developed by IBM and Microsoft.
Uniform Description, Discovery, and Integration (UDDI): Enables vendors to expose their proprietary XML schemas to each other through a distributed directory service.
XAML: Developed by IBM, Oracle, Hewlett- Packard, and Sun Microsystems, this emerging XML standard is designed to simplify the process of coordinating complex business transactions between multiple parties.
Implementation
XML requires much stricter adherence to formatting rules than HTML does. Specifically, XML documents must be
Well-formed: Complies with the general rules and syntax of XML. For example, in an XML document every opening tag requires a corresponding closing tag. In HTML you can get away with omitting closing tags (omitting the </p> in a <p></p> expression, for example), but not so in XML-the rules must be strictly followed. Another example is that all attributes within tags must be enclosed in quotes. For example, in HTML you could have either <font size=4> or <font size="4">, and both would be interpreted correctly by most browsers. In XML only the second of these statements would be considered acceptable in a well-formed document. Other essential aspects of XML formatting include case sensitivity (HTML is not case sensitive) and proper nesting of elements (HTML lets you get away with <b><p></p></b> instead of <p><b></b></p>).
Valid: Complies with the rules specified in the documents corresponding DTD or schema. In other words, the vocabulary of a particular XML dialect is limited to what is defined in that dialect's "dictionary."
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
XML standards have evolved considerably in the few years since XML originated, and they are still evolving. As a result, before implementing XML solutions, many businesses are adopting a "wait- and-see" approach until XML becomes a mature standard.
Although EDI is expensive and complex, those who have implemented EDI solutions often see little incentive to change, taking an "if it ain't broke, don't fix it" approach.
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)
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)
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:
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.121 address ,X.400
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
Application service provider (ASP): A company that offers software services to business customers across the Internet, particularly services involving outsourcing of Web and e-business applications. ASPs were the next to occur after ISPs, and the late 1990s saw hundreds of them appear-and most of them disappear a year or two later.
Business service provider (BSP): Basically an ASP that provides a wide range of online business services that include not just Web hosting and e-commerce services typical of ASPs but also customer relations management, desktop maintenance support, system integration and consulting services, and other value-added business services.
Caching service provider (CSP): A company that maintains caching servers that speed the transfer of information across the Internet's infrastructure and offers managed access to these servers for a fee.
Commercial service provider (also CSP): This broad term typically includes ISPs, online service providers, telephone and cable network operators, and similar companies.
Content service provider (again CSP): Helps organizations maintain fresh content on their portals without the need or cost of creating their own internal publishing house.
Full-service provider (FSP): Really just another name for BSP.
Hosting service provider (HSP): Basically an ISP that concentrates on Web hosting.
Management service provider (MSP): A company that manages the (IT) information technology infrastructure for other businesses.
Storage service provider (SSP): A company offering outsourced storage services to businesses.
Security service provider (also SSP): A company offering outsourced security services to businesses.
Wireless application service provider (WASP): An ASP that supports wireless business applications.
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)
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 servers: These form the server portion of the X Window System and are typically UNIX servers but can also be mainframe computers running UNIX.
X clients: These are typically 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 always 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: This is the protocol used for communication between X clients and X servers, and it enables the client to send keyboard and mouse data to the server and display the GUI on the client.
X window manager: Examples include OSF/Motif and K Desktop Environment (KDE), which implement windowing features such as menus, toolbars, and gadgets that provide the look and feel of a windowing GUI environment that allows X applications to run.
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