X

X is for XML

X.121: ITU-T standard defining an international numbering plan for public data networks.

X.21: ITU-T standard defining the interface between a Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE).

X.21 bis: ITU-T standard defining the use on public data networks of DTE which is designed for interfacing to synchronous V-Series modems..

X.24: ITU-T list of definitions for interchange circuits between DTE and DCE on public data networks.

X.25: ITU-T standard defining the interface between DTE and DCE for terminals operating in the packet mode and connected to public data networks by dedicated circuit.

X.28: A Triple-X protocol defining DTE/DCE interface for a start-stop mode Data Terminal Equipment accessing the PAD in a public data network situated in the same country.

X.29: A Triple-X protocol defining procedures for the exchange of control information and user data between a PAD facility and a packet mode DTE or another PAD.

X.3: A Triple-X protocol that defines a PAD.

X.400: An ITU-T standard for electronic mail and message handling.

X.500: An ITU-T standard for directory service.

X.509: In cryptography, X.509 is an ITU-T standard for PKI (Public Key Infrastructure). X.509 specifies, amongst other things, standard formats for public key certificates. X.509 is part of the hierarchical X.500 standard and thus assumes a strict hierarchical system of certificate authorities (CAs) for issuing the certificates. This is in contrast to web of trust models, like PGP, where anyone (not just special CAs) may sign (and thus attest to the validity) of others' key certificates. The X.500 system have never been fully implemented, so the IETF's public-key infrastructure working group have made extensive updates to the standard in order to make it work with the more loose organization of the Internet. In fact today X.509 certificate usually refers to the X.509 v3 certificate specified in RFC 3280.

x2: 56K modem technology from 3Com (U.S. Robotics). See also V.90 and K56Flex.

xDSL: A term referring to a variety of new Digital Subscriber Line technologies. Some of these varieties are asymmetric with different data rates in the downstream and upstream directions. Others are symmetric. See also ADSL, HDSL, RADSL, SDSL, VDSL and WDSL.

XHTML (Extensible HyperText Markup Language): XHTML is a reformulation of HTML 4.01 in XML, and combines the strength of HTML 4 with the power of XML. XHTML 1.0 is the first major change to HTML since HTML 4.0 was released in 1997. It brings the rigor of XML to Web pages and is the keystone in W3C's work to create standards that provide richer Web pages on an ever increasing range of browser platforms including cell phones, televisions, cars, wallet sized wireless communicators, kiosks, and desktops.

XMI, (XML Metadata Interchange), is a stream format for interchange of metadata including UML models created during analysis and design activities. It is useful for transferring the model from one step to the next as the design and coding progress, or for transferring from one design tool to another. See also UML and CORBA.

XML (eXtensible Markup Language): XML, a formal recommendation from the World Wide Web Consortium (W3C), is similar to the language of today's Web pages, the Hypertext Markup Language (HTML). Both XML and HTML contain markup symbols to describe the contents of a page or file. HTML, however, describes the content of a Web page (mainly text and graphic images) only in terms of how it is to be displayed and interacted with. For example, the letter "p" placed within markup tags starts a new paragraph. On the other hand, XML describes the content in terms of what data is being described, so XML can be used to define meta-data. This means that an XML file can be processed purely as data by a program or it can be stored with similar data on another computer or, like an HTML file, that it can be displayed. XML is "extensible" because, unlike HTML, the markup symbols are unlimited and self-defining. XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language (SGML), the standard for how to create a document structure.

XNS (Xerox Network System): The XNS protocols provide routing capability and support for both sequenced and connectionless packet delivery. Novell and 3Com 3Plus protocols use the lower layers of XNS for packet delivery. XNS includes the following protocols; IDP - Internet Datagram Protocol, RIP - Routing Information Protocol, PEP - Packet Exchange Protocol, and SPP - Sequenced Packet Protocol.

XSD (XML Schema Definition): XSD, a Recommendation of the World Wide Web Consortium (W3C), specifies how to formally describe the elements in an Extensible Markup Language (XML) document. This description can be used to verify that each item of content in a document adheres to the description of the element in which the content is to be placed. In general, a schema is an abstract representation of an object's characteristics and relationship to other objects. An XML schema represents the interrelationship between the attributes and elements of an XML object (for example, a document or a portion of a document). To create a schema for a document, you analyze its structure, defining each structural element as you encounter it. For example, within a schema for a document describing a Web site, you would define a Web site element, a Web page element, and other elements that describe possible content divisions within any page on that site. Just as in XML and HTML, elements are defined within a set of tags.

XSL (Extensible Stylesheet Language): XSL is a language for expressing stylesheets defined by W3C. It consists of three parts: XSL Transformations (XSLT): a language for transforming XML documents, the XML Path Language (XPath), an expression language used by XSLT to access or refer to parts of an XML document. (XPath is also used by the XML Linking specification). The third part is XSL Formatting Objects: an XML vocabulary for specifying formatting semantics. An XSL stylesheet specifies the presentation of a class of XML documents by describing how an instance of the class is transformed into an XML document that uses the formatting vocabulary.