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XX, x 1. symb. an unknown, situational, derivable, orarbitrary quantity. Commonly used in mathematicsand software programming, written both lower- anduppercase, and often italicized as in 4 +x= 10. Thesymbol is also often used in product identificationto indicate a family of products, e.g., xDSL for thefamily of Digital Subscriber Line services. 2. abbrev.cross. 3. abbrev. exchange. 4. abbrev. external.5. abbrev. trans- (prefix). 5. The USOC Federal Com-munications Commission (FCC) code for complexmultiline or series jack. 6. colloq. See X WindowSystem.

X axis, x axis A common convention for coordinatesystems (e.g., Cartesian coordinates) is to designatethe horizontal axis as the X axis. When graphing pro-cesses that may occur over time, the X axis is oftenused for the time variable.

X Consortium A group that continued the develop-ment and management of the X Window System,now part of the Open Group. See X Window Sys-tem.X cut A type of cut used with piezoelectric crystals.Crystals are used in radio wave detection and tim-ing applications, and their piezoelectric properties arepartly determined by their shape and size. An X cutcreates a crystal plate with the plane perpendicularto the crystals X axis. See crystal, detector, quartz,piezoelectric, X-ray goniometer, Y cut, Y bar.

X Files A highly popular evening UFO-paranormal-

FBI-spoof television drama series starring GillianAnderson and David Duchovny. Its mentioned herebecause some of the episodes featured interestingtelecommunications technologies and because someof the more imaginative episodes hinted at futuretechnologies that have yet to be devised. Science fic-tion themes have often been the forerunners of vi-able technologies that are invented years or decadeslater. The show was popular with techie gurus.

X Protocol A low-level client/server standard com-munications protocol that handles window manipu-lation routines for the graphical user interface (GUI)

X Window Systems. See X Window System.X Series Recommendations A set of ITU-T recom-mendations providing guidelines for interconnectingnetworks and network devices. These are availableas publications from the ITU-T for purchase and a

few may be downloadable from the Net. This is alarge category; some of the X Series Recommenda-tions of particular interest are listed in the followingcharts, organized into three categories. See Appen-dix C for a general list of the different categories.See also individual listings under G, I, Q, and V Se-ries Recommendations.

X Window System, The X Window System, XWindows, X, X.11 Hardware-independent founda-tion software for the development of graphical userinterfaces (GUIs) based upon a client/server model.The X Window System is a nonproprietary, distrib-uted, multitasking, network-transparent protocol thathas been implemented on many different Unix-basedsystems. Originally used as a graphics display pro-tocol for text-based UNIX platforms, developers arerecognizing and exploiting its ability to enable popu-

lar OSs to run on a UNIX workstation or, conversely,to run UNIX applications on popular hardware plat-forms, and to run applications from within Webbrowsers. Development tools such as Motif facili-tate the quick design of X GUIs.

The X Protocol is an X Windows System client/server protocol and the X server is a client/serverprocess that controls a display device on The X Win-dow System.

X Windows code for noncommercial purposes isfreely downloadable from The Open Group Web site.As of version X11R6.4, commercial users must pay

a license fee, which will be used by The Open Groupto continue support for these development efforts.See Athena project. http://www.opengroup.org/

X Window System 11 Release 6.x (X11R6.x) A sub-stantial initiative by The Open Group to enable theX Window System to be used to create and accessinteractive World Wide Web applications through theX Window System and a downloadable plugin. Ap-plications linked to the Web using X11R6.x canbe found, accessed, and executed with the same Webbrowsing utilities used to access current static HTMLdocuments. This may become a very significant

means of networking through the Internet. See XWindow System.

X Window System history X was originally devel-oped by Robert Scheifler and Ron Newman from theMassachusetts Institute of Technology (MIT) and

2002 by CRC Press LLC
http://www.opengroup.org/http://www.opengroup.org/

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ITU-T X Series Recommendations

Prevalent Formats

X.25 Definitions of the procedures forexchanging data between user devices(DTEs) and network nodes in a publicswitched packet data network (PSPDN)

in order to provide a common interfaceacross a variety of systems. X.25 is alayered packet transmissions protocolcommonly used in wide area networks(WANs). A version of X.25 specificallydesigned for packet radio has beendeveloped as AX.25.

X.400 An international ISO/ITU-T series ofstandards for electronic messagingarchitecture for the exchange of databetween computer systems. X.400 was

published by the ITU-T in 1984. Thestandard was jointly rewritten by ISOand ITU-T in 1988.

X.400 does not stipulate the formattingof data. It provides guidelines forinternetworking various messagingsystems, addressing individualmessages, and describing messagecontents. Within X.400 there are alsosubstandards and recommendations toX.400, some of which are: X.402describes the overall architecture; X.420

describes email transfer; X.435 definesthe electronic movement of ElectronicData Interchange (EDI); and X.440describes voice messaging. SeeElectronic Data Interchange.

X.445 Asynchronous Protocol Specification(APS). A commercially promotedmultiple media client/server extension ofthe X.400 standard that facilitates theexchange of digital data over publicphone networks rather than X.25standard leased lines.

X.500 A directory service protocol for buildingdistributed global directories. It wasdeveloped in response to a need todesign directories that would notexperience the same problems andbottlenecks that were developing withmany of the large databases beingaccessed by thousands or millions ofusers on the Internet. X.500 employsdecentralized maintenance, searchingcapabilities for complex queries,

homogenous global namespace, and astructured standards-based informationframework.

Recommendations of Particular Interest

X.1 International user classes of service in,and categories of access to, public datanetworks and integrated services digitalnetworks (ISDNs).

Includes information on access to leased

or switched circuits by data terminalequipment (DTEs) in various modes,access by facsimile terminals, and accessto Frame Relay systems.

X.6 Multicast service definition.

Service definitions and capabilities of amulticast service providing a commonmodel for the description of serviceelements. Interface specifications andprotocol elements are not specified byX.6.

X.31 Support of packet mode terminalequipment by an ISDN.

Service and signaling proceduresdefinitions operated at the S/T-referencepoint of an ISDN for subscribing packetmode terminal equipment and terminaladapter functionalities to supportexisting X.25 terminals at the R-reference point of the ISDN.

X.75 Packet-switched signaling systembetween public networks providing datatransmission services.

A description of packet-switchingsignaling systems among public datanetworks.

X.76 Network-to-network interface betweenpublic data networks providing theFrame Relay data transmission service.

A description of interfaceinterconnections between Frame Relaynetworks and public data networks.Layer, data transfer, and signalinginformation are provided.

X.77 Internetworking between PSPDNs viaB-ISDN.

Definitions of procedures forinternetworking that include referenceconfigurations, protocol stacks, andsignaling procedures.

X.121 International number plan for publicdata networks.

A description of the design,characteristics, and applications of thenumbering plan for public data

networks. The International NumberPlan was developed to facilitate thelinking of public data networks with the


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worldwide system. It describes countryidentification (assigned by the ITU-T),regional/local network identification,and a mechanism for integrating withother numbering plans. Guidance forefficient allocation of numbers is

included, along with eligibility criteriaand procedures. See Data NetworkIdentification Codes.

X.122 Numbering plan interworking for E.164and X.121 number plans.

Information on interworking with othernumbering plans.

Brief Listing of Further X Series Examples

X.2 International data transmission services

and optional user facilities in public datanetworks and ISDNs. X.3 defines a setof parameters used for regulating basicfunctions such as terminalcharacteristics, flow control, and dataforwarding.

X.3 Packet assembly/disassembly facility(PAD) in a public data network.

X.4 General structure of signals ofinternational alphabet No.5 code forcharacter-oriented data transmissionover public data networks.

X.5 Facsimile Packet Assembly/Disassemblyfacility (FPAD) in a public data network.

X.7 Technical characteristics of datatransmission services.

X.8 Multiaspect pad (MAP) framework andservice definition.

X.10 Categories of access for data terminalequipment (DTE) to public datatransmission services.

X.20 Interface between data terminal

equipment (DTE) and data circuit-terminating equipment (DCE) for start-stop transmission services on public datanetworks.

X.20 bis Use on public data networks of dataterminal equipment (DTE) designed forinterfacing to asynchronous duplex VSeries modems.

X.21 Interface between data terminalequipment and data circuit-terminatingequipment for synchronous operation onpublic data networks. A digital signalinginterface that includes specifications forphysical interface elements, characteralignment, and data transfer.

X.21 bis Use on public data networks of dataterminal equipment (DTE) designed forinterfacing to synchronous V-seriesmodems.

X.22 Multiplex DTE/DCE interface for userclasses 3-6.

X.24 List of definitions for interchangecircuits between data terminalequipment (DTE) and data circuit-terminating equipment (DCE) on publicdata networks.

X.25 Interface between Data TerminalEquipment (DTE) and Data Circuit-terminating Equipment (DCE) forterminals operating in the packet modeand connected to public data networksby a dedicated circuit.

X.28 DTE/DCE interface for a start-stopmode data terminal equipment accessingthe packet assembly/disassembly facility(PAD) in a public data network situatedin the same country.

X.29 Procedures for the exchange of controlinformation and user data between apacket assembly/disassembly (PAD)facility and a packet mode DTE oranother PAD.

X.30 Support of X.21-, X.21 bis-, and X.20

bis-based data terminal equipment(DTEs) by an integrated services digitalnetwork (ISDN).

X.32 Interface between Data TerminalEquipment (DTE) and Data Circuit-terminating Equipment (DCE) forterminals operating in the packet modeand accessing a packet-switched publicdata network through a public network.

X.33 Access to packet-switched datatransmission services via Frame

Relaying data transmission services.X.34 Access to packet-switched data

transmission services via B-ISDN.

X.35 Interface between a PSPDN and aprivate PSDN based upon X.25procedures and enhancements to define agateway function provided in thePSPDN.

X.36 Interface between data terminalequipment (DTE) and data circuit-terminating equipment (DCE) for public

data networks providing Frame Relaydata transmission service by a dedicatedcircuit.


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X Series Recommendations, cont.

Brief Listing of Further Examples, cont.

X.37 Encapsulation in X.25 packets ofvarious protocols including FrameRelay.

X.38 G3 facsimile equipment/DCEinterface for G3 facsimile equipmentaccessing the Facsimile PacketAssembly/Disassembly facility(FPAD) in a public data networksituated in the same country.

X.39 Procedures for the exchange ofcontrol information and user databetween a Facsimile PacketAssembly/Disassembly (FPAD)facility and a packet mode DataTerminal Equipment (DTE) or anotherFPAD.

X.42 Procedures and methods for accessinga Public Data Network from a DTEoperating under control of ageneralized polling protocol.

X.45 Interface between Data TerminalEquipment (DTE) and Data Circuit-terminating Equipment (DCE) forterminals operating in the packetmode and connected to public datanetworks, designed for efficiency at

higher speeds.

X.48 Procedures for the provision of a basicmulticast service for Data TerminalEquipment (DTEs) using 25.

X.49 Procedures for the provision of anextended multicast service for DataTerminal Equipment (DTEs) using 25.

X.50 Fundamental parameters of amultiplexing scheme for theinternational interface betweensynchronous data networks.

X.50 bis Fundamental parameters of a 48-kbpsuser data signaling rate transmissionscheme for the international interfacebetween synchronous data networks.

X.51 Fundamental parameters of amultiplexing scheme for theinternational interface betweensynchronous data networks using a10-bit envelope structure.

X.51 bis Fundamental parameters of a 48-kbpsuser data signaling rate transmission

scheme for the international interfacebetween synchronous data networksusing a 10-bit envelope structure.

X.52 Method of encoding anisochronoussignals into a synchronous user bearer.

X.53 Numbering of channels oninternational multiplex links at 64

kbps.

X.54 Allocation of channels oninternational multiplex links at 64kbps.

X.55 Interface between synchronous datanetworks using a 6 + 2 envelopestructure and single channel percarrier (SCPC) satellite channels.

X.56 Interface between synchronous datanetworks using an 8 + 2 envelopestructure and single channel percarrier (SCPC) satellite channels.

X.57 Method of transmitting a single lower-speed data channel on a 64 kbps datastream.

X.58 Fundamental parameters of amultiplexing scheme for theinternational interface betweensynchronous nonswitched datanetworks using no envelope structure.

X.60 Common channel signaling for circuit

switched data applications.X.61 Signaling System No. 7 data user

part.

X.70 Terminal and transit control signalingsystem for start-stop services oninternational circuits betweenanisochronous data networks.

X.71 Decentralized terminal and transitcontrol signaling system oninternational circuits betweensynchronous data networks.

X.80 Interworking of interexchangesignaling systems for circuit-switcheddata services.

X.81 Interworking between an ISDNcircuit-switched and a circuit-switched public data network(CSPDN).

X.82 Detailed arrangements forinterworking between CSPDNs andPSPDNs based on RecommendationT.70.

X.92 Hypothetical reference connectionsfor public synchronous data networks.


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Brief Listing of Further Examples, cont.

X.96 Call progress signals in public datanetworks.

X.110 International routing principles androuting plan for public data networks.

X.115 Definition of address translationcapability in public data networks.

X.116 Address translation registration andresolution protocol.

X.123 Mapping between escape codes andTOA/NPI for E.164/X.121 numberingplan interworking during thetransition period.

X.130 Call processing delays in public datanetworks when providing

international synchronous circuit-switched data services.

X.131 Call blocking in public data networkswhen providing internationalsynchronous circuit-switched dataservices.

X.134 Portion boundaries and packet-layerreference events: Basis for definingpacket-switched performanceparameters.

X.135 Speed of service (delay and

throughput) performance values forpublic data networks when providinginternational packet-switchedservices.

X.136 Accuracy and dependabilityperformance values for public datanetworks when providinginternational packet-switchedservices.

X.137 Availability performance values forpublic data networks when providing

international packet-switchedservices.

X.138 Measurement of performance valuesfor public data networks whenproviding international packet-switched services.

X.139 Echo, drop, generator, and test DTEsfor measurement of performancevalues in public data networks whenproviding international packet-switched services.

X.140 General quality of service parametersfor communication via public datanetworks.

X.141 General principles for the detectionand correction of errors in public datanetworks.

X.144 User information transfer performanceparameters for data networksproviding international Frame RelayPVC service.

X.145 Performance for data networksproviding international Frame RelaySVC service.

X.150 Principles of maintenance testing for

public data networks using dataterminal equipment (DTE) and datacircuit-terminating equipment (DCE)test loops.

X.160 Architecture for customer networkmanagement service for public datanetworks.

X.161 Definition of customer networkmanagement services for public datanetworks.

X.162 Definition of managementinformation for customer networkmanagement service for public datanetworks to be used with the CNMcinterface.

X.163 Definition of managementinformation for customer networkmanagement service for public datanetworks to be used with the CNMeinterface.

X.180 Administrative arrangements for

international closed user groups(CUGs).

X.181 Administrative arrangements for theprovision of international permanentvirtual circuits (PVCs).


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Jim Gettys of Digital Equipment Corporation (DEC)to provide a user interface for the Athena Project. Ithas been further developed by The X Consortium andis now trademarked and managed by The OpenGroup. See Athena project, X Window System.X Window System User Interface Toolkit XUI. Asuite of utilities that facilitates the implementationof user interfaces for hardware platforms installedwith the X Window System. See X Protocol, X Win-dow System.X9 standards An important set of standards definedby the American National Standards Institute (ANSI)for the financial industry. The X9 standards coversuch aspects as personal identification number (PIN)management, electronic transfer of funds, transactionprocessing, security mechanisms. A brief summaryof some of the X9 security-related standards includes

Title Description

X9.9 U.S. wholesale banking standard fortransaction authentication.

X9.17 Financial Institution KeyManagement standard for wholesaletransactions. Defines protocols forthe transfer of encryption keys usingsymmetric techniques.

X9.30 U.S. standards for digital signaturesbased upon the Digital SignatureAlgorithm (DSA) using the SHA-1hash algorithm.

X9.31 U.S. standards for digital signaturesbased upon the RSA algorithmbased upon the MDC-2 hashalgorithm.

X9.57 Certificate management encryptionschemes.

X9.42 A draft standard for key agreementbased upon the Diffie-Hellmanalgorithm.

X9.44 A draft standard for key transportbased upon the RSA algorithm.

See RSA Security Inc.

X.25A widely implemented, significant, connection-oriented, packet-based communications protocolused in local and wide area networks. The protocolwas developed in the mid-1970s, when analog net-working over noisy copper connections was opti-mized for voice rather than data communications. Ithelped to fulfill a growing need for a common lan-guage to interconnect local area networks that oftenused proprietary network protocols and for error

mechanisms that could overcome the problems as-sociated with marginal connections. In 1976, theCCITT (now the ITU) recommended X.25 for inter-national data exchange. It was approved and subse-quently revised every 4 years or so.

In general, X.25 is a three-layer model that includesa physical level, a link level, and a packet level thatare, in turn, associated with the lower physical, data,and network layers of the hierarchical Open SystemsInterconnect (OSI) model.

Basic Overview of X.25 Levels

Name Level # Description

Physical 1 The electromechanical, procedural,and functional interfaces betweenthe data terminal equipment (DTE)and the data circuit-terminatingequipment (DCE). This level isspecified by ITU-T X and V SeriesRecommendations that apply tomodems and interchange circuits(e.g., X.21).

Link 2 Also called the frame level, the linklevel ensures reliable data transferbetween the DTE and the DCE in anefficient, timely manner. Itsynchronizes transmitter/receiverinteractions, and detects and handleserrors. A number of link accessprotocols may be implemented at thelink level.

Packet 3 Also called the network level, thepacket level creates the network datachunks with appropriate control/

error information and user datapayload. Various virtual circuit (VC)and datagram services are handled atthis level.

Data is formatted into X-25 data packets by pack-etizing the data into smaller chunks and adding theappropriate protocol information into the header ofeach packet. Error mechanisms are also included toenable retransmission if packets are lost, to providea mechanism for reliable transfer.

X-10 Protocol A protocol for sending radio frequen-cies (RF) signals over power line carriers (PLCs).In other words, a radio frequency-emitting device canbe plugged into a wall socket and the protocol willenable device control signals to be sent through thewiring (over the alternating current power curve) tocompatible devices set to the same settings as thecontrolling devices. Lately, wireless X-10-compat-ible transmitters are being added to commercial cata-logs, with transmission ranges of up to about 100feet.

The X-10 Protocol and compatible components were

introduced in the late 1970s and since the 1990s,when electronic devices significantly dropped inprice, have become very popular for configuringhome and small office automation systems.

The basic X-10 categories of components are


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transmitters, receivers, and controllers (which aretransmitters with extra features). The controllers canbe AC-outlet devices that intercept and relay signalsor can be infrared or radio signal-based remote con-trols. The transmitters are a variety of motion detec-tors and audio or video transmitters, and the receiv-ers are responding devices such as outlet controllers(for turning on lamps, appliances, or VCRs) orchimes. Together the various types of componentscan be interfaced through wireless signals and build-ing wiring to control lamps, motion detectors,chimes, video cameras, and infrared devices throughstandalone controllers or computer software seriallinks.X-10 systems are relatively easy to install, althoughit can take some time to work out code correspon-dences and individual unit setup parameters if manycomponents are used. It is also important to makesure batteries are fresh and components are turnedon (some have on/off switches) if they are being usedin a security system. In fact, with X-10 systems, re-

dundancy is sometimes a good idea. There are com-puter interfaces and utility software packages forcontrolling X-10 systems and these offer much morepower and flexibility than noncomputer systems,including testing, scheduling, macro scripts, andmuch more.The trend in X-10 components has been to becomesmaller and more streamlined (e.g., socket rocketlight bulb controllers are now smaller than earlierdesigns). There are also new varieties of small, color,wireless transmitting video cameras that can be in-stalled to cycle through in series with a remote con-

trol especially designed for this purpose.This is an important protocol in terms of its poten-tial for data communications in smart houses, thatis, houses with a high degree of automation built in.There is the potential to design X-10-controllablemotors that could be used to pull curtains, lowerviewing screens, tip the dog food box over the bowl,lower the boat onto the floor in the garage, spin themobile over the babys crib at varying speeds, andmuch more. As far as commercial products go, weveonly seen the tip of the iceberg so far. See CEBus,LONWORKS.

X-10 Power Line Analyzer A commercial tool forhelping to install and troubleshoot X-10 systems. Itis a handheld device that plugs into a standard 110AC outlet or can be connected with jumper leads. Thetwo-line text LCD display indicates status. Its sim-pler to use and more appropriate than an oscilloscopefor most X-10 needs. It enables signal quality andinterference to be assessed, both of which may causeproblems with the reliable functioning of X-10 com-ponents. See X-10 Protocol.X-bandAn assigned spectrum in the microwave fre-quencies of approximately 8 to 12 GHz with wave-lengths of just a few centimeters. X-band signals arenot significantly hindered by the Earths atmosphereunless a lot of moisture (e.g., rain) is present. X-bandfrequencies are used by military satellites and deep-space vehicles. See band allocations for a chart ofassigned frequencies.

X-Bone A system designed to facilitate and automatethe rapid deployment and management of multipleoverlay networks. X-Bone is an overlay technologycombined with teleconferencing-style coordinationand management tools. X-Bone provides a virtualnetworking infrastructure that is configurable. WhileX-Bone is intended to be implemented with networksrunning more advanced systems, such as IPv6, someof the automatic tunneling services can be deployed

to a limited extent on IPv4 systems. See 6bone,Mbone, overlay network, X-Bone xd.X-Bone xd An X-Bone directory tool for performinga number of tasks including the coordination of re-source sharing at the local site, the support of localdaemons through authentication, configuration, andcreation of IP-encapsulation tunnels between dae-mons, and the provision of a user interface and APIfor users or programs wishing to manually param-eterize and override overlays. See X-Bone.X-dimension of recorded spot In facsimile trans-missions, a means of describing variation density in

terms of the minimum density. The largest center-to-center space between recorded spots is measuredin the direction of the recorded line. This can alsobe assessed perpendicular to the recorded line as theY-dimension of a recorded spot. The same principlescan be applied to assess the scanning spot.X-ray A radiant energy within the spectrum of highenergy, invisible, ionizing electromagnetic radiationthat ranges about 0.08 nm in wavelength, betweenultraviolet light and gamma rays. X-rays were some-what naively and irresponsibly used in early radiosignal experiments, and for extended imaging inside

objects and humans. These practices are now usedwith great care due to the damaging influence of X-rays on living cells. X-rays are used in many medi-cal, industrial, and fabrication applications. See X-ray goniometer; Roentgen, Wilhelm Konrad.X-ray goniometer An instrument for determiningthe position of the axes in a quartz crystal. X-raysare aimed at the atomic planes of the crystal and thereflected rays are evaluated. Since crystals are physi-cally manipulated to alter their oscillating propertiesand often cut in very thin slices, it is important toknow the orientation of the crystalline structure be-fore cutting. See quartz, X cut, Y cut.X-ray spectrometer An instrument that is used, bymeans of reflected rays and evaluation of the result-ing diffraction angles, to study the characteristics andcomposition of materials, including crystals. See X-ray goniometer.X/Open A global, independent organization of com-puter manufacturers founded in 1984. X/Open seeksto promote an open, multivendor Common Applica-tions Environment (CAE) to enhance applicationportability. This is a good concept, as it allows soft-ware developed by different vendors to run on a va-riety of platforms, leaving the choice of equipment

up to the individual purchaser. See Common Appli-cations Environment, Open Systems Interconnec-tion.

X/Open Federated Naming A naming mechanismfrom the X/Open group for developers to access


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network naming services and to provide integrationwith industry-accepted naming services such asX.500, Domain Name Service (DNS), DCE, and oth-ers.X/Open Portability Guide XPG. A guide docu-menting the X/Open common applications environ-ment system.

XA extended architecture.

XA-SMDS See Exchange Access SMDS.Xanadu, Project Xanadu A unified interconnecteddocument storage/access/retrieval/publishingenvironment. Designed over a period of decades,Nelson, the author, has entitled this a docuverse. TheXanadu scheme is a write-once/keep-always systemwith a hypertext-like file structure.XAPIA X.400 Application Program Interface Asso-ciation. See X Series Recommendations.

Xaw The Athena Widget set. A set of widgets dis-tributed with the X Window System, which beganas Project Athena. See Athena, X Window System.

Xbar abbrev. crossbar.XBase, XbaseA generic designation for applicationsthat read and/or write dBase-compatible files.XC abbrev. cross connect.

XCA extended communication adapter.

XcoralA multiwindow text editor for the X WindowSystem, that can be used in conjunction with amouse.

XCVR abbrev. transceiver.

xd See X-Bone xd.

XDF extended distance feature.XDL An object-oriented extension to the ITU-T-de-fined Specification and Design Language (SDL) fortelecommunications systems. See Specification andDesign Language.

XDMA Xing Distributed Media Architecture. Acommercial streaming media architecture for deliv-ery of live and on-demand audio-video from XingTechnology Corporation. It is built around standardssuch as TCP/IP and MPEG, and supports multicast-ing to multiple simultaneous users over local areanetworks (LANs) and wide area networks (WANs).XDMA can be implemented over ISDN networks forservices such as news and distance learning.

XDMCP X Display Manager Control Protocol. Aprotocol used to communicate between X terminalsand UNIX workstations.

XDR See External Data Representation.

xDSL Generic term for a variety of digital subscriberline technologies, which include ADSL, EDSL, andHDSL. See digital subscriber line and individual list-ings for further information.XENIX A Unix implementation best known as be-ing from the Santa Cruz Operation, Inc. (SCO), it wasoriginally codeveloped by International BusinessMachines (IBM) and Microsoft as XENIX-11 forIntel machines. Microsoft had licensed UNIX fromBell Laboratories and developed it into XENIX forDigital Equipments PDP-11, the predecessor to theVAX line. XENIX-11 was popular on the 80286/80386 line in the late 1980s. SCO is now marketing

UnixWare 7.

xerographic printer A printer that uses the samebasic electrostatic mechanisms and techniques as axerographic copier. The information is imaged ontoa drum with lasers, the printing medium is passedacross the drum and picks up the dry transfer toner,which then fuses the toner to the printing medium.See laser printer

Xerox Corporation One of the first companies tosee the commercial benefits of new photocopyingtechnology developed by Carlson in the early 1940s.When still a relatively new, small company, Xeroxtook a chance on the new photocopying inventionthat had been passed up by other companies. Xeroxis now known throughout the world for its technol-ogy, especially in the replication industry, and manypeople refer to all photocopies as xeroxes. Seephotocopy for further information and an illustrationof Carlsons patent.

Xerox Network Services XNS. A multilayer, dis-tributed file network architecture developed by theXerox Corporation which is somewhat similar toTCP/IP. Unlike many networks from other vendors,XNS permits a user to use files and devices from aremote machine as if they were on a local machine.XNS functions compatibly with the third and fourthlayers of the Open Systems Interconnection model(OSI).

Xerox PARC Xerox Corporations Palo Alto Re-search Center. This research center provided enor-mous impetus to early computer companies and soft-ware developers (e.g., Apple Computer Inc., Micro-soft Incorporated), especially those developing ob-

ject-oriented systems and graphical user interfaces(GUIs). PARC researchers invented mice and vari-ous laser printing technologies, developed Smalltalk;they generated one good idea after another through-out the 1970s and early 1980s, yet surprisingly fewof these were commercially implemented or mar-keted through Xerox. Charles Simonyi, one of theearly founding members of PARC, was the demon-strator of the Alto, a pioneer desktop computer thatinspired many of those fortunate enough to see it inthe early days. Later, Simonyi was hired by Micro-soft to move the company into graphical applications.

XFN See X/Open Federated Naming.XFR abbrev. transfer.

XFS X11 File System.

XGA 1. See extended graphics adapter. 2. See ex-tended graphics array.

XID exchange identification. In data networking,XIDs are request and response packets exchangedprior to communications between a router and a net-work host. XID is used for device discovery, addressconflict, resolution, and sniffing. The XID packetincludes the parameters of the serial device, and aconnection can only be negotiated if this configura-

tion is recognized by the host.XIP abbrev. execute in place. A means to accessmemory and execute code on PCMCIA cards with-out having to load them into system memory first.See PCMCIA.


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resume when it is convenient. This is a common wayof preventing a screenful of text information fromscrolling by so fast that it cannot be read in text win-dows that lack scroll bars for back-referencing.Xover abbrev. crossover.XPAD external packet assembler/disassembler.XPG See X/Open Portability Guide.XPM Extended Peripheral Module.

Xponder transponder.Xpress Transport Protocol XTP. A flexible, high-performance commercial multicast protocol to sup-port a wide variety of applications from wide areanetworks (WANs) to multimedia and realtime em-bedded systems. It was developed by an internationalgroup of representatives from academia, industry,and the government from 1987 to the early 1990sand was described by Strayer et al. in the early 1990s.It has been adopted as part of MIL-STD-2204 (Sur-vivable Adaptable Fiber Optic Embedded Network).XTP brings together the functionality of UDP, TCP,

and TP4, in addition to transport multicast, groupmanagement, Quality of Service (QoS) negotiationcapabilities, rate and burst control, and error and flowcontrol mechanisms. It is designed to operate overany network or datalink layer or may be implementeddirectly over an ATM AAL. It can run in parallel withother transport protocols and thus can increase func-tionality without giving up interoperability. It is ex-pressly intended to separate the communication para-digm from error control policies.The protocol sparked interest in the development ofa public domain version at Sandia National Labora-tories. This protocol is distinct from the lightweighteXpress Transfer Protocol that was originally devel-oped by Protocol Engines, Inc. See multicast, XTPForum.XRB transmit reference burst.xrefabbrev. cross reference.XRemote A serial transmission protocol for the XWindow System.XRF Extended Recovery Facility.XSG X.25 Service Group.XSI X/Open System Interface Specification.

XSMP X Session Manager Protocol.

XT 1. abbrev. crosstalk. 2. See IBM Personal Com-puter XT.

Xtal 1. abbrev. crystal.

Xtal Set SocietyA society founded in 1991 for build-ing and experimenting with radio electronics, par-ticularly crystal radio sets.

Xtalk abbrev. crosstalk.xterm A popular terminal emulator for the X Win-dow System has been ported to several other oper-ating systems. Xterm lets you have more than oneterminal window active at a time through a singlemodem, each with its own input/output process run-ning independently of the others. In other words, youcan be reading Internet newsgroups in one window,interacting in an Internet Relay Chat (IRC) channelin another, and downloading binary files in a thirdat the same time, which is very convenient.

XIWT See Cross-Industry Working Team.Xlib X Library. A program interface for the X Win-dows System.Xmission abbrev. transmission.Xmit abbrev. transmit.XML See Extensible Markup Language.XModem A widely used error-correcting file trans-fer data transmissions protocol developed by Ward

Christensen in the late 1970s. XModem utilizes 128-byte packets, so files of various lengths will be pad-ded to adjust the packet length and may be longerthan the original file. The filename is not sent withthe transmission. YModem, a successor to XModem,with support for longer data packets and file at-tributes, was developed by Chuck Forsberg.XModem is often used with computer modems totransfer files to and from bulletin board systems overtraditional phone lines. XModem is not fast, but ithas error correction and its reasonably reliable. Itswell supported, an important consideration sinceboth ends of the connection have to agree on a pro-tocol. Many service bureaus use XModem for filetransfers, however, check if they have ZModem,which is faster and capable of restarting an inter-rupted transmission from where it left off. See Ker-mit, YModem, ZModem.XModem-1K, XModem-K A variant of XModemthat manages data in 1K (1024-byte) packets. SeeXModem.XModem-CRC XModem with 16-bit cyclic redun-dancy checking (CRC) error detection mechanisms,instead of checksum. XModem-CRC can commu-nicate with XModem versions that use checksum for

error correction.XMP X/Open Management Protocol.XMS See Extended Memory Specification.XMT abbrev. transmit.XMTR abbrev. transmitter.XNMS MlCOMs commercial IBM-licensed Intel-based desktop computer packet data network (PDN)network management system software.XNS See Xerox Network Services.XO abbrev. crystal oscillator. See crystal detector,quartz.

XON/XOFF transmission on/transmission off.Common flow control signals used between twocommunicating devices or software programs, typi-cally through modems. Since many transmissionsmedia are inherently slow, there may be a delay be-tween receiving a block of data and resuming trans-missions. XON/XOFF signals allow the communi-cators to signal when to stop sending data and whento resume, in order to prevent loss or corruption.XON/XOFF is also known as software flow control.In newer high-speed modems, flow control may behandled by hardware, often in conjunction with spe-cific types of cables.

Flow control signals are not limited to modem com-munications. If a user is working on a terminal thatunderstands XON/XOFF commands, usually signi-fied by Ctrl-S (stop) and Ctrl-Q (resume), then it ispossible to suspend a listing or other activity and


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XTI X/Open Transport Interface. See X WindowSystem.

XTL See SunXTL.

XTL API See SunXTL.

XTL Provider Interface See SunXTL.

XTL Provider Library See SunXTL.

XTP 1. See eXpress Transfer Protocol. 2. See XpressTransport Protocol.

XTP Forum A nonprofit group promoting the de-velopment and distribution of Xpress Transport Pro-tocol (XTP). A number of significant vendors, in-cluding Apple Computer, Hughes Aircraft, LockheedMartin, Northrop-Grumman, Philips Research, andSilicon Graphics are involved in this effort, in col-laboration with XTP Forum Research Affiliates fromuniversities and military labs around the world. SeeXpress Transport Protocol.

XUI See X User Interface Toolkit.

XWA The callsign issued to a historically significantCanadian radio station, it stood for X-perimental

Wireless Apparatus. XWA was issued its first experi-

mental license by the Department of Naval Service,in 1918, following about 3 years of discussion (andpresumably some preliminary experiments).The first general broadcast licence was issued in Sep-tember 1919 to XWA as part of the Marconi Wire-less Telegraph Company. Broadcasts apparently be-gan in December 1919. The pioneer broadcasts wereprimarily musical and, since few consumer radiosexisted at the time, the listeners were amateur radioenthusiasts. In May 1920, XWA aired its first regu-larly scheduled broadcast and the radio industryevolved quickly, with XWA becoming station CFCFin November 1920. See CFCF, radio history.XWindows, XWS See X Window System.XXXXA designation for the last four digits in a tele-phone number, usually used when the numbers arenot yet known (or assigned). It represents any num-ber between 0 and 9.XY cutA means of angle-cutting a piezoelectric crys-tal so its electrical characteristics are between thoseof an X cut and a Y cut. See quartz, X-ray goniom-

eter, X cut, Y cut.

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