X
X
Symbol for reactance
X BAND
5200 to 10000MHz
X RECOMMENDATIONS
The CCITT documents that describe data communication network standards. Well-known ones include: X.25 Packet Switching standard, X.400 Message Handling System, and X.500 Directory Services.
X WINDOW SYSTEM (TM)
A popular window system developed by MIT and implemented on a number of workstations.
X.121
(In packet-switched networks) A CCITT recommendation that defines the international packet-switched networks numbering scheme.
X.21
(In PDNs) A CCITT recommendation that defines a digital interface, using the 15-pin connector described in V.11.
X.21 BIS
Used on public data networks of data terminal equipment which is designed for interfacing to synchronous V-series modems; equivalent to RS-232 and V.24
X.25
The CCITT standard network communications protocol for implementing the internetwork communications function specified by the third network layer of the ISO/OSI model. X.25 is a packet-switched protocol often used in wide-area network configurations. An X.25 network contains a number of packet switches that switch and route packets transmitted between two nodes, such as a terminal and host machine. Although data transfer takes place using packets, the appearance to higher layers of software is that a single continuous logical channel (or virtual circuit) exists between two nodes. Typical packet sizes of X.25 networks are 128 bytes, although other sizes are possible and can be negotiated at connection time. Although terminals are attached to the X.25 network through a Packet Assembler Disassembler (PAD), host machines can be attached directly or through synchronous modems. The X.25 network uses the X.25 protocols, which actually are three protocols. These protocols include the X.21 or X.21bis at the physical level, Link Access Procedure Balanced (LAPB) at the data link level, and X.25 Packet Level Protocol (PLP) at the network level. The X.25 protocol enables a maximum of 4095 virtual circuits to be time-multiplexed across a physical link between the node and an X.25 network. In actual practice, fewer virtual circuits are used because most physical links cannot support the aggregate data rate for 4095 virtual circuits. Typical transmission speeds of X.25 networks are 64 Kbps, but higher speeds are available. If two LANs are connected through an X.25 network, data transfer between the LANs is limited to this speed. Many LAN applications require data-transfer rates of at least 1 Mbps to run efficiently. File transfers and terminal emulation applications run efficiently across an X.25 network, but workstation applications run slowly. X.25 is identical to HDLC, with the exception of a routing field that is placed just before the data. The routing field is used to route each packet from node to node within a network that may have numerous redundant routes.
X.25 PAD
A device that permits communication between non-X.25 devices and the devices in an X.25 network.
X.28
Defines the interface between PADs and non-packet mode OTEs.
X.29
Defines the interface between PADs and packet mode OTEs or other PADs.
X.3
Describes the functions of the PAD and the various parameters which can be used to specify its mode of operation.
X/OPEN
A group of computer manufacturers that promotes the development of portable applications based on UNIX. They publish a document called the X/Open Portability Guide.
XDR
See eXternal Data Representation.
XEROX NETWORK SYSTEM
Xerox's network architecture, which provides the basis for both Novell's Universal Network Architecture and TCP/IP network protocols and packet formats.
XEROX NETWORK SYSTEM'S INTERNET TRANSPORT PROTOCOL
(XNS/ITP) In LAN technology, a special communications protocol used between networks. XNS/ITP functions at the 3rd and 4th layer of the OSI model. Similar to TCP/IP.
XGA
EXtended Graphics Adapter. IBM graphics standard that includes VGA and supports higher resolutions, up to 1024 pixels by 768 lines interlaced.
XID
Exchange Identification
XIP
See Execute in Place
XMM
See Extended Memory Manager.
XMS
See Extended Memory Specification.
XNET
Cross Net Debugger
XNS
See Xerox Network System.
XNS-IDP
Xerox NS IDP
XNS/ITP
See Xerox Network System's Internet Transport Protocol.
XON/XOFF
XON stands for "Transmitter On," and XOFF stands for "Transmitter Off." An asynchronous communication protocol for the receiving station to control the flow of data from the transmitting station. When the receiving station cannot continue to receive any more data, as when its buffer is full, it transmits an XOFF (DC3) control character that tells the sender to stop. When the receiving station is again able to process more data, it transmits an XON (DC1) control character that tells the sender to resume transmission. This protocol is also referred to as software handshaking, also called software flow control in a DTE. If the modem receiving data needs time to process the data or do some other task, it sends an XOFF signal to the host computer (or sending modem).