Glossary - X

X

Prefix used by the CCITT to identify standards concerning data networks. See also X.25, X.400, X.500.

X.121

A ITU-TSS packet switching network address standard. See X.25, NSAP, E.164.

X.25

One of the first - CCITT standardized - public (data) packet switching network protocols. Originally designed to operate over unreliable communications links, it supported both virtual circuit (VC) and datagram services. It specifies the protocols and functions of layers 1, 2 and 3 at the interface between users and the network for this type of network. See X.121.

X.400 (standard, messaging)

The international standard protocol for electronic mail. X.400 defines an electronic system for exchanging messages among store-and-forward mail systems.

X.500 (standard, communications)

The ITU-T and ISO international standard for electronic directory services that store and serve (to clients using a variety of interfaces) centrally-managed personal location information. The standard defines a hierarchical directory structure for administrative domains (organization, divisions, departments and workgroups).
Examples: white pages, Knowbot, WHOIS.

X.509 (standard, communications)

A security standard that defines a certification process by which users are authenticated.

XC

Cross-Connect (network)

XDF

Xrm Decrease Factor (ATM)
An ABR service parameter, XDF controls the decrease in ACR associated with Xrm. It is a power of two in range: [0, 1].

XDR

eXternal Data Representation
A standard for machine independent data structures developed by Sun Microsystems and defined in RFCs 1014 and 1832. It is similar to Abstract Syntax Notation One (ASN.1) [Source: RFC1208].

XML

eXtensible Markup Language (language, text, Internet)
An initiative from the W3C defining an "extremely simple" dialect of SGML suitable for use on the World-Wide Web.
Web site: http://www.w3.org/XML/.

XMS

eXtended Memory Specification (hardware)

XNS

Xerox Network Systems (network)
A protocol suite developed by Xerox Corporation to run on LAN and WAN networks, where the LANs are typically Ethernet. Implementations exist for both Xerox's workstations and 4.3BSD, and 4.3BSD-derived, systems.
XNS denotes not only the protocol stack, but also an architecture of standard programming interfaces, conventions, and service functions for authentication, directory, filing, email, and remote procedure call.
XNS is also the name of Xerox's implementation.

Xrm

An ABR service parameter, Xrm limits the number of forward RM-cells which may be sent in the absence of received backward RM-cells. The range is 0-255.

XTP

eXpress Transport Protocol (network)

X-Windows (standard)

X is the name for TCP/IP based network-oriented window systems. Network window systems allow a program to use a display on a different computer.
The most widely-implemented window system is X11 - a component of MIT's Project Athena.

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