X: X-XWindows



  1. An abbreviation for the word "cross," as in crossbar (Xbar). 5XB would be the abbreviation for a No. 5 Crossbar circuit switch.

  2. Generic. As in xDSL, generic Digital Subscriber Line, which is a family of access technologies including ISDN, ADSL, IDSL, HDSL, RADSL and SDSL. Also, as in 1000Base- CX, an IEEE standard for Gigabit Ethernet over some sort of Copper cable.

  3. In mathematics, an unknown quantity.

  4. The symbol designation for Reactance.

X Band

7 GHz and 8 GHz. Used by military satellites .

X Display Manager Control Protocol


X Rays

Electromagnetic radiation with a wavelength of under 100 ngstroms. When Wilhelm Roentgen discovered the X-ray in 1895, some journalists were convinced that the primary user of the revealing high-frequency radiation would be the "peeping Tom." The (pardon the expression) titillating publicity led to legal and mercantile steps, resulting in New Jersey passing a law banning "X-ray opera glasses ," and merchants in London selling X-ray proof feminine underwear.

X Recommendations

The ITU-T 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 Series

Recommendations drawn up by the ITU-T to establish communications interfaces for users' Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE). They govern the attachment of data terminals to public data networks (PDNs) and the Public Switched Telephone Network (PSTN). In short, a set of rules for interfacing terminals to networks.

X Terminal

A networked desktop machine that displays software applications which are running on a networked server. X terminals are allegedly cheaper and easier to network than PCs or workstations. They are said to be a forerunner to thin clients , also called network computers (NCs).

X Windows

X Windows is officially called X Window, or The X Window System, or X. Sometimes it is spelled with an hyphen, e.g.X-Window. It is used primarily with UNIX systems, but not exclusively. X was originally designed to be an industry-standard network windowing system, and because UNIX is widely used, X has become the windowing standard for UNIX.

X-10 Protocol

A protocol found in home automation. You can use X-10 to control conforming black boxes and devices. The protocol is a command signal that rides on the AC 60 cycle sine power curve. The signal is a series of 120 kHz pulses sent on the "zero crossing " of each cycle. The signal is in a binary fashion, transmitting the letter code, unit code and command for a device. All receivers monitor the power line waiting for a command to respond. The limit of number of unique codes available is 256. This is derived by having 16 letter codes and 16 unit codes, hence 16x16=256 "addresses." More than one device can share an address. If you decide that every time you select "A1" on, you want all the front lights to come on, then you can place all the receivers to the same address. The system is easy to use and very flexible.


Horizontal axis on a graph or chart.


A term used to describe any database application capable of generating custom programs with dBASE-compatible code.

X-Dimension Of Recorded Spot

In facsimile, the center-to-center distance between two recorded spots measured in the direction of the recorded line. This term applies to facsimile equipment that responds to a constant density in the subject copy by yielding a succession of discrete recorded spots.

X-Dimension Of Scanning Spot

In facsimile, the center-to-center distance between two scanning spots measured in the direction of the scanning line on the subject copy. The numerical value of this term will depend upon the type of system used.


A flow control protocol for asynchronous serial transmission. Flow control is a method of adjusting information flow. For example, in transmitting between a computer and a printer, the computer sends the information to be printed at 9600 baud. That's several times faster than the printer can print. The printer, however, has a small memory. The computer dumps to the memory, called a buffer, at 9600 baud. When it fills up, the printer signals the computer that it is full and please stop sending. When the buffer is ready to receive again, the printer (which also has a small computer in it) sends a signal to the desktop computer (the one doing the printing) to please start sending again. X-OFF means turn the transmitter off (xmit in Ham radio terms). It is the ASCII character Control-S. X-ON means turn the transmitter on. It is the ASCII character Control-Q. You can use these characters with many microcomputer functions. For example, if you do DIR in MS-DOS and you want to stop the fast rush of files, then type Control-S.

X- Open

An international consortium of computer vendors working to create an internationally supported vendor-independent Common Applications Environment based on industry standards.


When X-rays were discovered by Wilhelm Roentgen in 1895, some journalists were convinced that the primary user of the revealing shortwave radiation would be the "peeping Tom." The titillating publicity led to legal and mercantile steps. Two examples: A law introduced in New Jersey forbade the use of "X-ray opera glasses," and merchants in London sold X-ray-proof underwear to modest ladies. See X-Ray Lithography.

X-Ray Lithography

A lithographic process in which X-rays, rather than light or electron beams, are used to transfer circuit patterns to a silicon wafer. The advantage of X- rays is their shorter wavelengths , which reduce diffraction and yield greater resolution and finer line widths of features. This allows more transistors to be packed onto a chip.

X-Series Recommendations

Set of data telecommunications protocols and interfaces defined by the ITU-T.


The UNIX equivalent of Windows. What Windows is to MS-DOS, X- Windows is to Unix. A network-based windowing system that provides a program interface for graphic window displays. X-Windows permits graphics produced on one networked station to be displayed on another. Almost all UNIX graphical interfaces, including Motif and OpenLook, are based on X-Windows. X-Windows is a networked window system developed and specified by the MIT X Consortium. Members of the X Consortium include IBM, DEC, Hewlett-Packard and Sun Microsystems. Sun Microsystems has been contracted by the MIT X Consortium to implement PEX (PHIGS Extensions to X), which will be the standard networking protocol for sending PHIGS (Programmers Hierarchical Graphics System) graphics commands through X-Windows. Some people spell it X-Windows and some spell it X Windows.


A specific variety of electromechanical switch. Does the same things as a Stronger step-by-step switch but in a horizontal plane. It's so called because it's a two motion switch with horizontal and vertical movements. The first pulse sends the switch horizontally to the right place, then the next pulse sends it vertically up to the right place and so on, until it has switched the call through. One of the most reliable switches ever produced. Unfortunately, it's slow, space-consuming and unable to be programmed with many new customer pleasing features.


CCITT specification that defines classes of service in a packet-switched network, such as virtual-circuit, datagram, and fast-packet services.


CCITT recommendation that specifies routing protocol for international PDN services accessing similar switched PDNs.


The dominant EDI standard in the U.S. today. IN 1979 ANSI chartered the Accredited Standards Committee (ASC) X.12 to develop uniform standards for electronic interchange of business transactions. Pronounced "ex twelve" ” the dot (X.12) was formally dropped by the standards board.


International Numbering Plan for public data networks. X.121 defines the numbering system used by data devices operating in the packet mode. X.121 is used by ITU-T X.25 packet-switched networks and has been proposed by several computer vendors as the future universal addressing scheme. X.121 comprises the CCITT recommendations that specifies the international numbering plan for PDNs. The numbering plan uniquely identifies the world zone, country or geographical area, network, and subscriber.

X.121 Address

A standard O/R (Originator/Recipient) attribute that allows Telex terminals to be identified in the context of store-and forward communications.


Call setup and clear down times for international connection to synchronous PDNs.


Grade of service over international connections to PDNs.


DTE and DCE test loops in public data networks.

X.2 CCITT recommendation that specifies the international interface for special and optional service facilities (features) available to PDNs.


Asynchronous communications interface definitions between data terminal equipment (DTE) and data circuit terminating equipment (DCE) for start-stop transmission services on public switched telephone networks.

X.20 bis

Used on public data networks of data terminal equipment (DTE) that is designed for interfacing to asynchronous duplex V-series modems.


A series of ITU (International Telegraph and Telephone Consultative Committee) recommendations that defines the type of service offered by specific layers in the OSI (Open Systems Interconnection) Model, and defines the protocol to be used at those layers . ITU is now the ITU.

X.21 Interface between Data Terminal Equipment (DTE) and DATA Circuit-Terminating Equipment (DCE) for synchronous operation on public data networks. Specifies protocols for all three lowest layers of OSI. Efficient transfer of data bit streams (unblocked) between the DTE and DCE. Only used for link establishment/disestablishment and other connection control functions. Applicable to both leased lines and circuit switching.

X.21 bis

CCITT recommendation that specifies the interface for DTE synchronous V- series modems (similar to RS-232-C). Used on public switched telephone networks of data terminal equipment (DTE) that is designed for interfacing to synchronous V-series modems.

X.21 TSS

Specification for Layer 1 interface used in the X.25 packet-switching protocol and in certain types of circuit-switched data transmissions.

X.215/X.225 CCITT Layer 5 protocol with "Call Set Up" to provide "sessions" between host computers. Access validation can be provided for security. Re-establishes failed transport connections. Synchronizes transfer of different kinds of information.

X.216/X.226, X.209 CCITT Layer 6 Presentation Services and Protocol negotiates syntax and format (ASCII , Compressed Data, Encrypted data). X.209 Abstract Syntax Notation One to specify format or to switch from one format to another for a different session activity.


A ITU-T standard associated with the transport layer of the Open Systems Interconnect (OSI) architecture used in networks employing circuit-switched techniques.


A ITU-T standard associated with the session layer of the Open Systems Interconnect (OSI) architecture used in networks employing circuit-switched techniques.


A ITU-T standard associated with the Open Systems Interconnect (OSI) architecture that defines specific presentation layer services used with circuit-switched network services.


List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit terminating equipment (DCE) on public switched telephone networks.


From its beginning as an international standards recommendation from ITU-T, the term X.25 has come to represent a common reference point by which mainframe computers, word processors, mini-computers, VDUs, microcomputers and a wide variety of specialized terminal equipment from many manufacturers can be made to work together over a type of data communications network called a packet switched network. On a packet switched data network (private or public), the data to be transmitted is cut up into fixed size blocks (i.e. packets). Each block has a header with the network address of the sender and that of the destination. As the block enters the network, the number of bits in the block are put through some mathematical functions (an algorithm) to produce a check sum. The check sum is attached as a "trailer" to the packet as it enters the network.

Packets may travel different routes through the network. But, ultimately, the packets are routed by the network to the node where the destination computer or terminal is located. At the destination, the packet is disassembled. The bits are put through the same algorithm, and if the digits computed are the same as the ones attached as the trailer, there are no detected errors. An ACK, or acknowledgement, is then sent to the transmitting end. If the check sum does not match, a NAK, or Negative Acknowledgement is sent back, and the packet is retransmitted. In this manner, high speed, low error rate information can be transmitted around the country using shared telecommunications circuits on public or private data networks. X.25 is the protocol providing devices with direct connection to a packet switched network. These devices are typically larger computers, mainframes, minicomputers, etc. Word processors, personal computers, workstations, dumb terminals, etc. do not support the X.25 packet switching protocols unless they are connected to the network via PADs ” Packet Assembler/Disassemblers. A PAD converts between the protocol used by the smaller device and the X.25 protocol. This conversion is performed on both outgoing (from the network) and incoming data (to the network), so the transmission looks transparent to the terminal.

Here is a more technical definition: X.25 is the ITU-T Standards Recommendation (1976) for the protocol which provides the user interface into the original packet switched network. The basic concept of packet switching is one of a highly flexible, shared network in support of interactive computer communications across a wide area network. Previously, large numbers of users spread across a wide area and with only occasional communications requirements, had no cost-effective means of sharing access from their computer terminals to computer resources for time-share applications. In specific, the issue revolved around the fact that asynchronous communications are bursty in nature. In other words, data transmission is in bursts of keystrokes or data file transfers. Further, there is lots of idle time on the circuit between transmissions in either direction of relatively small amounts of data. Additionally, those early networks consisted of analog, twisted pair facilities, which offered very poor error performance and relatively little bandwidth.

Existing circuit-switched networks certainly offered the required flexibility, as users could dial-up the various host computers on which the desired database resided. Through a low- speed modem, which was quite expensive at the time, data could be passed over the analog network, although error performance was less than desirable. However, the cost of the connection was significant, as the calls were billed based on the entire duration of the connection, even though the circuit was idle much of the time. Dedicated circuits could address the imbalance between cost and usage, as costs are not usage-sensitive and as dedicated circuits could be shared among multiple users through a concentrator. However, dedicated circuits were expensive, especially in long-haul applications, and involved long implementation delays. Further, users tended not to be concentrated in locations where they could make effective use of dedicated circuits on a shared basis. Finally, large numbers of dedicated circuits would be required to establish connectivity to the various hosts . Packet switching solved many of those problems, in the context of the limitations of the existing networks ”namely, their analog, twisted pair nature. Packet-switched networks support low- speed, asynchronous, conversational and bursty communications between computer systems. As packet-switched network usage can be billed to the user on the basis of the number of packets transmitted, such networks are very cost-effective for low-volume, interactive data communications. This cost advantage comes from the fact that the bursty nature of the such interactive applications allows large volumes of data transmissions from multiple users to be aggregated in order to share network facilities and bandwidth. Further, packet-switched networks perform the process of error detection and correction at each of the packet switches, or nodes; thereby, the integrity of the transmitted data is improved considerably.

Information is transported and switched through the network on the basis of packets, which are structured in a standard data format by a Packet Assembler/Disassembler (PAD). The PAD, which functionally sits between the user host computer or terminal and the edge of the carrier network, executes the X.25 protocol by assembling (forming) the packets on the transmit side and disassembling the packets on the receiving side of the communication. The packet format comprises a header, the payload, and a trailer. Each packet is of a fixed maximum size, typically containing 128B (B=Byte or octet) or 256B of payload (user data); packet sizes of up to 4,096B can be accommodated in some networks. The typical upper limit of a packet is 512B or 1,024B, with the latter packet size being used in many airline reservation networks. If a large file is being transmitted, the PAD fragments the file which streams out of the transmitting device, carving it into packets of the specified size. If only a few bytes are being transmitted (e.g., a few keyboard commands or mouse clicks), the PAD gathers them up over a short, pre-determined period of time, and adds the necessary number of "stuff bits" to fill the packet payload.

The payload data is encapsulated by a beginning flag (8 bits) in the header, and an ending flag (8 bits) in the trailer. These flags serve to the distinguish each packet from other packets traveling the same path . The beginning flag also serves as synchronizing bits in order that the packet nodes ( intelligent switches) and the receiving terminal equipment might synchronize on the rate of transmission. This approach reduces overhead in data transmission, thereby improving the efficiency of transmission. The header also includes a packet address field of 8 bits (4 bits for the calling DTE and 4 bits for the called DTE) prepended to the data in order that the various packet nodes might route each packet on its way over an appropriate path, ultimately to the target device. Control data (8-16 bits) in the header includes the packet sequence number in order that the target node and terminal equipment are able to identify errored, corrupted or lost packets, and to resequence the packets should they arrive out of order. Additionally, the header control data includes the number of the virtual circuit (4 bits) and virtual channel (8 bits) over which the data will travel, if a path has been preordained. Finally, error control data is included in the form of a highly reliable CRC check (16 bits), which comprises the trailer. On the receiving side of the transmission, the PAD disassembles the packet, stripping away the header, trailer and stuff bits in order to get to the actual data being transmitted. See also ATM, CRC, Frame Relay, NBMA, Packet and X.25 network.

X.25 Network

Any network that implements the internationally accepted ITU-T standard governing the operation of packet-switching networks. The X.25 standard describes a switched communications service where call setup times are relatively fast. The standard also defines how data streams are to be assembled into packets, controlled, routed, and protected as they cross the network. See X.25.


Defines the Terminal-Pad Interface. DTE/DCE interface for start-stop-mode data terminal equipment accessing the packet assembly/disassembly facility (PAD) in a public switched telephone networks situated in the same country.


Procedures for the exchange of control information (handshaking) and user data between a packet assembly/disassembly facility (PAD) and a packet mode DTE or another PAD.


ITU-T recommendation describing the operation of a Packet Assembly/Disassembly (PAD) device or facility in a public data network. X.3 defines a set of 18 parameters that regulate basic functions performed by a PAD to control an asynchronous terminal. The setting of these parameters governs such characteristics as terminal speed, terminal display, flow control, break handling and data forwarding conditions, and so on.


Support of X.20 bis, X.21 and X.21 bis DTEs by an ISDN line.


CCITT recommendation that specifies the essential interaction between elements of customer interfaces, interexchange signaling systems, and other network functions specifically related to the use of international user facilities and network utilities.


Support of packet mode DTEs by an ISDN system. This standard describes two modes of operation packetized data in the B-channel and packetized data over the D-channel.


CCITT recommendation for an interface between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) operating in a packet mode (i.e. X.25) and accessing a packet switched public data network via a public switched telephone network or a circuit switched public data network. X.32 describes the functional and procedural aspects of the DTE/DCE interface for DTEs accessing a packet switched public data network via a public switched network.


ITU-T recommendation for the access of Group 3 facsimile equipment to the Facsimile Packet Assembly/Disassembly (FPAD) facility in public data networks situated in the same country.


ITU-T recommendation for the exchange of control information and user data between a Facsimile Packet Assembly/Disassembly (FPAD) facility and a packet mode Data Terminal Equipment (DTE) or another pad, for international networking.


The ANSI committee responsible for the creation and perpetuation of Fiber Channel standards.


The ANSI committee responsible for the creation and perpetuation of FDDI Standards.


International Alphabet No.5 for character oriented data.


X.400 is an international standard which enables disparate electronic mail systems to exchange messages. Although each e-mail system may operate internally with its own, proprietary set of protocols, the X.400 protocol acts as a translating software making communication between the electronic mail systems possible. The result is that users can now reach beyond people on their same e-mail system to the universe of users of interconnected systems. One problem with e-mail sent between X.400 networks is that the sender's name is not sent. (I kid you not.) This was one element of the protocol the committees forgot ! If your message crosses an X.400 network, remember to sign your name. The X.400 standard itself is an overview which is broken down under subsequent numbers:


A standard for electronic mail exchange; developed by the CCITT.


Overall Architecture.


Conformance testing.


Abstract service definition conventions.


Encoded information type conversion rules.


CCITT recommendation that specifies protocol for formatting a digital message. The format specifies such things as the fields and character length (must be in bytes).


Message transfer system.


Message store.


Protocol specifications.


Interpersonal messaging system. An IPM format specification using the X.400 transfer protocol. In addition to text, it also allows CAD/CAM, graphics, Fax, and other electronic information.


An EDI (Electronic Data Interchange) format specification based on the X.400 transfer protocol. It can also allow for CAD/CAM, graphics, Fax, and other electronic information to accompany an EDI interchange.


A VM (Voice Messaging) format specification, using the X.400 transfer protocol. In addition to voice, it can also contain CAD/CAM, graphics, fax, and other electronic information.

X.445 The X.445 standard, or APS (Asynchronous Protocol Specification), lets X.400 clients and servers exchange all types of digital data over public telephone networks rather than over X.25 leased lines, which are required today. Among those backing the spec are Intel, AT&T, Microsoft Corp., Lotus and Isocor. X445 is an extension of the X.400 standard. X.400 provides an option to other messaging backbones such as System Network Architectural Distribution Services and SMTP. It supports multimedia data traffic including text, binary files, E-mail, voice images, and sound.

In its December 19, 1994 issue, PC Week said that the X.445 standard, which earlier this month gained final approval from the International Telecommunications Union, should ease messaging dramatically by allowing users to exchange X.400 data traffic over standard telephone networks.


Fundamental parameters of multiplexing scheme for the international interface between synchronous data networks.


The ITU-T international standard designation for a directory standard that permits applications such as electronic mail to access information which can either be central or distributed. The X.500 standard for directory services provides the means to consolidate e-mail directory information through central servers situated at strategic points throughout the network. These X.500 servers then exchange directory information so each server can keep all its local mail directory information current. With X.500, any e-mail user, whether on OpenVMS, Macintosh, DOS, or UNIX workstations, can be listed in a central directory that can be accessed using an X.500-compatible user agent.


A cryptography term. The part of the ITU-T X.500 Recommendation which deals with Authentication Frameworks for Directories. Within X.509 is a specification for a certificate which binds an entity's distinguished name to its public key through the use of a digital signature. Also contains the distinguished name of the certificate issuer.


Fundamental parameters of multiplexing scheme for the international interface between synchronous data networks using 10-bit envelop structure.

X.51 bis

Fundamental parameters of a 48Kbit/s transmission scheme for the international interface between synchronous data networks using a 10-bit envelope structure.


Fundamental parameters of multiplexing scheme for the international interface between synchronous data networks using a 10-bit envelope structure.


Common channel signaling for circuit switched data applications.


Signaling system no. 7 ” data user part.


Terminal and transit control signaling for asynchronous services on international circuits between anisochronous data networks.


Management Framework for Open Systems Interconnection (OSI) for CCITT Applications.


Decentralized terminal and transit control signaling on international circuits between synchronous data networks.


An international standard and CCITT recommendation for linking X.25 packet switched networks. X.75 defines the connection between public networks, i.e. for a gateway between X.25 networks. X.75 defines terminal and transit call control procedure and data transfer system between packet switched public data networks.


Interworking of inter-exchange signals for circuit switched data services.


Hypothetical reference connections for synchronous PDNs in a packet switched network.


ITU-T specification dealing with a number of internal packet-switched network parameters such as packet size limitations and service restrictions.


Call progress signals in PDNs.


A group of computer manufacturers that promotes the development of portable applications based on UNIX. Formed in 1984, X/Open is dedicated to the identification, agreement and wide-scale adoption of information technology standards which reduce issues of incompatibility and which help users realize the benefits of open information systems. In February 1996, X/Open and the Open Software Foundation (OSF) consolidated to form The Open Group (www.opengroup.org). See The Open Group.


See Blind Dialing.


X10 is a powerline carrier protocol that allows compatible devices throughout the home to communicate with each other via the existing 110 volt AC (alternating current) electricity wiring in the house. With X10 it is possible to control lights and virtually any other electrical device from anywhere in the house with no additional wiring. What Can I Control? According to the X10 proponents, Lighting is the most popular control category. Starter kits begin at $10 or so and are plug-and-play. You can also control Security Systems & Access Control; Home Theater & Entertainment; Phone Systems; Thermostats for adjusting room temperatures ; Irrigation Have your sprinklers turn on only when it's dry. Because X10 devices don't use dedicated wiring, they can mysteriously stop working. Here's why. When X10 modules suddenly stop responding, it's usually due to something that was recently plugged into the electricity lines. Some electrical devices, like computers, televisions, and A/V gear, which have nothing to do with X10, may cause interference. Since the X10 signal goes everywhere in your home, some devices will have more effect on the signal strength. Power supplies in the following devices were designed to kill electrical noise, however, X10 signal looks like noise: Televisions , computers and monitors , battery chargers for cell phones and laptop computers, satellite receivers, surge-protection power strips , computer UPS'.


ANSI (American national Standards Institute) standard that is the dominant EDI (Electronic Data Interchange) standard in the U.S. today; designed to support cross-industry exchange of business transactions. Standard specifies the vocabulary (dictionary) and format for electronic business transactions.


A pre-standard 56-Kbps modem solution developed by US Robotics, x2 was one of two proprietary (read non-standard) approaches for running data over dial-up phone lines at up to 56 Kbps (actually 53.3 Kbps) one way and up to 33.6 Kbps the other way. The standard was developed for use on the Internet, with the 53.3 Kbps channel flowing downstream (i.e., to you) and the 33.6 Kbps channel running upstream (i.e., away from you). The logic is that at 53.3 Kbps, Web pages fill a lot faster on your screen. x2 was developed by US Robotics. K56flex, the competing proprietary 56-Kbps solution, was developed by Rockwell Semiconductor and Lucent Technologies. x2 was not compatible with K56flex. In other words, a K56flex modem could not talk to a x2 modem at full speed. The modems can, however, communicate at speeds up to 33.6 Kbps, which is the maximum speed of the predecessor V.34+ modems. In February 1998, the ITU-T finalized the specification for V.90, the international standard for 56-Kbps modems. While V.90 is an amalgamation of x2 and K56flex, neither conforms to V.90, and not all x2 and K56flex modems are upgradable to V.90. See also 56 Kbps Modem (for a much longer explanation), K56flex, V.90, V.91, and V.92.


Bit sequencing of ASCII in serial-by-bit data transmission.


Character structure and character parity sense for serial-by-data communications in ASCII.


Synchronous high speed data signaling rates between data terminal equipment and data circuit terminating equipment.


Code extension techniques for use with 7-bit coded character set of ASCII.


Determination of the performance of data communications systems.


Determination of performance of data communications systems that use bit oriented control procedures.


Data encryption algorithm.


A networked GUI developed at the Massachusetts Institute of Technology (MIT) as part of Project Athena. Based on a client/server architecture it displays information from multiple networked hosts on a single workstation.


Transaction management protocol.


Exchange Access SMDS. An access service provided by a local exchange carrier to an interexchange carrier. It enables the delivery of a customer's data over local and long distance SMDS networks.


An idyllic place in which "did Kubla Khan a stately pleasure -dome decree," according to Samuel T. Coleridge in his poem (1798) "Kubla Khan." Jump forward to 1941, when Orson Wells released "Citizen Kane." Charles Foster Kane (allegedly William Randolph Hearst) built Xanadu, an elaborate palace (allegedly modeled on the San Simeon mansion of William Randolph Hearst). The place (Coleridge) didn't exist, and the mansion (Wells) brought only heartache. Jump forward to 1980, when Ted Nelson starts the project and the company Xanadu, a system for the network sale of documents with automatic royalty on every byte ” hypertext and copyright protection (and royalty) on every byte. Nelson is a genius, but his Xanadu never (yet) was successful (more heartache). His "hypertext" concept is what we now see on the World Wide Web. See also Hypertext; Nelson, Ted; and World Wide Web.


X.400 Application Program Interface Association. Microsoft has a new concept. It's called an Enterprise Messaging Server (EMS). The idea is to allow users to transparently access the messaging engine from within desktop applications to route messages, share files, or retrieve reference data. According to Microsoft, corporate developers will be able to add capabilities using Visual Basic and access EMS by writing either to the X.400 Application Program Interface Association's (XAPIA's) Common Mail Calls (CMC) or to Microsoft's Messaging API (MAPI). (ftp://nemo.ncsl.nist.gov/pub/olw/dssig/xapla/) See MAPI.


XAUI (pronounced "Zowie") is an interface employed by 10GbE (10 Gigabit Ethernet). "X" denotes the Roman numeral for 10, implying 10 Gbps, and "AUI" is derived from Ethernet Attachment Unit Interface. The XAUI is an interface extender for the XGMII (10 gigabit Media Independent Interface), a 74-signal wide interface comprising one 32- bit wide data path for the transmit direction and one for receive direction used to attach the Ethernet MAC (Media Access Control Layer) to the PHY (PHYsical Layer). The XAUI is a self-clocked serial bus evolved directly from the GbE 1000BASE-X PHY. The XAUI interface speed is 3.125Gbps (gigabits per second) and makes use of four serial lanes that operate in parallel over a WWDM optical fiber in order to achieve aggregate transmission speed of 10 Gbps. As is the case with 1000BASE-X, 10GbE employs the 8B10B transmission code in order to ensure signal integrity through the copper circuitry of PCBs (Printed Circuit Boards ). See also 10GbE.


Crossbar. The second generation of automatic voice switching systems, Xbar switches are voice circuit switches that can be characterized as electromagnetic in nature.

First installed in 1938, they quickly rendered obsolete electromechanical SxS (Step-by- Step) switches, and in turn were rendered obsolete by ECC (Electronic Common Control) systems. The first common control switches, Xbar systems also sometimes are known as Xreed (Crossreed). They rely on a common, centralized set of programmed logic to set up, maintain, and tear down conversation paths, and to provide various features to end users. An end user's request for dial tone is recognized by a "marker," which directs a "sender" to store the dialed digits. At that point, a "translator" is directed to route the call by reserving a path through a switching matrix. The switching matrix is a centrally -controlled network of electromagnetic switches which work with magnets and which interconnect horizontal and vertical paths to establish a path from port to port. Xbar switches served in various capacities , including COs (Central Offices), tandems, and PBXs. See also Xbar and ECC.


Cross connect.


See DD.


X Display Manager Control Protocol. Protocol used to communicate between X terminals and workstations running the UNIX operating system.


Ron Stadler of Panasonic dreamed this one up. It stands for eXtra Device Port. It's an analog RJ-11 equipped port on the back of a Panasonic digital telephone, which is driven by Panasonic's Digital Super Hybrid switch. The XDP is an extension line completely separate to your digital voice line. You can be speaking on the phone while receiving or sending a fax or while sending or receiving data. Or plug a cordless phone or answering machine into the XTP.


External Data Representation. Standard for machine-independent data structures developed by Sun Microsystems. Similar to BER.


A generic ” the letter x means generic ” term for Digital Subscriber Line equipments and services, including ADSL, HDSL, IDSL, SDSL and VDSL. xDSL technologies provide extremely high bandwidth over the twisted-pair that runs from your phone company's central office to your office or home. Some xDSL lines are symmetrical (the same bandwidth in both directions). Some xDSL lines (for example, ADSL) are asymmetrical ” different bandwidth (and thus speed) in both directions. Many xDSL loops are already installed. xDSL technology is being installed by local phones in order to provide faster access for their subscribers to the Internet and also (and most importantly) to counter the competition from the CATV industry with its cable modem. See ADSL, G.990, G-Lite, HDSL, IDSL, RADSL, SDSL, Splitter, Splitterless and VDSL for more detailed explanations .


xDSL Element Manager System


Microsoft name for a 16-bit microcomputer operating system derived from AT&T Bell Labs' UNIX. See also Linux.

Xerographic Recording

Recording by action of a light spot on an electrically charged photoconductive insulating surface where the latent image is developed with a resinous powder.

Xerox Network Services

XNS. An old multilayer protocol system developed by Xerox and adopted, at least in part, by Novell and other vendors. XNS is one of the many distributed- file-system protocols that allow network stations to use other computers files and peripherals as if they were local. XNS is used by some companies on Ethernet LANs. In local area networking technology, special communications protocol used between networks. XNS/ITP functions at the 3rd and 4th layer of the Open Systems Interconnection (OSI) model. Similar to transmission control protocol/internet protocol (TCP/IP), which, of course, is now much more popular, since it is the foundation of the Internet. See Internet.

Xerox Network Systems

XNS. A peer-to-peer communications standard, developed by Xerox and designed for Ethernets.

Xerox Subsidy

Euphemism for swiping free photocopies from one's workplace.


X/Open Federated Naming. Used in Sun's Enterprise Server. Provides enterprise directory name Service for simplified access to and federation among multiple naming services. Works with Distributed Computing Environment (DCE), ONC, and Internet Domain Name Service (DNS).


Part of Sun Microsystems' ENOS Networking Solutions. It's a secure repository of network information.




eXtended Graphics Array. A new IBM level of video graphics which has a screen resolution of 1,024 dots horizontally by 768 vertically, yielding 786,432 possible bits of information on one screen, more than two and a half times what is possible with VGA. See also Monitor.


Part of Sun Microsystems' Imaging and Graphics Solutions. Solaris' graphics library for 2-D/3-D applications development.


XHTML 1.0 is a reformulation of HTML 4.0 as an XML 1.0 application. The hybrid language allows users to migrate from HTML to XML as users can create documents in HTML but also mix in XML functions. For example, complex documents can be created in HTML but can incorporate things like mathematical equations and music from XML. See XML.

XID Frame

A High-Level Data Link Control (HDLC) transmission frame used to transfer operational parameters between two or more stations.


Part of Sun Microsystems' Imaging and Graphics Solutions, XIL is an open Imaging API used for image enhancement, scaling, compression, color conversion, and display.


eXecute-In-Place. Refers to specification for directly executing code from a PCMCIA Card without first having to load it into system memory.


Cross-Industry Working Team. About 28 companies (and growing) whose work centers on the Clinton-Gore administration's document "National Information Infrastructure ” Agenda for Action." The document spells out policy initiatives required to achieve the benefits of widespread, convenient and affordable access to existing and future information resources. In short, the Information SuperHighway.


A registered trademark of modem manufacturer, MegaHertz, for one of the most innovative ideas in PCMCIA modems. XJACK is the world's tiniest female RJ-11. It's about a quarter of an inch wide and the most convenient way of attaching your laptop's modem to the phone network.


xDSL Line Card, in Line Concentrating Module.


X.25/X.75/X.75' Link Interface Unit.


eXtensible Linking Language. A XML linking language which provides for multidirectional linking of documents written in XML. Unlike HTML-based sites, XML sites with XLL support are capable of linkage at the object level, rather than at the page level, only. See XML.

XLR Connector

"X"-Series "L"ockheed "R"ubber. It's a three pin connector used in all sorts of applications, including video conferencing. It was originally used by the Lockheed Corp. on fighter plane's dashboards. It's now also known as a "QG" connector or Quick Ground, because the grounding pin is longer than the other two pins making the connection grounded first.


XM Radio is a company which launched two satellites (one called Rock, the other called Roll) in the S-band. These satellites transmit music and talk (some commercial-free) to monthly paying subscribers on earth sitting at home or traveling in cars and equipped with the appropriate satellite antenna. The service is known as Satellite Digital Audio Receiver Services (SDARS). XM's competitor is called Sirius.


eXtended Memory specificAtion. Interface that lets DOS programs cooperatively use extended memory in 80286 and higher computers. One such driver is Microsoft's HIMEM.SYS, which manages extended memory and HMA (high memory area), a 64k block just above 1Mb.


XGMII is a 74-signal wide interface comprising one 32-bit wide data path for the transmit direction and one for receive direction used to attach the 10GbE (10 Gigabit Ethernet) MAC (Media Access Control Layer) to the PHY (PHYsical Layer). The "X" is from the Roman numeral for 10, implying 10 Mbps, and the "GMII" stands for Gigabit Media Independent Interface. See 10GbE for more detail.




eXtensible Markup Language. XML, the language is a standard way of tagging data so it can be read and interpreted by a variety of Web browsers, by a variety of software, servers, and clients, regardless of how it was created. The vast bulk of the largest companies in the world use XML for electronic transactions with their customers or their suppliers, including using XML for EDI (Electronic Data Interchange). XML allows companies to automatically order from and sell to each other ” without having to have a human in between physically translating between the different systems, or worse , physically entering the information again into another incompatible computer system. Developed by the W3C (World Wide Web Consortium), XML is a pared-down version of SGML, designed especially for Web documents. It enables designers to create their own customized tags to provide functionality not available with HTML. For example, XML supports links that point to multiple documents, as opposed to HTML links, which can reference just one destination each. Whether XML eventually supplants HTML as the standard web formatting specification depends a lot on whether it is supported by future Web browsers. The World Wide Web Consortium released the first spec on XML in February, 1998. According to Inter@ctive Week, a weekly newspaper covering the Internet, companies are seizing upon the ability of XML to allow structured exchanges of data between machines attached to the Web. That will enable one Web server to talk to another Web server, meaning manufacturers and merchants can begin to quickly swap data, such as pricing, stock-keeping numbers, transaction terms and product descriptions. See also HTML, OFX, SGML and the three definitions following.

XML Attributes

XML attributes are used to provide more information, often meta- information, about an XML element. They are made up of the name of the attribute, an equals (=) sign, and the value of the attribute, where the value is surrounded by quotes. See XML.

XML Elements

XML elements are the basic building blocks of an XML document or data stream. There are three basic elements in the XML protocol: message, presence and iq. See XML.

XML Namespace

An XML namespace provides a simple method for qualifying element and attribute names used in XML documents by associating them with namespaces identified by a URI reference. The XML Namespace specification is currently a recommendation with the W3C. See XML.

XML Schema

XML Schema is a language for writing rules that constrain the kinds of elements that can appear in in documents and the ways in which they can be sequenced , grouped and nested. XML Schema is still a relatively new specification. The W3C Recommendation for XML Schema was published in May, 2001. See XML.


Extensible Markup Language (XML), a Web markup language, combines with scripting languages and electronic data interexchange to form XML/EDI. XML/EDI may be the means to bridge EDI into Internet Electronic Commerce (EC) by making the existing EDI (Electronic Data Interchange) knowledge base more readily usable to the Internet EC developers. See HTML.


Also called "Christiansen Protocol". An error-correcting file transfer, data transmission protocol created by Ward Christiansen of Chicago for transmitting files between PCs. A file might be anything ” a letter, an article, a sales call report, a Lotus 1-2-3 spreadsheet. The XModem protocol sends information in 128-byte blocks of data. Error control is provided through checksums, which are performed on each data block, and transmitted along with it. If the result does not check out at the other end, the computer at the other end sends a NAK (Negative AcKnowledgement), requesting that the transmitting device re-transmit that block. If the block checks out, the computer sends an ACK (Acknowledgement). In this way, relatively error-free transmission can be accomplished.

XModem was first used by computer hobbyists and then by business users of PCs. If you're buying a telecommunications software program for your PC ” IBM, Compaq, Apple, etc. ” it's a good idea to buy a program with XModem. It's among the most common data communications protocols. But it's not the fastest , just the most common. AT&T Mail supports XModem protocol. So does TELECONNECT Magazine's own E-mail InfoBoard system. MCI Mail does not support XModem protocol. We don't know why. There are many variations of XModem including XModem 1K (which uses blocks of 1,025 bytes), Modem7, YModem, Y-Modem-G, and ZModem. Most common communications software packages only support (i.e. will handle) the original version of XModem (checksum) and the newer CRC variation. A study in Byte Magazine (March, 1989) showed ZModem to be a far more efficient file transfer protocol than XModem, YModem, or W/XModem. The author now tends to use ZModem more commonly. It is supported by most on-line services, such as CompuServe, etc. See also Data Compression Protocols, Error Controls Protocols, File Transfer Protocol, YModem and ZModem.


Xmodem-1K is an error-correcting file transfer, data transmission protocol for transmitting files between PCs. It is essentially Xmodem CRC with 1K (1024 byte) packets. On some systems and bulletin boards it may also be referred to as Ymodem.


Cyclic Redundancy Checking is added to XMODEM frames for increased reliability of errors detection. See XMODEM.


X/Open Management Protocol; an API and software interface specified in the Open Software Foundation's Distributed Management Environment.


eXtensible Messaging and Presence Protocol. XMPP is the part of the XML Protocol that supports Instant Messaging functionality.


An acronym for eXtended Memory Specification. To run this standard, your system must have 350K of extended memory. XMS creates the HMA (High Memory Area), then governs access to and the allocation of the remainder of extended memory.


xDSL Line Card in AccessNode Express.


Xerox Network System. The LAN architecture developed at the Xerox Palo Alto Research Center (Parc). It is a five-layer architecture of protocols and was the foundation of the OSI seven-layer model. It has been adopted in part by Novell and other vendors. XNS is one of the many distributed file system protocols that allow network stations to use other computers files and peripherals as if they were local. XNS is used by some companies on Ethernet LANs.


Crystal Oscillator.


XON/XOFF are standard ASCII control characters used to tell an intelligent device to stop or resume transmitting data. In most systems typing <Ctrl>-S sends the XOFF character, i.e. to stop transmitting. Some devices understand <Ctrl>-Q as XON, i.e. start transmitting again. Others interpret the pressing of any key after <Ctrl>-S as XON.


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. X/Open is the correct spelling.

XOR Gate

A digital device which outputs a high state only if either of its inputs are high (but not both).


An eXternal Packet Assembler/Disassembler.


A set of protocols developed by British Telecom Tymnet to allow asynchronous terminals to connect to an X.25 packet-switched network.


  1. Cross-phase Modulation.

  2. eXtended Peripheral Module.


A Private Line provided by a long distance carrier in the U.S.


Transmit Reference Burst.


Tool that lets you grab the mouse and keyboard of another Windows based machine and control them with your local mouse and keyboard. It creates a form of remote control of other machines. All mouse- and keyboard-actions on the local machine are forwarded to the remote display.


X.25 Service Group.


eXtensible Stylesheet Language. An extension of XML which allows the automatic transformation of XML-based data into HTML and other presentation formats. Thereby, the presentation format can vary from the underlying data structure. See XML.


Abbreviation for crosstalk.


The Sun Solaris Teleservices Application Programming Interface. See Teleservices for a long explanation.


The Sun Solaris Teleservices protocol. The XTELS protocol is essentially the XTEL provider protocol with extensions to support multiple clients and multiple XTEL providers. See Teleservices for a long explanation.


A window-based tool used to configure providers in the Sun Solaris Teleservices platform. Basically a graphical user interface. See TELESERVICES for a long explanation.


Sun Microsystems' "Teleservices" architecture for Solaris. XTL is the foundation library for applications using or controlling telecom data streams. It is used for computer telephony application development. The XTL subsystem and API includes call control functions. It establishes a call or connection, and data stream access methods to control the flow of data over that connection.


Part of the Sun XTL Teleservices architecture. An object-oriented interface accessed using the C++ language. The API includes XtlProvider, XtlCall, XtlCallState, and XtlMonitor objects. These base objects define the command and callback methods of XTL teleservices. There are three methods types on these objects: synchronous requests , asynchronous commands, and asynchronous events.

XTL Call Object

Part of Sun Microsystems' XTL Teleservices architecture. C++ objects created by developers with the XTL API. Each XtlCall object corresponds to a telephone call. An XtlCall object has command methods to query the current state of a call or request a change in state (to put a call on hold). An XtlCall object also has callback methods for the asynchronous notification of state changes.

XTL Provider Configuration Database

Part of the Sun XTL Teleservices architecture. Means of registering third-party "providers" in an XTL-based system. The database shows the existence of each provider, how to invoke it (used by the XTL system internally) and describes the specific capabilities of that provider.

XTL Provider Interface

Part of Sun Microsystems' XTL Teleservices architecture. The means for third-party developers and Independent Hardware Vendors (IHVs) to integrate provider, technology or call- type-specific features into an XTL environment. Developers use the XTL Provider Library (MPI) to do this.

XTL Provider Library

Part of Sun Microsystems' XTL Teleservices architecture. Isolates the provider code from the intricacies of the XTL system services. The library supplies interfaces to the provider database, the server messaging system for commands and callbacks, and the means to make device data streams accessible to applications.

XTL System Services

Part of Sun Microsystems' XTL Teleservices architecture. Acts as the intermediary between the application view of a call object and the provider's implementation of the call. The XTL server, along with the application and provider libraries, handles the interprocess message passing, object identification and creation, call ownership, security, and asynchronous event notification. The XTL subsystem also manages a database of available providers and helps manage data stream routing and access.


A Sun Microsystems term for a GUI tool supplied to browse and edit the provider configuration database in an XTL environment.


The last four digits of a telephone number are called the "line number." A ten- digit telephone number in the U.S., for example, follows the format NXX-NXX-XXXX, where N must be a number other than "0" or "1," and X can be any number.


See X-Windows.

Newton[ap]s Telecom Dictionary
Newton[ap]s Telecom Dictionary
ISBN: 979387345
Year: 2004
Pages: 133

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