Prefacedbillings.com/networking/Routing_Terms.pdf · Supports speeds of up to 100 Mbps over multimode fiber. See also TAXI 4B/5B. 4-byte/5-byte local fiber See 4B/5B local fiber

Preface vii

PrefaceComputer networks have become a vital tool in the daily operations of businesses aroundthe world. For example, employees in an accounting department can use a commondatabase to access and share customer account information using DECnet. Using anAppleTalk network, Macintosh users in a marketing department can share productbulletins, data sheets, and slide presentations. In an engineering department, Sunworkstation users can share product specifications using TCP/IP over Ethernet. And in acompany’s manufacturing department, IBM devices attached to a Token Ring network canprocess real-time data about material availability and fill orders sent over links from remoteoffices.

This glossary assembles and defines the terms and acronyms used in the internetworkingindustry. Many of the definitions have yet to be standardized, and many terms have severalmeanings. Multiple definitions and acronym expressions are included where they apply.

The first part of this guide contains terms and acronyms that are commonly used in theinternetworking industry. The second part of this guide lists terms and acronyms that arespecific to Cisco Systems and Cisco IOS.

This guide also appears on the Cisco documentation CD-ROM.

While many product names and descriptions are included in this glossary, you areencouraged to get more specific information from the appropriate vendor. For informationabout Cisco products, refer to theCisco Product Catalog.

We hope that this glossary adds to your understanding of internetworking technologies andspecific Cisco terms. Suggestions for new terms or acronyms and their associateddefinitions can be submitted by sending an e-mail to [email protected].

Internetworking Terms and Acronyms, June 1999viii

C H A P T E R

Numerics 1-1

1

Numerics10Base2

10-Mbps baseband Ethernet specification using 50-ohm thin coaxial cable. 10Base2,which is part of the IEEE 802.3 specification, has a distance limit of 606.8 feet(185 meters) per segment. See alsoCheapernet, Ethernet, IEEE 802.3, andThinnet.

10Base510-Mbps baseband Ethernet specification using standard (thick) 50-ohm basebandcoaxial cable. 10Base5, which is part of the IEEE 802.3 baseband physical layerspecification, has a distance limit of 1640 feet (500 meters) per segment. See alsoEthernet andIEEE 802.3.

10BaseF10-Mbps baseband Ethernet specification that refers to the 10BaseFB, 10BaseFL, and10BaseFP standards for Ethernet over fiber-optic cabling. See also10BaseFB,10BaseFL, 10BaseFP, andEthernet.

10BaseFB10-Mbps baseband Ethernet specification using fiber-optic cabling. 10BaseFB is part ofthe IEEE 10BaseF specification. It is not used to connect user stations, but insteadprovides a synchronous signaling backbone that allows additional segments andrepeaters to be connected to the network. 10BaseFB segments can be up to 1.24 miles(2000 meters) long. See also10BaseF andEthernet.

Internetworking Terms and Acronyms, June 19991-2

10BaseFL10-Mbps baseband Ethernet specification using fiber-optic cabling. 10BaseFL is part ofthe IEEE 10BaseF specification and, while able to interoperate with FOIRL, is designedto replace the FOIRL specification. 10BaseFL segments can be up to 3280 feet(1000 meters) long if used with FOIRL, and up to 1.24 miles (2000 meters) if10BaseFL is used exclusively. See also10BaseF, Ethernet, andFOIRL.

10BaseFP10-Mbps fiber-passive baseband Ethernet specification using fiber-optic cabling.10BaseFP is part of the IEEE 10BaseF specification. It organizes a number ofcomputers into a star topology without the use of repeaters. 10BaseFP segments can beup to 1640 feet (500 meters) long. See also10BaseF andEthernet.

10BaseT10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling(Category 3, 4, or 5): one pair for transmitting data and the other for receiving data.10BaseT, which is part of the IEEE 802.3 specification, has a distance limit ofapproximately 328 feet (100 meters) per segment. See alsoEthernet andIEEE 802.3.

10Broad3610-Mbps broadband Ethernet specification using broadband coaxial cable. 10Broad36,which is part of the IEEE 802.3 specification, has a distance limit of 2.24 miles(3600 meters) per segment. See alsoEthernet andIEEE 802.3.

100BaseFX100-Mbps baseband Fast Ethernet specification using two strands of multimodefiber-optic cable per link. To guarantee proper signal timing, a 100BaseFX link cannotexceed 1312 feet (400 meters) in length. Based on the IEEE 802.3 standard. See also100BaseX, Fast Ethernet, andIEEE 802.3.

100BaseT100-Mbps baseband Fast Ethernet specification using UTP wiring. Like the 10BaseTtechnology on which it is based, 100BaseT sends link pulses over the network segmentwhen no traffic is present. However, these link pulses contain more information thanthose used in 10BaseT. Based on the IEEE 802.3 standard. See also10BaseT, FastEthernet, andIEEE 802.3.

Numerics 1-3

100BaseT4100-Mbps baseband Fast Ethernet specification using four pairs of Category 3, 4, or 5UTP wiring. To guarantee proper signal timing, a 100BaseT4 segment cannot exceed328 feet (100 meters) in length. Based on the IEEE 802.3 standard. See alsoFastEthernet andIEEE 802.3.

100BaseTX100-Mbps baseband Fast Ethernet specification using two pairs of either UTP or STPwiring. The first pair of wires is used to receive data; the second is used to transmit. Toguarantee proper signal timing, a 100BaseTX segment cannot exceed 328 feet(100 meters) in length. Based on the IEEE 802.3 standard. See also100BaseX, FastEthernet, andIEEE 802.3.

100BaseX100-Mbps baseband Fast Ethernet specification that refers to the 100BaseFX and100BaseTX standards for Fast Ethernet over fiber-optic cabling. Based on theIEEE 802.3 standard. See also100BaseFX, 100BaseTX, Fast Ethernet, andIEEE 802.3.

100VG-AnyLAN100-Mbps Fast Ethernet and Token Ring media technology using four pairs ofCategory 3, 4, or 5 UTP cabling. This high-speed transport technology, developed byHewlett-Packard, can operate on existing 10BaseT Ethernet networks. Based on theIEEE 802.12 standard. See alsoIEEE 802.12.

1822Historic term that refers to the original ARPANET host-to-IMP interface. Thespecifications are in BBN report 1822. Seehost andIMP.

24th channel signalingSee24th channel signaling.

2B1Q2 binary 1 quaternary. Encoding scheme that provides a 2 bits per baud, 80-kbaud persecond, 160-kbps transfer rate. The most common signaling method on ISDN Uinterfaces. This protocol is defined in detail in 1988 ANSI spec T1.601.

370 block mux channelSeeblock multiplexer channel.


4B/5B local fiber4-byte/5-byte local fiber. Fiber channel physical media used for FDDI and ATM.Supports speeds of up to 100 Mbps over multimode fiber. See alsoTAXI 4B/5B.

4-byte/5-byte local fiberSee4B/5B local fiber.

6BONEThe internet’s experimental IPv6 network.

8-byte/10-byte local fiberSee8B/10B local fiber.

802.xSet of IEEE standards for the definition of LAN protocols.

822Short form of RFC 822. Refers to the format of Internet style e-mail as defined in RFC822.

8B/10B local fiber8-byte/10-byte local fiber. Fiber channel physical media that supports speeds up to149.76 Mbps over multimode fiber.

C H A P T E R

A 2-1

2

AA&B bit signaling

Procedure used in T1 transmission facilities in which each of the 24 T1 subchannelsdevotes 1 bit of every sixth frame to the carrying of supervisory signaling information.Also called 24th channel signaling.

AAAauthentication, authorization, and accounting. Pronounced “triple a."

AALATM adaptation layer. Service-dependent sublayer of the data link layer. The AALaccepts data from different applications and presents it to the ATM layer in the form of48-byte ATM payload segments. AALs consist of two sublayers: CS and SAR. AALsdiffer on the basis of the source-destination timing used, whether they use CBR or VBR,and whether they are used for connection-oriented or connectionless mode data transfer.At present, the four types of AAL recommended by the ITU-T are AAL1, AAL2,AAL3/4, and AAL5. See alsoAAL1, AAL2, AAL3/4, AAL5, ATM, ATM layer, CS, andSAR.

AAL1ATM adaptation layer. One of four AALs recommended by the ITU-T. AAL1 is usedfor connection-oriented, delay-sensitive services requiring constant bit rates, such asuncompressed video and other isochronous traffic. See alsoAAL.

AAL2ATM adaptation layer 2. One of four AALs recommended by the ITU-T. AAL2 is usedfor connection-oriented services that support a variable bit rate, such as someisochronous video and voice traffic. See alsoAAL.


AAL3/4ATM adaptation layer 3/4. One of four AALs (merged from two initially distinctadaptation layers) recommended by the ITU-T. AAL3/4 supports both connectionlessand connection-oriented links, but is primarily used for the transmission of SMDSpackets over ATM networks. See alsoAAL.

AAL5ATM adaptation layer 5. One of four AALs recommended by the ITU-T. AAL5supports connection-oriented VBR services and is used predominantly for the transferof classical IP over ATM and LANE traffic. AAL5 uses SEAL and is the least complexof the current AAL recommendations. It offers low bandwidth overhead and simplerprocessing requirements in exchange for reduced bandwidth capacity anderror-recovery capability. See alsoAAL andSEAL.

AARPAppleTalk Address Resolution Protocol. Protocol in the AppleTalk protocol stack thatmaps a data-link address to a network address.

AARP probe packetsPackets transmitted by AARP that determine if a randomly selected node ID is beingused by another node in a nonextended AppleTalk network. If the node ID is not beingused, the sending node uses that node ID. If the node ID is being used, the sending nodechooses a different ID and sends more AARP probe packets. See alsoAARP.

ABCD signaling4-bit telephony line signaling coding in which each letter represents 1 of the 4 bits. Thisis often associated with CAS or robbed-bit signaling on a T1 or E1 telephony trunk.

ABMAsynchronous Balanced Mode. HDLC (and derivative protocol) communication modesupporting peer-oriented, point-to-point communications between two stations, whereeither station can initiate transmission.

ABR1. available bit rate. QoS class defined by the ATM Forum for ATM networks. ABR isused for connections that do not require timing relationships between source anddestination. ABR provides no guarantees in terms of cell loss or delay, providing onlybest-effort service. Traffic sources adjust their transmission rate in response to

A 2-3

information they receive describing the status of the network and its capability tosuccessfully deliver data. Compare withCBR, UBR, andVBR.

2. area border router. Router located on the border of one or more OSPF areas thatconnects those areas to the backbone network. ABRs are considered members of boththe OSPF backbone and the attached areas. They therefore maintain routing tablesdescribing both the backbone topology and the topology of the other areas.

Abstract Syntax Notation OneSeeASN.1.

access deviceHardware component used in your signaling controller system: access server or mux.

access listList kept by routers to control access to or from the router for a number of services (forexample, to prevent packets with a certain IP address from leaving a particular interfaceon the router).

access method1. Generally, the way in which network devices access the network medium.

2. Software within an SNA processor that controls the flow of information through anetwork.

access serverCommunications processor that connects asynchronous devices to a LAN or WANthrough network and terminal emulation software. Performs both synchronous andasynchronous routing of supported protocols. Sometimes called a network accessserver. See alsocommunication server.

access unitSeeAU.


accounting managementOne of five categories of network management defined by ISO for management of OSInetworks. Accounting management subsystems are responsible for collecting networkdata relating to resource usage. See alsoconfiguration management, fault management,performance management, andsecurity management.

ACDautomatic call distribution. Device or service that automatically reroutes calls tocustomers in geographically distributed locations served by the same CO. See alsoCO.

ACELPalgebraic code excited linear prediction.

ACFAdvanced Communications Function. A group of SNA products that providesdistributed processing and resource sharing. See alsoACF/NCP.

ACF/NCPAdvanced Communications Function/Network Control Program. The primary SNANCP. ACF/NCP resides in the communications controller and interfaces with the SNAaccess method in the host processor to control network communications. See alsoACFandNCP.

ACKSeeacknowledgment.

acknowledgmentNotification sent from one network device to another to acknowledge that some event(for example, receipt of a message) occurred. Sometimes abbreviated ACK. Compareto NAK.

ACOMTerm used in G.165, “General Characteristics of International Telephone Connectionsand International Telephone Circuits: Echo Cancellers.” ACOM is the combined lossachieved by the echo canceller, which is the sum of the echo return loss, echo return lossenhancement, and nonlinear processing loss for the call.

A 2-5

ACRallowed cell rate. Parameter defined by the ATM Forum for ATM traffic management.ACR varies between the MCR and the PCR, and is dynamically controlled usingcongestion control mechanisms. See alsoMCR andPCR.

ACSEassociation control service element. OSI convention used to establish, maintain, orterminate a connection between two applications.

active hubMultiported device that amplifies LAN transmission signals.

active monitorDevice responsible for managing a Token Ring. A network node is selected to be theactive monitor if it has the highest MAC address on the ring. The active monitor isresponsible for such management tasks as ensuring that tokens are not lost, or thatframes do not circulate indefinitely. See alsoring monitor andstandby monitor.

ActiveXMicrosoft’s Windows-specific non-Java technique for writing applets. ActiveX appletstake considerably longer to download than the equivalent Java applets; however, theymore fully exploit the features of Windows 95. ActiveX is sometimes said to be asuperset of Java. See alsoapplet, Java.

ADadministrative domain. Group of hosts, routers, and networks operated and managed bya single organization.

adapterSeeNIC.

adaptive differential pulse code modulationSeeADPCM.

adaptive routingSeedynamic routing.


ADCCPAdvanced Data Communications Control Protocol. ANSI standard bit-oriented datalink control protocol.

addressData structure or logical convention used to identify a unique entity, such as a particularprocess or network device.

addressed call modeMode that permits control signals and commands to establish and terminate calls inV.25bis. See alsoV.25bis.

address mappingTechnique that allows different protocols to interoperate by translating addresses fromone format to another. For example, when routing IP over X.25, the IP addresses mustbe mapped to the X.25 addresses so that the IP packets can be transmitted by the X.25network. See alsoaddress resolution.

address maskBit combination used to describe which portion of an address refers to the network orsubnet and which part refers to the host. Sometimes referred to simply as mask. See alsosubnet mask.

address resolutionGenerally, a method for resolving differences between computer addressing schemes.Address resolution usually specifies a method for mapping network layer (Layer 3)addresses to data link layer (Layer 2) addresses. See alsoaddress mapping.

Address Resolution ProtocolSeeARP.

address translation gatewaySeeATG (address translation gateway) in the “Cisco Systems Terms and Acronyms”section.

A 2-7

adjacencyRelationship formed between selected neighboring routers and end nodes for thepurpose of exchanging routing information. Adjacency is based upon the use of acommon media segment.

adjacent nodes1. In SNA, nodes that are connected to a given node with no intervening nodes.

2. In DECnet and OSI, nodes that share a common network segment (in Ethernet, FDDI,or Token Ring networks).

ADMAdd Drop Multiplexer. In OSS, a multiplexer that allows a signal to be added into ordropped out of a SONET span. See alsoSONET.

ADMDAdministration Management Domain. X.400 Message Handling System public carrier.The ADMDs in all countries worldwide together provide the X.400 backbone. See alsoPRMD.

administrative distanceRating of the trustworthiness of a routing information source. Administrative distanceis often expressed as a numerical value between 0 and 255. The higher the value, thelower the trustworthiness rating.

Administrative DomainSeeAD.

administrative weightSeeAW andPTSP.

admission controlSeetraffic policing.

ADPCMadaptive differential pulse code modulation. Process by which analog voice samples areencoded into high-quality digital signals.


ADSLasymmetric digital subscriber line. One of four DSL technologies. ADSL is designed todeliver more bandwidth downstream (from the central office to the customer site) thanupstream. Downstream rates range from 1.5 to 9 Mbps, while upstream bandwidthranges from 16 to 640 kbps. ADSL transmissions work at distances up to 18,000 feet(5,488 meters) over a single copper twisted pair. See alsoHDSL, SDSL, andVDSL.

ADSUATM DSU. Terminal adapter used to access an ATM network via an HSSI-compatibledevice. See alsoDSU.

Advanced Communications FunctionSeeACF.

Advanced Communications Function/Network Control ProgramSeeACF/NCP.

Advanced CoS Managementadvanced class-of-service management. Essential for delivering the required QoS to allapplications. Cisco switches contain per-VC queuing, per-VC rate scheduling, multipleCoS queuing, and egress queuing. This enables network managers to refine connectionsto meet specific application needs. Formerly called FairShare and OptiClass.

Advanced Data Communications Control ProtocolSeeADCCP.

Advanced Intelligent NetworkSee AIN.

Advanced Peer-to-Peer NetworkingSeeAPPN.

Advanced Program-to-Program CommunicationSeeAPPC.

Advanced Research Projects AgencySeeARPA.

A 2-9

Advanced Research Projects Agency NetworkSeeARPANET.

advertisingRouter process in which routing or service updates are sent at specified intervals so thatother routers on the network can maintain lists of usable routes.

AEPAppleTalk Echo Protocol. Used to test connectivity between two AppleTalk nodes. Onenode sends a packet to another node and receives a duplicate, or echo, of that packet.

AFIauthority and format identifier. Portion of an NSAP-format ATM address that identifiesthe type and format of the IDI portion of an ATM address. See alsoIDI andNSAP.

AFPAppleTalk Filing Protocol. Presentation-layer protocol that allows users to share datafiles and application programs that reside on a file server. AFP supports AppleShare andMac OS File Sharing.

agent1. Generally, software that processes queries and returns replies on behalf of anapplication.

2. In NMSs, process that resides in all managed devices and reports the values ofspecified variables to management stations.

AINAdvanced Intelligent Network. In SS7, an expanded set of network services madeavailable to the user, and under user control, that requires improvement in networkswitch architecture, signaling capabilities, and peripherals. See alsoSS7.

AIOAsynchronous input/output.

AIPSeeAIP (ATM Interface Processor) in the “Cisco Systems Terms and Acronyms”section.


AISalarm indication signal. In a T1 transmission, an all-ones signal transmitted in lieu ofthe normal signal to maintain transmission continuity and to indicate to the receivingterminal that there is a transmission fault that is located either at, or upstream from, thetransmitting terminal. See alsoT1.

alarmSNMP message notifying an operator or administrator of a network problem. See alsoevent andtrap.

alarm indication signalSeeAIS.

a-lawITU-T companding standard used in the conversion between analog and digital signalsin PCM systems. A-law is used primarily in European telephone networks and is similarto the North American mu-law standard. See alsocompanding andmu-law.

algorithmWell-defined rule or process for arriving at a solution to a problem. In networking,algorithms are commonly used to determine the best route for traffic from a particularsource to a particular destination.

aliasSeeentity.

alignment errorIn IEEE 802.3 networks, an error that occurs when the total number of bits of a receivedframe is not divisible by eight. Alignment errors are usually caused by frame damagedue to collisions.

A-linkSS7 access link. Dedicated SS7 signaling link not physically associated with anyparticular link carrying traffic.

allowed cell rateSeeACOM.

A 2-11

all-rings explorer packetSeeall-routes explorer packet.

all-routes explorer packetExplorer packet that traverses an entire SRB network, following all possible paths to aspecific destination. Sometimes called all-rings explorer packet. See alsoexplorerpacket, local explorer packet, andspanning explorer packet.

ALO transactionATP transaction in which the request is repeated until a response is received by therequester or until a maximum retry count is reached. This recovery mechanism ensuresthat the transaction request is executed at least once. See alsoATP.

alternate mark inversionSeeAMI.

AMamplitude modulation. Modulation technique whereby information is conveyed throughthe amplitude of the carrier signal. Compare withFM andPAM. See alsomodulation.

AMAAutomatic Messaging Accounting. In OSS, the automatic collection, recording, andprocessing of information relating to calls for billing purposes.

AMADNSAMA Data Networking System. In OSS, the next generation (formerly Bellcore) systemfor the collection and transport of AMA data from central office switches to a billingsystem. See alsoAMA.

AMATPSAMA Teleprocessing System. In OSS, the Bellcore legacy system for collecting andtransporting AMA data from central office switches to a billing system. The AMATPSconsists of an AMA transmitter and a collector. See alsoAMA.

American National Standards InstituteSeeANP.


American Standard Code for Information InterchangeSeeASCII.

AMIalternate mark inversion. Line-code type used on T1 and E1 circuits. In AMI, zeros arerepresented by 01 during each bit cell, and ones are represented by 11 or 00, alternately,during each bit cell. AMI requires that the sending device maintain ones density. Onesdensity is not maintained independently of the data stream. Sometimes called binarycoded alternate mark inversion. Compare withB8ZS. See alsoones density.

amplitudeMaximum value of an analog or a digital waveform.

amplitude modulationSeeAM.

analog transmissionSignal transmission over wires or through the air in which information is conveyedthrough the variation of some combination of signal amplitude, frequency, and phase.

ANIautomatic number identification. SS7 (signaling system 7) feature in which a series ofdigits, either analog or digital, are included in the call, identifying the telephone numberof the calling device. In other words, ANI identifies the number of the calling party.

anonymous FTPAllows a user to retrieve documents, files, programs, and other archived data fromanywhere on the Internet without having to establish a userid and password. By usingthe special userid of anonymous, the network user will bypass local security checks andwill have access to publicly accessible files on the remote system. SeeFTP.

ANPautomatic numbering plan.

ANSIAmerican National Standards Institute. Voluntary organization composed of corporate,government, and other members that coordinates standards-related activities, approvesU.S. national standards, and develops positions for the United States in international

A 2-13

standards organizations. ANSI helps develop international and U.S. standards relatingto, among other things, communications and networking. ANSI is a member of the IECand the ISO. See alsoIEC andISO.

ANSI X3T9.5SeeX3T9.5.

anycastIn ATM, an address that can be shared by multiple end systems. An anycast address canbe used to route a request to a node that provides a particular service.

AOWAsia and Oceania Workshop. One of the three regional OSI Implementors Workshops.See alsoEWOS.

APaRTSeeAPaRT(Automated Packet Recognition/Translation) in the “Cisco Systems Termsand Acronyms” section.

APCadjacent point code. The point code of the next hop in the system for the bearerchannels; usually it is the STP (signal transfer point).

APIApplication Programming Interface. Specification of function-call conventions thatdefines an interface to a service.

APNICAsia Pacific Network Information Center. Nonprofit Internet registry organization forthe Asia Pacific region. The other Internet registries are currently IANA, RIPE NCC andInterNIC.

Apollo DomainProprietary network protocol suite developed by Apollo Computer for communicationon proprietary Apollo networks.


APPCAdvanced Program-to-Program Communication. IBM SNA system software thatallows high-speed communication between programs on different computers in adistributed computing environment. APPC establishes and tears down connectionsbetween communicating programs. It consists of two interfaces: programming anddata-exchange. The programming interface replies to requests from programs requiringcommunication; the data-exchange interface establishes sessions between programs.APPC runs on LU 6.2 devices. See alsoLU 6.2.

appletSmall program, often used in the context of a Java-based program, that is compiled andembedded in an HTML page. SeeActiveX andJava.

AppleTalkSeries of communications protocols designed by Apple Computer consisting of twophases. Phase 1, the earlier version, supports a single physical network that can haveonly one network number and be in one zone. Phase 2, supports multiple logicalnetworks on a single physical network and allows networks to be in more than one zone.See also zone.

AppleTalk Address Resolution ProtocolSeeAARP.

AppleTalk Filing ProtocolSeeAFP.

AppleTalk Echo ProtocolSeeAEP.

AppleTalk Remote AccessSeeARA.

AppleTalk Session ProtocolSeeASP.

AppleTalk Transaction ProtocolSeeATP.

A 2-15

AppleTalk Update-Based Routing ProtocolSeeAURP.

AppleTalk zoneSeezone.

applicationProgram that performs a function directly for a user. FTP and Telnet clients areexamples of network applications.

application layerLayer 7 of the OSI reference model. This layer provides services to applicationprocesses (such as e-mail, file transfer, and terminal emulation) that are outside of theOSI model. The application layer identifies and establishes the availability of intendedcommunication partners (and the resources required to connect with them),synchronizes cooperating applications, and establishes agreement on procedures forerror recovery and control of data integrity. Corresponds roughly with thetransactionservices layerin the SNA model. See alsodata-link layer, network layer, physical layer,PQ, session layer, andtransport layer.

application programming interfaceSeeAPI.

APPNAdvanced Peer-to-Peer Networking. Enhancement to the original IBM SNAarchitecture. APPN handles session establishment between peer nodes, dynamictransparent route calculation, and traffic prioritization for APPC traffic. Compare withAPPN+. See alsoAPPC.

APPN+Next-generation APPN that replaces the label-swapping routing algorithm with sourcerouting. Also called high-performance routing. See alsoAPPN.

APSautomatic protection switching. SONET switching mechanism that routes traffic fromworking lines to protect them in case of a line card failure or fiber cut.


ARAAppleTalk Remote Access. Protocol that provides Macintosh users direct access toinformation and resources at a remote AppleTalk site.

ArchieSystem that provides lists of anonymous FTP archives. SeeGopher, WAIS, andWorldWide Web.

ARCnetAttached Resource Computer Network. 2.5-Mbps token-bus LAN developed in the late1970s and early 1980s by Datapoint Corporation.

areaLogical set of network segments (CLNS-, DECnet-, or OSPF-based) and their attacheddevices. Areas are usually connected to other areas via routers, making up a singleautonomous system. See alsoautonomous system.

area border routerSeeABR.

ARINAmerican Registry for Internet Numbers. Nonprofit organization established for thepurpose of administrating and registrating IP numbers to the geographical areascurrently managed by Network Solutions (InterNIC). Those areas include, but are notlimited to, North America, South America, South Africa, and the Caribbean.

ARMasynchronous response mode. HDLC communication mode involving one primarystation and at least one secondary station, where either the primary or one of thesecondary stations can initiate transmissions. See alsoprimary station andsecondarystation.

ARPAddress Resolution Protocol. Internet protocol used to map an IP address to a MACaddress. Defined in RFC 826. Compare withRARP. See alsoproxy ARP.

A 2-17

ARPAAdvanced Research Projects Agency. Research and development organization that ispart of DoD. ARPA is responsible for numerous technological advances incommunications and networking. ARPA evolved into DARPA, and then back intoARPA again (in 1994). See alsoDARPA.

ARPANETAdvanced Research Projects Agency Network. Landmark packet-switching networkestablished in 1969. ARPANET was developed in the 1970s by BBN and funded byARPA (and later DARPA). It eventually evolved into the Internet. The term ARPANETwas officially retired in 1990. See alsoARPA, BBN, DARPA, andInternet.

ARQautomatic repeat request. Communication technique in which the receiving devicedetects errors and requests retransmissions.

ARUalarm relay unit.

ASSeeautonomous system.

ASAMATM subscriber access multiplexer. A telephone central office multiplexer that supportsSDL ports over a wide range of network interfaces. An ASAM sends and receivessubscriber data (often Internet services) over existing copper telephone lines,concentrating all traffic onto a single high-speed trunk for transport to the Internet or theenterprise intranet. This device is similar to a DSLAM (different manufacturers usedifferent terms for similar devices).

ASBRautonomous system boundary router. ABR located between an OSPF autonomoussystem and a non-OSPF network. ASBRs run both OSPF and another routing protocol,such as RIP. ASBRs must reside in a nonstub OSPF area. See alsoABR, non-stub area,andOSPF.


ASCIIAmerican Standard Code for Information Interchange. 8-bit code for characterrepresentation (7 bits plus parity).

ASCUagent-set control unit.

ASIATM Service Interface.

ASNauxiliary signal network.

ASN.1Abstract Syntax Notation One. OSI language for describing data types independent ofparticular computer structures and representation techniques. Described by ISOInternational Standard 8824. See alsoBER, basic encoding rules.

ASP1. AppleTalk Session Protocol. Protocol that uses ATP to provide session establishment,maintenance, and teardown, as well as request sequencing. See alsoATP.

2. Telecommunications: Auxiliary signal path. Link between TransPaths that allowsthem to exchange signaling information that is incompatible with the PSTN backbonenetwork; used to provide feature transparency.

assigned numbersRFC [STD2] documents the currently assigned values from several series of numbersused in network protocol implementations. This RFC is updated periodically, andcurrent information can be obtained from the IANA. If you are developing a protocol orapplication that will require the use of a link, socket, port, protocol, and so forth, contactthe IANA to receive a number assignment. SeeIANA and STD.

association control service elementSeeACSE.

A 2-19

associative memoryMemory that is accessed based on its contents, not on its memory address. Sometimescalled content addressable memory (CAM).

ASTautomatic spanning tree. Function that supports the automatic resolution of spanningtrees in SRB networks, providing a single path for spanning explorer frames to traversefrom a given node in the network to another. AST is based on the IEEE 802.1 standard.SeeIEEE 802.1 andSRB.

ASTAAdvanced Software Technology and Algorithms. Component of the HPCC programintended to develop software and algorithms for implementation on high-performancecomputer and communications systems. See alsoHPCC.

asyncSubset of tty.

Asynchronous Balanced ModeSeeABM.

asynchronous response modeSeeARM.

asynchronous time-division multiplexingSeeATDM.

Asynchronous Transfer ModeSeeATM.

asynchronous transmissionTerm describing digital signals that are transmitted without precise clocking. Suchsignals generally have different frequencies and phase relationships. Asynchronoustransmissions usually encapsulate individual characters in control bits (called start andstop bits) that designate the beginning and end of each character. Compare withisochronous transmission, plesiochronous transmission, andsynchronous transmission.


ATCPAppleTalk Control Protocol. Protocol that establishes and configures AppleTalk overPPP, as defined in RFC 1378. See alsoPPP.

ATDMasynchronous time-division multiplexing. Method of sending information thatresembles normal TDM, except that time slots are allocated as needed rather thanpreassigned to specific transmitters. Compare withFDM, statistical multiplexing, andTDM.

ATGSeeATG (address translation gateway) in the “Cisco Systems Terms and Acronyms”section.

ATHattention hangup.

at-least-once transactionSeeALO transaction.

ATMAsynchronous Transfer Mode. International standard for cell relay in which multipleservice types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells.Fixed-length cells allow cell processing to occur in hardware, thereby reducing transitdelays. ATM is designed to take advantage of high-speed transmission media such asE3, SONET, and T3.

ATM adaptation layerSeeAAL.

ATM adaptation layer 1SeeAAL1.


ATM adaptation layer 3/4SeeAAL3/4.

A 2-21


ATM ARP serverDevice that provides address-resolution services to LISs when running classical IP overATM. See alsoLIS.

ATM data service unitSeeADSU.

ATM endpointPoint in an ATM network where an ATM connection is initiated or terminated. ATMendpoints include ATM-attached workstations, ATM-attached servers, ATM-to-LANswitches, and ATM routers.

ATM ForumInternational organization jointly founded in 1991 by Cisco Systems,NET/ADAPTIVE, Northern Telecom, and Sprint that develops and promotesstandards-based implementation agreements for ATM technology. The ATM Forumexpands on official standards developed by ANSI and ITU-T, and developsimplementation agreements in advance of official standards.

ATM interface processorSeeAIS.

ATM layerService-independent sublayer of the data link layer in an ATM network. The ATM layerreceives the 48-byte payload segments from the AAL and attaches a 5-byte header toeach, producing standard 53-byte ATM cells. These cells are passed to the physical layerfor transmission across the physical medium. See alsoAAL.

ATMMATM management. Process that runs on an ATM switch that controls VCI translationand rate enforcement. See alsoATM andVCD.

ATM managementSeeATMM.


ATM networkSeeATM network in the “Cisco Systems Terms and Acronyms” section.

ATM NICSeeATM network interface card in the “Cisco Systems Terms and Acronyms” section.

ATM service interface.SeeASCU.

ATM UNISeeUNI.

ATM user-user connectionConnection created by the ATM layer to provide communication between two or moreATM service users, such as ATMM processes. Such communication can beunidirectional, using one VCC, or bidirectional, using two VCCs. See alsoATM layer,ATMM, andVCC.

ATPAppleTalk Transaction Protocol. Transport-level protocol that provides a loss-freetransaction service between sockets. The service allows exchanges between two socketclients in which one client requests the other to perform a particular task and to reportthe results. ATP binds the request and response together to ensure the reliable exchangeof request-response pairs.

Attached Resource Computer NetworkSeeARCnet.

attachment unit interfaceSeeAUI.

attenuationLoss of communication signal energy.

attributeForm of information items provided by the X.500 Directory Service. The directoryinformation base consists of entries, each containing one or more attributes. Eachattribute consists of a type identifier together with one or more values.

A 2-23

AUaccess unit. Device that provides ISDN access to PSNs. See alsoPSN.

AUIattachment unit interface. IEEE 802.3 interface between an MAU and a NIC. The termAUI can also refer to the rear panel port to which an AUI cable might attach. Also calledtransceiver cable. See alsoIEEE 802.3, MAU, andNIC.

AUPacceptable use policy. Many transit networks have policies that restrict the use to whichthe network can be put. Enforcement of AUPs varies with the network.

AURPAppleTalk Update-Based Routing Protocol. Method of encapsulating AppleTalk trafficin the header of a foreign protocol, allowing the connection of two or morediscontiguous AppleTalk internetworks through a foreign network (such as TCP/IP) toform an AppleTalk WAN. This connection is called an AURP tunnel. In addition to itsencapsulation function, AURP maintains routing tables for the entire AppleTalk WANby exchanging routing information between exterior routers. See alsoAURP tunnelandexterior router.

AURP tunnelConnection created in an AURP WAN that functions as a single, virtual data linkbetween AppleTalk internetworks physically separated by a foreign network (a TCP/IPnetwork, for example). See alsoAURP.

AUSMATM user service module.

authenticationIn security, the verification of the identity of a person or process.

authority zoneAssociated with DNS, an authority zone is a section of the domain-name tree for whichone name server is the authority. See alsoDNS.

Automated Packet Recognition/TranslationSeeAPaRT in the “Cisco Systems Terms and Acronyms” section.


automatic call distributionSeeACD.

automatic call reconnectFeature permitting automatic call rerouting away from a failed trunk line.

automatic protection switchingSeeAPS.

automatic repeat requestSeeARQ.

Automatic Routing ManagementFormerly AutoRoute. Connection-oriented mechanism used in Cisco WAN switches toprovide connectivity across the network. Switches perform a connection admissioncontrol (CAC) function on all types of connections in the network. Distributed networkintelligence enables the CAC function to automatically route and reroute connectionsover optimal paths, while guaranteeing the required QoS.

automatic spanning treeSeeAST.

autonomous confederationGroup of autonomous systems that rely on their own network reachability and routinginformation more than they rely on that received from other autonomous systems orconfederations.

autonomous switchingSeeautonomous switching in the “Cisco Systems Terms and Acronyms” section.

autonomous systemCollection of networks under a common administration sharing a common routingstrategy. Autonomous systems are subdivided by areas. An autonomous system must beassigned a unique 16-bit number by the IANA. Sometimes abbreviated as AS. See alsoarea andIANA.

autonomous system boundary routerSeeASAM.

A 2-25

autoreconfigurationProcess performed by nodes within the failure domain of a Token Ring network. Nodesautomatically perform diagnostics in an attempt to reconfigure the network around thefailed areas. See alsofailure domain.

available bit rateSeeABR.

average rateAverage rate, in kilobits per second (kbps), at which a given virtual circuit will transmit

AVMATM voice multiplexer.

AWadministrative weight. Value set by the network administrator to indicate the desirabilityof a network link. One of four link metrics exchanged by PTSPs to determine theavailable resources of an ATM network.


C H A P T E R

B 3-1

3

BB8ZS

binary 8-zero substitution. Line-code type, used on T1 and E1 circuits, in which aspecial code is substituted whenever 8 consecutive zeros are sent over the link. Thiscode is then interpreted at the remote end of the connection. This technique guaranteesones density independent of the data stream. Sometimes calledbipolar 8-zerosubstitution. Compare withAMI. See alsoones density.

backbonePart of a network that acts as the primary path for traffic that is most often sourced from,and destined for, other networks.

back endNode or software program that provides services to a front end. See alsoclient, FRF.11,andserver.

backoffThe (usually random) retransmission delay enforced by contentious MAC protocolsafter a network node with data to transmit determines that the physical medium isalready in use.

backplanePhysical connection between an interface processor or card and the data buses and thepower distribution buses inside a chassis.

back pressurePropagation of network congestion information upstream through an internetwork.


backward explicit congestion notificationSeeBECN.

backward learningAlgorithmic process used for routing traffic that surmises information by assumingsymmetrical network conditions. For example, if node A receives a packet from node Bthrough intermediate node C, the backward-learning routing algorithm will assume thatA can optimally reach B through C.

balanced configurationIn HDLC, a point-to-point network configuration with two combined stations.

balanced, unbalancedSeebalun.

balunbalanced, unbalanced. Device used for matching impedance between a balanced and anunbalanced line, usually twisted-pair and coaxial cable.

bandwidthDifference between the highest and lowest frequencies available for network signals.The term is also used to describe the rated throughput capacity of a given networkmedium or protocol.

bandwidth allocationSeebandwidth reservation.

bandwidth reservationProcess of assigning bandwidth to users and applications served by a network. Involvesassigning priority to different flows of traffic based on how critical and delay-sensitivethey are. This makes the best use of available bandwidth, and if the network becomescongested, lower-priority traffic can be dropped. Sometimes calledbandwidthallocation. See alsocall leg.

Banyan VINESSeeVINES.

B 3-3

BARRNetBay Area Regional Research Network. Regional network serving the San Francisco BayArea. The BARRNet backbone is composed of four University of California campuses(Berkeley, Davis, Santa Cruz, and San Francisco), Stanford University, LawrenceLivermore National Laboratory, and NASA Ames Research Center. BARRNet is nowpart of BBN Planet. See alsoBBN Planet.

basebandCharacteristic of a network technology where only one carrier frequency is used.Ethernet is an example of a baseband network. Also called narrowband. Contrast withbroadband.

bashBourne-again shell. Interactive UNIX shell based on the traditional Bourne shell, butwith increased functionality. See alsoroot account.

basic encoding rulesSeeBER.

Basic Rate InterfaceSeeBRI.

Basic Research and Human ResourcesSeeBRHR.

baudUnit of signaling speed equal to the number of discrete signal elements transmitted persecond. Baud is synonymous with bits per second (bps) if each signal elementrepresents exactly 1 bit.

Bay Area Regional Research NetworkSeeBARRNet.

BBNBolt, Beranek, and Newman, Inc. High-technology company located in Massachusettsthat developed and maintained the ARPANET (and later, the Internet) core gatewaysystem. See alsoBBN Planet.


BBN PlanetSubsidiary company of BBN that operates a nationwide Internet access networkcomposed in part by the former regional networks BARRNet, NEARNET, andSURAnet. See alsoBARRNet, BBN, NEARNET, andSURAnet.

BcCommitted Burst. Negotiated tariff metric in Frame Relay internetworks. Themaximum amount of data (in bits) that a Frame Relay internetwork is committed toaccept and transmit at the CIR. See alsoBe andCIR.

B channelbearer channel. In ISDN, a full-duplex, 64-kbps channel used to send user data.Compare toD channel, E channel, andH channel.

BCPBest Current Practices. The newest subseries of RFCs that are written to describe BCPsin the Internet. Rather than specifying a protocol, these documents specify the best waysto use the protocols and the best ways to configure options to ensure interoperabilitybetween various vendors’ products.

BDCSBroadband Digital Cross-Connect System. SONET DCS capable of cross-connectingDS-3, STS-1 and STS-3c signals. See alsoDCS.

Beexcess burst. Negotiated tariff metric in Frame Relay internetworks. The number of bitsthat a Frame Relay internetwork will attempt to transmit after Bc is accommodated. Bedata is, in general, delivered with a lower probability than Bc data because Be data canbe marked as DE by the network. See alsoBc andDE.

beaconFrame from a Token Ring or FDDI device indicating a serious problem with the ring,such as a broken cable. A beacon frame contains the address of the station assumed tobe down. See alsofailure domain.

bearer channelSeeB channel.

B 3-5

Because It’s Time NetworkSeeBITNET.

BECNbackward explicit congestion notification. Bit set by a Frame Relay network in framestraveling in the opposite direction of frames encountering a congested path. DTEreceiving frames with the BECN bit set can request that higher-level protocols take flowcontrol action as appropriate. Compare withFE.

Bell Communications ResearchSeeBellcore.

BellcoreBell Communications Research. Organization that performs research and developmenton behalf of the RBOCs.

Bellman-Ford routing algorithmSeedistance vector routing algorithm.

Bell operating companySeeBOC.

BER1. bit error rate. Ratio of received bits that contain errors.

2. basic encoding rules. Rules for encoding data units described in the ISO ASN.1standard. See alsoASN.1.

Berkeley Internet Name DomainSeeBIND.

Berkeley Standard DistributionSeeBSD.

BERTbit error rate tester. Device that determines the BER on a given communicationschannel. See alsoBER, bit error rate.


best-effort deliveryDescribes a network system that does not use a sophisticated acknowledgment systemto guarantee reliable delivery of information.

BGPBorder Gateway Protocol. Interdomain routing protocol that replaces EGP. BGPexchanges reachability information with other BGP systems. It is defined by RFC 1163.See alsoBGP4 andEGP.

BGP4BGP Version 4. Version 4 of the predominant interdomain routing protocol used on theInternet. BGP4 supports CIDR and uses route aggregation mechanisms to reduce thesize of routing tables. See alsoBGP andCIDR.

BIAburned-in MAC address.

BICIBroadband Inter-Carrier Interface. ITU-T standard that defines the protocols andprocedures needed for establishing, maintaining, and terminating broadband switchedvirtual connections between public networks.

BIGASeeBIGA (Bus Interface Gate Array) in the “Cisco Systems Terms and Acronyms”section.

big-endianMethod of storing or transmitting data in which the most significant bit or byte ispresented first. Compare withlittle-endian.

binaryNumbering system characterized by ones and zeros (1 = on, 0 = off).

binary 8-zero substitutionSeeB8ZS.

binary coded alternate mark inversionSeeAMI.

B 3-7

binary synchronous communicationSeeBSC.

Binary Synchronous Communication ProtocolSeebisync.

BINDBerkeley Internet Name Domain. Implementation of DNS developed and distributed bythe University of California at Berkeley (United States). Many Internet hosts run BIND,which is the ancestor of many commercial BIND implementations.

BinHexBinary Hexadecimal. Method for converting binary files into ASCII for transmission byapplications, such as e-mail, that can only handle ASCII.

BIPbit interleaved parity. In ATM, a method used to monitor errors on a link. A check bit orword is sent in the link overhead for the previous block or frame. Bit errors in thepayload can then be detected and reported as maintenance information.

biphase codingBipolar coding scheme originally developed for use in Ethernet. Clocking informationis embedded into and recovered from the synchronous data stream without the need forseparate clocking leads. The biphase signal contains no direct current energy.

bipolarElectrical characteristic denoting a circuit with both negative and positive polarity.Contrast withunipolar.

bipolar 8-zero substitutionSeeB8ZS.

BISDNBroadband ISDN. ITU-T communication standards designed to handle high-bandwidthapplications such as video. BISDN currently uses ATM technology over SONET-basedtransmission circuits to provide data rates from 155 to 622 Mbps and beyond. Contrastwith N-ISDN. See alsoBRI, ISDN, andPRI.


bisyncBinary Synchronous Communication Protocol. Character-oriented data-link protocolfor applications. Contrast with Synchronous Data Link Control (SDLC).

bitBinary digit used in the binary numbering system. Can be 0 or 1.

bit error rateSeeBER.

bit error rate testerSeeBERT.

bit interleaved paritySeeBIP.

BITNET“Because It’s Time” Networking Services. Low-cost, low-speed academic networkconsisting primarily of IBM mainframes and 9600-bps leased lines. BITNET is nowpart of CREN. See alsoCREN.

BITNET IIIDial-up service providing connectivity for members of CREN. See alsoCREN.

bit-oriented protocolClass of data link layer communication protocols that can transmit frames regardless offrame content. Unlike byte-oriented protocols, bit-oriented protocols providefull-duplex operation and are more efficient and reliable. Compare withbyte-orientedprotocol.

bit rateSpeed at which bits are transmitted, usually expressed in bits per second.

bits per secondAbbreviatedbps. See alsobit rate.

B 3-9

black holeRouting term for an area of the internetwork where packets enter, but do not emerge,due to adverse conditions or poor system configuration within a portion of the network.

blockingIn a switching system, a condition in which no paths are available to complete a circuit.The term is also used to describe a situation in which one activity cannot begin untilanother is completed.

block multiplexer channelIBM-style channel that implements the FIPS-60 channel, a U.S. channel standard. Thischannel is also referred to asOEMI channel and370 block mux channel.

blowerInternal cooling fan used in larger router and switch chassis.

BLSRBidirectional Line Switch Ring. SONET ring architecture that provides working andprotection fibers between nodes. If the working fiber between nodes is cut, traffic isautomatically routed onto the protection fiber. See alsoSONET.

BNC connectorStandard connector used to connect IEEE 802.3 10Base2 coaxial cable to an MAU.

BNIBroadband Network Interface.

BNMBroadband Network Module.

BNNboundary network node. In SNA terminology, a subarea node that provides boundaryfunction support for adjacent peripheral nodes. This support includes sequencing,pacing, and address translation. Also calledboundary node.

BOCBell operating company. Twenty-two local phone companies formed by the breakup ofAT&T. SeeRBOC.


Bolt, Beranek, and Newman, Inc.SeeBBN.

BOOTPBootstrap Protocol. Protocol used by a network node to determine the IP address of itsEthernet interfaces, in order to affect network booting.

boot programmable read-only memorySeeboot PROM.

boot PROMboot programmable read-only memory. Chip mounted on a printed circuit board used toprovide executable boot instructions to a computer device.

Bootstrap ProtocolSeeBOOTP.

border gatewayRouter that communicates with routers in other autonomous systems.

Border Gateway ProtocolSeeBGP.

boundary functionCapability of SNA subarea nodes to provide protocol support for attached peripheralnodes. Typically found in IBM 3745 devices.

boundary network nodeSeeBNN.

boundary nodeSeeBNN.

BPDUBridge Protocol Data Unit. Spanning-Tree Protocol hello packet that is sent out atconfigurable intervals to exchange information among bridges in the network. See alsoPDU.

B 3-11

BPIbaseline privacy interface.

bpsbits per second.

BPVbipolar violation.

BPX Service NodeSeeBPX Service Node in the “Cisco Systems Terms and Acronyms” section.

break-out/break-inSeeBOBI in the “Cisco Systems Terms and Acronyms” section.

BRHRBasic Research and Human Resources. Component of the HPCC program designed tosupport research, training, and education in computer science, computer engineering,and computational science. See alsoHPCC.

BRFbridge relay function.

BRIBasic Rate Interface. ISDN interface composed of two B channels and one D channelfor circuit-switched communication of voice, video, and data. Compare withPRI. SeealsoBISDN, ISDN, andN-ISDN.

bridgeDevice that connects and passes packets between two network segments that use thesame communications protocol. Bridges operate at the data link layer (Layer 2) of theOSI reference model. In general, a bridge will filter, forward, or flood an incomingframe based on the MAC address of that frame. See alsorelay.

bridge forwardingProcess that uses entries in a filtering database to determine whether frames with a givenMAC destination address can be forwarded to a given port or ports. Described in theIEEE 802.1 standard. See alsoIEEE 802.1.


bridge groupBridging feature that assigns network interfaces to a particular spanning-tree group.Bridge groups can be compatible with the IEEE 802.1 or the DEC specification.

bridge numberNumber that identifies each bridge in an SRB LAN. Parallel bridges must have differentbridge numbers.

bridge protocol data unitSeeBPDU.

bridge static filteringProcess in which a bridge maintains a filtering database consisting of static entries. Eachstatic entry equates a MAC destination address with a port that can receive frames withthis MAC destination address and a set of ports on which the frames can be transmitted.Defined in the IEEE 802.1 standard. See alsoIEEE 802.1.

broadband1. Transmission system that multiplexes multiple independent signals onto one cable.

2. Telecommunications terminology: Any channel having a bandwidth greater than avoice-grade channel (4 kHz).

3. LAN terminology: A coaxial cable on which analog signaling is used. Also calledwideband. Contrast withbaseband.

Broadband ISDNSeeBISDN.

Broadband Network InterfaceSeeBNI.

Broadband Network ModuleSeeBNM.

Broadband Switch ModuleSeeBXM.

B 3-13

broadcastData packet that will be sent to all nodes on a network. Broadcasts are identified by abroadcast address. Compare withmulticast andunicast. See alsobroadcast address.

broadcast addressSpecial address reserved for sending a message to all stations. Generally, a broadcastaddress is a MAC destination address of all ones. Compare withmulticast addressandunicast address. See alsobroadcast.

broadcast and unknown serverSeeBUS.

broadcast domainSet of all devices that will receive broadcast frames originating from any device withinthe set. Broadcast domains are typically bounded by routers because routers do notforward broadcast frames.

broadcast searchPropagation of a search request to all network nodes if the location of a resource isunknown to the requester. See alsodirected search.

broadcast stormUndesirable network event in which many broadcasts are sent simultaneously across allnetwork segments. A broadcast storm uses substantial network bandwidth and,typically, causes network time-outs.

brouterConcatenation of “bridge” and “router.” Used to refer to devices which perform bothbridging and routing functions.

browserGUI-based hypertext client application, such as Internet Explorer, Mosaic, andNetscape Navigator, used to access hypertext documents and other services located oninnumerable remote servers throughout the WWW and Internet. See alsohypertext,Internet, Mosaic, andWWW.


BSCbinary synchronous communication. Character-oriented data link layer protocol forhalf-duplex applications. A form of telecommunication line control that uses a standardset of transmission control characters and control character sequences, for binarysynchronous transmission of binary-coded data between stations. Often referred tosimply as .

BSDBerkeley Standard Distribution. Term used to describe any of a variety of UNIX-typeoperating systems based on the UC Berkeley BSD operating system.

BSTUNBlock Serial Tunneling.

BTburst tolerance. Parameter defined by the ATM Forum for ATM traffic management. ForVBR connections, BT determines the size of the maximum burst of contiguous cells thatcan be transmitted. See alsoVBR.

BTUBritish thermal units.

BTWby the way. One of many short-hand phrases used in chat sessions and e-mailconversations. See alsoIMHO.

bufferStorage area used for handling data in transit. Buffers are used in internetworking tocompensate for differences in processing speed between network devices. Bursts of datacan be stored in buffers until they can be handled by slower processing devices.Sometimes referred to as apacket buffer.

buildCreate flat files that are ready for use by the signaling controller database.

burst toleranceSeeBSTUN.

B 3-15

BUSbroadcast and unknown server. Multicast server used in ELANs that is used to floodtraffic addressed to an unknown destination and to forward multicast and broadcasttraffic to the appropriate clients. See alsoELAN.

bus1. Common physical signal path composed of wires or other media across which signalscan be sent from one part of a computer to another. Sometimes called highway.

2. Seebus topology.

bus and tag channelIBM channel, developed in the 1960s, incorporating copper multiwire technology.Replaced by the ESCON channel. See alsoESCON channel andparallel channel.

Bus Interface Gate ArraySeeBIGA in the “Cisco Systems Terms and Acronyms” section.

bus topologyLinear LAN architecture in which transmissions from network stations propagate thelength of the medium and are received by all other stations. Compare withringtopology, star topology, andtree topology.

BVIBridge Group Virtual Interface.

BX.25AT&T implementation of X.25. See alsoX.25.

BXMBroadband Switch Module.

bypass modeOperating mode on FDDI and Token Ring networks in which an interface has beenremoved from the ring.


bypass relayAllows a particular Token Ring interface to be shut down and thus effectively removedfrom the ring.

byteTerm used to refer to a series of consecutive binary digits that are operated upon as aunit (for example, an 8-bit byte).

byte-oriented protocolClass of data-link communications protocols that use a specific character from the usercharacter set to delimit frames. These protocols have largely been replaced bybit-oriented protocols. Compare withbit-oriented protocol.

byte reversalProcess of storing numeric data with the least-significant byte first. Used for integersand addresses on devices with Intel microprocessors.

C H A P T E R

C 4-1

4

CCA

1. certification authority.

2. Telecommunications: call appearance.

cableTransmission medium of copper wire or optical fiber wrapped in a protective cover.

cable rangeRange of network numbers that is valid for use by nodes on an extended AppleTalknetwork. The cable range value can be a single network number or a contiguoussequence of several network numbers. Node addresses are assigned based on the cablerange values.

cable televisionSeeCATV.

CACconnection admission control. Set of actions taken by each ATM switch duringconnection setup in order to determine whether a connection’s requested QoS willviolate the QoS guarantees for established connections. CAC is also used when routinga connection request through an ATM network.

cachingForm of replication in which information learned during a previous transaction is usedto process later transactions.


CAFcontrollable ATM fabric.

cagePiece of hardware into which cards are installed.

California Education and Research Federation NetworkSeeCERFnet.

Call Detail RecordSeeCDR in the “Cisco Systems Terms and Acronyms” section.

call legDiscrete segment of a call connection. A call leg is a logical connection between therouter and either a telephony endpoint over a bearer channel, or another endpoint usinga session protocol.

call priorityPriority assigned to each origination port in circuit-switched systems. This prioritydefines the order in which calls are reconnected. Call priority also defines which callscan or cannot be placed during a bandwidth reservation. See alsobandwidthreservation.

call reference valueSeeCRV.

call setup timeTime required to establish a switched call between DTE devices.

CAMcontent-addressable memory. Seeassociative memory. See alsoCAM in the “CiscoSystems Terms and Acronyms” section.

Canadian Standards AssociationSeeCSA.

C 4-3

CAPCompetitive Access Provider. Independent company providing localtelecommunications services mainly to business customers in competition with anarea’s BOC or IOC. Teleport and MFS are the two major CAPs operating in majormetropolitan areas in the United States. See alsoBOC andIOC.

carrierElectromagnetic wave or alternating current of a single frequency, suitable formodulation by another, data-bearing signal. See alsomodulation.

Carrier DetectSeeCD.

carrier sense multiple access collision detectSeeCSI.

CASchannel associated signaling.

Category 1 cablingOne of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 1cabling is used for telephone communications and is not suitable for transmitting data.Compare withCategory 2 cabling, Category 3 cabling, Category 4 cabling, andCategory 5 cabling. See alsoEIA/TIA-586 andUTP.

Category 2 cablingOne of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 2cabling is capable of transmitting data at speeds up to 4 Mbps. Compare withCategory 1 cabling, Category 3 cabling, Category 4 cabling, andCategory 5 cabling.See alsoEIA/TIA-586 andUTP.

Category 3 cablingOne of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 3cabling is used in 10BaseT networks and can transmit data at speeds up to 10 Mbps.Compare withCategory 1 cabling, Category 2 cabling, Category 4 cabling, andCategory 5 cabling. See alsoEIA/TIA-586 andUTP.


Category 4 cablingOne of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 4cabling is used in Token Ring networks and can transmit data at speeds up to 16 Mbps.Compare withCategory 1 cabling, Category 2 cabling, Category 3 cabling, andCategory 5 cabling. See alsoEIA/TIA-586 andUTP.

Category 5 cablingOne of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 5cabling can transmit data at speeds up to 100 Mbps. Compare withCategory 1 cabling,Category 2 cabling, Category 3 cabling, andCategory 4 cabling. See alsoEIA/TIA-586andUTP.

catenetNetwork in which hosts are connected to diverse networks, which themselves areconnected with routers. The Internet is a prominent example of a catenet.

CATVcable television. Communication system where multiple channels of programmingmaterial are transmitted to homes using broadband coaxial cable. Formerly calledCommunity Antenna Television.

CBACContext-based Access Control. Protocol that provides internal users with secure accesscontrol for each application and for all traffic across network perimeters. CBACenhances security by scrutinizing both source and destination addresses and by trackingeach application’s connection status.

CBDSConnectionless Broadband Data Service. European high-speed, packet-switched,datagram-based WAN networking technology. Similar to SMDS. See alsoSMDS.

CBRconstant bit rate. QoS class defined by the ATM Forum for ATM networks. CBR is usedfor connections that depend on precise clocking to ensure undistorted delivery. Comparewith ABR, UBR, andVBR.

CCBcall control block.

C 4-5

CCITTConsultative Committee for International Telegraph and Telephone. Internationalorganization responsible for the development of communications standards. Now calledthe ITU-T. SeeITU-T.

CCOTcross office transfer time.

CCRcommitment, concurrency, and recovery. OSI application service element used to createatomic operations across distributed systems. Used primarily to implement two-phasecommit for transactions and nonstop operations.

CCScommon channel signaling. Signaling system used in telephone networks that separatessignaling information from user data. A specified channel is exclusively designated tocarry signaling information for all other channels in the system. See alsoSS7.

CDCarrier Detect. Signal that indicates whether an interface is active. Also, a signalgenerated by a modem indicating that a call has been connected.

CDDICopper Distributed Data Interface. Implementation of FDDI protocols over STP andUTP cabling. CDDI transmits over relatively short distances (about 90 yards[100 meters]), providing data rates of 100 Mbps using a dual-ring architecture toprovide redundancy. Based on the ANSI TPPMD standard. Compare withFDDI.

CDFchannel definition format. Technology for “push” applications on the World Wide Web.CDF is an application of XML. SeeXML.

CDPSeeCDP (Cisco Discovery Protocol) in the “Cisco Systems Terms and Acronyms”section.


CDPDCellular Digital Packet Data. Open standard for two-way wireless data communicationover high-frequency cellular telephone channels. Allows data transmissions between aremote cellular link and a NAP. Operates at 19.2 Kbps.

CDRcall detail record.

CDVcell delay variation. Component of cell transfer delay, which is induced by buffering andcell scheduling. CDV is a QoS delay parameter associated with CBR and VBR service.See alsoCBR andVBR.

CDVTcell delay variation tolerance. In ATM, a QoS parameter for managing traffic that isspecified when a connection is set up. In CBR transmissions, CDVT determines thelevel of jitter that is tolerable for the data samples taken by the PCR. See alsoCBRandPCR.

CEFSeeCEF in the in the “Cisco Systems Terms and Acronyms” section.

cellBasic data unit for ATM switching and multiplexing. Cells contain identifiers thatspecify the data stream to which they belong. Each cell consists of a 5-byte header and48 bytes of payload. See alsocell relay.

cell delay variationSeeCDV.

cell delay variation toleranceSeeCDVT.

cell loss prioritySeeCLP.

cell loss ratioSeeCLR.

C 4-7

cell payload scramblingTechnique using an ATM switch to maintain framing on some medium-speed edge andtrunk interfaces.

cell relayNetwork technology based on the use of small, fixed-size packets, or cells. Because cellsare fixed-length, they can be processed and switched in hardware at high speeds. Cellrelay is the basis for many high-speed network protocols including ATM, IEEE 802.6,and SMDS. See alsocell.

cells per secondAbbreviated cps.

cell transfer delaySeeCTD.

Cellular Digital Packet DataSeeCDPD.

cellular radioTechnology that uses radio transmissions to access telephone-company networks.Service is provided in a particular area by a low-power transmitter.

CELPcode excited linear prediction compression. Compression algorithm used in low bit-ratevoice encoding. Used in ITU-T Recommendations G.728, G.729, G.723.1.

central officeSeeCO.

CentrexLEC service that provides local switching applications similar to those provided by anonsite PBX. With Centrex, there is no onsite switching; all customer connections goback to the CO. See alsoCO andLEC.

CEPTConférence Européenne des Postes et des Télécommunications. Association of the26 European PTTs that recommends communication specifications to the ITU-T.


CERcell error ratio. In ATM, the ratio of transmitted cells that have errors to the total cellssent in a transmission for a specific period of time.

CERFnetCalifornia Education and Research Federation Network. TCP/IP network, based inSouthern California, that connects hundreds of higher-education centers internationallywhile also providing Internet access to subscribers. CERFnet was founded in 1988 bythe San Diego Supercomputer Center and General Atomics, and is funded by the NSF.

CERNEuropean Laboratory for Particle Physics. Birthplace of the World Wide Web.

CERTComputer Emergency Response Team. Chartered to work with the Internet communityto facilitate its response to computer security events involving Internet hosts, to takeproactive steps to raise the community’s awareness of computer security issues, and toconduct research targeted at improving the security of existing systems. The U.S. CERTis based at Carnegie Mellon University in Pittsburgh (United States), Regional CERTsare, like NICs, springing up in different parts of the world.

CEScircuit emulation service. Enables users to multiplex or concentrate multiple circuitemulation streams for voice and video with packet data on a single high-speed ATM linkwithout a separate ATM access multiplexer.

CFRADSeeCisco FRAD in the “Cisco Systems Terms and Acronyms” section.

CGICommon Gateway Interface. Set of rules that describe how a Web server communicateswith another application running on the same computer and how the application (calleda CGI program) communicates with the Web server. Any application can be a CGIprogram if it handles input and output according to the CGI standard.

chainingSNA concept in which RUs are grouped together for the purpose of error recovery.

C 4-9

Challenge Handshake Authentication ProtocolSeeCHAP.

channel1. Communication path. Multiple channels can be multiplexed over a single cable incertain environments.

2. In IBM, the specific path between large computers (such as mainframes) and attachedperipheral devices.

3. Specific frequency allocation and bandwidth. Downstream channels are used fortelevision in the United States are 6 MHz wide.

channel-attachedPertaining to attachment of devices directly by data channels (input/output channels) toa computer.

channel definition format.SeeCDF.

Channel Interface ProcessorSeeCIP in the “Cisco Systems Terms and Acronyms” section.

channel service unitSeeCSU.

channelized E1Access link operating at 2.048 Mbps that is subdivided into 30 B-channels and1 D-channel. Supports DDR, Frame Relay, and X.25. Compare withchannelized T1.

channelized T1Access link operating at 1.544 Mbps that is subdivided into 24 channels (23 B-channelsand 1 D-channel) of 64 Kbps each. The individual channels or groups of channelsconnect to different destinations. Supports DDR, Frame Relay, and X.25. Also calledfractional T1. Compare withchannelized E1.


CHAPChallenge Handshake Authentication Protocol. Security feature supported on linesusing PPP encapsulation that prevents unauthorized access. CHAP does not itselfprevent unauthorized access, it merely identifies the remote end. The router or accessserver then determines whether that user is allowed access. Compare toPAP.

chat scriptString of text that defines the login “conversation” that occurs between two systems.Consists of expect-send pairs that define the string that the local system expects toreceive from the remote system and what the local system should send as a reply.

CheapernetIndustry term used to refer to the IEEE 802.3 10Base2 standard or the cable specified inthat standard. Compare withThinnet. See also10Base2, Ethernet, andIEEE 802.3.

checksumMethod for checking the integrity of transmitted data. A checksum is an integer valuecomputed from a sequence of octets taken through a series of arithmetic operations. Thevalue is recomputed at the receiving end and compared for verification.

child peer groupPeer group for which another peer group is the parent peer group. See alsoLGN, peergroup, andparent peer group.

choke packetPacket sent to a transmitter to tell it that congestion exists and that it should reduce itssending rate.

CIAclassical IP over ATM. Specification for running IP over ATM in a manner that takesfull advantage of the features of ATM. Defined in RFC 1577.

CICNetRegional network that connects academic, research, nonprofit, and commercialorganizations in the Midwestern United States. Founded in 1988, CICNet was a part ofthe NSFNET and was funded by the NSF until the NSFNET dissolved in 1995. See alsoNSFNET.

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CICSCustomer Information Control System. IBM application subsystem allowingtransactions entered at remote terminals to be processed concurrently by userapplications.

CID1. craft interface device. Terminal or PC-based interface that enables the performanceof local maintenance operations.

2. channel ID. Designates the Frame Relay subchannel ID for Voice over Frame Relay.

CIDRclassless interdomain routing. Technique supported by BGP4 and based on routeaggregation. CIDR allows routers to group routes together in order to cut down on thequantity of routing information carried by the core routers. With CIDR, several IPnetworks appear to networks outside the group as a single, larger entity. With CIDR, IPaddresses and their subnet masks are written as 4 octets, separated by periods, followedby a forward slash and a 2-digit number that represents the subnet mask. See alsoBGP4.

CIPSeeCIP (Channel Interface Processor) in the “Cisco Systems Terms and Acronyms”section.

CIRcommitted information rate. Rate at which a Frame Relay network agrees to transferinformation under normal conditions, averaged over a minimum increment of time.CIR, measured in bits per second, is one of the key negotiated tariff metrics. See alsoBc.

circuitCommunications path between two or more points.

circuit groupGrouping of associated serial lines that link two bridges. If one of the serial links in acircuit group is in the spanning tree for a network, any of the serial links in the circuitgroup can be used for load balancing. This load-balancing strategy avoids data orderingproblems by assigning each destination address to a particular serial link.


circuit steeringMechanism used by some ATM switches to eavesdrop on a virtual connection and copyits cells to another port where an ATM analyzer is attached. Also known asportsnooping.

circuit switchingSwitching system in which a dedicated physical circuit path must exist between senderand receiver for the duration of the “call.” Used heavily in the telephone companynetwork. Circuit switching can be contrasted withcontention andtoken passing as achannel-access method, and withmessage switching andpacket switching as aswitching technique.

C-ISUPSeeC-ISUP in the “Cisco Systems Terms and Acronyms” section. See alsoISUP.

ciscoBus controllerSeeSP in the “Cisco Systems Terms and Acronyms” section.

Cisco Discovery ProtocolSeeCDP in the “Cisco Systems Terms and Acronyms” section.

Cisco FRADSeeCisco FRAD in the “Cisco Systems Terms and Acronyms” section.

Cisco Frame Relay access deviceSeeCisco FRAD in the “Cisco Systems Terms and Acronyms” section.

CiscoFusionSeeCiscoFusion in the “Cisco Systems Terms and Acronyms” section.

Cisco Internetwork Operating System softwareSeeCisco IOS in the “Cisco Systems Terms and Acronyms” section.

Cisco IOSSeeCisco IOS in the “Cisco Systems Terms and Acronyms” section.

Cisco Link ServicesSeeCLS in the “Cisco Systems Terms and Acronyms” section.

C 4-13

Cisco Link Services InterfaceSeeCLSI in the “Cisco Systems Terms and Acronyms” section.

Cisco-trunk (private line) callSeeCisco-trunk (private line) callin the “Cisco Systems Terms and Acronyms” section.

CiscoViewSeeCisco-trunk (private line) callin the “Cisco Systems Terms and Acronyms” section.

CIXCommercial Internet Exchange. A connection point between the commercial Internetservice providers. Pronounced “kicks.” SeeFIX andGIX.

Class A stationSeeDAS.

Class B stationSeeSAS.

classical IP over ATMSeeCIA.

classless interdomain routingSeeCIDR.

class of serviceSeeCoS.

CLAWCommon Link Access for Workstations. Data link layer protocol used bychannel-attached RISC System/6000 series systems and by IBM 3172 devices runningTCP/IP off-load. CLAW improves efficiency of channel use and allows the CIP toprovide the functionality of a 3172 in TCP/IP environments and support direct channelattachment. The output from TCP/IP mainframe processing is a series of IP datagramsthat the router can switch without modifications.

Clear To SendSeeCTS.


clear channelChannel that uses out-of-band signaling (as opposed to in-band signaling), so thechannel’s entire bit rate is available.

CLECcompetitive local exchange carrier. Company that builds and operates communicationnetworks in metropolitan areas and provides its customers with an alternative to thelocal telephone company. SeeCAP.

CLI1. command line interface. Interface that allows the user to interact with the operatingsystem by entering commands and optional arguments. The UNIX operating system andDOS provide CLIs. Compare withGUI.

2. Command Language Interpreter. Basic Cisco IOS configuration and managementinterface.

clientNode or software program (front-end device) that requests services from a server. Seealsoback end, FRF.11, andserver.

client/server computingTerm used to describe distributed computing (processing) network systems in whichtransaction responsibilities are divided into two parts: client (front end) and server (backend). Both terms (client and server) can be applied to software programs or actualcomputing devices. Also calleddistributed computing (processing). Compare withpeer-to-peer computing. See alsoRPC.

client-server modelCommon way to describe network services and the model user processes (programs) ofthose services. Examples include the nameserver/nameresolver paradigm of the DNSand fileserver/file-client relationships such as NFS and diskless hosts.

CLNPConnectionless Network Protocol. OSI network layer protocol that does not require acircuit to be established before data is transmitted. See alsoCLNS.

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CLNSConnectionless Network Service. OSI network layer service that does not require acircuit to be established before data is transmitted. CLNS routes messages to theirdestinations independently of any other messages. See alsoCLNP.

CLPcell loss priority. Field in the ATM cell header that determines the probability of a cellbeing dropped if the network becomes congested. Cells with CLP = 0 are insured traffic,which is unlikely to be dropped. Cells with CLP = 1 are best-effort traffic, which mightbe dropped in congested conditions in order to free up resources to handle insuredtraffic.

CLRcell loss ratio. In ATM, the ratio of discarded cells to cells that are successfullytransmitted. CLR can be set as a QoS parameter when a connection is set up.

CLTPConnectionless Transport Protocol. Provides for end-to-end Transport data addressing(via Transport selector) and error control (via checksum), but cannot guarantee deliveryor provide flow control. The OSI equivalent of UDP.

cluster controller1. Generally, an intelligent device that provides the connections for a cluster ofterminals to a data link.

2. In SNA, a programmable device that controls the input/output operations of attacheddevices. Typically, an IBM 3174 or 3274 device.

CMI1. coded mark inversion. ITU-T line coding technique specified for STS-3ctransmissions. Also used in DS-1 systems. See alsoDS-1 andSTS-3c.

2. control mode idle?


CMIPCommon Management Information Protocol. OSI network management protocolcreated and standardized by ISO for the monitoring and control of heterogeneousnetworks. See alsoCMIS.

CMISCommon Management Information Services. OSI network management serviceinterface created and standardized by ISO for the monitoring and control ofheterogeneous networks. See alsoCMIP.

CMNSConnection-Mode Network Service. Extends local X.25 switching to a variety of media(Ethernet, FDDI, Token Ring). See alsoCONP.

CMTconnection management. FDDI process that handles the transition of the ring throughits various states (off, active, connect, and so on), as defined by the ANSI X3T9.5specification.

CMTScable modem termination system. Any DOCSIS-compliant headend cable router, suchas the Cisco uBR7246.

COcentral office. Local telephone company office to which all local loops in a given areaconnect and in which circuit switching of subscriber lines occurs.

coaxial cableCable consisting of a hollow outer cylindrical conductor that surrounds a single innerwire conductor. Two types of coaxial cable are currently used in LANs: 50-ohm cable,which is used for digital signaling, and 75-ohm cable, which is used for analog signalingand high-speed digital signaling.

C 4-17

codeccoder-decoder.

1. Integrated circuit device that typically uses pulse code modulation to transformanalog signals into a digital bit stream and digital signals back into analog signals.

2. In Voice over IP, Voice over Frame Relay, and Voice over ATM, a DSP softwarealgorithm used to compress/decompress speech or audio signals.

coded mark inversionSeeCMI.

coder-decoderSeeCODEC.

codingElectrical techniques used to convey binary signals.

CO FRADcentral office frame relay access device.

CO-IPXConnection Oriented IPX. Native ATM protocol based on IPX under development byNovell.

collapsed backboneNondistributed backbone in which all network segments are interconnected by way ofan internetworking device. A collapsed backbone might be a virtual network segmentexisting in a device such as a hub, a router, or a switch.

collisionIn Ethernet, the result of two nodes transmitting simultaneously. The frames from eachdevice impact and are damaged when they meet on the physical media. See alsocollision domain.

collision detectionSeeCSI.


collision domainIn Ethernet, the network area within which frames that have collided are propagated.Repeaters and hubs propagate collisions; LAN switches, bridges and routers do not. Seealsocollision.

command line interfaceSeeCLI.

Committed BurstSeeBc.

committed information rateSeeCIR.

common carrierLicensed, private utility company that supplies communication services to the public atregulated prices.

common channel signalingSeeCCS.

Common Gateway InterfaceSeeCGI.

Common Link Access for WorkstationsSeeCLAW.

Common Management Information ProtocolSeeCMIP.

Common Management Information ServicesSeeCMIS.

common part convergence sublayerSeeCPCS.

Common Programming Interface for CommunicationsSeeCPI-C.

C 4-19

common transport semanticSeeCTS.

communicationTransmission of information.

communication controllerIn SNA, a subarea node (such as an IBM 3745 device) that contains an NCP.

communication serverCommunications processor that connects asynchronous devices to a LAN or WANthrough network and terminal emulation software. Performs only asynchronous routingof IP and IPX. Compare withaccess server.

communications linePhysical link (such as wire or a telephone circuit) that connects one or more devices toone or more other devices.

communityIn SNMP, a logical group of managed devices and NMSs in the same administrativedomain.

Community Antenna TelevisionNow known as CATV. SeeCATV.

community nameSeecommunity string.

community stringText string that acts as a password and is used to authenticate messages sent between amanagement station and a router containing an SNMP agent. The community string issent in every packet between the manager and the agent. Also called acommunity name.

compandingContraction derived from the opposite processes of compression and expansion. Part ofthe PCM process whereby analog signal values are logically rounded to discretescale-step values on a nonlinear scale. The decimal step number is then coded in its


binary equivalent prior to transmission. The process is reversed at the receiving terminalusing the same nonlinear scale. Compare withcompression andexpansion. See alsoa-law andmu-law.

complete sequence number PDUSeeCSNP.

Compressed Serial Link Internet ProtocolSeeCSI.

compressionThe running of a data set through an algorithm that reduces the space required to storeor the bandwidth required to transmit the data set. Compare withcompanding andexpansion.

Computer Science NetworkSeeCSNET.

concentratorSeehub.

CONFconfiguration failure. Resource is OOS because its provisioning information isinconsistent.

Conférence Européenne des Postes et des TélécommunicationsSeeCEPT.

configuration direct VCCIn ATM, a bi-directional point-to-point VCC set up by a LEC to an LES. One of threecontrol connections defined by Phase 1 LANE. Compare withcontrol distribute VCCandcontrol direct VCC.

configuration managementOne of five categories of network management defined by ISO for management of OSInetworks. Configuration management subsystems are responsible for detecting anddetermining the state of a network. See alsoaccounting management, faultmanagement, performance management, andsecurity management.

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configuration registerSeeconfiguration register in the “Cisco Systems Terms and Acronyms” section.

configuration tool1. Service management tool with a GUI.

2. Element management service tool with a GUI.

congestionTraffic in excess of network capacity.

congestion avoidanceMechanism by which an ATM network controls traffic entering the network to minimizedelays. In order to use resources most efficiently, lower-priority traffic is discarded atthe edge of the network if conditions indicate that it cannot be delivered.

congestion collapseCondition in which the retransmission of frames in an ATM network results in little orno traffic successfully arriving at the destination. Congestion collapse frequently occursin ATM networks composed of switches that do not have adequate and effectivebuffering mechanisms complimented by intelligent packet discard or ABR congestionfeedback mechanisms.

connection admission controlSeeCAC.

connectionlessTerm used to describe data transfer without the existence of a virtual circuit. Comparewith connection-oriented. See alsovirtual circuit.

Connectionless Broadband Data ServiceSeeCBDS.

Connectionless Network ProtocolSeeCLNP.

Connectionless Network ServiceSeeCLNS.


connection managementSeeCMT.

Connection-Mode Network ServiceSeeCMNS.

connection-orientedTerm used to describe data transfer that requires the establishment of a virtual circuit.See alsoconnectionless andvirtual circuit.

Connection-Oriented Network ProtocolSeeCONP.

CONPConnection-Oriented Network Protocol. OSI protocol providing connection-orientedoperation to upper-layer protocols. See alsoCMNS.

CONSconnection-oriented network service.

consoleDTE through which commands are entered into a host.

constant bit rateSeeCBR.

Consultative Committee for International Telegraph and TelephoneSeeCCITT.

content-addressable memorySeeassociative memory.

contentionAccess method in which network devices compete for permission to access the physicalmedium. Compare withcircuit switching andtoken passing.

Context-based Access ControlSeeCCB.

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control direct VCCIn ATM, a bidirectional VCC set up by a LEC to a LES. One of three controlconnections defined by Phase 1 LANE. Compare withconfiguration direct VCC andcontrol distribute VCC.

control distribute VCCIn ATM, a unidirectional VCC set up from a LES to a LEC. One of three controlconnections defined by Phase 1 LANE. Typically, the VCC is a point-to-multipointconnection. Compare withconfiguration direct VCC andcontrol direct VCC.

control pointSeeCP.

convergenceSpeed and ability of a group of internetworking devices running a specific routingprotocol to agree on the topology of an internetwork after a change in that topology.

convergence sublayerSeeCS.

conversationIn SNA, an LU 6.2 session between two transaction programs.

cookiePiece of information sent by a Web server to a Web browser that the browser is expectedto save and send back to the Web server whenever the browser makes additional requestsof the Web server.

Cooperation for Open Systems Interconnection Networking in EuropeSeeCOSINE.

COOSCommanded OOS. A resource is OOS because it was entered as a command. See alsoOOS in the “Cisco Systems Terms and Acronyms” section.

Copper Distributed Data InterfaceSeeCDDI.


COPSCommon Open Policy Service. Quality-of-service (QoS) policy exchange protocolproposed as an IETF standard for communicating network QoS policy information.

CORBACommon Object Request Broker Architecture. OMG’s answer to the need forinteroperability among the rapidly proliferating number of hardware and softwareproducts available today. Simply stated, CORBA allows applications to communicatewith one another no matter where they are located or who has designed them. SeeIIOP.

core gatewayPrimary routers in the Internet.

core routerIn a packet-switched star topology, a router that is part of the backbone and that servesas the single pipe through which all traffic from peripheral networks must pass on itsway to other peripheral networks.

Corporation for Open SystemsSeeCOS.

Corporation for Research and Educational NetworkingSeeCREN.

CoSclass of service. Indication of how an upper-layer protocol requires a lower-layerprotocol to treat its messages. In SNA subarea routing, COS definitions are used bysubarea nodes to determine the optimal route to establish a given session. A COSdefinition comprises a virtual route number and a transmission priority field. Also calledToS.

COSCorporation for Open Systems. Organization that promulgates the use of OSI protocolsthrough conformance testing, certification, and related activities.

C 4-25

COSINECooperation for Open Systems Interconnection Networking in Europe. Europeanproject financed by the EC to build a communication network between scientific andindustrial entities in Europe. The project ended in 1994.

costArbitrary value, typically based on hop count, media bandwidth, or other measures, thatis assigned by a network administrator and used to compare various paths through aninternetwork environment. Cost values are used by routing protocols to determine themost favorable path to a particular destination: the lower the cost, the better the path.Sometimes calledpath cost. See alsorouting metric.

COTContinuity Test. Requirement of the SS7 protocol specifications. It tests the bearerchannels’ status using either loopback or tone detection and generation. Used to testindividual DS0 channels via either loopback or tone detection and generation.

count to infinityProblem that can occur in routing algorithms that are slow to converge, in which routerscontinuously increment the hop count to particular networks. Typically, some arbitraryhop-count limit is imposed to prevent this problem.

CP1. control point. In SNA networks, element that identifies the APPN networkingcomponents of a PU 2.1 node, manages device resources, and provides services to otherdevices. In APPN, CPs are able to communicate with logically adjacent CPs by way ofCP-to-CP sessions. See alsoEN andNN.

2. Telecommunications: control processor.

CPCcalling party category.


CPCS1. common part convergence sublayer. One of the two sublayers of any AAL. The CPCSis service-independent and is further divided into the CS and the SAR sublayers. TheCPCS is responsible for preparing data for transport across the ATM network, includingthe creation of the 48-byte payload cells that are passed to the ATM layer. See alsoAAL,ATM layer, CS, SAR, andSSCS.

2. Telecommunications: call processing control system.

CPEcustomer premises equipment. Terminating equipment, such as terminals, telephones,and modems, supplied by the telephone company, installed at customer sites, andconnected to the telephone company network.

CPI-Ccommon programming interface for communications. Platform-independent APIdeveloped by IBM and used to provide portability in APPC applications. See alsoAPPC.

CPNIEcalled party number information element.

CPPSeeCPP(Combinet Proprietary Protocol) in the “Cisco Systems Terms and Acronyms”section.

cpscells per second.

CQcustom queuing.

craft interface deviceSeeCID.

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crankbackA mechanism used by ATM networks when a connection setup request is blockedbecause a node along a selected path cannot accept the request. In this case, the path isrolled back to an intermediate node, which attempts to discover another path to the finaldestination using GCAC. See alsoGCAC.

CRCcyclic redundancy check. Error-checking technique in which the frame recipientcalculates a remainder by dividing frame contents by a prime binary divisor andcompares the calculated remainder to a value stored in the frame by the sending node.

CRENCorporation for Research and Educational Networking. The result of a merger ofBITNET and CSNET. CREN is devoted to providing Internet connectivity to itsmembers, which include the alumni, students, faculty, and other affiliates ofparticipating educational and research institutions, via BITNET III. See alsoBITNET,BITNET III, andCSNET.

CRFConcentrator Relay Function CRMcell rate margin. One of three link attributesexchanged using PTSPs to determine the available resources of an ATM network. CRMis a measure of the difference between the effective bandwidth allocation per trafficclass as the allocation for sustainable cell rate.

CRLcertificate revocation list.

cross talkInterfering energy transferred from one circuit to another.

CRVcall reference value. Number carried in all Q.931 (I.451) messages that provides anidentifier for each ISDN call.

CSconvergence sublayer. One of the two sublayers of the AAL CPCS, which is responsiblefor padding and error checking. PDUs passed from the SSCS are appended with an8-byte trailer (for error checking and other control information) and padded, if


necessary, so that the length of the resulting PDU is divisible by 48. These PDUs arethen passed to the SAR sublayer of the CPCS for further processing. See alsoAAL,CPCS, SAR, andSSCS.

CSACanadian Standards Association. Canadian agency that certifies products that conformto Canadian national safety standards.

CS-ACELPConjugate Structure Algebraic Code Excited Linear Prediction. CELP voicecompression algorithm providing 8 Kbps, or 8:1 compression, standardized in ITU-TRecommendation G.729.

CSIcalled subscriber identification. An identifier whose coding format contains thetelephone number from a remote fax terminal.

CSLIPCompressed Serial Link Internet Protocol. Extension of SLIP that, when appropriate,allows just header information to be sent across a SLIP connection, reducing overheadand increasing packet throughput on SLIP lines. See alsoSLIP.

CSMcall switching module.

CSMA/CDcarrier sense multiple access collision detect. Media-access mechanism wherein devicesready to transmit data first check the channel for a carrier. If no carrier is sensed for aspecific period of time, a device can transmit. If two devices transmit at once, a collisionoccurs and is detected by all colliding devices. This collision subsequently delaysretransmissions from those devices for some random length of time. CSMA/CD accessis used by Ethernet and IEEE 802.3.

CSNETComputer Science Network. Large internetwork consisting primarily of universities,research institutions, and commercial concerns. CSNET merged with BITNET to formCREN. See alsoBITNET andCREN.

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CSNPcomplete sequence number PDU. PDU sent by the designated router in an OSPFnetwork to maintain database synchronization.

CSUchannel service unit. Digital interface device that connects end-user equipment to thelocal digital telephone loop. Often referred to together with DSU, asCSU/DSU. See alsoDSU.

CTDcell transfer delay. In ATM, the elapsed time between a cell exit event at the source UNIand the corresponding cell entry event at the destination UNI for a particular connection.The CTD between the two points is the sum of the total inter-ATM node transmissiondelay and the total ATM node processing delay.

CTIcomputer telephony integration. Name given to the merger of traditionaltelecommunications (PBX) equipment with computers and computer applications. Theuse of Caller ID to automatically retrieve customer information from a database is anexample of a CTI application.

CTS1. Clear To Send. Circuit in the EIA/TIA-232 specification that is activated when DCEis ready to accept data from a DTE.

2. common transport semantic. Cornerstone of the IBM strategy to reduce the numberof protocols on networks. CTS provides a single API for developers of network softwareand enables applications to run over APPN, OSI, and TCP/IP.

CUcoding unit. Type of access device. (Seeaccess device.)

Customer Information Control SystemSeeCICS.

customer premises equipmentSeeCPE.


cut-through packet switchingPacket switching approach that streams data through a switch so that the leading edgeof a packet exits the switch at the output port before the packet finishes entering theinput port. A device using cut-through packet switching reads, processes, and forwardspackets as soon as the destination address is looked up and the outgoing portdetermined. Also known ason-the-fly packet switching. Compare withstore andforward packet switching.

CxBusSeeCxBus(Cisco Extended Bus) in the “Cisco Systems Terms and Acronyms” section.

CyberspaceTerm coined by William Gibson in his fantasy novelNeuromancer to describe the“world” of computers and the society that gathers around them. Often used to refer tothe Internet, the World Wide Web, or some combination thereof.

cycles per secondSeehertz.

cyclic redundancy checkSeeCRC.

C H A P T E R

D 5-1

5

DD4 framing

SeeSF.

DACdual-attached concentrator. FDDI or CDDI concentrator capable of attaching to bothrings of an FDDI or CDDI network. It can also be dual-homed from the master ports ofother FDDI or CDDI concentrators.

DACSDigital Access and Crossconnect System. AT&T’s term for a digital crossconnectsystem.

DAPDirectory Access Protocol. Protocol used between a DUA and a DSA in an X.500directory system. SeeLDAP.

DARPADefense Advanced Research Projects Agency. U.S. government agency that fundedresearch for and experimentation with the Internet. Evolved from ARPA, and then, in1994, back to ARPA. See alsoARPA.

DARPA InternetObsolete term referring to the Internet. SeeInternet.


DAS1. dual attachment station. Device attached to both the primary and the secondary FDDIrings. Dual attachment provides redundancy for the FDDI ring: if the primary ring fails,the station can wrap the primary ring to the secondary ring, isolating the failure andretaining ring integrity. Also called aClass A station. Compare withSAS.

2. dynamically assigned socket. Socket that is dynamically assigned by DDP uponrequest by a client. In an AppleTalk network, the sockets numbered 128 to 254 areallocated as DASs.

DATABASE2SeeDB2.

database objectPiece of information that is stored in a database.

data bus connectorSeeDB connector.

data channelSeeD channel.

data circuit-terminating equipmentSeeDCE.

data communications equipmentSeeDCE.

Data Country CodeSeeDCC.

data direct VCCIn ATM, a bi-directional point-to-point VCC set up between two LECs. One of threedata connections defined by Phase 1 LANE. Data direct VCCs do not offer any type ofQOS guarantee, so they are typically used for UBR and ABR connections. Comparewith control distribute VCC andcontrol direct VCC.

D 5-3

Data Encryption StandardSeeDES.

Data Exchange InterfaceSeeDXI.

data flow control layerLayer 5 of the SNA architectural model. This layer determines and manages interactionsbetween session partners, particularly data flow. Corresponds to thesession layerof theOSI model. See alsodata-link control layer, path control layer, physical control layer,presentation services layer, transaction services layer, andtransmission control layer.

datagramLogical grouping of information sent as a network layer unit over a transmissionmedium without prior establishment of a virtual circuit. IP datagrams are the primaryinformation units in the Internet. The termscell, frame, message, packet, andsegmentare also used to describe logical information groupings at various layers of the OSIreference model and in various technology circles.

Datagram Delivery ProtocolSeeDDP.

DatakitAT&T proprietary packet switching system widely deployed by the RBOCs.

data-link connection identifierSeeDLCI.

data-link control layerLayer 2 in the SNA architectural model. Responsible for the transmission of data overa particular physical link. Corresponds roughly to thedata-link layerof the OSI model.See alsodata flow control layer, path control layer, physical control layer, presentationservices layer, transaction services layer, andtransmission control layer.

data-link layerLayer 2 of the OSI reference model. Provides reliable transit of data across a physicallink. The data-link layer is concerned with physical addressing, network topology, linediscipline, error notification, ordered delivery of frames, and flow control. The IEEE


divided this layer into two sublayers: the MAC sublayer and the LLC sublayer.Sometimes simply calledlink layer. Roughly corresponds to thedata-link control layerof the SNA model. See alsoapplication layer, LLC, MAC, network layer, physical layer,PQ, session layer, andtransport layer.

data-link switchingSeeDLSw.

data-link switching plusSeeDLSw+ in the “Cisco Systems Terms and Acronyms” section.

Data Movement ProcessorSeeDMP in the “Cisco Systems Terms and Acronyms” section.

Data Network Identification CodeSeeDNIC.

data set readySeeDSR.

data service unitSeeDSU.

data sinkNetwork equipment that accepts data transmissions.

data streamAll data transmitted through a communications line in a single read or write operation.

data terminal equipmentSeeDTE.

data terminal readySeeDTR.

dBdecibels.

D 5-5

DB2IBM relational database management system.

DB connectordata bus connector. Type of connector used to connect serial and parallel cables to a databus. DB connector names are in the format DB-x, wherex represents the number ofwires within the connector. Each line is connected to a pin on the connector, but in manycases, not all pins are assigned a function. DB connectors are defined by variousEIA/TIA standards.

dBmdecibles per milliwatt.

DCADefense Communications Agency. U.S. government organization responsible for DDNnetworks such as MILNET. Now calledDISA. SeeDISA.

DCCData Country Code. One of two ATM address formats developed by the ATM Forumfor use by private networks. Adapted from the subnetwork model of addressing in whichthe ATM layer is responsible for mapping network layer addresses to ATM addresses.Compare withICD.

DCE1. data communications equipment (EIA expansion).

2. data circuit-terminating equipment (ITU-T expansion). Devices and connections of acommunications network that comprise the network end of the user-to-networkinterface. The DCE provides a physical connection to the network, forwards traffic, andprovides a clocking signal used to synchronize data transmission between DCE andDTE devices. Modems and interface cards are examples of DCE. Compare withDTE.

DCOMDistributed Component Object Model. Protocol that enables software components tocommunicate directly over a network. Developed by Microsoft and previously calledNetwork OLE, DCOM is designed for use across multiple network transports, includingInternet protocols such as HTTP. SeeIIOP.


DCSDigital Crossconnect System. Network element providing automatic cross-connectionof a digital signal or its constituent parts.

DCTdiscrete cosine transform.

D channel1. data channel. Full-duplex, 16-kbps (BRI) or 64-kbps (PRI) ISDN channel. Comparewith B channel, E channel, andH channel.

2. In SNA, a device that connects a processor and main storage with peripherals.

DDMdistributed data management. Software in an IBM SNA environment that providespeer-to-peer communication and file sharing. One of three SNA transaction services.See alsoDIA andSNADS.

DDNDefense Data Network. U.S. military network composed of an unclassified network(MILNET) and various secret and top-secret networks. DDN is operated andmaintained byDISA. See alsoDISA andMILNET.

DDPDatagram Delivery Protocol. AppleTalk network layer protocol that is responsible forthe socket-to-socket delivery of datagrams over an AppleTalk internetwork.

DDRdial-on-demand routing. Technique whereby a router can automatically initiate andclose a circuit-switched session as transmitting stations demand. The router spoofskeepalives so that end stations treat the session as active. DDR permits routing overISDN or telephone lines using an external ISDN terminal adaptor or modem.

DEdiscard eligible. Seetagged traffic.

D 5-7

deadlock1. Unresolved contention for the use of a resource.

2. In APPN, when two elements of a process each wait for action by or a response fromthe other before they resume the process.

decibelsAbbreviateddB.

DECnetGroup of communications products (including a protocol suite) developed andsupported by Digital Equipment Corporation. DECnet/OSI (also calledDECnet Phase V) is the most recent iteration and supports both OSI protocols andproprietary Digital protocols. Phase IV Prime supports inherent MAC addresses thatallow DECnet nodes to coexist with systems running other protocols that have MACaddress restrictions. See alsoDNA.

DECnet routingProprietary routing scheme introduced by Digital Equipment Corporation in DECnetPhase III. In DECnet Phase V, DECnet completed its transition to OSI routing protocols(ES-IS and IS-IS).

decryptionReverse application of an encryption algorithm to encrypted data, thereby restoring thatdata to its original, unencrypted state. See alsoencryption.

dedicated LANNetwork segment allocated to a single device. Used in LAN switched networktopologies.

dedicated lineCommunications line that is indefinitely reserved for transmissions, rather thanswitched as transmission is required. See alsoleased line.

de facto standardStandard that exists by nature of its widespread use. Compare withde jure standard. Seealsostandard.


default routeRouting table entry that is used to direct frames for which a next hop is not explicitlylisted in the routing table.

Defense Advanced Research Projects AgencySeeDARPA.

Defense Communications AgencySeedBm.

Defense Data NetworkSeeDDN.

Defense Information Systems AgencySeeDISA.

Defense Intelligence AgencySeeDIA.

de jure standardStandard that exists because of its approval by an official standards body. Compare withde facto standard. See alsostandard.

DEKdata encryption key. Used for the encryption of message text and for the computation ofmessage integrity checks (signatures).

delayTime between the initiation of a transaction by a sender and the first response receivedby the sender. Also, the time required to move a packet from source to destination overa given path.

demand priorityMedia access method used in 100VG-AnyLAN that uses a hub that can handle multipletransmission requests and can process traffic according to priority, making it useful forservicing time-sensitive traffic such as multimedia and video. Demand priorityeliminates the overhead of packet collisions, collision recovery, and broadcast traffictypical in Ethernet networks. See also100VG-AnyLAN.

D 5-9

demarcDemarcation point between carrier equipment and CPE.

demodulationProcess of returning a modulated signal to its original form. Modems performdemodulation by taking an analog signal and returning it to its original (digital) form.See alsomodulation.

demultiplexingSeparating of multiple input streams that were multiplexed into a common physicalsignal back into multiple output streams. See alsomultiplexing.

dense mode PIMSeePIM dense mode.

Department of DefenseSeeDoD.

DoD Intelligence Information System Network Security for Information ExchangeSeeDNSIX.

Dependent LUSeeDLU.

Dependent LU RequesterSeeDLUR.

Dependent LU ServerSeeDLUS.

DES1. Data Encryption Standard. Standard cryptographic algorithm developed by the U.S.National Bureau of Standards.

2. destination end station.


designated bridgeBridge that incurs the lowest path cost when forwarding a frame from a segment to theroot bridge.

designated routerOSPF router that generates LSAs for a multiaccess network and has other specialresponsibilities in running OSPF. Each multiaccess OSPF network that has at least twoattached routers has a designated router that is elected by the OSPF Hello protocol. Thedesignated router enables a reduction in the number of adjacencies required on amultiaccess network, which in turn reduces the amount of routing protocol traffic andthe size of the topological database.

destination addressAddress of a network device that is receiving data. See alsosource address.

destination MACSeeDMAC.

destination service access pointSeeDSAP.

deterministic load distributionTechnique for distributing traffic between two bridges across a circuit group.Guarantees packet ordering between source-destination pairs and always forwardstraffic for a source-destination pair on the same segment in a circuit group for a givencircuit-group configuration.

Deutsche Industrie NormSeeDIN.

Deutsche Industrie Norm connectorSeeDIN connector.

deviceSeenode.

D 5-11

DHCPDynamic Host Configuration Protocol. Provides a mechanism for allocating IPaddresses dynamically so that addresses can be reused when hosts no longer need them.

DIADocument Interchange Architecture. Defines the protocols and data formats needed forthe transparent interchange of documents in an SNA network. One of three SNAtransaction services. See alsoDDM andSNADS.

dial backupFeature that provides protection against WAN downtime by allowing the networkadministrator to configure a backup serial line through a circuit-switched connection.

dial-on-demand routingSeeDDR.

dial-up lineCommunications circuit that is established by a switched-circuit connection using thetelephone company network.

differential encodingDigital encoding technique whereby a binary value is denoted by a signal change ratherthan a particular signal level.

differential Manchester encodingDigital coding scheme where a mid-bit-time transition is used for clocking, and atransition at the beginning of each bit time denotes a zero. This coding scheme is usedby IEEE 802.5 and Token Ring networks.

Diffusing Update AlgorithmSeeDUAL in the “Cisco Systems Terms and Acronyms” section.

Digital Network ArchitectureSeeDNA.

digital signal level 0SeeDS-0.




Dijkstra’s algorithmSeeSPF.

DINDeutsche Industrie Norm. German national standards organization.

DIN connectorDeutsche Industrie Norm connector. Multipin connector used in some Macintosh andIBM PC-compatible computers, and on some network processor panels.

directed searchSearch request sent to a specific node known to contain a resource. A directed search isused to determine the continued existence of the resource and to obtain routinginformation specific to the node. See alsobroadcast search.

directed treeLogical construct used to define data streams or flows. The origin of a data stream is theroot. Data streams are unidirectional branches directed away from the root and towardtargets, and targets are the leaves of the directed tree.

direct memory accessSeeDMA.

directory servicesServices that help network devices locate service providers.

DISADefense Information Systems Agency. Formerly DCA. U.S. military organizationresponsible for implementing and operating military information systems, including theDDN. See alsoDDN anddBm.

D 5-13

discard eligibleSeeDE.

discovery architectureAPPN software that enables a machine configured as an APPN EN to automatically findprimary and backup NNs when the machine is brought onto an APPN network.

discovery modeMethod by which an AppleTalk interface acquires information about an attachednetwork from an operational node and then uses this information to configure itself.Also called dynamic configuration.

Distance Vector Multicast Routing ProtocolSeeDVMRP.

distance vector routing algorithmClass of routing algorithms that iterate on the number of hops in a route to find ashortest-path spanning tree. Distance vector routing algorithms call for each router tosend its entire routing table in each update, but only to its neighbors. Distance vectorrouting algorithms can be prone to routing loops, but are computationally simpler thanlink state routing algorithms. Also calledBellman-Ford routing algorithm. See alsolink-state routing algorithmandSPF.

distortion delayProblem with a communication signal resulting from nonuniform transmission speedsof the components of a signal through a transmission medium. Also calledgroup delay.

distributed computing (processing)Seeclient/server computing.

Distributed Data ManagementSeeDDM.

Distributed Queue Dual BusSeeDQDB.

Distributed Relational Database ArchitectureSeeDRDA.


DITDirectory Information Tree. Global tree of entries corresponding to information objectsin the OSI X.500 Directory.

DLCIdata-link connection identifier. Value that specifies a PVC or SVC in a Frame Relaynetwork. In the basic Frame Relay specification, DLCIs are locally significant(connected devices might use different values to specify the same connection). In theLMI extended specification, DLCIs are globally significant (DLCIs specify individualend devices). See alsoLMI.

DLLdynamic link library.

DLSwdata-link switching. Interoperability standard, described in RFC 1434, that provides amethod for forwarding SNA and NetBIOS traffic over TCP/IP networks using data-linklayer switching and encapsulation. DLSw uses SSP instead of SRB, eliminating themajor limitations of SRB, including hop-count limits, broadcast and unnecessary traffic,timeouts, lack of flow control, and lack of prioritization schemes. See alsoSRBandSSP(Switch-to-Switch Protocol).

DLSw+SeeDLSw+ (data-link switching plus) in the “Cisco Systems Terms and Acronyms”section.

DLUDependent LU. LU that depends on the SSCP to provide services for establishingsessions with other LUs. See alsoLU andSSCP.

DLURDependent LU Requester. Client half of the Dependent LU Requestor/Serverenhancement to APPN. The DLUR component resides in APPN ENs and NNs thatsupport adjacent DLUs by securing services from the DLUS. See alsoAPPN, DLU, andDLUS.

D 5-15

DLUR nodeIn APPN networks, an EN or NN that implements the DLUR component. See alsoDLUR.

DLUSDependent LU Server. Server half of the Dependent LU Requestor/Server enhancementto APPN. The DLUS component provides SSCP services to DLUR nodes over anAPPN network. See alsoAPPN, DLU, andDLUR.

DLUS nodeIn APPN networks, a NN that implements the DLUS component. See alsoDLUS.

DMAdirect memory access. Transfer of data from a peripheral device, such as a hard diskdrive, into memory without that data passing through the microprocessor. DMAtransfers data into memory at high speeds with no processor overhead.

DMACdestination MAC. The MAC address specified in the Destination Address field of apacket. Compare withSMAC. See alsoMAC address.

DMPSeeDMP (Data movement processor) in the “Cisco Systems Terms and Acronyms”section.

DNDistinguished Name. Global, authoritative name of an entry in the OSI Directory(X.500).

DNADigital Network Architecture. Network architecture developed by Digital EquipmentCorporation. The products that embody DNA (including communications protocols) arecollectively referred to as DECnet. See alsoDECnet.

DNICData Network Identification Code. Part of an X.121 address. DNICs are divided intotwo parts: the first specifying the country in which the addressed PSN is located and thesecond specifying the PSN itself. See alsoX.121.


DNSDomain Name System. System used in the Internet for translating names of networknodes into addresses. See alsoauthority zone.

DNSIXDepartment of Defense Intelligence Information System Network Security forInformation Exchange. Collection of security requirements for networking defined bythe U.S. Defense Intelligence Agency.

DOCSISData-over-Cable Service Interface Specifications. Defines technical specifications forequipment at both subscriber locations and cable operators’ headends. Adoption ofDOCSIS will accelerate deployment of data-over-cable services and ensureinteroperability of equipment throughout system operators’ infrastructures.

Document Interchange ArchitectureSeeDIA.

DoDDepartment of Defense. U.S. government organization that is responsible for nationaldefense. The DoD has frequently funded communication protocol development.

domain1. In the Internet, a portion of the naming hierarchy tree that refers to general groupingsof networks based on organization-type or geography.

2. In SNA, an SSCP and the resources it controls.

3. In IS-IS, a logical set of networks.

DomainNetworking system developed by Apollo Computer (now part of Hewlett-Packard) foruse in its engineering workstations.

Domain Name SystemSeeDNS.

D 5-17

domain specific partSeeDSP.

dot addressRefers to the common notation for IP addresses in the formn.n.n.nwhere neach numbern represents, in decimal, 1 byte of the 4-byte IP address. Also calleddotted notationandfour-part dotted notation.

dotted decimal notationSyntactic representation for a 32-bit integer that consists of four 8-bit numbers writtenin base 10 with periods (dots) separating them. Used to represent IP addresses in theInternet, as in 192.67.67.20. Also calleddotted quad notation.

dotted notationSeedot address.

downlink stationSeeground station.

downstream physical unitSeeDSPU.

DQDBDistributed Queue Dual Bus. Data-link layer communication protocol, specified in theIEEE 802.6 standard, designed for use in MANs. DQDB, which permits multiplesystems to interconnect using two unidirectional logical buses, is an open standard thatis designed for compatibility with carrier transmission standards, and is aligned withemerging standards for BISDN. SIP is based on DQDB. See alsoMAN.

DRAMdynamic random-access memory. RAM that stores information in capacitors that mustbe periodically refreshed. Delays can occur because DRAMs are inaccessible to theprocessor when refreshing their contents. However, DRAMs are less complex and havegreater capacity than SRAMs. See alsoSRAM.

DRDADistributed Relational Database Architecture. IBM proprietary architecture.


dropPoint on a multipoint channel where a connection to a networked device is made.

drop cableCable that connects a network device (such as a computer) to a physical medium. A typeof AUI. See alsoAUI.

DRPSeeDRP (Director Response Protocol) in the “Cisco Systems Terms and Acronyms”section.

DS-0digital signal level 0. Framing specification used in transmitting digital signals over asingle channel at 64-kbps on a T1 facility. Compare withDS-1 andDS-3.

DS-1digital signal level. Framing specification used in transmitting digital signals at1.544-Mbps on a T1 facility (in the United States) or at 2.108-Mbps on an E1 facility(in Europe). Compare withDS-0 andDS-3. See alsoE1 andT1.

DS-1 domestic trunk interfaceSeeDS-1/DTI.

DS-1/DTIDS-1 domestic trunk interface. Interface circuit used for DS-1 applications with24 trunks.

DS-3digital signal level 3. Framing specification used for transmitting digital signals at44.736 Mbps on a T3 facility. Compare withDS-0 andDS-1. See alsoE3 andT.120.

DSADirectory System Agent. Software that provides the X.500 Directory Service for aportion of the directory information base. Generally, each DSA is responsible for thedirectory information for a single organization or organizational unit.

D 5-19

DSAPdestination service access point. SAP of the network node designated in the Destinationfield of a packet. Compare withSSAP. See alsoSAP (service access point).

DSLdigital subscriber line. Public network technology that delivers high bandwidth overconventional copper wiring at limited distances. There are four types of DSL: ADSL,HDSL, SDSL, and VDSL. All are provisioned via modem pairs, with one modemlocated at a central office and the other at the customer site. Because most DSLtechnologies do not use the whole bandwidth of the twisted pair, there is roomremaining for a voice channel. See alsoADSL, HDSL, SDSL, andVDSL.

DSPdomain specific part. Part of an NSAP-format ATM address that contains an areaidentifier, a station identifier, and a selector byte. See alsoNSAP.

DSPUdownstream physical unit. In SNA, a PU that is located downstream from the host. SeealsoDSPU concentration in the “Cisco Systems Terms and Acronyms” section.

DSPU concentrationSeeDSPU concentration in the “Cisco Systems Terms and Acronyms” section.

DSRdata set ready. EIA/TIA-232 interface circuit that is activated when DCE is powered upand ready for use.

DSUdata service unit. Device used in digital transmission that adapts the physical interfaceon a DTE device to a transmission facility such as T1 or E1. The DSU is also responsiblefor such functions as signal timing. Often referred to together with CSU, asCSU/DSU.See alsoCSU.

DSX-1Crossconnection point for DS-1 signals.


DTEdata terminal equipment. Device at the user end of a user-network interface that servesas a data source, destination, or both. DTE connects to a data network through a DCEdevice (for example, a modem) and typically uses clocking signals generated by theDCE. DTE includes such devices as computers, protocol translators, and multiplexers.Compare withDCE.

DTLdesignated transit list. List of nodes and optional link IDs that completely specify a pathacross a single PNNI peer group.

DTMFdual tone multifrequency. Use of two simultaneous voice-band tones for dialing (suchas touch tone).

DTRdata terminal ready. EIA/TIA-232 circuit that is activated to let the DCE know when theDTE is ready to send and receive data.

DUADirectory User Agent. Software that accesses the X.500 Directory Service on behalf ofthe directory user. The directory user can be a person or another software element.

DUALSeeDUAL (Diffusing update algorithm) in the “Cisco Systems Terms and Acronyms”section.

dual-attached concentratorSeeDAC.

dual attachment stationSeeDAS.

dual counter-rotating ringsNetwork topology in which two signal paths, whose directions are opposite each other,exist in a token-passing network. FDDI and CDDI are based on this concept.

D 5-21

dual-homed stationDevice attached to multiple FDDI rings to provide redundancy.

dual homingNetwork topology in which a device is connected to the network by way of twoindependent access points (points of attachment). One access point is the primaryconnection, and the other is a standby connection that is activated in the event of afailure of the primary connection.

Dual IS-ISSeeIntegrated IS-IS.

dual tone multifrequencySeeDTMF.

DVMRPDistance Vector Multicast Routing Protocol. Internetwork gateway protocol, largelybased on RIP, that implements a typical dense mode IP multicast scheme. DVMRP usesIGMP to exchange routing datagrams with its neighbors. See alsoIGMP.

DXIData Exchange Interface. ATM Forum specification, described in RFC 1483, thatdefines how a network device such as a bridge, router, or hub can effectively act as anFEP to an ATM network by interfacing with a special DSU that performs packetsegmentation and reassembly.

dynamic adaptive routingAutomatic rerouting of traffic based on a sensing and analysis of current actual networkconditions, not including cases of routing decisions taken on predefined information.

dynamic address resolutionUse of an address resolution protocol to determine and store address information ondemand.

Dynamic Buffer ManagementFrame Relay and ATM service modules are equipped with large buffers and the patentedDynamic Buffer Management scheme for allocating and scaling traffic entering orleaving a node on a per-VC basis. The WAN switch dynamically assigns buffers to


individual virtual circuits based upon the amount of traffic present and service-levelagreements. This deep pool of available buffers readily accommodates large bursts oftraffic into the node.

dynamic configurationSeediscovery mode.

Dynamic IISPDynamic Interim-Interswitch Signaling Protocol. Basic call routing protocol thatautomatically reroutes ATM connections in the event of link failures. Dynamic IISP isan interim solution until PNNI Phase 1 is completed. Contrast withIISP.

dynamic random-access memorySeeDRAM.

dynamic routingRouting that adjusts automatically to network topology or traffic changes. Also calledadaptive routing.

C H A P T E R

E 6-1

6

EE&M

recEive and transMit (or ear and mouth). Trunking arrangement generally used fortwo-way switch-to-switch or switch-to-network connections. Cisco’s analog E&Minterface is an RJ-48 connector that allows connections to PBX trunk lines (tie lines).E&M is also available on E1 and T1 digital interfaces.

E1Wide-area digital transmission scheme used predominantly in Europe that carries dataat a rate of 2.048 Mbps. E1 lines can be leased for private use from common carriers.Compare withT1. See alsoDS-1.

E.1641. ITU-T recommendation for international telecommunication numbering, especiallyin ISDN, BISDN, and SMDS. An evolution of standard telephone numbers.

2. Name of the field in an ATM address that contains numbers in E.164 format.

E2ALegacy protocols for providing OAM&P functions between a network element and anoperations support system. See alsoOAM&P.

E3Wide-area digital transmission scheme used predominantly in Europe that carries dataat a rate of 34.368 Mbps. E3 lines can be leased for private use from common carriers.Compare withT.120. See alsoDS-3.

early packet discardSeeEPD.


early token releaseTechnique used in Token Ring networks that allows a station to release a new token ontothe ring immediately after transmitting, instead of waiting for the first frame to return.This feature can increase the total bandwidth on the ring. See alsoToken Ring.

EARNEuropean Academic Research Network. European network connecting universities andresearch institutes. EARN merged with RARE to form TERENA. See alsoRARE andTERENA.

EBCDICextended binary coded decimal interchange code. Any of a number of coded charactersets developed by IBM consisting of 8-bit coded characters. This character code is usedby older IBM systems and telex machines. Compare withASCII.

EBONEEuropean Backbone. Pan-European network backbone service.

E channelecho channel. 64-kbps ISDN circuit-switching control channel. The E channel wasdefined in the 1984 ITU-T ISDN specification, but was dropped in the 1988specification. Compare withB channel, D channel, andH channel.

ECEuropean Community.

echo channelSeeE channel.

echoplexMode in which keyboard characters are echoed on a terminal screen upon return of asignal from the other end of the line indicating that the characters were receivedcorrectly.

ECMAEuropean Computer Manufacturers Association. Group of European computer vendorswho have done substantial OSI standardization work.

E 6-3

edge device1. Physical device that is capable of forwarding packets between legacy interfaces (suchas Ethernet and Token Ring) and ATM interfaces based on data-link and network layerinformation. An edge device does not participate in the running of any network layerrouting protocol, but it obtains forwarding descriptions using the route distributionprotocol.

2. Any device that is not an ATM switch that can connect to an ATM switch.

EDIelectronic data interchange. Electronic communication of operational data such asorders and invoices between organizations.

EDIFACTElectronic Data Interchange for Administration, Commerce, and Transport. Dataexchange standard administered by the United Nations to be a multi-industry EDIstandard.

EECMend-to-end call manager.

EEPROMelectrically erasable programmable read-only memory. EPROM that can be erasedusing electrical signals applied to specific pins. See alsoEPROM.

EFCIExplicit Forward Congestion Indication. In ATM, one of the congestion feedbackmodes allowed by ABR service. A network element in an impending congestion stateor in a congested state can set the EFCI. The destination end-system can implement aprotocol that adaptively lowers the cell rate of the connection based on the value of theEFCI. See alsoABR.

EFFElectronic Frontier Foundation. Foundation established to address social and legalissues arising from the impact on society of the increasingly pervasive use of computersas the means of communication and information distribution.


EGPExterior Gateway Protocol. Internet protocol for exchanging routing informationbetween autonomous systems. Documented in RFC 904. Not to be confused with thegeneral termexterior gateway protocol. EGP is an obsolete protocol that was replacedby BGP. See alsoBGP.

EIAElectronic Industries Association. Group that specifies electrical transmissionstandards. The EIA and TIA have developed numerous well-known communicationsstandards, including EIA/TIA-232 and EIA/TIA-449. See alsoTIA.

EIA-530Refers to two electrical implementations of EIA/TIA-449: RS-422 (for balancedtransmission) and RS-423 (for unbalanced transmission). See alsoRS-422, RS-423, andEIA/TIA-449.

EIA/TIA-232Common physical layer interface standard, developed by EIA and TIA, that supportsunbalanced circuits at signal speeds of up to 64 kbps. Closely resembles the V.24specification. Formerly called asRS-232.

EIA/TIA-449Popular physical layer interface developed by EIA and TIA. Essentially, a faster (up to2 Mbps) version of EIA/TIA-232 capable of longer cable runs. Formerly called RS-449.See alsoEIA-530.

EIA/TIA-586Standard that describes the characteristics and applications for various grades of UTPcabling. See alsoCategory 1 cabling, Category 2 cabling, Category 3 cabling,Category 4 cabling, Category 5 cabling, andUTC.

EIGRPSeeEnhanced IGRP in the “Cisco Systems Terms and Acronyms” section.

EIPSeeEIP (Ethernet Interface Processor) in the “Cisco Systems Terms and Acronyms”section.

E 6-5

EISAExtended Industry-Standard Architecture. 32-bit bus interface used in PCs, PC-basedservers, and some UNIX workstations and servers. See alsoISA.

ELANemulated LAN. ATM network in which an Ethernet or Token Ring LAN is emulatedusing a client-server model. ELANs are composed of an LEC, an LES, a BUS, and anLECS. Multiple ELANs can exist simultaneously on a single ATM network. ELANs aredefined by the LANE specification. See alsoBUS, LANE, LEC, LECS, andLES.

ELAPEtherTalk Link Access Protocol. Link-access protocol used in an EtherTalk network.ELAP is built on top of the standard Ethernet data link layer.

electromagnetic interferenceSeeEMI.

electromagnetic pulseSeeEMP.

electrically erasable programmable read-only memorySeeEECM.

electronic data interchangeSeeEDI.

Electronic Data Interchange for Administration, Commerce, and TransportSeeEDIFACT.

Electronic Frontier FoundationSeeEFF.

Electronic Industries AssociationSeeEIA.

electronic mailSeee-mail.


Electronic Messaging AssociationSeeEMA.

electrostatic dischargeSeeESD.

ELMIEnhanced Local Management Interface.

EMA1. Enterprise Management Architecture. Digital Equipment Corporation networkmanagement architecture, based on the OSI network management model.

2. Electronic Messaging Association. Forum devoted to standards and policy work,education, and development of electronic messaging systems such as e-mail, voice mail,and facsimile.

e-mailelectronic mail. Widely used network application in which text messages aretransmitted electronically between end users over various types of networks usingvarious network protocols.

EMIelectromagnetic interference. Interference by electromagnetic signals that can causereduced data integrity and increased error rates on transmission channels.

EMIFESCON Multiple Image Facility. Mainframe I/O software function that allows oneESCON channel to be shared among multiple logical partitions on the same mainframe.See alsoESCON.

EMPelectromagnetic pulse. Caused by lightning and other high-energy phenomena. Capableof coupling enough energy into unshielded conductors to destroy electronic devices. SeealsoTempest.

emulated LANSeeELAN.

E 6-7

emulation modeFunction of an NCP that enables it to perform activities equivalent to those performedby a transmission control unit.

ENend node. APPN end system that implements the PU 2.1, provides end-user services,and supports sessions between local and remote CPs. ENs are not capable of routingtraffic and rely on an adjacent NN for APPN services. Compare withNN. See alsoCP.

encapsulationWrapping of data in a particular protocol header. For example, Ethernet data is wrappedin a specific Ethernet header before network transit. Also, when bridging dissimilarnetworks, the entire frame from one network is simply placed in the header used by thedata link layer protocol of the other network. See alsotunneling.

encapsulation bridgingCarries Ethernet frames from one router to another across disparate media, such as serialand FDDI lines. Contrast withtranslational bridging.

encoderDevice that modifies information into the required transmission format.

encryptionApplication of a specific algorithm to data so as to alter the appearance of the datamaking it incomprehensible to those who are not authorized to see the information. Seealsodecryption.

end nodeSeeEN.

end of transmissionSeeEOT.

endpointH.323 terminal or gateway. An endpoint can call and be called. It generates andterminates the information stream.


end pointDevice at which a virtual circuit or virtual path begins or ends.

end systemSeeES.

End System-to-Intermediate SystemSeeES-IS.

Energy Sciences NetworkSeeESnet.

Enhanced IGRPSeeEnhanced IGRP in the “Cisco Systems Terms and Acronyms” section.

Enhanced Interior Gateway Routing ProtocolSeeEnhanced IGRP in the “Cisco Systems Terms and Acronyms” section.

Enhanced Monitoring ServicesSeeEnhanced Monitoring Services in the “Cisco Systems Terms and Acronyms”section.

Enterprise Management ArchitectureSeeEMA.

enterprise networkLarge and diverse network connecting most major points in a company or otherorganization. Differs from a WAN in that it is privately owned and maintained.

Enterprise System ConnectionSeeESCON.

Enterprise System Connection channelSeeESCON channel.

entityGenerally, an individual, manageable network device. Sometimes called an alias.

E 6-9

entity identifierThe unique address of an NVEs socket in a node on an AppleTalk network. The specificformat of an entity identifier is network-dependent. See alsoNVE.

entity nameName that an NVE can assign to itself. Although not all NVEs have names, NVEs canpossess several names (or aliases). An entity name is made up of three character strings:object, entity type, and zone. For example: Bldg 2 LaserJet 5:LaserWriter@Bldg 2Zone. See alsoNVE.

entity typePart of an entity name that describes the entity’s class. For example, LaserWriter orAFPServer. See alsoentity name.

EOMend of message. Indicator that identifies the last ATM cell containing information froma data packet that was segmented.

EOTend of transmission. Generally, a character that signifies the end of a logical group ofcharacters or bits.

EPDearly packet discard. Mechanism used by some ATM switches for discarding a completeAAL5 frame when a threshold condition, such as imminent congestion, is met. EPDprevents congestion that would otherwise jeopardize the switch’s ability to properlysupport existing connections with a guaranteed service. Compare withTPD.

EPROMerasable programmable read-only memory. Nonvolatile memory chips that areprogrammed after they are manufactured, and, if necessary, can be erased by somemeans and reprogrammed. Compare withEECM andPROM.

equalizationTechnique used to compensate for communications channel distortions.


ERexplicit rate. In ATM, an RM cell used to limit the ACR for a transmission to a specificvalue. Usually the source initially sets the ER initially to a requested rate, such as thePCR. Later, any network element in the path can reduce the ER to a value that theelement can sustain. See alsoACOM, PCR, andRLM.

erasable programmable read-only memorySeeEPROM.

error controlTechnique for detecting and correcting errors in data transmissions.

error-correcting codeCode having sufficient intelligence and incorporating sufficient signaling information toenable the detection and correction of many errors at the receiver.

error-detecting codeCode that can detect transmission errors through analysis of received data based on theadherence of the data to appropriate structural guidelines.

ES1. end system. Generally, an end-user device on a network.

2. end system. Nonrouting host or node in an OSI network.

ESIend system identifier. Identifier that distinguishes multiple nodes at the same level whenthe lower level peer group is partitioned (usually an IEEE 802 address).

ESCONEnterprise System Connection. IBM channel architecture that specifies a pair offiber-optic cables, with either LEDs or lasers as transmitters, and a signaling rate of200 Mbps.

E 6-11

ESCON channelIBM channel for attaching mainframes to peripherals such as storage devices, backupunits, and network interfaces. This channel incorporates fiber channel technology. TheESCON channel replaces the bus and tag channel. Compare withparallel channel. Seealsobus and tag channel.

ESCON Multiple Image FacilitySeeEMIF.

ESDelectrostatic discharge. Discharge of stored static electricity that can damage electronicequipment and impair electrical circuitry, resulting in complete or intermittent failures.

ESFExtended Superframe. Framing type used on T1 circuits that consists of 24 frames of192 bits each, with the 193rd bit providing timing and other functions. ESF is anenhanced version of SF. See alsoSF.

ES-ISEnd System-to-Intermediate System. OSI protocol that defines how end systems (hosts)announce themselves to intermediate systems (routers). See alsoIS-IS.

ESMTPExtended Simple Mail Transfer Protocol. Extended version of the Simple Mail TransferProtocol (SMTP), which includes additional functionality such as delivery notificationand session delivery. ESMTP is described in RFC 1869, SMTP Service Extensions.

ESnetEnergy Sciences Network. Data communications network managed and funded by theU.S. Department of Energy Office of Energy Research (DOE/OER). Interconnects theDOE to educational institutions and other research facilities.

ESPExtended Services Processor.


ESSElectronic Switching System. AT&T’s term for an electronic central office switch. A5ESS is AT&T’s digital central office for end office applications. A 4ESS is its digitalcentral office for toll center application.

EthernetBaseband LAN specification invented by Xerox Corporation and developed jointly byXerox, Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CDand run over a variety of cable types at 10 Mbps. Ethernet is similar to the IEEE 802.3series of standards. See also10Base2, 10Base5, 10BaseF, 10BaseT, 10Broad36, FastEthernet, andIEEE 802.3.

Ethernet Interface ProcessorSeeEIP in the “Cisco Systems Terms and Acronyms” section.

ethernet meltdownEvent that causes saturation, or near saturation, on an Ethernet. It usually results fromillegal or misrouted packets and typically lasts only a short time.

EtherTalkApple Computer’s data-link product that allows an AppleTalk network to be connectedby Ethernet cable.

EtherTalk Link Access ProtocolSeeELAP.

ETSIEuropean Telecommunication Standards Institute. Organization created by theEuropean PTTs and the EC to propose telecommunications standards for Europe.

EUnetEuropean Internet. European commercial Internet service provider. EUnet is designedto provide e-mail, news, and other Internet services to European markets.

European Academic Research NetworkSeeEARN.

E 6-13

European Computer Manufacturers AssociationSeeECMA.

European Telecommunication Standards InstituteSeeETSI.

European InternetSeeEUnet.

eventNetwork message indicating operational irregularities in physical elements of a networkor a response to the occurrence of a significant task, typically the completion of arequest for information. See alsoalarm andtrap.

EWOSEuropean Workshop for Open Systems. The OSI Implementors Workshop for Europe.

Excess BurstSeeBe.

excess rateIn ATM, traffic in excess of the insured rate for a given connection. Specifically, theexcess rate equals the maximum rate minus the insured rate. Excess traffic is deliveredonly if network resources are available and can be discarded during periods ofcongestion. Compare withinsured rate andmaximum rate.

exchange identificationSeeXID.

EXECSeeEXEC in the “Cisco Systems Terms and Acronyms” section.

expansionThe process of running a compressed data set through an algorithm that restores the dataset to its original size. Compare withcompanding andcompression.


expedited deliveryOption set by a specific protocol layer telling other protocol layers (or the same protocollayer in another network device) to handle specific data more rapidly.

explicit routeIn SNA, a route from a source subarea to a destination subarea, as specified by a list ofsubarea nodes and transmission groups that connect the two.

explicit forward congestion indicationSeeEFCI.

explicit rateSeeER.

explorer frameFrame sent out by a networked device in a SRB environment to determine the optimalroute to another networked device.

explorer packetGenerated by an end station trying to find its way through a SRB network. Gathers ahop-by-hop description of a path through the network by being marked (updated) byeach bridge that it traverses, thereby creating a complete topological map. See alsoall-routes explorer packet, local explorer packet, andspanning explorer packet.

Extended Binary Coded Decimal Interchange CodeSeeEBCDIC.

Extended Industry-Standard ArchitectureSeeEISA.

Extended Services ProcessorSeeESP in the “Cisco Systems Terms and Acronyms” section.

Extended Superframe FormatSeeESF.

E 6-15

exterior gateway protocolAny internetwork protocol used to exchange routing information between autonomoussystems. Not to be confused with Exterior Gateway Protocol (EGP), which is aparticular instance of an exterior gateway protocol.

Exterior Gateway ProtocolSeeEGP.

exterior routerRouter connected to an AURP tunnel, responsible for the encapsulation anddeencapsulation of AppleTalk packets in a foreign protocol header (for example, IP).See alsoAURP andAURP tunnel.

EXZexcessive zeros.


C H A P T E R

F 7-1

7

Ffailure domain

Area in which a failure occurred in a Token Ring, defined by the information containedin a beacon. When a station detects a serious problem with the network (such as a cablebreak), it sends a beacon frame that includes the station reporting the failure, its NAUN,and everything in between. Beaconing in turn initiates a process calledautoreconfiguration. See alsoautoreconfiguration, beacon, andNAUN.

fallbackMechanism used by ATM networks when rigorous path selection does not generate anacceptable path. The fallback mechanism attempts to determine a path by selectivelyrelaxing certain attributes, such as delay, in order to find a path that meets some minimalset of desired attributes.

fan-out unitDevice that allows multiple devices on a network to communicate using a singlenetwork attachment.

fantailPanel of I/O connectors that attaches to an equipment rack, providing easy access fordata connections to a networking.

FAQfrequently asked questions. Usually appears in the form of a “read-me” file in a varietyof Internet forums. New users are expected to read the FAQ before participating innewsgroups, bulletin boards, video conferences, and so on.

FARNETFederation of American Research NETworks.


Fast EthernetAny of a number of 100-Mbps Ethernet specifications. Fast Ethernet offers a speedincrease ten times that of the 10BaseT Ethernet specification, while preserving suchqualities as frame format, MAC mechanisms, and MTU. Such similarities allow the useof existing 10BaseT applications and network management tools on Fast Ethernetnetworks. Based on an extension to the IEEE 802.3 specification. Compare withEthernet. See also100BaseFX, 100BaseT, 100BaseT4, 100BaseTX, 100BaseX, andIEEE 802.3.

Fast Ethernet Interface ProcessorSeeFEIP in the “Cisco Systems Terms and Acronyms” section.

Fast Sequenced TransportSeeFST in the “Cisco Systems Terms and Acronyms” section.

Fast Serial Interface ProcessorSeeFSIP in the “Cisco Systems Terms and Acronyms” section.

fast switchingSeefast switching in the “Cisco Systems Terms and Acronyms” section.

fault managementOne of five categories of network management defined by ISO for management of OSInetworks. Fault management attempts to ensure that network faults are detected andcontrolled. See alsoaccounting management, configuration management, performancemanagement, andsecurity management.

FCCFederal Communications Commission. U.S. government agency that supervises,licenses, and controls electronic and electromagnetic transmission standards.

FCFSfirst come first served.

FCSframe check sequence. Extra characters added to a frame for error control purposes.Used in HDLC, Frame Relay, and other data link layer protocols.

F 7-3

FDDIFiber Distributed Data Interface. LAN standard, defined by ANSI X3T9.5, specifying a100-Mbps token-passing network using fiber-optic cable, with transmission distances ofup to 2 km. FDDI uses a dual-ring architecture to provide redundancy. Compare withCDDI andFDDI II .

FDDI IIANSI standard that enhances FDDI. FDDI II provides isochronous transmission forconnectionless data circuits and connection-oriented voice and video circuits. Comparewith FDDI.

FDDI Interface ProcessorSeeFIP in the “Cisco Systems Terms and Acronyms” section.

FDDITalkApple Computer’s data-link product that allows an AppleTalk network to be connectedby FDDI cable.

FDMfrequency-division multiplexing. Technique whereby information from multiplechannels can be allocated bandwidth on a single wire based on frequency. Compare withATDM, statistical multiplexing, andTDM.

FEFast Ethernet.

FECNforward explicit congestion notification. Bit set by a Frame Relay network to informDTE receiving the frame that congestion was experienced in the path from source todestination. DTE receiving frames with the FECN bit set can request that higher-levelprotocols take flow-control action as appropriate. Compare withBECN.

Federal Communications CommissionSeeFCC.

Federal Networking CouncilSeeFNC.


FEIPSeeFEIP (Fast Ethernet Interface Processor) in the “Cisco Systems Terms andAcronyms” section.

FEPfront-end processor. Device or board that provides network interface capabilities for anetworked device. In SNA, typically an IBM 3745 device.

FGDFeature Group-D.

Fiber Distributed Data InterfaceSeeFDDI.

fiber-optic cablePhysical medium capable of conducting modulated light transmission. Compared withother transmission media, fiber-optic cable is more expensive, but is not susceptible toelectromagnetic interference, and is capable of higher data rates. Sometimes calledoptical fiber.

fiber-optic interrepeater linkSeeFOIRL.

FID0format indicator 0. One of several formats that an SNA TH can use. An FID0 TH is usedfor communication between an SNA node and a non-SNA node. See alsoTH.

FID1format indicator. One of several formats that an SNA TH can use. An FID1 THencapsulates messages between two subarea nodes that do not support virtual andexplicit routes. See alsoTH.

FID2format indicator 2. One of several formats that an SNA TH can use. An FID2 TH is usedfor transferring messages between a subarea node and a PU 2, using local addresses. SeealsoTH.

F 7-5

FID3format indicator 3. One of several formats that an SNA TH can use. An FID3 TH is usedfor transferring messages between a subarea node and a PU 1, using local addresses. SeealsoTH.

FID4format indicator 4. One of several formats that an SNA TH can use. An FID4 THencapsulates messages between two subarea nodes that are capable of supporting virtualand explicit routes. See alsoTH.

field replaceable unitHardware component that can be removed and replaced on-site. Typicalfield-replaceable units include cards, power supplies, and chassis components.

file transferCategory of popular network applications that allow files to be moved from one networkdevice to another.

File Transfer, Access, and ManagementSeeFTAM.

File Transfer ProtocolSeeFTP.

filterGenerally, a process or device that screens network traffic for certain characteristics,such as source address, destination address, or protocol, and determines whether toforward or discard that traffic based on the established criteria.

fingerSoftware tool for determining whether a person has an account at a particular Internetsite. Many sites do not allow incoming finger requests.

FIPSeeFIP (FDDI Interface Processor) in the “Cisco Systems Terms and Acronyms”section.


firewallRouter or access server, or several routers or access servers, designated as a bufferbetween any connected public networks and a private network. A firewall router usesaccess lists and other methods to ensure the security of the private network.

firmwareSoftware instructions set permanently or semipermanently in ROM.

FIXFederal Internet Exchange. Connection point between the North Americangovernmental internets and the Internet. The FIXs are named after their geographicregion, as in FIX West (Mountain View, California) and FIX East (College Park,Maryland). SeeCIX, GIX, andMAE.

flappingRouting problem where an advertised route between two nodes alternates (flaps) backand forth between two paths due to a network problem that causes intermittent interfacefailures.

Flash memoryNonvolatile storage that can be electrically erased and reprogrammed so that softwareimages can be stored, booted, and rewritten as necessary. Flash memory was developedby Intel and is licensed to other semiconductor companies.

flash updateRouting update sent asynchronously in response to a change in the network topology.Compare withrouting update.

flat addressingScheme of addressing that does not use a logical hierarchy to determine location. Forexample, MAC addresses are flat, so bridging protocols must flood packets throughoutthe network in order to deliver the packet to the appropriate location. Compare withhierarchical addressing.

F-linkSS7 fully associated link. An SS7 signaling link directly associated with a link carryingtraffic.

F 7-7

floodingTraffic passing technique used by switches and bridges in which traffic received on aninterface is sent out all of the interfaces of that device except the interface on which theinformation was originally received.

flowStream of data traveling between two endpoints across a network (for example, fromone LAN station to another). Multiple flows can be transmitted on a single circuit.

flow controlTechnique for ensuring that a transmitting entity, such as a modem, does not overwhelma receiving entity with data. When the buffers on the receiving device are full, a messageis sent to the sending device to suspend the transmission until the data in the buffers hasbeen processed. In IBM networks, this technique is called pacing.

flowspecIn IPv6, the traffic parameters of a stream of IP packets between two applications. SeealsoIPv6.

FLTFull Line Terminal. Multiplexer that terminates a SONET span. See alsoSONET.

FMfrequency modulation. Modulation technique in which signals of different frequenciesrepresent different data values. Compare withAM andPAM. See alsomodulation.

FNCFederal Networking Council. Group responsible for assessing and coordinating U.S.federal agency networking policies and needs.

FOIRLfiber-optic interrepeater link. Fiber-optic signaling methodology based on theIEEE 802.3 fiber-optic specification. FOIRL is a precursor of the 10BaseFLspecification, which is designed to replace it. See also10BaseFL.

format indicator 0SeeFID0.






forward channelCommunications path carrying information from the call initiator to the called party.

forward delay intervalAmount of time an interface spends listening for topology change information after thatinterfaceis activated for bridging and before forwarding actually begins.

forward explicit congestion notificationSeeFE.

forwardingProcess of sending a frame toward its ultimate destination by way of an internetworkingdevice.

FOTSFiber Optics Transmission Systems. Vendor-proprietary fiber-optic transmissionequipment.

Fourier transformTechnique used to evaluate the importance of various frequency cycles in a time seriespattern.

four-part dotted notationSeedot address.

F 7-9

FQDNfully qualified domain name. FQDN is the full name of a system, rather than just its hostname. For example, aldebaran is a host name, and aldebaran.interop.com is an FQDN.

fractional T1Seechannelized T1.

FRADFrame Relay access device. Any network device that provides a connection between aLAN and a Frame Relay WAN. See alsoCisco FRAD (Cisco Frame Relay accessdevice) andFRAS (Frame Relay access support) in the “Cisco Systems Terms andAcronyms” section.

fragmentPiece of a larger packet that has been broken down to smaller units.

fragmentationProcess of breaking a packet into smaller units when transmitting over a networkmedium that cannot support the original size of the packet. See alsoreassembly.

frameLogical grouping of information sent as a data link layer unit over a transmissionmedium. Often refers to the header and trailer, used for synchronization and errorcontrol, that surround the user data contained in the unit. The termscell, datagram,message, packet, andsegmentare also used to describe logical information groupings atvarious layers of the OSI reference model and in various technology circles.

frame check sequenceSeeFCFS.

frame forwardingMechanism by which frame-based traffic, such asHDLC andSDLC, traverses an ATMnetwork.


Frame RelayIndustry-standard, switched data link layer protocol that handles multiple virtualcircuits using HDLC encapsulation between connected devices. Frame Relay is moreefficient than X.25, the protocol for which it is generally considered a replacement. SeealsoX.25.

Frame Relay access deviceSeeFRAD.

Frame Relay access supportSeeFRAS in the “Cisco Systems Terms and Acronyms” section.

Frame Relay bridgingBridging technique, described in RFC 1490, that uses the same spanning-tree algorithmas other bridging functions, but allows packets to be encapsulated for transmissionacross a Frame Relay network.

frame switchSeeLAN switch.

FRASSeeFRAS(Frame Relay access support) in the “Cisco Systems Terms and Acronyms”section.

FRASMFrame Relay access service module.

freenetCommunity-based bulletin board system with e-mail, information services, interactivecommunications, and conferencing.

free-trade zonePart of an AppleTalk internetwork that is accessible by two other parts of theinternetwork that are unable to directly access one another.

frequencyNumber of cycles, measured in hertz, of an alternating current signal per unit time.

F 7-11

frequency-division multiplexingSeeFDM.

frequency modulationSeeFM.

FRFFrame Relay Forum. An association of corporate members consisting of vendors,carriers, users and consultants committed to the implementation of Frame Relay inaccordance with national and international standards. See www.frforum.com.

FRF.11Frame Relay Forum implementation agreement for Voice over Frame Relay (v1.0 May1997). This specification defines multiplexed data, voice, fax, DTMF digit-relay andCAS/Robbed-bit signaling frame formats, but does not include call setup, routing oradministration facilities. See www.frforum.com.

FRF.11 Annex CSeeFRF.12.

FRF11-trunkPoint to point permanent voice connection (private line) conforming to the FRF.11specification.

FRF.12The FRF.12 Implementation Agreement (also known as FRF.11 Annex C) wasdeveloped to allow long data frames to be fragmented into smaller pieces andinterleaved with real-time frames. In this way, real-time voice and non real-time dataframes can be carried together on lower speed links without causing excessive delay tothe real-time traffic. See www.frforum.com.

FRMRFrame REJECT.

front endNode or software program that requests services of a back end. See alsoback end, client,andserver.


front-end processorSeeFEP.

FSIPSeeFSIP(Fast Serial Interface Processor) in the “Cisco Systems Terms and Acronyms”section.

FSTSeeFST (Fast Sequenced Transport) in the “Cisco Systems Terms and Acronyms”section.

FTAMFile Transfer, Access, and Management. In OSI, an application layer protocoldeveloped for network file exchange and management between diverse types ofcomputers.

FTPFile Transfer Protocol. Application protocol, part of the TCP/IP protocol stack, used fortransferring files between network nodes. FTP is defined in RFC 959.

full duplexCapability for simultaneous data transmission between a sending station and a receivingstation. Compare withhalf duplex andsimplex.

full meshTerm describing a network in which devices are organized in a mesh topology, with eachnetwork node having either a physical circuit or a virtual circuit connecting it to everyother network node. A full mesh provides a great deal of redundancy, but because it canbe prohibitively expensive to implement, it is usually reserved for network backbones.See alsomesh andpartial mesh.

fully qualified domain nameSeeFQDN.

FUNIframe user network interface.

F 7-13

FuzzballDigital Equipment Corporation LSI-11 computer system running IP gateway software.The NSFnet used these systems as backbone packet switches.

FXOForeign Exchange Office. An FXO interface connects to the Public Switched TelephoneNetwork’s (PSTN) central office and is the interface offered on a standard telephone.Cisco’s FXO interface is an RJ-11 connector that allows an analog connection to bedirected at the PSTN’s central office or to a station interface on a PBX.

FXSForeign Exchange Station. An FXS interface connects directly to a standard telephoneand supplies ring, voltage, and dial tone. Cisco’s FXS interface is an RJ-11 connectorthat allows connections to basic telephone service equipment, keysets, and PBXes.


C H A P T E R

G 8-1

8

GG.703/G.704

ITU-T electrical and mechanical specifications for connections between telephonecompany equipment and DTE using BNC connectors and operating at E1 data rates.

G.711Describes the 64-kbps PCM voice coding technique. In G.711, encoded voice is alreadyin the correct format for digital voice delivery in the PSTN or through PBXs. Describedin the ITU-T standard in its G-series recommendations.

G.723.1Describes a compression technique that can be used for compressing speech or audiosignal components at a very low bit rate as part of the H.324 family of standards. ThisCODEC has two bit rates associated with it: 5.3 and 6.3 kbps. The higher bit rate isbased on ML-MLQ technology and provides a somewhat higher quality of sound. Thelower bit rate is based on CELP and provides system designers with additionalflexibility. Described in the ITU-T standard in its G-series recommendations.

G.726Describes ADPCM coding at 40, 32, 24, and 16 kbps. ADPCM-encoded voice can beinterchanged between packet voice, PSTN, and PBX networks if the PBX networks areconfigured to support ADPCM. Described in the ITU-T standard in its G-seriesrecommendations.

G.728Describes a 16-kbps low-delay variation of CELP voice compression. CELP voicecoding must be translated into a public telephony format for delivery to or through thePSTN. Described in the ITU-T standard in its G-series recommendations.


G.729Describes CELP compression where voice is coded into 8-kbps streams. There are twovariations of this standard (G.729 and G.729 Annex A) that differ mainly incomputational complexity; both provide speech quality similar to 32-kbps ADPCM.Described in the ITU-T standard in its G-series recommendations.

G.804ITU-T framing standard that defines the mapping of ATM cells into the physicalmedium.

Gatekeeper1. Component of an H.323 conferencing system that performs call address resolution,admission control, and subnet bandwidth management.

2. Telecommunications: H.323 entity on a LAN that provides address translation andcontrol access to the LAN for H.323 terminals and gateways. The gatekeeper canprovide other services to the H.323 terminals and gateways, such as bandwidthmanagement and locating gateways. A gatekeeper maintains a registry of devices in themultimedia network. The devices register with the gatekeeper at startup and requestadmission to a call from the gatekeeper.

gatewayIn the IP community, an older term referring to a routing device. Today, the termrouteris used to describe nodes that perform this function, andgateway refers to aspecial-purpose device that performs an application layer conversion of informationfrom one protocol stack to another. Compare withrouter.

Gateway Discovery ProtocolSeeGDP in the “Cisco Systems Terms and Acronyms” section.

gateway hostIn SNA, a host node that contains a gateway SSCP.

gateway NCPNCP that connects two or more SNA networks and performs address translation to allowcross-network session traffic.

G 8-3

Gateway-to-Gateway ProtocolSeeGGP.

GBgigabyte. Approximately 1,000,000,000 bytes.

GBpsgigabytes per second.

Gbgigabit. Approximately 1,000,000,000 bits.

Gbpsgigabits per second.

GCACgeneric connection admission control. In ATM, a PNNI algorithm designed for CBRand VBR connections. Any node can use GCAC to calculate the expected CACbehavior of another node given than node’s advertised link metrics and the QoS of aconnection setup request. See alsoCAC.

GCRAgeneric cell rate algorithm. In ATM, an algorithm that defines conformance with respectto the traffic contract of the connection. For each cell arrival, the GCRA determineswhether the cell conforms to the traffic contract.

GDPSeeGDP (Gateway Discovery Protocol) in the “Cisco Systems Terms and Acronyms”section.

generic connection admission controlSeeGCAC.

generic routing encapsulationSeeGRE in the “Cisco Systems Terms and Acronyms” section.

Get Nearest ServerSeeGNS.


GGPGateway-to-Gateway Protocol. MILNET protocol specifying how core routers(gateways) should exchange reachability and routing information. GGP uses adistributed shortest-path algorithm.

GHzgigahertz.

gigabitAbbreviated Gb.

gigabits per secondAbbreviated Gbps.

gigabyteAbbreviated GB.

gigabytes per secondAbbreviated GBps.

gigahertzAbbreviated GHz.

GIXGlobal Internet eXchange. Common routing exchange point which allows pairs ofnetworks to implement agreed-upon routing policies. The GIX is intended to allowmaximum connectivity to the Internet for networks all over the world. SeeCIX, FIX, andMAE.

gleaningProcess by which a router automatically derives AARP table entries from incomingpackets. Gleaning speeds up the process of populating the AARP table. See alsoAARP.

G 8-5

GNSGet Nearest Server. Request packet sent by a client on an IPX network to locate thenearest active server of a particular type. An IPX network client issues a GNS requestto solicit either a direct response from a connected server or a response from a routerthat tells it where on the internetwork the service can be located. GNS is part of the IPXSAP. See alsoIPX andSAP (Service Advertisement Protocol).

goodputGenerally referring to the measurement of actual data successfully transmitted from thesender(s) to receiver(s). This is often a more useful measurement than the number ofATM cells per second throughput of an ATM switch if that switch is experiencing cellloss that results in many incomplete, and therefore unusable, frames arriving at therecipient.

Gopherdistributed document delivery system. The Internet Gopher allows a neophyte user toaccess various types of data residing on multiple hosts in a seamless fashion.

GOSIPGovernment OSI Profile. U.S. government procurement specification for OSI protocols.Through GOSIP, the government mandates that all federal agencies standardize on OSIand implement OSI-based systems as they become commercially available.

Government OSI ProfileSeeGOSIP.

grade of serviceMeasure of telephone service quality based on the probability that a call will encountera busy signal during the busiest hours of the day.

graphical user interfaceSeeGUI.

GRESeeGRE(generic routing encapsulation) in the “Cisco Systems Terms and Acronyms”section.


ground stationCollection of communications equipment designed to receive signals from (and usuallytransmit signals to) satellites. Also called a downlink station.

Group 3Standard created by the International Telecommunications Union Telecommunications(ITU-T) relating to fax devices. A Group 3 fax device is a digital machine containing a14400 baud modem that can transmit an 8 1/2 by 11 inch page in approximately20 seconds with a resolution of either 203 by 98 dots per inch (dpi) or 203 by 196 dpi(fine), using Huffman code to compress fax data. Group 3 faxes use a standard dial-uptelephone line for transmission.

group addressSeemulticast address.

group delaySeedistortion delay.

GSSGeneric Service State.

guard bandUnused frequency band between two communications channels that provides separationof the channels to prevent mutual interference.

GUIgraphical user interface. User environment that uses pictorial as well as textualrepresentations of the input and output of applications and the hierarchical or other datastructure in which information is stored. Conventions such as buttons, icons, andwindows are typical, and many actions are performed using a pointing device (such asa mouse). Microsoft Windows and the Apple Macintosh are prominent examples ofplatforms using a GUI.

C H A P T E R

H 9-1

9

HH.225.0

An ITU standard that governs H.225.0 session establishment and packetization. H.225.0actually describes several different protocols: RAS, use of Q.931, and use of RTP.

H.245An ITU standard that governs H.245 endpoint control.

H.320Suite of ITU-T standard specifications for videoconferencing over circuit-switchedmedia such as ISDN, fractional T-1, and switched-56 lines.

H.323Extension of ITU-T standard H.320 that enables videoconferencing over LANs andother packet-switched networks, as well as video over the Internet.

H.323 RASregistration, admission, and status. The RAS signaling protocol performs registration,admissions, bandwidth changes, and status and disengage procedures between the VoIPgateway and the gatekeeper.

hairpinTelephony term that means to send a call back in the direction that it came from. Forexample, if a call cannot be routed over IP to a gateway that is closer to the targettelephone, the call is typically sent back out the local zone, back the way it came from.


half duplexCapability for data transmission in only one direction at a time between a sendingstation and a receiving station. BSC is an example of a half-duplex protocol. See alsoBSC. Compare withfull duplex andsimplex.

handshakeSequence of messages exchanged between two or more network devices to ensuretransmission synchronization.

hardware addressSeeMAC address.

HBD3Line code type used on E1 circuits.

H channelhigh-speed channel. Full-duplex ISDN primary rate channel operating at 384 Kbps.Compare withB channel, D channel, andE channel.

HDLCHigh-Level Data Link Control. Bit-oriented synchronous data link layer protocoldeveloped by ISO. Derived from SDLC, HDLC specifies a data encapsulation methodon synchronous serial links using frame characters and checksums. See alsoSDLC.

HDSLhigh-data-rate digital subscriber line. One of four DSL technologies. HDSL delivers1.544 Mbps of bandwidth each way over two copper twisted pairs. Because HDSLprovides T1 speed, telephone companies have been using HDSL to provision localaccess to T1 services whenever possible. The operating range of HDSL is limited to12,000 feet (3658.5 meters), so signal repeaters are installed to extend the service.HDSL requires two twisted pairs, so it is deployed primarily for PBX networkconnections, digital loop carrier systems, interexchange POPs, Internet servers, andprivate data networks. Compare withADSL, SDSL, andVDSL.

headendEnd point of a broadband network. All stations transmit toward the headend; theheadend then transmits toward the destination stations.

H 9-3

headerControl information placed before data when encapsulating that data for networktransmission. Compare withtrailer. See alsoPCI.

heartbeatSeeSQE.

HECheader error control. Algorithm for checking and correcting an error in an ATM cell.Using the fifth octet in the ATM cell header, ATM equipment will check for an error andcorrect the contents of the header. The check character is calculated using a CRCalgorithm allowing a single bit error in the header to be corrected or multiple errors tobe detected.

HELLOInterior routing protocol used principally by NSFnet nodes. HELLO allows particularpacket switches to discover minimal delay routes. Not to be confused with theHelloprotocol.

hello packetMulticast packet that is used by routers for neighbor discovery and recovery. Hellopackets also indicate that a client is still operating and network-ready.

Hello protocolProtocol used by OSPF systems for establishing and maintaining neighborrelationships. Not to be confused with HELLO.

HEPnetHigh-Energy Physics Network. Research network that originated in the United States,but that has spread to most places involved in high-energy physics. Well-known sitesinclude Argonne National Laboratory, Brookhaven National Laboratory, LawrenceBerkeley Laboratory, and the SLAC.

hertzMeasure of frequency. Abbreviated Hz. Synonymous with cycles per second.


heterogeneous networkNetwork consisting of dissimilar devices that run dissimilar protocols and in many casessupport dissimilar functions or applications.

HFChybrid fiber-coaxial. Technology being developed by the cable TV industry to providetwo-way, high-speed data access to the home using a combination of fiber optics andtraditional coaxial cable.

hierarchical addressingScheme of addressing that uses a logical hierarchy to determine location. For example,IP addresses consist of network numbers, subnet numbers, and host numbers, which IProuting algorithms use to route the packet to the appropriate location. Compare withflataddressing.

hierarchical routingThe complex problem of routing on large networks can be simplified by reducing thesize of the networks. This is accomplished by breaking a network into a hierarchy ofnetworks, where each level is responsible for its own routing.

High-Energy Physics NetworkSeeHEPnet.

High-Level Data Link ControlSeeHDLC.

High Performance Computing and CommunicationsSeeHPCC.

High Performance Computing SystemsSeeHPCS.

High-Performance Parallel InterfaceSeeHIPPI.

High Performance RoutingSeeHPR.

H 9-5

High-Speed Communications InterfaceSeeHSCI in the “Cisco Systems Terms and Acronyms” section.

High-Speed Serial InterfaceSeeHSSI.

highwaySeebus.

HIPSeeHIP (HSSI Interface Processor) in the “Cisco Systems Terms and Acronyms”section.

HIPPIHigh-Performance Parallel Interface. High-performance interface standard defined byANSI. HIPPI is typically used to connect supercomputers to peripherals and otherdevices.

holddownState into which a route is placed so that routers will neither advertise the route noraccept advertisements about the route for a specific length of time (the holddownperiod). Holddown is used to flush bad information about a route from all routers in thenetwork. A route is typically placed in holddown when a link in that route fails.

homologationConformity of a product or specification to international standards, such as ITU-T, CSA,TUV, UL, or VCCI. Enables portability across company and international boundaries.

hookflashShort on-hook period usually generated by a telephone-like device during a call toindicate that the telephone is attempting to perform a dial-tone recall from a PBX.Hookflash is often used to perform call transfer.

hopPassage of a data packet between two network nodes (for example, between tworouters). See alsohop count.


hop countRouting metric used to measure the distance between a source and a destination. RIPuses hop count as its sole metric. See alsohookflash andRIP.

hop offPoint at which a call transitions from H.323 to non-H.323, typically at a gateway.

hostComputer system on a network. Similar to node, except that host usually implies acomputer system, whereas node generally applies to any networked system, includingaccess servers and routers. See alsonode.

host addressSeehost number.

host nameName given to a machine. SeeFQDN.

host nodeSNA subarea node that contains an SSCP. See alsoSSCP.

host numberPart of an IP address that designates which node on the subnetwork is being addressed.Also called ahost address.

Hot Standby Router ProtocolSeeHSRP in the “Cisco Systems Terms and Acronyms” section.

hot swappingSeeOIR andpower-on servicing.

HPCCHigh-Performance Computing and Communications. U.S. government funded programadvocating advances in computing, communications, and related fields. The HPCC isdesigned to ensure U.S. leadership in these fields through education, research anddevelopment, industry collaboration, and implementation of high-performancetechnology. See also the five components of the HPCC:ASTA, BRHR, HPCS, IITA, andNREN.

H 9-7

HPCSHigh-Performance Computing Systems. Component of the HPCC program designed toensure U.S. technological leadership in high-performance computing through researchand development of computing systems and related software. See alsoHPCC.

HPRHigh-Performance Routing. Second-generation routing algorithm for APPN. HPRprovides a connectionless layer with nondisruptive routing of sessions around linkfailures, and a connection-oriented layer with end-to-end flow control, error control, andsequencing. Compare toISR. See alsoAPPN.

HSCISeeHSCI (High-Speed Communications Interface) in the “Cisco Systems Terms andAcronyms” section.

HSRPSeeHSRP(Hot Standby Router Protocol) in the “Cisco Systems Terms and Acronyms”section.

HSSIHigh-Speed Serial Interface. Network standard for high-speed (up to 52 Mbps) serialconnections over WAN links.

HSSI Interface ProcessorSeeHIP in the “Cisco Systems Terms and Acronyms” section.

HTTPHypertext Transfer Protocol. The protocol used by Web browsers and Web servers totransfer files, such as text and graphic files.

HTMLHypertext Markup Language. Simple hypertext document formatting language that usestags to indicate how a given part of a document should be interpreted by a viewingapplication, such as a Web browser. See alsohypertext andWeb browser.


hub1. Generally, a term used to describe a device that serves as the center of a star-topologynetwork.

2. Hardware or software device that contains multiple independent but connectedmodules of network and internetwork equipment. Hubs can be active (where they repeatsignals sent through them) or passive (where they do not repeat, but merely split, signalssent through them).

3. In Ethernet and IEEE 802.3, an Ethernet multiport repeater, sometimes called aconcentrator.

hybrid networkInternetwork made up of more than one type of network technology, including LANsand WANs.

hyperlinkPointer within a hypertext document that points (links) to another document, which mayor may not also be a hypertext document.

hypertextElectronically-stored text that allows direct access to other texts by way of encodedlinks. Hypertext documents can be created using HTML, and often integrate images,sound, and other media that are commonly viewed using a browser. See alsoHTML andbrowser.

Hypertext Transfer ProtocolSeeHTTP.

Hypertext Markup LanguageSeeHTML.

HzSeehertz.

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IIAB

Internet Architecture Board. Board of internetwork researchers who discuss issuespertinent to Internet architecture. Responsible for appointing a variety ofInternet-related groups such as the IANA, IESG, and IRSG. The IAB is appointed bythe trustees of the ISOC. See alsoIANA, IESG, IRSG, andISOC.

IAHCInternet International Ad Hoc Committee. Coalition of participants from the broadInternet community, working to satisfy the requirement for enhancements to theInternet’s global DNS. Organizations naming members to the committee includeInternet Society (ISOC), Internet Assigned Numbers Authority (IANA), InternetArchitecture Board (IAB), Federal Networking Council (FNC), InternationalTelecommunication Union (ITU), International Trademark Association (INTA), andWorld Intellectual Property Organization (WIPO).

IANAInternet Assigned Numbers Authority. Organization operated under the auspices of theISOC as a part of the IAB. IANA delegates authority for IP address-space allocation anddomain-name assignment to the InterNIC and other organizations. IANA also maintainsa database of assigned protocol identifiers used in the TCP/IP stack, includingautonomous system numbers. See alsoICP cell, ISOC, andInterNIC.

ICDInternational Code Designator. One of two ATM address formats developed by the ATMForum for use by private networks. Adapted from the subnetwork model of addressingin which the ATM layer is responsible for mapping network layer addresses to ATMaddresses. Compare withDCC.


ICMPInternet Control Message Protocol. Network layer Internet protocol that reports errorsand provides other information relevant to IP packet processing. Documented inRFC 792.

ICMP Router Discovery ProtocolSeeIRDP.

ICP cellIMA control protocol cell used for aligning the cells in multiple links.

ICRinitial cell rate.

I-DInternet-Draft. Working documents of the IETF, from its Areas and Working Groups.They are valid for a maximum of 6 months and might be updated, replaced, or obsoletedby other documents at any time. Very often, I-Ds are precursors to RFCs.

IDIinitial domain identifier. Portion of an NSAP or NSAP-format ATM address thatspecifies the address allocation and administration authority. See alsoNSAP.

IDNInternational Data Number. SeeX.121.

IDPinitial domain part. Part of a CLNS address that contains an authority and formatidentifier, and a domain identifier.

IDPRInterdomain Policy Routing. Interdomain routing protocol that dynamically exchangespolicies between autonomous systems. IDPR encapsulates interautonomous systemtraffic and routes it according to the policies of each autonomous system along the path.IDPR is currently an IETF proposal. See alsopolicy-based routing.

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IDRPIS-IS Interdomain Routing Protocol. OSI protocol that specifies how routerscommunicate with routers in different domains.

IEinformation element.

IECInternational Electrotechnical Commission. Industry group that writes and distributesstandards for electrical products and components.

IEEEInstitute of Electrical and Electronics Engineers. Professional organization whoseactivities include the development of communications and network standards. IEEELAN standards are the predominant LAN standards today.

IEEE 802.1IEEE specification that describes an algorithm that prevents bridging loops by creatinga spanning tree. The algorithm was invented by Digital Equipment Corporation. TheDigital algorithm and the IEEE 802.1 algorithm are not exactly the same, nor are theycompatible. See alsospanning tree, spanning-tree algorithm, andSpanning-TreeProtocol.

IEEE 802.12IEEE LAN standard that specifies the physical layer and the MAC sublayer of the datalink layer. IEEE 802.12 uses the demand priority media-access scheme at 100 Mbpsover a variety of physical media. See also100VG-AnyLAN.

IEEE 802.2IEEE LAN protocol that specifies an implementation of the LLC sublayer of the datalink layer. IEEE 802.2 handles errors, framing, flow control, and the network layer(Layer 3) service interface. Used in IEEE 802.3 and IEEE 802.5 LANs. See alsoIEEE802.3 andIEEE 802.5.

IEEE 802.3IEEE LAN protocol that specifies an implementation of the physical layer and the MACsublayer of the data link layer. IEEE 802.3 uses CSMA/CD access at a variety of speedsover a variety of physical media. Extensions to the IEEE 802.3 standard specify


implementations for Fast Ethernet. Physical variations of the original IEEE 802.3specification include10Base2, 10Base5, 10BaseF, 10BaseT, and10Broad36. Physicalvariations forFast Ethernet include100BaseT, 100BaseT4, and100BaseX.

IEEE 802.4IEEE LAN protocol that specifies an implementation of the physical layer and the MACsublayer of the data link layer. IEEE 802.4 uses token-passing access over a bustopology and is based on the token bus LAN architecture. See alsotoken bus.

IEEE 802.5IEEE LAN protocol that specifies an implementation of the physical layer and MACsublayer of the data link layer. IEEE 802.5 uses token passing access at 4 or 16 Mbpsover STP cabling and is similar to IBM Token Ring. See alsoToken Ring.

IEEE 802.6IEEE MAN specification based on DQDB technology. IEEE 802.6 supports data ratesof 1.5 to 155 Mbps. See alsoDQDB.

IEPGInternet Engineering Planning Group. Group, primarily composed of Internet serviceoperators, whose goal is to promote a globally coordinated Internet operatingenvironment. Membership is open to all.

IESGInternet Engineering Steering Group. Organization, appointed by the IAB, that managesthe operation of the IETF. See alsoICP cell andIETF.

IETFInternet Engineering Task Force. Task force consisting of over 80 working groupsresponsible for developing Internet standards. The IETF operates under the auspices ofISOC. See alsoISOC.

IFIPInternational Federation for Information Processing. Research organization thatperforms OSI prestandardization work. Among other accomplishments, IFIPformalized the original MHS model. See alsoMHS.

I 10-5

I-frameInformation frame. One of three SDLC frame formats. See alsoS-frame andU-frame.

IGMPInternet Group Management Protocol. Used by IP hosts to report their multicast groupmemberships to an adjacent multicast router. See alsomulticast router.

IGPInterior Gateway Protocol. Internet protocol used to exchange routing informationwithin an autonomous system. Examples of common Internet IGPs include IGRP,OSPF, and RIP. See alsoOSPF andRIP. See alsoIGRP (Interior Gateway RoutingProtocol) in the “Cisco Systems Terms and Acronyms” section.

IGRPSeeIGRP (Interior Gateway Routing Protocol) in the “Cisco Systems Terms andAcronyms” section.

IIHIS-IS Hello. Message sent by all IS-IS systems to maintain adjacencies. See alsoIS-IS.

IINRENInteragency Interim National Research and Education Network. Evolving operatingnetwork system. Near term research and development activities will provide for thesmooth evolution of this networking infrastructure into the future gigabit NREN.

IIOPInternet Inter-ORB Protocol. Protocol used in the CORBA framework for accessingobjects across the Internet. See alsoCORBA.

IISPInterim-Interswitch Signaling Protocol. ATM signaling protocol for inter-switchcommunication using manually configured prefix tables. When a signaling request isreceived by a switch, the switch checks the destination ATM address against the prefixtable and notes the port with the longest prefix match. It then forwards the signalingrequest across that port using UNI procedures. IISP is an interim solution until PNNIPhase 1 is completed. Formerly known as PNNI Phase 0. Contrast withDynamic IISP.


IITAInformation Infrastructure Technology and Applications. Component of the HPCCprogram intended to ensure U.S. leadership in the development of advanced informationtechnologies. See alsoHPCC.

IKEInternet Key Exchange

ILMIInterim Local Management Interface. Specification developed by the ATM Forum forincorporating network-management capabilities into the ATM UNI.

IMAinverse multiplexing over ATM. Standard protocol defined by the ATM Forum in 1997.

IMA groupPhysical links grouped to form a higher-bandwidth logical link whose rate isapproximately the sum of the individual link rates.

IMAPInternet Message Access Protocol. Method of accessing e-mail or bulletin boardmessages kept on a mail server that can be shared. IMAP permits client electronic mailapplications to access remote message stores as if they were local without actuallytransferring the message.

IMHO“In My Humble Opinion.” One of many short-form phrases seen in e-mail messages,newsgroups, and so on.

IMPinterface message processor. Old name for ARPANET packet switches. See alsoARPANET andpacket switch.

IMTInter-Machine Trunk.

IN/AINIntelligent Network/Advanced Intelligent Network

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INAInformation Networking Architecture. Bellcore object-oriented architecture for themanagement of ATM and SONET equipment and services in an operating companyenvironment.

INASoftBellcore implementation of INA. See alsoINA.

INBInstall Busy. Entity has just been created but has not been commanded In-Service orOut-of-Service yet.

in-band signalingTransmission within a frequency range normally used for information transmission.Compare without-of-band signaling.

Industry-Standard ArchitectureSeeISA.

information elementIn ATM, the portion of a signaling packet that carries information, such as addresses,used in the UNI specification. See alsoUNI.

Information Infrastructure Technology and ApplicationsSeeIITA.

infraredElectromagnetic waves whose frequency range is above that of microwaves, but belowthat of the visible spectrum. LAN systems based on this technology represent anemerging technology.

INEIntelligent Network Element. Network element that can be provisioned from a remoteOSS.

initial domain identifierSeeIDI .


initial domain partSeeIDP.

INOCInternet Network Operations Center. BBN group that in the early days of the Internetmonitored and controlled the Internet core gateways (routers). INOC no longer exists inthis form.

input/outputSeeIN/AIN.

Institute of Electrical and Electronics EngineersSeeIEEE.

insured burstIn an ATM network, the largest burst of data above the insured rate that will betemporarily allowed on a PVC and not tagged by the traffic policing function fordropping in the case of network congestion. The insured burst is specified in bytes orcells. Compare withmaximum burst. See alsoinsured rate.

insured rateLong-term data throughput, in bits or cells per second, that an ATM network commitsto support under normal network conditions. The insured rate is 100 percent allocated;the entire amount is deducted from the total trunk bandwidth along the path of thecircuit. Compare withexcess rate andmaximum rate. See alsoinsured burst.

insured trafficTraffic within the insured rate specified for an ATM PVC. This traffic should not bedropped by the network under normal network conditions. See alsoCLP andinsuredrate.

INTAPInteroperability Technology Association for Information Processing. Technicalorganization that has the official charter to develop Japanese OSI profiles andconformance tests.

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Integrated IS-ISRouting protocol based on the OSI routing protocol IS-IS, but with support for IP andother protocols. Integrated IS-IS implementations send only one set of routing updates,making it more efficient than two separate implementations. Formerly called DualIS-IS. Compare withIS-IS.

Integrated Services Digital NetworkSeeISDN.

Integrated Services InternetIETF proposal for enhancing IP to allow it to support integrated or multimedia services,including traffic management mechanisms that closely match the traffic managementmechanisms of ATM. An example is RSVP.

Intelligent QoS Management SuiteComposed of Automatic Routing Management, Advanced CoS Management,Optimized Bandwidth Management, and Dynamic Buffer Management. Formerlycalled Advanced Networking Features.

interarea routingTerm used to describe routing between two or more logical areas. Compare withintra-area routing.

Interdomain Policy RoutingSeeIDPR.

interface1. Connection between two systems or devices.

2. In routing terminology, a network connection.

3. In telephony, a shared boundary defined by common physical interconnectioncharacteristics, signal characteristics, and meanings of interchanged signals.

4. Boundary between adjacent layers of the OSI model.

interface message processorSeeIMP.


interface processorSeeinterface processor in the “Cisco Systems Terms and Acronyms” section.

interferenceUnwanted communication channel noise.

Interim Local Management InterfaceSeeILMI .

Interior Gateway ProtocolSeeIGP.

Interior Gateway Routing ProtocolSeeIGRP in the “Cisco Systems Terms and Acronyms” section.

intermediate routing nodeSeeIRN.

Intermediate Session RoutingSeeISR.

intermediate systemSeeIS.

Intermediate System-to-Intermediate SystemSeeIS-IS.

International Code DesignatorSeeICD.

International Data NumberSeeX.121.

International Electrotechnical CommissionSeeIEC.

International Federation for Information ProcessingSeeIFIP.

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International Organization for StandardizationSeeISO.

International Standards OrganizationErroneous expansion of the acronym ISO. SeeISO.

International Telecommunication Union Telecommunication Standardization SectorSeeITU-T.

InternetLargest global internetwork, connecting tens of thousands of networks worldwide andhaving a “culture” that focuses on research and standardization based on real-life use.Many leading-edge network technologies come from the Internet community. TheInternet evolved in part from ARPANET. At one time, called the DARPA Internet. Notto be confused with the general term internet. See alsoARPANET.

internetShort for internetwork. Not to be confused with the Internet. Seeinternetwork.

Internet Architecture BoardSeeICP cell.

Internet addressSeeIP address.

Internet Assigned Numbers AuthoritySeeIANA.

Internet Control Message ProtocolSeeICMP.

Internet-DraftSeeI-D.

Internet Engineering Planning GroupSeeIEPG.


Internet Engineering Steering GroupSeeIESG.

Internet Engineering Task ForceSeeIETF.

Internet Group Management ProtocolSeeIGMP.

Internet Message Access ProtocolSeeIMAP.

Internet Network Operations CenterSeeINOC.

Internet protocolAny protocol that is part of the TCP/IP protocol stack. SeeIP. See alsoTCP/IP.

Internet Protocol (IP, IPv4)Network layer for the TCP/IP protocol suite. Internet Protocol (version 4) is aconnectionless, best-effort packet switching protocol. Defined in RFC 791.

Internet Protocol (IPng, IPv6)SeeIPv6.

Internet RegistrySeeIR.

Internet Relay ChatSeeIRC.

Internet Research Steering GroupSeeIRSG.

Internet Research Task ForceSeeIRTF.

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Internet service providerSeeISP.

Internet SocietySeeISOC.

Internet telephonyGeneric term used to describe various approaches to running voice telephony over IP.

internetworkCollection of networks interconnected by routers and other devices that functions(generally) as a single network. Sometimes called an internet, which is not to beconfused with the Internet.

internetworkingGeneral term used to refer to the industry devoted to connecting networks together. Theterm can refer to products, procedures, and technologies.

Internetwork Packet ExchangeSeeIPX.

InterNICOrganization that serves the Internet community by supplying user assistance,documentation, training, registration service for Internet domain names, and otherservices. Formerly called NIC.

interoperabilityAbility of computing equipment manufactured by different vendors to communicatewith one another successfully over a network.

Inter-Switching System InterfaceSeeISSI.

Inter-Switch LinkSeeISL in the “Cisco Systems Terms and Acronyms” section.

intra-area routingTerm used to describe routing within a logical area. Compare withinterarea routing.


Inverse Address Resolution ProtocolSeeInverse ARP.

Inverse ARPInverse Address Resolution Protocol. Method of building dynamic routes in a network.Allows an access server to discover the network address of a device associated with avirtual circuit.

inverse multiplexingProcess whereby physical links are grouped to form a higher-bandwidth logical linkwhose rate is approximately the sum of the individual link rates.

I/Oinput/output.

IOCindependent operating company. Independently owned company providing localtelephone services to residential and business customers in a geographic area not servedby an RBOC.

IOCCI/O channel controller.

IONLInternal Organization of the Network Layer. OSI standard for the detailed architectureof the network layer. Basically, it partitions the network layer into subnetworksinterconnected by convergence protocols (equivalent to internet working protocols),creating what the Internet community calls a catenet or internet.

IOSSeeCisco IOS in the “Cisco Systems Terms and Acronyms” section.

IPInternet Protocol. Network layer protocol in the TCP/IP stack offering a connectionlessinternetwork service. IP provides features for addressing, type-of-service specification,fragmentation and reassembly, and security. Defined in RFC 791.

I 10-15

IP address32-bit address assigned to hosts using TCP/IP. An IP address belongs to one of fiveclasses (A, B, C, D, or E) and is written as 4 octets separated by periods (dotted decimalformat). Each address consists of a network number, an optional subnetwork number,and a host number. The network and subnetwork numbers together are used for routing,while the host number is used to address an individual host within the network orsubnetwork. A subnet mask is used to extract network and subnetwork information fromthe IP address. CIDR provides a new way of representing IP addresses and subnetmasks. Also called an Internet address. See alsoCIDR, IP, andsubnet mask.

IP datagramFundamental unit of information passed across the Internet. Contains source anddestination addresses along with data and a number of fields that define such things asthe length of the datagram, the header checksum, and flags to indicate whether thedatagram can be (or was) fragmented.

IPCinterprocess communication.

IPCPIP Control Protocol. Protocol that establishes and configures IP over PPP. See alsoIPandPPP.

IP multicastRouting technique that allows IP traffic to be propagated from one source to a numberof destinations or from many sources to many destinations. Rather than sending onepacket to each destination, one packet is sent to a multicast group identified by a singleIP destination group address.

IPngSeeIPv6.

IPv6IP version 6. Replacement for the current version of IP (version 4). IPv6 includessupport for flow ID in the packet header, which can be used to identify flows. Formerlycalled IPng (next generation).


IP Security OptionSeeIPSO.

IPSOIP Security Option. U.S. government specification that defines an optional field in theIP packet header that defines hierarchical packet security levels on a per interface basis.

IPXInternetwork Packet Exchange. NetWare network layer (Layer 3) protocol used fortransferring data from servers to workstations. IPX is similar to IP and XNS.

IPXCPIPX Control Protocol. Protocol that establishes and configures IPX over PPP. See alsoIPX andPPP.

IPXWANIPX wide-area network. Protocol that negotiates end-to-end options for new links.When a link comes up, the first IPX packets sent across are IPXWAN packetsnegotiating the options for the link. When the IPXWAN options are successfullydetermined, normal IPX transmission begins. Defined by RFC 1362.

IRInternet Registry. IR was delegated the responsibility of network address andautonomuous system identifiers from the IANA, which has the discretionary authorityto delegate portions of its responsibility.

IRBintegrated routing and bridging

Integrated Services Digital Network (ISDN) User Part. An upper-layer applicationsupported by SS7 for connection set up and tear down.

IRCInternet Relay Chat. World-wide “party line” protocol that allows one to converse withothers in real time. IRC is structured as a network of servers, each of which acceptsconnections from client programs, one per user.

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IRDPICMP Router Discovery Protocol. Protocol that enables a host to determine the addressof a router that it can use as a default gateway. Similar to ES-IS, but used with IP. SeealsoES-IS.

IRNintermediate routing node. In SNA, a subarea node with intermediate routing capability.

IRSGInternet Research Steering Group. Group that is part of the IAB and oversees theactivities of the IRTF. See alsoICP cell andIRTF.

IRTFInternet Research Task Force. Community of network experts that considersInternet-related research topics. The IRTF is governed by the IRSG and is considered asubsidiary of the IAB. See alsoICP cell andIRSG.

IS1. intermediate system. Routing node in an OSI network.

2. Telecommunications: In-Service. Entity is fully operational and capable of providingservice to a requesting entity.

ISAIndustry-Standard Architecture. 16-bit bus used for Intel-based personal computers. SeealsoEISA.

isarithmic flow controlFlow control technique that permits travel through the network. Isarithmic flow controlis not commonly implemented.

ISDNIntegrated Services Digital Network. Communication protocol, offered by telephonecompanies, that permits telephone networks to carry data, voice, and other sourcetraffic. See alsoBISDN, BRI, N-ISDN, andPRI.


IS-ISIntermediate System-to-Intermediate System. OSI link-state hierarchical routingprotocol based on DECnet Phase V routing, whereby ISs (routers) exchange routinginformation based on a single metric, to determine network topology. Compare withIntegrated IS-IS. See alsoES-IS andOSPF.

IS-IS HelloSeeIIH .

IS-IS Interdomain Routing ProtocolSeeIDRP.

ISLSeeISL (Inter-Switch Link) in the “Cisco Systems Terms and Acronyms” section.

ISMinternetwork status monitor .

ISOInternational Organization for Standardization. International organization that isresponsible for a wide range of standards, including those relevant to networking. ISOdeveloped the OSI reference model, a popular networking reference model.

ISO 3309HDLC procedures developed by ISO. ISO 3309:1979 specifies the HDLC framestructure for use in synchronous environments. ISO 3309:1984 specifies proposedmodifications to allow the use of HDLC in asynchronous environments as well.

ISO 9000Set of international quality-management standards defined by ISO. The standards,which are not specific to any country, industry, or product, allow companies todemonstrate that they have specific processes in place to maintain an efficient qualitysystem.

I 10-19

ISOCInternet Society. International nonprofit organization, founded in 1992, that coordinatesthe evolution and use of the Internet. In addition, ISOC delegates authority to othergroups related to the Internet, such as the IAB. ISOC is headquartered in Reston,Virginia, (United States). See alsoICP cell.

isochronous transmissionAsynchronous transmission over a synchronous data link. Isochronous signals require aconstant bit rate for reliable transport. Compare withasynchronous transmission,plesiochronous transmission, andsynchronous transmission.

ISODEISO development environment. Large set of libraries and utilities used to developupper-layer OSI protocols and applications.

ISO development environmentSeeISODE.

ISPInternet service provider. Company that provides Internet access to other companies andindividuals.

ISRIntermediate Session Routing. Initial routing algorithm used in APPN. ISR providesnode-to-node connection-oriented routing. Network outages cause sessions to failbecause ISR cannot provide nondisruptive rerouting around a failure. ISR was replacedby HPR. Compare withHPR. See alsoAPPN.

ISSIInter-Switching System Interface. Standard interface between SMDS switches.

ISUPISDN User Part.


ITU-TInternational Telecommunication Union Telecommunication Standardization Sector.International body that develops worldwide standards for telecommunicationstechnologies. The ITU-T carries out the functions of the former CCITT. See alsoCCITT.

IVRinteractive voice response. Term used to describe systems that provide information inthe form of recorded messages over telephone lines in response to user input in the formof spoken words or more commonly DTMF signaling. Examples include banks thatallow you to check your balance from any telephone and automated stock quotesystems.

IXCinter-exchange carrier. Common carrier providing long distance connectivity betweenLATAs. The three major IXCs are AT&T, MCI, and Sprint, but several hundred IXCsoffer long distance service in the United States.

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Jjabber

1. Error condition in which a network device continually transmits random, meaninglessdata onto the network.

2. In IEEE 802.3, a data packet whose length exceeds that prescribed in the standard.

JANETJoint Academic Network. X.25 WAN connecting university and research institutions inthe United Kingdom.

Japan UNIX NetworkSeeJUNET.

JavaObject-oriented programming language developed at Sun Microsystems to solve anumber of problems in modern programming practice. The Java language is usedextensively on World-Wide Web, particularly for applets.

JDBCJava Database Connectivity. Java API that enables Java programs to execute SQLstatements. This allows Java programs to interact with any SQL-compliant database.Since nearly all relational database management systems (DBMSs) support SQL, andbecause Java itself runs on most platforms, JDBC makes it possible to write a singledatabase application that can run on different platforms and interact with differentDBMSs. JDBC is similar to ODBC, but is designed specifically for Java programs,whereas ODBC is language-independent. JDBC was developed by JavaSoft, asubsidiary of Sun Microsystems. SeeODBC.


jitterAnalog communication line distortion caused by the variation of a signal from itsreference timing positions. Jitter can cause data loss, particularly at high speeds.

John von Neumann Computer NetworkSeeJvNCnet.

Joint Academic NetworkSeeJANET.

JPEGJoint Photographics Expert Group. Graphic file format that was adopted as a standardby the ITU-T and the ISO. JPEG is most often used to compress still images using DCTanalysis.

jumperElectrical switch consisting of a number of pins and a connector that can be attached tothe pins in a variety of different ways. Different circuits are created by attaching theconnector to different pins.

JUNETJapan UNIX Network. Nationwide, noncommercial network in Japan, designed topromote communication between Japanese and other researchers.

JvNCnetJohn von Neumann Computer Network. Regional network, owned and operated byGlobal Enterprise Services, Inc., composed of T1 and slower serial links providingmidlevel networking services to sites in the Northeastern United States.

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KKA9Q

Popular implementation of TCP/IP and associated protocols for amateur packet radiosystems.

Karn’s algorithmAlgorithm that improves round-trip time estimations by helping transport layerprotocols distinguish between good and bad round-trip time samples.

KBkilobyte. Approximately 1,000 bytes.

Kbkilobit. Approximately 1,000 bits.

kBpskilobytes per second.

kbpskilobits per second.

keepalive intervalPeriod of time between each keepalive message sent by a network device.

keepalive messageMessage sent by one network device to inform another network device that the virtualcircuit between the two is still active.


KerberosDeveloping standard for authenticating network users. Kerberos offers two key benefits:it functions in a multivendor network, and it does not transmit passwords over thenetwork.

KermitPopular file-transfer and terminal-emulation program.

kilobitAbbreviated Kb.

kilobits per secondAbbreviated kbps.

kilobyteAbbreviated KB.

kilobytes per secondAbbreviated kBps.

kVAkilovoltampere.

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LL2F Protocol

Layer 2 Forwarding Protocol. Protocol that supports the creation of secure virtualprivate dial-up networks over the Internet.

label swappingRouting algorithm used by APPN in which each router that a message passes throughon its way to its destination independently determines the best path to the next router.

LANlocal-area network. High-speed, low-error data network covering a relatively smallgeographic area (up to a few thousand meters). LANs connect workstations,peripherals, terminals, and other devices in a single building or other geographicallylimited area. LAN standards specify cabling and signaling at the physical and data linklayers of the OSI model. Ethernet, FDDI, and Token Ring are widely used LANtechnologies. Compare withMAN andWAN.

LANELAN emulation. Technology that allows an ATM network to function as a LANbackbone. The ATM network must provide multicast and broadcast support, addressmapping (MAC-to-ATM), SVC management, and a usable packet format. LANE alsodefines Ethernet and Token Ring ELANs. See alsoELAN.

LANE UNILANE User-Network Interface.

LAN emulationSeeLANE.


LAN Emulation ClientSeeLEC.

LAN Emulation Configuration ServerSeeLECS.

LAN Emulation ServerSeeLES.

LAN ManagerDistributed NOS, developed by Microsoft, that supports a variety of protocols andplatforms. See alsoNOS.

LAN Manager for UNIXSeeLM/X.

LAN Network ManagerSeeLNM.

LAN ServerServer-based NOS developed by IBM and derived from LNM. See alsoLNM.

LAN switchHigh-speed switch that forwards packets between data-link segments. Most LANswitches forward traffic based on MAC addresses. This variety of LAN switch issometimes called a frame switch. LAN switches are often categorized according to themethod they use to forward traffic: cut-through packet switching or store-and-forwardpacket switching. Multilayer switches are an intelligent subset of LAN switches.Compare withmultilayer switch. See alsocut-through packet switching andstore andforward packet switching.

LAPBLink Access Procedure, Balanced. Data link layer protocol in the X.25 protocol stack.LAPB is a bit-oriented protocol derived from HDLC. See alsoHDLC andX.25.

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LAPDLink Access Procedure on the D channel. ISDN data link layer protocol for theD channel. LAPD was derived from the LAPB protocol and is designed primarily tosatisfy the signaling requirements of ISDN basic access. Defined by ITU-TRecommendations Q.920 and Q.921.

LAPMLink Access Procedure for Modems. ARQ used by modems implementing the V.42protocol for error correction. See alsoARQ andV.42.

laserlight amplification by stimulated emission of radiation. Analog transmission device inwhich a suitable active material is excited by an external stimulus to produce a narrowbeam of coherent light that can be modulated into pulses to carry data. Networks basedon laser technology are sometimes run over SONET.

LATlocal-area transport. A network virtual terminal protocol developed by DigitalEquipment Corporation.

LATAlocal access and transport area. Geographic telephone dialing area serviced by a singlelocal telephone company. Calls within LATAs are called local calls. There are well over100 LATAs in the United States.

latency1. Delay between the time a device requests access to a network and the time it isgranted permission to transmit.

2. Delay between the time a device receives a frame and the time that frame is forwardedout the destination port.

Layer 3 SwitchingEmerging Layer 3 switching technology that integrates routing with switching to yieldvery high routing throughput rates in the millions-of-packets- per-second range. Themovement to Layer 3 switching is designed to address the downsides of the current


generation of layer 2 switches, which are functionally equivalent to bridges. Thesedownsides for a large, flat network include being subject to broadcast storms, spanningtree loops, and address limitations.

LCIlogical channel identifier. SeeVCN.

LCNlogical channel number. SeeVCN.

LCPlink control protocol. Protocol that establishes, configures, and tests data-linkconnections for use by PPP. See alsoPPP.

LCVline code violation. Occurrence of a BPV or EXZ error event.

LDAPLightweight Directory Access Protocol. Protocol that provides access for managementand browser applications that provide read/write interactive access to the X.500Directory.

LDCELPlow-delay CELP. CELP voice compression algorithm providing 16 Kbps, or 4:1compression. Standardized in ITU-T Recommendation G.728.

leaf internetworkIn a star topology, an internetwork whose sole access to other internetworks in the staris through a core router.

leaky bucketIn ATM, a metaphor for the GCRA, which is used for conformance checking of cellflows from a user or network. The hole in the bucket represents the sustained rate atwhich cells can be accommodated, and the bucket depth represents the tolerance for cellbursts over a period of time. See alsoGCRA.

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learning bridgeBridge that performs MAC address learning to reduce traffic on the network. Learningbridges manage a database of MAC addresses and the interfaces associated with eachaddress. See alsoMAC address learning.

leased lineTransmission line reserved by a communications carrier for the private use of acustomer. A leased line is a type of dedicated line. See alsodedicated line.

LE_ARPLAN Emulation Address Resolution Protocol. Protocol that provides the ATM addressthat corresponds to a MAC address.

LEC1. LAN Emulation Client. Entity in an end system that performs data forwarding,address resolution, and other control functions for a single ES within a single ELAN.An LEC also provides a standard LAN service interface to any higher-layer entity thatinterfaces to the LEC. Each LEC is identified by a unique ATM address, and isassociated with one or more MAC addresses reachable through that ATM address. SeealsoELAN andLES.

2. local exchange carrier. Local or regional telephone company that owns and operatesa telephone network and the customer lines that connect to it.

LECSLAN Emulation Configuration Server. Entity that assigns individual LANE clients toparticular ELANs by directing them to the LES that corresponds to the ELAN. There islogically one LECS per administrative domain, and this serves all ELANs within thatdomain. See alsoELAN.

LEDlight emitting diode. Semiconductor device that emits light produced by convertingelectrical energy. Status lights on hardware devices are typically LEDs.


LEN nodelow-entry networking node. In SNA, a PU 2.1 that supports LU protocols, but whose CPcannot communicate with other nodes. Because there is no CP-to-CP session betweena LEN node and its NN, the LEN node must have a statically defined image of the APPNnetwork.

LESLAN Emulation Server. Entity that implements the control function for a particularELAN. There is only one logical LES per ELAN, and it is identified by a unique ATMaddress. See alsoELAN.

Level 1 routerDevice that routes traffic within a single DECnet or OSI area.

Level 2 routerDevice that routes traffic between DECnet or OSI areas. All Level 2 routers must forma contiguous network.

LGNlogical group node. The node that represents its peer group in the peer group’s parentpeer group. See alsoparent peer group andpeer group.

light amplification by stimulated emission of radiationSeelaser.

light emitting diodeSeeLED.

limited resource linkResource defined by a device operator to remain active only when being used.

limited-route explorer packetSeespanning explorer packet.

line1. In SNA, a connection to the network.

2. Seelink.

L 13-7

line cardAny I/O card that can be inserted in a modular chassis.

line code typeOne of a number of coding schemes used on serial lines to maintain data integrity andreliability. The line code type used is determined by the carrier service provider. See alsoAMI, B8ZS, andHBD3.

line code violationSeeLCV.

line conditioningUse of equipment on leased voice-grade channels to improve analog characteristics,thereby allowing higher transmission rates.

line driverInexpensive amplifier and signal converter that conditions digital signals to ensurereliable transmissions over extended distances.

line of sightCharacteristic of certain transmission systems such as laser, microwave, and infraredsystems in which no obstructions in a direct path between transmitter and receiver canexist.

line printer daemonSeeLPD.

line turnaroundTime required to change data transmission direction on a telephone line.

linkNetwork communications channel consisting of a circuit or transmission path and allrelated equipment between a sender and a receiver. Most often used to refer to a WANconnection. Sometimes referred to as a line or a transmission link.

Link Access Procedure, BalancedSeeLAPB.


Link Access Procedure for ModemsSeeLAPM.

Link Access Procedure on the D channelSeeLAPD.

Link Control ProtocolSeeLCP.

link layerSeedata-link layer.

link-layer addressSeeMAC address.

link-state advertisementSeeLSA.

link-state packetSeeLSA.

link-state routing algorithmRouting algorithm in which each router broadcasts or multicasts information regardingthe cost of reaching each of its neighbors to all nodes in the internetwork. Link statealgorithms create a consistent view of the network and are therefore not prone to routingloops, but they achieve this at the cost of relatively greater computational difficulty andmore widespread traffic (compared with distance vector routing algorithms). Comparewith distance vector routing algorithm. See alsoDijkstra’s algorithm.

LISlogical IP subnet. A group of IP nodes (such as hosts and routers) that connects to asingle ATM network and belongs to the same IP subnet.

listservAutomated mailing list distribution system originally designed for the Bitnet/EARNnetwork. Allows users to add or delete themselves from mailing lists without (other)human intervention.

L 13-9

little-endianMethod of storing or transmitting data in which the least significant bit or byte ispresented first. Compare withbig-endian.

LLAPLocalTalk Link Access Protocol. Link-level protocol that manages node-to-nodedelivery of data on a LocalTalk network. LLAP manages bus access, provides anode-addressing mechanism, and controls data transmission and reception, ensuringpacket length and integrity. See alsoLocalTalk.

LLCLogical Link Control. Higher of the two data link layer sublayers defined by the IEEE.The LLC sublayer handles error control, flow control, framing, and MAC-sublayeraddressing. The most prevalent LLC protocol is IEEE 802.2, which includes bothconnectionless and connection-oriented variants. See alsodata-link layer andMAC.

LLC2Logical Link Control, type 2. Connection-oriented OSI LLC-sublayer protocol. SeealsoLLC.

LMILocal Management Interface. Set of enhancements to the basic Frame Relayspecification. LMI includes support for a keepalive mechanism, which verifies that datais flowing; a multicast mechanism, which provides the network server with its localDLCI and the multicast DLCI; global addressing, which gives DLCIs global rather thanlocal significance in Frame Relay networks; and a status mechanism, which provides anon-going status report on the DLCIs known to the switch. Known asLMT in ANSIterminology.

LMTSeeLMI.

LM/XLAN Manager for UNIX. Monitors LAN devices in UNIX environments.


LNNILAN Emulation Network-to-Network Interface. Supports communication between theserver components within a single ELAN. Phase 1 LANE protocols do not allow for thestandard support of multiple LESs or BUSs within an ELAN. Phase 2 addresses theselimitations.

LNMLAN Network Manager. SRB and Token Ring management package provided by IBM.Typically running on a PC, it monitors SRB and Token Ring devices, and can pass alertsup to NetView.

load balancingIn routing, the ability of a router to distribute traffic over all its network ports that arethe same distance from the destination address. Good load-balancing algorithms useboth line speed and reliability information. Load balancing increases the use of networksegments, thus increasing effective network bandwidth.

local access and transport areaSeeLATA.

local acknowledgmentMethod whereby an intermediate network node, such as a router, responds toacknowledgments for a remote end host. Use of local acknowledgments reducesnetwork overhead and, therefore, the risk of time-outs. Also known aslocal termination.

local adjacencySeelocal adjacency in the “Cisco Systems Terms and Acronyms” section.

local-area networkSeeLAN.

local-area transportSeeLAT.

local bridgeBridge that directly interconnects networks in the same geographic area.

L 13-11

local exchange carrierSeeLEC.

local explorer packetPacket generated by an end system in an SRB network to find a host connected to thelocal ring. If the local explorer packet fails to find a local host, the end system produceseither a spanning explorer packet or an all-routes explorer packet. See alsoall-routesexplorer packet, explorer packet, andspanning explorer packet.

local loopLine from the premises of a telephone subscriber to the telephone company CO.

Local Management InterfaceSeeLMI.

LocalTalkApple Computer’s proprietary baseband protocol that operates at the data link andphysical layers of the OSI reference model. LocalTalk uses CSMA/CD and supportstransmissions at speeds of 230.4 Kbps.

LocalTalk Link Access ProtocolSeeLLAP.

local terminationSeelocal acknowledgment.

local traffic filteringProcess by which a bridge filters out (drops) frames whose source and destination MACaddresses are located on the same interface on the bridge, thus preventing unnecessarytraffic from being forwarded across the bridge. Defined in the IEEE 802.1 standard. SeealsoIEEE 802.1.

logical addressSeenetwork address.


logical channelNondedicated, packet-switched communications path between two or more networknodes. Packet switching allows many logical channels to exist simultaneously on asingle physical channel.

logical channel identifierSeeLCI.

logical channel numberSeeLCN.

logical group nodeSeeLGN.

Logical Link ControlSeeLLC.

Logical Link Control, type 2SeeLLC2.

logical unitSeeLU.

Logical Unit 6.2SeeLU 6.2.

loopRoute where packets never reach their destination, but simply cycle repeatedly througha constant series of network nodes.

loopback testTest in which signals are sent and then directed back toward their source from somepoint along the communications path. Loopback tests are often used to test networkinterface usability.

lossyCharacteristic of a network that is prone to lose packets when it becomes highly loaded.

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low-entry networking nodeSeeLEN node.

LPDline printer daemon. Protocol used to send print jobs between UNIX systems.

LSAlink-state advertisement. Broadcast packet used by link-state protocols that containsinformation about neighbors and path costs. LSAs are used by the receiving routers tomaintain their routing tables. Sometimes called aLSP.

LSPlink-state packet. SeeLSA.

LUlogical unit. Primary component of SNA, an NAU that enables end users tocommunicate with each other and gain access to SNA network resources.

LU 6.2Logical Unit 6.2. In SNA, an LU that provides peer-to-peer communication betweenprograms in a distributed computing environment. APPC runs on LU 6.2 devices. SeealsoAPPC.

LUNILAN Emulation User-to-Network Interface. The ATM Forum standard for LANemulation on ATM networks. Defines the interface between the LEC and the LANEmulation Server components. See alsoBUS, LES, andLECS.


L 13-15

C H A P T E R

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1 4

MMAC

Media Access Control. Lower of the two sublayers of the data link layer defined by theIEEE. The MAC sublayer handles access to shared media, such as whether tokenpassing or contention will be used. See alsodata-link layer andLLC.

MAC addressStandardized data link layer address that is required for every port or device thatconnects to a LAN. Other devices in the network use these addresses to locate specificports in the network and to create and update routing tables and data structures. MACaddresses are 6 bytes long and are controlled by the IEEE. Also known as ahardwareaddress, MAC-layer address, andphysical address. Compare withnetwork address.

MAC address learningService that characterizes a learning bridge, in which the source MAC address of eachreceived packet is stored so that future packets destined for that address can beforwarded only to the bridge interface on which that address is located. Packets destinedfor unrecognized addresses are forwarded out every bridge interface. This scheme helpsminimize traffic on the attached LANs. MAC address learning is defined in theIEEE 802.1 standard. See alsolearning bridge andMAC address.

MacIPNetwork layer protocol that encapsulates IP packets in DDP packets for transmissionover AppleTalk. MacIP also provides proxy ARP services. See alsoDDP andproxyARP.

MAC-layer addressSeeMAC address.

M 14-2

MAEmetropolitan access exchange. One of a number of Internet exchange points. Examplesinclude MAE West and MAE East. See alsoCIX, FIX, andGIX.

mail bridgeMail gateway that forwards e-mail between two or more networks while ensuring thatthe messages it forwards meet certain administrative criteria. A mail bridge is simply aspecialized form of mail gateway that enforces an administrative policy with regard towhat mail it forwards.

mail gatewayMachine that connects two or more electronic mail systems (especially dissimilar mailsystems on two different networks) and transfers messages between them. Sometimesthe mapping and translation can be quite complex, and generally it requires astore-and-forward scheme whereby the message is received from one systemcompletely before it is transmitted to the next system after suitable translations.

mail exchange recordSeeMX record.

mail exploderPart of an e-mail delivery system that allows a message to be delivered to a list ofaddressees. Mail exploders are used to implement mailing lists. Users send messages toa single address (for example, [email protected]), and the mail exploder takes careof delivery to the individual mailboxes in the list.

Maintenance Operation ProtocolSeeMOP.

MANmetropolitan-area network. Network that spans a metropolitan area. Generally, a MANspans a larger geographic area than a LAN, but a smaller geographic area than a WAN.Compare withLAN andWAN.

managed objectIn network management, a network device that can be managed by a networkmanagement protocol.


Management Information BaseSeeMIB.

management servicesSNA functions distributed among network components to manage and control an SNAnetwork.

Manchester encodingDigital coding scheme, used by IEEE 802.3 and Ethernet, in which a mid-bit-timetransition is used for clocking, and a 1 is denoted by a high level during the first half ofthe bit time.

Manufacturing Automation ProtocolSeeMAP.

MAPManufacturing Automation Protocol. Network architecture created by General Motorsto meet the specific needs of the factory floor. MAP specifies a token-passing LANsimilar to IEEE 802.4. See alsoIEEE 802.4.

MARSMulticast Address Resolution Server. Mechanism for supporting IP multicast. A MARSserves a group of nodes (known as a cluster); each node in the cluster is configured withthe ATM address of the MARS. The MARS supports multicast through multicastmessages of overlaid point-to-multipoint connections or through multicast servers.

MartianHumorous term applied to packets that turn up unexpectedly on the wrong networkbecause of bogus routing entries. Also used as a name for a packet that has an altogetherbogus (nonregistered or ill-formed) Internet address.

maskSeeaddress mask andsubnet mask.

MAUmedia attachment unit. Device used in Ethernet and IEEE 802.3 networks that providesthe interface between the AUI port of a station and the common medium of the Ethernet.The MAU, which can be built into a station or can be a separate device, performs

M 14-4

physical layer functions including the conversion of digital data from the Ethernetinterface, collision detection, and injection of bits onto the network. Sometimes referredto as amedia access unit, also abbreviatedMAU, or as atransceiver. In Token Ring, aMAU is known as amultistation access unitand is usually abbreviatedMSAUto avoidconfusion. See alsoAUI andMSAU.

maximum burstSpecifies the largest burst of data above the insured rate that will be allowed temporarilyon an ATM PVC, but will not be dropped at the edge by the traffic policing function,even if it exceeds the maximum rate. This amount of traffic will be allowed onlytemporarily; on average, the traffic source needs to be within the maximum rate.Specified in bytes or cells. Compare withinsured burst. See alsomaximum rate.

maximum rateMaximum total data throughput allowed on a given virtual circuit, equal to the sum ofthe insured and uninsured traffic from the traffic source. The uninsured data might bedropped if the network becomes congested. The maximum rate, which cannot exceedthe media rate, represents the highest data throughput the virtual circuit will ever deliver,measured in bits or cells per second. Compare withexcess rate andinsured rate. Seealsomaximum burst.

maximum transmission unitSeeMTU.

MBmegabyte. Approximately 1,000,000 bytes.

Mbmegabit. Approximately 1,000,000 bits.

MBSmaximum burst size. In an ATM signaling message, burst tolerance is conveyed throughthe MBS, which is coded as a number of cells. The burst tolerance together with theSCR and the GCRA determine the MBS that can be transmitted at the peak rate and stillbe in conformance with the GCRA. See alsoSCP andGCRA.


MBONEmulticast backbone. Multicast backbone of the Internet. MBONE is a virtual multicastnetwork composed of multicast LANs and the point-to-point tunnels that interconnectthem.

Mbpsmegabits per second.

MCAmicro channel architecture. Bus interface commonly used in PCs and some UNIXworkstations and servers.

MCDVmaximum cell delay variation. In an ATM network, the maximum two-point CDVobjective across a link or node for the specified service category. One of four linkmetrics exchanged using PTSPs to determine the available resources of an ATMnetwork. There is one MCDV value for each traffic class. See alsoCDV andPTSP.

MCLRmaximum cell loss ratio. In an ATM network, the maximum ratio of cells that do notsuccessfully transit a link or node compared with the total number of cells that arrive atthe link or node. One of four link metrics exchanged using PTSPs to determine theavailable resources of an ATM network. The MCLR applies to cells in the CBR andVBR traffic classes whose CLP bit is set to zero. See alsoCBR, CLP, PTSP, andVBR.

MCNSMultimedia Cable Network System Partners Ltd. Consortium of cable companiesproviding service to the majority of homes in the United States and Canada. Thisconsortium drives a standard with the goal of having interoperable cable modems.

MCRminimum cell rate. Parameter defined by the ATM Forum for ATM traffic management.MCR is defined only for ABR transmissions, and specifies the minimum value for theACR. See alsoABR (available bit rate), ACOM, andPCR.

M 14-6

MCTDmaximum cell transfer delay. In an ATM network, the sum of the MCDV and the fixeddelay component across the link or node. One of four link metrics exchanged usingPTSPs to determine the available resources of an ATM network. There is one MCTDvalue for each traffic class. See alsoMCDV andPTSP.

MDmediation device. Device that provides protocol translation and concentration oftelemetry information originating from multiple network elements and transport to anOSS. See alsoOSS.

MD5Message Digest 5. Algorithm used for message authentication in SNMP v.2. MD5verifies the integrity of the communication, authenticates the origin, and checks fortimeliness. See alsoSNMP2.

MDLThe TransPath Message Definition Language. High-level language used to specifyprotocols and protocol conversion operations on the TransPath.

MDNmessage disposition notification. Message returned to the originator of an e-mailmessage indicating that the e-mail message has been opened. Specifications for MDNare described in RFC 2298.

mediaPlural of medium. Various physical environments through which transmission signalspass. Common network media include twisted-pair, coaxial, and fiber-optic cable, andthe atmosphere (through which microwave, laser, and infrared transmission occurs).Sometimes calledphysical media.

Media Access ControlSeeMAC.

media access unitSeeMAU.


media attachment unitSeeMAU.

media interface connectorSeeMIC.

media rateMaximum traffic throughput for a particular media type.

mediumSeemedia.

megabitAbbreviated Mb. Approximately 1,000,000 bits.

megabits per secondAbbreviated Mbps.

megabyteAbbreviated MB. Approximately 1,000,000 bytes.

MEL CASMercury Exchange Limited (MEL) Channel Associated Signaling. A voice signalingprotocol used primarily in the United Kingdom.

meshNetwork topology in which devices are organized in a manageable, segmented mannerwith many, often redundant, interconnections strategically placed between networknodes. See alsofull mesh andpartial mesh.

messageApplication layer (Layer 7) logical grouping of information, often composed of anumber of lower-layer logical groupings such as packets. The termsdatagram, frame,packet, andsegment are also used to describe logical information groupings at variouslayers of the OSI reference model and in various technology circles.

message handling systemSeeMHS.

M 14-8

Message Digest 5SeeMD5.

Message Queuing InterfaceSeeMQI.

message switchingSwitching technique involving transmission of messages from node to node through anetwork. The message is stored at each node until such time as a forwarding path isavailable. Contrast withcircuit switching andpacket switching.

message unitUnit of data processed by any network layer.

metasignalingProcess running at the ATM layer that manages signaling types and virtual circuits.

meteringSeetraffic shaping.

metricSeerouting metric.

metropolitan-area networkSeeMAN.

MFTmultiflex trunk module.

MHSmessage handling system. ITU-T X.400 recommendations that provide messagehandling services for communications between distributed applications. NetWare MHSis a different (though similar) entity that also provides message-handling services. SeealsoIFIP.


MIBManagement Information Base. Database of network management information that isused and maintained by a network management protocol such as SNMP or CMIP. Thevalue of a MIB object can be changed or retrieved using SNMP or CMIP commands,usually through a GUI network management system. MIB objects are organized in a treestructure that includes public (standard) and private (proprietary) branches.

MICmedia interface connector. FDDIde facto standard connector.

MICAModem ISDN channel aggregation. Modem module and card used in the Cisco AS5300universal access servers. A MICA modem provides an interface between an incomingor outgoing digital call and an Integrated Services Digital Network (ISDN) telephoneline; the call does not have to be converted to analog, as it does with a conventionalmodem and an analog telephone line. Each line can accommodate, or aggregate, up to24 (T1) or 30 (E1)calls.

microfilterDevice that prevents data frequencies (intended for a data device such as a router) fromtraveling over the telephone line and interfering with telephone calls.

MIDmessage identifier. In ATM, used to identify ATM cells that carry segments from thesame higher-layer packet.

micro channel architectureSeeMCA.

microcodeTranslation layer between machine instructions and the elementary operations of acomputer. Microcode is stored in ROM and allows the addition of new machineinstructions without requiring that they be designed into electronic circuits when newinstructions are needed.

microsegmentationDivision of a network into smaller segments, usually with the intention of increasingaggregate bandwidth to network devices.

M 14-10

microwaveElectromagnetic waves in the range 1 to 30 GHz. Microwave-based networks are anevolving technology gaining favor due to high bandwidth and relatively low cost.

mid-level networkmid-level networks. Makes up the second level of the Internet hierarchy. They are thetransit networks that connect the stub networks to the backbone networks. Also referredto asregionals.

midsplitBroadband cable system in which the available frequencies are split into two groups:one for transmission and one for reception.

MIImedia independent interface. Standard specification for the interface between networkcontroller chips and their associated media interface chip(s). The MII automaticallysenses 10- and 100-MHz Ethernet speeds.

Military NetworkSeeMILNET.

millions of instructions per secondSeemips.

MILNETMilitary Network. Unclassified portion of the DDN. Operated and maintained by theDISA. See alsoDDN andDISA.

MIMEMultipurpose Internet Mail Extension. MIME. Sstandard for transmitting non-text data(or data that cannot be represented in plain ASCII code) in Internet mail, such as binary,foreign language text (such as Russian or Chinese), audio, or video data. MIME isdefined in RFC 2045.

minimum cell rateSeeMCR.


MIPSeeMIP (MultiChannel Interface Processor) in the “Cisco Systems Terms andAcronyms” section.

mipsmillions of instructions per second. Number of instructions executed by a processor persecond.

MIXmultiservice interchange.

MLPMultilink PPP. Method of splitting, recombining, and sequencing datagrams acrossmultiple logical data links.

MMFmultimode fiber.

MMLTransPath Man-Machine Language.

MMoIPMultimedia Mail over IP.

MMoIP dial peerMultimedia Mail over Internet Protocol dial peer. Dial peer specific to Store andForward Fax. The MMoIP dial peer is the vehicle you use to assign particular linecharacteristics (such as a destination telephone number) to the connection between theCisco router or access server and the SMTP mail server during on-ramp faxing.

MMPMultichassis Multilink PPP. Extends MLP support across multiple routers and accessservers. MMP enables multiple routers and access servers to operate as a single, largedial-up pool, with a single network address and ISDN access number. MMP correctlyhandles packet fragmenting and reassembly when a user connection is split between twophysical access devices.

M 14-12

modemmodulator-demodulator. Device that converts digital and analog signals. At the source,a modem converts digital signals to a form suitable for transmission over analogcommunication facilities. At the destination, the analog signals are returned to theirdigital form. Modems allow data to be transmitted over voice-grade telephone lines.

modem eliminatorDevice allowing connection of two DTE devices without modems.

modulationProcess by which the characteristics of electrical signals are transformed to representinformation. Types of modulation include AM, FM, and PAM. See alsoAM, FM, andPAM.

modulator-demodulatorSeemodem.

monomode fiberSeesingle-mode fiber.

MOPMaintenance Operation Protocol. Digital Equipment Corporation protocol that providesa way to perform primitive maintenance operations on DECnet systems. For example,MOP can be used to download a system image to a diskless station.

MosaicPublic-domain WWW browser, developed at the NCSA. See alsobrowser.

MOSPFMulticast OSPF. Intradomain multicast routing protocol used in OSPF networks.Extensions are applied to the base OSPF unicast protocol to support IP multicastrouting.


MPEGMotion Picture Experts Group. Standard for compressing video. MPEG1is a bit streamstandard for compressed video and audio optimized to fit into a bandwidth of 1.5 Mbps.MPEG2 isintended for higher quality video-on-demand applications and runs at datarates between 4 and 9 Mbps.MPEG4is a low-bit-rate compression algorithm intendedfor 64-kbps connections.

MPLSMultiprotocol Label Switching. Emerging industry standard upon which tag switchingis based.

MPOAMultiprotocol over ATM. ATM Forum standardization effort specifying how existingand future network-layer protocols such as IP, IPv6, AppleTalk, and IPX run over anATM network with directly attached hosts, routers, and multilayer LAN switches.

MQIMessage Queuing Interface. International standard API that provides functionalitysimilar to that of the RPC interface. In contrast to RPC, MQI is implemented strictly atthe application layer. See alsoRPC.

MSAUmultistation access unit. Wiring concentrator to which all end stations in a Token Ringnetwork connect. The MSAU provides an interface between these devices and the TokenRing interface of a router. Sometimes abbreviatedMAU.

MSOmultiple service operator. Cable service provider that also provides other services suchas data and/or voice telephony.

MTA1. Message Transfer Agent. OSI application process used to store and forward messagesin the X.400 Message Handling System. Equivalent to Internet mail agent.

2. Mail Transfer Agent. Software that implements SMTP and provides storage for mailmessages to be forwarded or delivered to a local user. MTAs implement SMTP(RFC 821).

M 14-14

MTBFmeantime between failure.

MTUmaximum transmission unit. Maximum packet size, in bytes, that a particular interfacecan handle.

MUDmulti-user dungeon. Adventure, role playing games, or simulations played on theInternet. Players interact in real time and can change the “world” in the game as theyplay it. Most MUDs are based on the Telnet protocol.

mu-lawNorth American companding standard used in conversion between analog and digitalsignals in PCM systems. Similar to the European a-law. See alsoa-law andcompanding.

multiaccess networkNetwork that allows multiple devices to connect and communicate simultaneously.

multicastSingle packets copied by the network and sent to a specific subset of network addresses.These addresses are specified in the Destination Address Field. Compare withbroadcastandunicast.

multicast addressSingle address that refers to multiple network devices. Synonymous with group address.Compare withbroadcast address andunicast address. See alsomulticast.

multicast backboneSeeMBONE.

multicast forward VCCVCC set up by the BUS to the LEC as a leaf in a point-to-multipoint connection. SeealsoBUS, LEC (LAN Emulation Client), andVCC.

multicast groupDynamically determined group of IP hosts identified by a single IP multicast address.


Multicast OSPFSeeMOSPF.

multicast routerRouter used to send IGMP query messages on their attached local networks. Hostmembers of a multicast group respond to a query by sending IGMP reports noting themulticast groups to which they belong. The multicast router takes responsibility forforwarding multicast datagrams from one multicast group to all other networks thathave members in the group. See alsoIGMP.

multicast send VCCIn an ATM network, a bi-directional point-to-point VCC set up by a LEC to a BUS. Oneof three data connections defined by Phase 1 LANE. Compare withcontrol distributeVCC andcontrol direct VCC. See alsoBUS, LEC (LAN Emulation Client), andVCC.

multicast serverEstablishes a one-to-many connection to each device in a VLAN, thus establishing abroadcast domain for each VLAN segment. The multicast server forwards incomingbroadcasts only to the multicast address that maps to the broadcast address.

MultiChannel Interface ProcessorSeeMIP in the “Cisco Systems Terms and Acronyms” section.

multidrop lineCommunications line with multiple cable access points. Sometimes called a multipointline.

multihomed hostHost attached to multiple physical network segments in an OSI CLNS network.

multihomingAddressing scheme in IS-IS routing that supports assignment of multiple areaaddresses.

multilayer switchSwitch that filters and forwards packets based on MAC addresses and networkaddresses. A subset of LAN switch. Compare withLAN switch.

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multimode fiberOptical fiber supporting propagation of multiple frequencies of light. See alsosingle-mode fiber.

multiple domain networkSNA network with multiple SSCPs. See alsoSSCP.

multiplexingScheme that allows multiple logical signals to be transmitted simultaneously across asingle physical channel. Compare withdemultiplexing.

Multipoint.1. Line or channel connecting three or more different service points.

2. Circuit that has points served by three or more switches. Single communicationschannel (typically a leased telephone circuit) to which two or more stations or logicalunits are attached although only one can transmit at a time. Such arrangements usuallyrequire a polling mechanism under the control of a master station to ensure that only onedevice transmits at a time.

multipoint control unitEndpoint on the LAN that provides the capability for three or more terminals andgateways to participate in a multipoint conference.

multipoint lineSeemultidrop line.

Multiprotocol over ATMSeeMPOA.

Multipurpose Internet Mail ExtensionSeeMIME.

multistation access unitSeeMSAU.

multi-user dungeonSeeMUD.


multivendor networkNetwork using equipment from more than one vendor. Multivendor networks posemany more compatibility problems than single-vendor networks. Compare withsingle-vendor network.

muxmultiplexing device. Combines multiple signals for transmission over a single line. Thesignals are demultiplexed, or separated, at the receiving end.

MX recordmail exchange record. DNS resource record type indicating which host can handlee-mail for a particular domain.

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C H A P T E R

N 15-1

1 5

NNACS

NetWare Asynchronous Communication Services. Novell software that supportsNovell’s AIO and NASI programming interfaces. NACS promotes the sharing ofcommunications resources such as modems, asynchronous hosts, and X.25 networkservices.

NADFNorth American Directory Forum. Collection of organizations that offer, or plan tooffer, public directory services in North America, based on the CCITT X.500Recommendations.

NADNnearest active downstream neighbor. In Token Ring or IEEE 802.5 networks, the closestdownstream network device from any given device that is still active.

Nagle’s algorithmActually two separate congestion control algorithms that can be used in TCP-basednetworks. One algorithm reduces the sending window; the other limits small datagrams.

NAKnegative acknowledgment. Response sent from a receiving device to a sending deviceindicating that the information received contained errors. Compare toacknowledgment.

Name Binding ProtocolSeeNBNS.


name cachingMethod by which remotely discovered host names are stored by a router for use in futurepacket-forwarding decisions to allow quick access.

name resolutionGenerally, the process of associating a name with a network location.

name serverServer connected to a network that resolves network names into network addresses.

namespaceCommonly distributed set of names in which all names are unique.

NANOGNorth American Network Operator’s Group. Primary forum for information exchangeamong U.S. exchange point participants, Internet service providers, and end users.

NANPNorth American Numbering Plan.

NAPnetwork access point. Location for interconnection of Internet service providers in theUnited States for the exchange of packets.

NARPNBMA Address Resolution Protocol. Functional subset of NHRP that returns only theaddress mappings of nodes that are directly connected to the NBMA network. Comparewith NHRP.

narrowbandSeebaseband.

Narrowband ISDNSeeN-ISDN.

N 15-3

NASnetwork access server. Cisco platform (or collection of platforms such as an AccessPathsystem which interfaces between the packet world (e.g. the Internet) and the circuitworld (e.g. the PSTN). (Seeaccess device.)

NASnetwork access server. A NASI

NetWare Asynchronous Support Interface.

NATNetwork Address Translation. Mechanism for reducing the need for globally unique IPaddresses. NAT allows an organization with addresses that are not globally unique toconnect to the Internet by translating those addresses into globally routable addressspace. Also known asNetwork Address Translator.

National Bureau of StandardsSeeNBS.

National Institute of Standards and TechnologySeeNIST.

National Research and Education NetworkSeeNREN.

National Science FoundationSeeNSF.

National Science Foundation NetworkSeeNSFNET.

native client interface architectureSeeNCIA in the “Cisco Systems Terms and Acronyms” section.


NAUnetwork addressable unit. SNA term for an addressable entity. Examples include LUs,PUs, and SSCPs. NAUs generally provide upper-level network services. Compare withpath control network.

NAUNnearest active upstream neighbor. In Token Ring or IEEE 802.5 networks, the closestupstream network device from any given device that is still active.

NBFCPNetBIOS Frames Control Protocol. Protocol that establishes and configures NetBIOSover PPP. See alsoNetBIOS andPPP.

NBMAnonbroadcast multiaccess. Term describing a multiaccess network that either does notsupport broadcasting (such as X.25) or in which broadcasting is not feasible (forexample, an SMDS broadcast group or an extended Ethernet that is too large). See alsomultiaccess network.

NBNSNetBIOS Name Service.

NBPName Binding Protocol. AppleTalk transport-level protocol that translates a characterstring name into the DDP address of the corresponding socket client. NBP enablesAppleTalk protocols to understand user-defined zones and device names by providingand maintaining translation tables that map names to their corresponding socketaddresses.

NBSNational Bureau of Standards. Organization that was part of the U.S. Department ofCommerce. Now known as NIST. See alsoNIST.

NCIASeeNCIA (native client interface architecture) in the “Cisco Systems Terms andAcronyms” section.

N 15-5

NCP1. Network Control Program. In SNA, a program that routes and controls the flow ofdata between a communications controller (in which it resides) and other networkresources.

2. Network Control Protocol. Series of protocols for establishing and configuringdifferent network layer protocols, such as for AppleTalk over PPP. See alsoPPP.

NCP/Token Ring InterconnectionSeeNTRI.

NCSANational Center for Supercomputing Applications.

NDISnetwork driver interface specification. Microsoft specification for a generic, hardware-and protocol-independent device driver for NICs.

NEnetwork element. In OSS, a single piece of telecommunications equipment used toperform a function or service integral to the underlying network.

nearest active upstream neighborSeeNAUN.

NEARNETRegional network in New England (United States) that links Boston University, HarvardUniversity, and MIT. Now part of BBN Planet. See alsoBBN Planet.

NEBSNetwork Equipment Building Systems. In OSS, the Bellcore requirement for equipmentdeployed in a central office environment. Covers spatial, hardware, crafts personinterface, thermal, fire resistance, handling and transportation, earthquake andvibration, airborne contaminants, grounding, acoustical noise, illumination, EMC, andESD requirements.

negative acknowledgmentSeeNAK.


neighboring routersIn OSPF, two routers that have interfaces to a common network. On multiaccessnetworks, neighbors are dynamically discovered by the OSPF Hello protocol.

NEMSNetwork Element Management Server.

NETnetwork entity title. Network addresses, defined by the ISO network architecture, andused in CLNS-based networks.

netShort forNetwork.

NetBEUINetBIOS Extended User Interface. Enhanced version of the NetBIOS protocol used bynetwork operating systems such as LAN Manager, LAN Server, Windows forWorkgroups, and Windows NT. NetBEUI formalizes the transport frame and addsadditional functions. NetBEUI implements the OSI LLC2 protocol. See alsoLLC2andOSI.

NetBIOSNetwork Basic Input/Output System. API used by applications on an IBM LAN torequest services from lower-level network processes. These services might includesession establishment and termination, and information transfer.

netiquetteA pun on “etiquette” referring to proper behavior on a network.

NETscoutSeeNETscout in the “Cisco Systems Terms and Acronyms” section.

NetViewIBM network management architecture and related applications. NetView is a VTAMapplication used for managing mainframes in SNA networks. See alsoVTAM.

N 15-7

NetWarePopular distributed NOS developed by Novell. Provides transparent remote file accessand numerous other distributed network services.

NetWare Link Services ProtocolSeeNLSP.

NetWare Loadable ModuleSeeNLM.

NetworkThe highest level of your signaling controller system. You have only one network,within which you create your sites.

networkCollection of computers, printers, routers, switches, and other devices that are able tocommunicate with each other over some transmission medium.

network access pointSeeNAP.

network access serverSeeaccess server andNAS.

network addressNetwork layer address referring to a logical, rather than a physical, network device.Also called aprotocol address. Compare withMAC address.

Network Address TranslationSeeNAT.

Network Address TranslatorSeeNAT.

network addressable unitSeeNAU.


network administratorPerson responsible for the operation, maintenance, and management of a network. Seealsonetwork operator.

network analyzerHardware or software device offering various network troubleshooting features,including protocol-specific packet decodes, specific preprogrammed troubleshootingtests, packet filtering, and packet transmission.

Network Basic Input/Output SystemSeeNetBIOS.

network byte orderInternet-standard ordering of the bytes corresponding to numeric values.

Network Control ProgramSeeNCP.

network driver interface specificationSeeNDIS.

network entity titleSeeNET.

Network File SystemSeeNFS.

Network IndicatorDetermines the type of call that is being placed: 0 = international, 1 = reserved, 2=national, and 3 = national spare.

Network Information CenterSeeInterNIC.

Network Information ServiceSeeNIS.

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network interfaceBoundary between a carrier network and a privately-owned installation.

network interface cardSeeNIC.

network layerLayer 3 of the OSI reference model. This layer provides connectivity and path selectionbetween two end systems. The network layer is the layer at which routing occurs.Corresponds roughly with thepath control layer of the SNA model. See alsoapplication layer, data-link layer, physical layer, PQ, session layer, andtransportlayer.

network managementGeneric term used to describe systems or actions that help maintain, characterize, ortroubleshoot a network.

Network Management ProcessorSeeNMP.

network management systemSeeNMS.

network management vector transportSeeNMVT.

Network-to-Network InterfaceSeeNNI.

network nodeSeeNN.

network node interfaceSeeNNI.

Network Node ServerSNA NN that provides resource location and route selection services for ENs, LENnodes, and LUs that are in its domain.


network numberPart of an IP address that specifies the network to which the host belongs.

network operating systemSeeNOS.

Network Operations CenterSeeNOC.

network operatorPerson who routinely monitors and controls a network, performing such tasks asreviewing and responding to traps, monitoring throughput, configuring new circuits,and resolving problems. See alsonetwork administrator.

network service access pointSeeNSAP.

network termination device 1SeeNT-1.

Network Time ProtocolSeeNTP.

network-visible entitySeeNVE.

Next Hop Resolution ProtocolSeeNHRP.

NFSNetwork File System. As commonly used, a distributed file system protocol suitedeveloped by Sun Microsystems that allows remote file access across a network. Inactuality, NFS is simply one protocol in the suite. NFS protocols include NFS, RPC,XDR, and others. These protocols are part of a larger architecture that Sun refers to asONC. See alsoONC.

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NHRPNext Hop Resolution Protocol. Protocol used by routers to dynamically discover theMAC address of other routers and hosts connected to a NBMA network. These systemscan then directly communicate without requiring traffic to use an intermediate hop,increasing performance in ATM, Frame Relay, SMDS, and X.25 environments.

NHSNext Hop Server. Server defined by the NHRP protocol that maintains next-hopresolution cache tables containing the IP-to-ATM address mappings of associated nodesand nodes that are reachable through routers served by the NHS.

NIC1. network interface card. Board that provides network communication capabilities toand from a computer system. Also called anadapter. See alsoAUI.

2. Network Information Center. Organization whose functions have been assumed bythe InterNIC. SeeInterNIC.

NISNetwork Information Service. Protocol developed by Sun Microsystems for theadministration of network-wide databases. The service essentially uses two programs:one for finding a NIS server and one for accessing the NIS databases.

N-ISDNNarrowband ISDN. Communication standards developed by the ITU-T for basebandnetworks. Based on 64-kbps B channels and 16- or 64-kbps D channels. Contrast withBISDN. See alsoBRI, ISDN, andPRI.

NISTNational Institute of Standards and Technology. U.S. government organization thatsupports and catalogs a variety of standards. Formerly the NBS. See alsoNBS.

NLMNetWare Loadable Module. Individual program that can be loaded into memory andfunction as part of the NetWare NOS.


NLSPNetWare Link Services Protocol. Link-state routing protocol based on IS-IS. See alsoIS-IS.

NMANetwork Management and Analysis. Bellcore OSS providing alarm surveillance andperformance monitoring of intelligent network elements.

NMPSeeNMP (Network Management Processor) in the “Cisco Systems Terms andAcronyms” section.

NMSnetwork management system. System responsible for managing at least part of anetwork. An NMS is generally a reasonably powerful and well-equipped computer suchas an engineering workstation. NMSs communicate with agents to help keep track ofnetwork statistics and resources.

NMVTnetwork management vector transport. SNA message consisting of a series of vectorsconveying network management specific information.

NNnetwork node. SNA intermediate node that provides connectivity, directory services,route selection, intermediate session routing, data transport, and network managementservices to LEN nodes and ENs. The NN contains a CP that manages the resources ofboth the NN itself and those of the ENs and LEN nodes in its domain. NNs provideintermediate routing services by implementing the APPN PU 2.1 extensions. Comparewith EN. See alsoCP.

NNI1. Network-to-Network Interface. ATM Forum standard that defines the interfacebetween two ATM switches that are both located in a private network or are both locatedin a public network. The interface between a public switch and private one is defined bythe UNI standard. Also, the standard interface between two Frame Relay switchesmeeting the same criteria. Compare withUNI.

N 15-13

2. network node interface.

NOAnature of address.

NOCnetwork operations center. Organization responsible for maintaining a network.

node1. Endpoint of a network connection or a junction common to two or more lines in anetwork. Nodes can be processors, controllers, or workstations. Nodes, which vary inrouting and other functional capabilities, can be interconnected by links, and serve ascontrol points in the network. Node is sometimes used generically to refer to any entitythat can access a network, and is frequently used interchangeably with device. See alsohost.

2. H.323 entity that uses RAS to communicate with the gatekeeper (for example, anendpoint such as a terminal, proxy, or gateway).

3. In SNA, the basic component of a network and the point at which one or morefunctional units connect channels or data circuits.

node

noiseUndesirable communications channel signals.

nonbroadcast multiaccessSeeNBMA.

nonextended networkAppleTalk Phase 2 network that supports addressing of up to 253 nodes and only 1 zone.

nonreturn to zeroSeeNRZ.


nonreturn to zero invertedSeeNRZI.

nonseed routerIn AppleTalk, a router that must first obtain, and then verify, its configuration with aseed router before it can begin operation. See alsoseed router.

non-stub areaResource-intensive OSPF area that carries a default route, static routes, intra-arearoutes, interarea routes, and external routes. Nonstub areas are the only OSPF areas thatcan have virtual links configured across them, and are the only areas that can contain anASBR. Compare withstub area. See alsoASAM andOSPF.

nonvolatile random-access memorySeeNVRAM.

normal response modeSeeNRM.

Northwest NetNSF-funded regional network serving the Northwestern United States, Alaska,Montana, and North Dakota. Northwest Net connects all major universities in the regionas well as many leading industrial concerns.

NOSnetwork operating system. Generic term used to refer to what are really distributed filesystems. Examples of NOSs include LAN Manager, NetWare, NFS, and VINES.

Novell IPXSeeIPX.

NPInumber plan identification.

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NRENNational Research and Education Network. Component of the HPCC program designedto ensure U.S. technical leadership in computer communications through research anddevelopment efforts in state-of-the-art telecommunications and networkingtechnologies. See alsoHPCC.

NRMnormal response mode. HDLC mode for use on links with one primary station and oneor more secondary stations. In this mode, secondary stations can transmit only if theyfirst receive a poll from the primary station.

NRZnonreturn to zero. Signals that maintain constant voltage levels with no signaltransitions (no return to a zero-voltage level) during a bit interval. Compare withNRZI.

NRZInonreturn to zero inverted. Signals that maintain constant voltage levels with no signaltransitions (no return to a zero-voltage level), but interpret the presence of data at thebeginning of a bit interval as a signal transition and the absence of data as no transition.Compare withNRZ.

NSAPnetwork service access point. Network addresses, as specified by ISO. An NSAP is thepoint at which OSI Network Service is made available to a transport layer (Layer 4)entity.

NSFNational Science Foundation. U.S. government agency that funds scientific research inthe United States. The now-defunct NSFNET was funded by the NSF. See alsoNSFNET.

NSFNETNational Science Foundation Network. Large network that was controlled by the NSFand provided networking services in support of education and research in the UnitedStates, from 1986 to 1995. NSFNET is no longer in service.


NT-1network termination 1. In ISDN, a device that provides the interface between customerpremises equipment and central office switching equipment.

NTPNetwork Time Protocol. Protocol built on top of TCP that assures accurate localtime-keeping with reference to radio and atomic clocks located on the Internet. Thisprotocol is capable of synchronizing distributed clocks within milliseconds over longtime periods.

NTRINCP/Token Ring Interconnection. Function used by ACF/NCP to support TokenRing-attached SNA devices. NTRI also provides translation from Token Ring-attachedSNA devices (PUs) to switched (dial-up) devices.

null modemSmall box or cable used to join computing devices directly, rather than over a network.

NVEnetwork-visible entity. Resource that is addressable through a network. Typically, anNVE is a socket client for a service available in a node.

NVRAMnonvolatile RAM. RAM that retains its contents when a unit is powered off.

NYSERNetNetwork in New York (United States) with a T1 backbone connecting NSF, manyuniversities, and several commercial concerns.

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OOAM cell

operation, administration, and maintenance cell. ATM Forum specification for cellsused to monitor virtual circuits. OAM cells provide a virtual circuit-level loopback inwhich a router responds to the cells, demonstrating that the circuit is up, and the routeris operational.

OAM&Poperations administration maintenance and provisioning.

OARnetOhio Academic Resources Network. Internet service provider that connects a numberof U.S. sites, including the Ohio supercomputer center in Columbus, Ohio.

object instanceNetwork management term referring to an instance of an object type that has beenbound to a value.

OCoptical carrier. Series of physical protocols (OC-1, OC-2, OC-3, and so forth), definedfor SONET optical signal transmissions. OC signal levels put STS frames ontomultimode fiber-optic line at a variety of speeds. The base rate is 51.84 Mbps (OC-1);each signal level thereafter operates at a speed divisible by that number (thus, OC-3 runsat 155.52 Mbps). See alsoSONET, STS-1, andSTS-3c.

OCCoriginating call control.


OCLCOnline Computer Library Catalog. Nonprofit membership organization offeringcomputer-based services to libraries, educational organizations, and their users.

octet8 bits. In networking, the term octet is often used (rather than byte) because somemachine architectures employ bytes that are not 8 bits long.

ODAOpen Document Architecture. ISO standard that specifies how documents arerepresented and transmitted electronically. Formerly calledOffice DocumentArchitecture.

ODBCOpen DataBase Connectivity. Standard application programming interface foraccessing data in both relational and nonrelational database management systems.Using this application programming interface, database applications can access datastored in database management systems on a variety of computers even if each databasemanagement system uses a different data storage format and programming interface.ODBC is based on the call level interface specification of the X/Open SQL AccessGroup and was developed by Digitial Equipment Corporation, Lotus, Microsoft, andSybase. Contrast withJDBC.

ODIOpen Data-Link Interface. Novell specification providing a standardized interface forNICs (network interface cards) that allows multiple protocols to use a single NIC. SeealsoNIC.

OEMI channelSeeblock multiplexer channel.

Office Document ArchitectureSeeODA.

Ohio Academic Resources NetworkSeeOARnet.

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OIMOSI Internet Management. Group tasked with specifying ways in which OSI networkmanagement protocols can be used to manage TCP/IP networks.

OIRonline insertion and removal. Feature that permits the addition, replacement, or removalof cards without interrupting the system power, entering console commands, or causingother software or interfaces to shut down. Sometimes calledhot swapping or power-onservicing.

OLOother local operator.

OMGObject Management Group.

ONCOpen Network Computing. Distributed applications architecture designed by SunMicrosystems, currently controlled by a consortium led by Sun. The NFS protocols arepart of ONC. See alsoNFS.

ones densityScheme that allows a CSU/DSU to recover the data clock reliably. The CSU/DSUderives the data clock from the data that passes through it. In order to recover the clock,the CSU/DSU hardware must receive at least one 1 bit value for every 8 bits of data thatpass through it. Also calledpulse density.

online insertion and removalSeeOIR.

on-the-fly packet switchingSeecut-through packet switching.

OOS1. Out-of-Service.

2. Telecommunications: Out-of-Service signaling.


OPCown point code. Point code of the Cisco SC2200 signaling controller.

open architectureArchitecture with which third-party developers can legally develop products and forwhich public domain specifications exist.

open circuitBroken path along a transmission medium. Open circuits will usually prevent networkcommunication.

open database connectivitySeeODBC.

Open Data-Link InterfaceSeeODI.

Open Document ArchitectureSeeODA.

Open GroupGroup formed in February 1996 by the consolidation of the two leading open systemsconsortia: X/Open Company Ltd (X/Open) and the Open Software Foundation (OSF).

Open Network ComputingSeeONC.

Open Shortest Path FirstSeeOSPF.

Open System InterconnectionSeeOSI.

Open System Interconnection reference modelSeeOSI reference model.

Operation, Administration, and Maintenance cellSeeOAM cell.

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OPS/INEOperations Provisioning System/lntelligent Network Element. Bellcore OSS thatprovides provisioning services for intelligent network elements. See alsoOSS.

Optical CarrierSeeOC.

optical fiberSeefiber-optic cable.

Optimized Bandwidth ManagementCisco wide-area switches ensure fair and cost-efficient bandwidth utilization usingvarious techniques. ABR and Optimized Banwidth Management are used for ATM andFrame Relay traffic. ABR is a standards-based ATM traffic management mechanism,and ForeSight is Cisco’s implementation that mirrors ABR capabilities for Frame Relaytraffic. ABR and Optimized Bandwidth Management optimize real-time trafficperformance and throughput, and minimize data loss. Bandwidth management for voiceis achieved through the use of standards-based voice compression and silencesuppression mechanisms for circuit data services. Formerly called ForeSight.

Organizational Unique IdentifierSeeOUI.

OSFOpen Software Foundation. Group responsible for the Distributed ComputingEnvironment (DCE) and the Distributed Management Environment (DME). SeeDCE.

OSIOpen System Interconnection. International standardization program created by ISOand ITU-T to develop standards for data networking that facilitate multivendorequipment interoperability.

OSI Internet ManagementSeeOIM.


OSI network addressAddress, consisting of up to 20 octets, used to locate an OSI Transport entity. Theaddress is formatted into two parts: an Initial Domain Part that is standardized for eachof several addressing domains and a Domain Specific Part that is the responsibility ofthe addressing authority for that domain.

OSI presentation addressAddress used to locate an OSI Application entity. It consists of an OSI Network Addressand up to three selectors, one each for use by the transport, session, and presentationentities.

OSI reference modelOpen System Interconnection reference model. Network architectural model developedby ISO and ITU-T. The model consists of seven layers, each of which specifiesparticular network functions such as addressing, flow control, error control,encapsulation, and reliable message transfer. The lowest layer (the physical layer) isclosest to the media technology. The lower two layers are implemented in hardware andsoftware, while the upper five layers are implemented only in software. The highestlayer (the application layer) is closest to the user. The OSI reference model is useduniversally as a method for teaching and understanding network functionality. Similarin some respects toSNA. Seeapplication layer, data-link layer, network layer, physicallayer, PQ, session layer, andtransport layer.

OSINETInternational association designed to promote OSI in vendor architectures.

OSPFOpen Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as asuccessor to RIP in the Internet community. OSPF features include least-cost routing,multipath routing, and load balancing. OSPF was derived from an early version of theIS-IS protocol. See alsoIGP, IS-IS, andRIP. See alsoEnhanced IGRP andIGRP(Interior Gateway Routing Protocol) in the “Cisco Systems Terms and Acronyms”section.

O 16-7

OSSOperations Support System. Network management system supporting a specificmanagement function, such as alarm surveillance and provisioning, in a carrier network.Many OSSs are large centralized systems running on mainframes or minicomputers.Common OSSs used within an RBOC includeNMA, OPS/INE, andTIRKS.

OUIOrganizational Unique Identifier. 3 octets assigned by the IEEE in a block of 48-bitLAN addresses.

outframeMaximum number of outstanding frames allowed in an SNA PU 2 server at any time.

out-of-band signalingTransmission using frequencies or channels outside the frequencies or channelsnormally used for information transfer. Out-of-band signaling is often used for errorreporting in situations in which in-band signaling can be affected by whatever problemsthe network might be experiencing. Contrast within-band signaling.


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PPABX

private automatic branch exchange. Telephone switch for use inside a corporation.PABX is the preferred term in Europe, while PBX is used in the United States.

pacingSeeflow control.

packetLogical grouping of information that includes a header containing control informationand (usually) user data. Packets are most often used to refer to network layer units ofdata. The termsdatagram, frame, message, andsegmentare also used to describe logicalinformation groupings at various layers of the OSI reference model and in varioustechnology circles. See alsoPDU.

packet assembler/disassemblerSeePAD.

packet bufferSeebuffer.

packet internet groperSeeping.

packet level protocolSeePLP.


packet switchWAN device that routes packets along the most efficient path and allows acommunications channel to be shared by multiple connections. Formerly called an IMP.See alsoIMP.

packet switch exchangeSeePSE.

packet-switched data networkSeePSN.

packet-switched networkSeePSN.

packet switchingNetworking method in which nodes share bandwidth with each other by sendingpackets. Compare withcircuit switching andmessage switching. See alsoPSN.

packet-switching nodeSeepacket switch.

PADpacket assembler/disassembler. Device used to connect simple devices (likecharacter-mode terminals) that do not support the full functionality of a particularprotocol to a network. PADs buffer data and assemble and disassemble packets sent tosuch end devices.

Palo Alto Research CenterSeePARC.

PAMpulse amplitude modulation. Modulation scheme where the modulating wave is causedto modulate the amplitude of a pulse stream. Compare withAM andFM. See alsomodulation.

P 17-3

PAPPassword Authentication Protocol. Authentication protocol that allows PPP peers toauthenticate one another. The remote router attempting to connect to the local router isrequired to send an authentication request. Unlike CHAP, PAP passes the password andhost name or username in the clear (unencrypted). PAP does not itself preventunauthorized access, but merely identifies the remote end. The router or access serverthen determines if that user is allowed access. PAP is supported only on PPP lines.Compare withCHAP.

parallel channelChannel that uses bus and tag cables as a transmission medium. Compare withESCONchannel. See alsobus and tag channel.

parallelismIndicates that multiple paths exist between two points in a network. These paths mightbe of equal or unequal cost. Parallelism is often a network design goal: if one path fails,there is redundancy in the network to ensure that an alternate path to the same pointexists.

parallel transmissionMethod of data transmission in which the bits of a data character are transmittedsimultaneously over a number of channels. Compare withserial transmission.

PARCPalo Alto Research Center. Research and development center operated by XEROX. Anumber of widely-used technologies were originally conceived at PARC, including thefirst personal computers and LANs.

PARC Universal ProtocolSeePUP.

parent peer groupIn ATM, a peer group that acts as a “parent” to a subordinate peer group. Organizingpeer groups hierarchically reduces the exchange of PTSPs. See alsochild peer group,peer group, andPTSP.


parity checkProcess for checking the integrity of a character. A parity check involves appending abit that makes the total number of binary 1 digits in a character or word (excluding theparity bit) either odd (forodd parity) or even (foreven parity).

partial meshNetwork in which devices are organized in a mesh topology, with some network nodesorganized in a full mesh, but with others that are only connected to one or two othernodes in the network. A partial mesh does not provide the level of redundancy of a fullmesh topology, but is less expensive to implement. Partial mesh topologies are generallyused in the peripheral networks that connect to a fully meshed backbone. See alsofullmesh andmesh.

Password Authentication ProtocolSeePAP.

path control layerLayer 3 in the SNA architectural model. This layer performs sequencing services relatedto proper data reassembly. The path control layer is also responsible for routing.Corresponds roughly with thenetwork layer of the OSI model. See alsodata flowcontrol layer, data-link control layer, physical control layer, presentation serviceslayer, transaction services layer, andtransmission control layer.

path control networkSNA concept that consists of lower-level components that control the routing and dataflow through an SNA network and handle physical data transmission between SNAnodes. Compare withNAU.

path costSeecost.

path nameFull name of a DOS, Mac OS, or UNIX file or directory, including all directory andsubdirectory names. Consecutive names in a path name are typically separated by abackslash (\) for DOS, a colon (:) for Mac OS, and a forward slash (/) for UNIX.

payloadPortion of a cell, frame, or packet that contains upper-layer information (data).

P 17-5

payload type identifierSeePTI.

PBXprivate branch exchange. Digital or analog telephone switchboard located on thesubscriber premises and used to connect private and public telephone networks.

PCIprotocol control information. Control information added to user data to comprise anOSI packet. The OSI equivalent of the term header. See alsoheader.

PCMpulse code modulation. Transmission of analog information in digital form throughsampling and encoding the samples with a fixed number of bits.

PCRpeak cell rate. Parameter defined by the ATM Forum for ATM traffic management. InCBR transmissions, PCR determines how often data samples are sent. In ABRtransmissions, PCR determines the maximum value of the ACR. See alsoABR(available bit rate), ACOM, andCBR.

PCS1. Personal Communications Service. Advanced network architecture that providespersonal, terminal, and service mobility. In the United States, PCS spectrum has beenallocated for broadband, narrowband, and unlicensed services.

2. port concentrator switch.

PDNpublic data network. Network operated either by a government (as in Europe) or by aprivate concern to provide computer communications to the public, usually for a fee.PDNs enable small organizations to create a WAN without all the equipment costs oflong-distance circuits.

PDUprotocol data unit. OSI term for packet. See alsoBPDU andpacket.


peak cell rateSeePCR.

peak rateMaximum rate, in kilobits per second, at which a virtual circuit can transmit.

peer-to-peer computingCalls for each network device to run both client and server portions of an application.Also describes communication between implementations of the same OSI referencemodel layer in two different network devices. Compare withclient/server computing.

peer groupCollection of ATM nodes that share identical topological databases and exchange fulllink state information with each other. Peer groups are arranged hierarchically toprevent excessive PTSP traffic. See alsoparent peer group andPTSP.

peer group leaderSeePGL.

PEMprivacy enhanced mail. Internet e-mail that provides confidentiality, authentication, andmessage integrity using various encryption methods. Not widely deployed in theInternet.

performance managementOne of five categories of network management defined by ISO for management of OSInetworks. Performance management subsystems are responsible for analyzing andcontrolling network performance including network throughput and error rates. See alsoaccounting management, configuration management, fault management, andsecuritymanagement.

peripheral nodeIn SNA, a node that uses local addresses and is therefore not affected by changes tonetwork addresses. Peripheral nodes require boundary function assistance from anadjacent subarea node.

permanent virtual circuitSeePVC.

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permanent virtual connectionSeePVC.

permanent virtual pathSeePVP.

permit processingSeetraffic policing.

Personal Communications ServiceSeePCS.

P/Fpoll/final bit. Bit in bit-synchronous data link layer protocols that indicates the functionof a frame. If the frame is a command, a 1 in this bit indicates a poll. If the frame is aresponse, a 1 in this bit indicates that the current frame is the last frame in the response.

PGLpeer group leader. In ATM, a node in a peer group that performs the functions of theLGN. Peer group leaders exchange PTSPs with peer nodes in the parent peer group toinform those nodes of the peer group’s attributes and reachability and to propagateinformation about the parent group and the parent group’s parents to the nodes in thepeer group. See alsopeer group andPTSP.

PGPPretty Good Privacy. Public-key encryption application that allows secure file andmessage exchanges. There is some controversy over the development and use of thisapplication, in part due to U.S. national security concerns.

phaseLocation of a position on an alternating wave form.

phase shiftSituation in which the relative position in time between the clock and data signals of atransmission becomes unsynchronized. In systems using long cables at highertransmission speeds, slight variances in cable construction, temperature, and otherfactors can cause a phase shift, resulting in high error rates.


PHY1. physical sublayer. One of two sublayers of the FDDI physical layer. See alsoPMD.

2. physical layer. In ATM, the physical layer provides for the transmission of cells overa physical medium that connects two ATM devices. The PHY is comprised of twosublayers: PMD and TC. See alsoPMD andTC.

physical addressSeeMAC address.

physical control layerLayer 1 in the SNA architectural model. This layer is responsible for the physicalspecifications for the physical links between end systems. Corresponds to thephysicallayer of the OSI model. See alsodata flow control layer, data-link control layer, pathcontrol layer, presentation services layer, transaction services layer, andtransmissioncontrol layer.

physical layerLayer 1 of the OSI reference model. The physical layer defines the electrical,mechanical, procedural, and functional specifications for activating, maintaining, anddeactivating the physical link between end systems. Corresponds with thephysicalcontrol layer in the SNA model. See alsoapplication layer, data-link layer, networklayer, PQ, session layer, andtransport layer.

physical layer convergence procedureSeePLCP.

physical layer interface moduleSeePLIM in the “Cisco Systems Terms and Acronyms” section.

physical mediaSeemedia.

physical mediumSeemedia.

physical medium dependentSeePMD.

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physical sublayerSeePHY.

physical unitSeePU.

Physical Unit 2SeePU 2.

Physical Unit 2.1SeePU 2.1.



Physics NetworkSeePHYSNET.

PHYSNETPhysics Network. Group of many DECnet-based physics research networks, includingHEPnet. See alsoHEPnet.

piggybackingProcess of carrying acknowledgments within a data packet to save network bandwidth.

PIMProtocol Independent Multicast. Multicast routing architecture that allows the additionof IP multicast routing on existing IP networks. PIM is unicast routing protocolindependent and can be operated in two modes: dense and sparse. See alsoPIM densemode andPIM sparse mode.

PIM dense modeOne of the two PIM operational modes. PIM dense mode is data-driven and resemblestypical multicast routing protocols. Packets are forwarded on all outgoing interfacesuntil pruning and truncation occurs. In dense mode, receivers are densely populated, and


it is assumed that the downstream networks want to receive and will probably use thedatagrams that are forwarded to them. The cost of using dense mode is its defaultflooding behavior. Sometimes called dense mode PIM or PIM DM. Contrast withPIMsparse mode. See alsoPIM.

PIM DMSeePIM dense mode.

PIM SMSeePIM sparse mode.

PIM sparse modeOne of the two PIM operational modes. PIM sparse mode tries to constrain datadistribution so that a minimal number of routers in the network receive it. Packets aresent only if they are explicitly requested at the RP (rendezvous point). In sparse mode,receivers are widely distributed, and the assumption is that downstream networks willnot necessarily use the datagrams that are sent to them. The cost of using sparse modeis its reliance on the periodic refreshing of explicit join messages and its need for RPs.Sometimes called sparse mode PIM or PIM SM. Contrast withPIM dense mode. SeealsoPIM andrendezvous point.

pingpacket internet groper. ICMP echo message and its reply. Often used in IP networks totest the reachability of a network device.

ping-pongingPhrase used to describe the actions of a packet in a two-node routing loop.

PINXprivate integrated services network exchange. A PBX or key system which, in a BRIvoice application, uses QSIG signaling.

PKIpublic key infrastructure.

plain old telephone serviceSeePOTS.

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PLARprivate line, automatic ringdown. Leased voice circuit that connects two singleendpoints together. When either telephone handset is taken off-hook, the remotetelephone automatically rings.

PLCPphysical layer convergence procedure. Specification that maps ATM cells into physicalmedia, such as T3 or E3, and defines certain management information.

plesiochronous transmissionTerm describing digital signals that are sourced from different clocks of comparableaccuracy and stability. Compare withasynchronous transmission, isochronoustransmission, andsynchronous transmission.

PLIMSeePLIM (physical layer interface module) in the “Cisco Systems Terms andAcronyms” section.

PLPpacket level protocol. Network layer protocol in the X.25 protocol stack. Sometimescalled X.25 Level 3 and X.25 Protocol. See alsoX.25.

PLSPPNNI link state packets.

PLUPrimary Logical Unit. The LU that is initiating a session with another LU. See alsoLU.

PMDphysical medium dependent. Sublayer of the FDDI physical layer that interfacesdirectly with the physical medium and performs the most basic bit transmissionfunctions of the network. See alsoPHY.


PNNI1. Private Network-Network Interface. ATM Forum specification for distributingtopology information between switches and clusters of switches that is used to computepaths through the network. The specification is based on well-known link-state routingtechniques and includes a mechanism for automatic configuration in networks in whichthe address structure reflects the topology.

2. Private Network Node Interface. ATM Forum specification for signaling to establishpoint-to-point and point-to-multipoint connections across an ATM network. Theprotocol is based on the ATM Forum UNI specification with additional mechanisms forsource routing, crankback, and alternate routing of call setup requests.

PNNI Link State PacketsSeePLSP.

PNNI topology state elementSeePTSE.

PNOPublic Network Operator. See alsoPTT.

POETpacket over E3/T3

point-to-multipoint connectionOne of two fundamental connection types. In ATM, a point-to-multipoint connection isa unidirectional connection in which a single source end-system (known as a root node)connects to multiple destination end-systems (known as leaves). Compare withpoint-to-point connection.

point of presenceSeePOP.

point-to-point connectionOne of two fundamental connection types. In ATM, a point-to-point connection can bea unidirectional or bidirectional connection between two ATM end-systems. Comparewith point-to-multipoint connection.

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Point-to-Point ProtocolSeePPP.

poison reverse updatesRouting updates that explicitly indicate that a network or subnet is unreachable, ratherthan implying that a network is unreachable by not including it in updates. Poisonreverse updates are sent to defeat large routing loops.

POLProvisioning Object Library

policy-based routingSeepolicy routing.

policy routingRouting scheme that forwards packets to specific interfaces based on user-configuredpolicies. Such policies might specify that traffic sent from a particular network shouldbe forwarded out one interface, while all other traffic should be forwarded out anotherinterface.

poll/final bitSeeP/F.

pollingAccess method in which a primary network device inquires, in an orderly fashion,whether secondaries have data to transmit. The inquiry occurs in the form of a messageto each secondary that gives the secondary the right to transmit.

POMProvisioning Object Manager

POP1. point of presence. In OSS, a physical location where an interexchange carrierinstalled equipment to interconnect with anLEC (local exchange carrier).

2. Post Office Protocol. Protocol that client e-mail applications use to retrieve mail froma mail server.


port1. Interface on an internetworking device (such as a router).

2. In IP terminology, an upper-layer process that receives information from lower layers.Ports are numbered, and each numbered port is associated with a specific process. Forexample, SMTP is associated with port 25. A port number is also called a well-knownaddress.3. To rewrite software or microcode so that it will run on a different hardware platformor in a different software environment than that for which it was originally designed.

port concentrator switchSeePCS.

port snoopingSeecircuit steering.

POSIPromoting Conference for OSI. Group of executives from the six major Japanesecomputer manufacturers and Nippon Telephone and Telegraph that sets policies andcommits resources to promote OSI.

POSTpower-on self test. Set of hardware diagnostics that runs on a hardware device when thatdevice is powered up.

Post Office ProtocolSeePOP.

Post, Telephone, and TelegraphSeePTT.

POTSplain old telephone service. SeePSTN.

POTS dial peerDial peer connected via a traditional telephony network. POTS peers point to aparticular voice port on a voice network device.

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POTS splitterA device (or one part of a larger device) that enables both an DSL data device (forexample, a Cisco 1400 series router) and a standard analog device (such as a telephone)to share the same ADSL line.

power-on self testSeePOST.

power-on servicingFeature that allows faulty components to be diagnosed, removed, and replaced while therest of the device continues to operate normally. Sometimes abbreviated POS.Sometimes called hot swapping. See alsoOIR.

PPPPoint-to-Point Protocol. Successor to SLIP that provides router-to-router andhost-to-network connections over synchronous and asynchronous circuits. WhereasSLIP was designed to work with IP, PPP was designed to work with several networklayer protocols, such as IP, IPX, and ARA. PPP also has builtin security mechanisms,such as CHAP and PAP. PPP relies on two protocols: LCP and NCP. See alsoCHAP,LCP, NCP, PAP, andSLIP.

PQpriority queuing.

presentation layerLayer 6 of the OSI reference model. This layer ensures that information sent by theapplication layer of one system will be readable by the application layer of another. Thepresentation layer is also concerned with the data structures used by programs andtherefore negotiates data transfer syntax for the application layer. Corresponds roughlywith thepresentation services layer of the SNA model. See alsoapplication layer,data-link layer, network layer, physical layer, session layer, andtransport layer.

presentation services layerLayer 6 of the SNA architectural model. This layer provides network resourcemanagement, session presentation services, and some application management.Corresponds roughly with thePQ of the OSI model. See alsodata flow control layer,data-link control layer, path control layer, physical control layer, transaction serviceslayer, andtransmission control layer.


Pretty Good PrivacySeePGP.

PRIPrimary Rate Interface. ISDN interface to primary rate access. Primary rate accessconsists of a single 64-Kbps D channel plus 23 (T1) or 30 (E1) B channels for voice ordata. Compare withBRI. See alsoBISDN, ISDN, andN-ISDN.

primarySeeprimary station.

Primary LUSeePLU.

Primary Rate InterfaceSeePRI.

primary ringOne of the two rings that make up an FDDI or CDDI ring. The primary ring is the defaultpath for data transmissions. Compare withsecondary ring.

primary stationIn bit-synchronous data link layer protocols such as HDLC and SDLC, a station thatcontrols the transmission activity of secondary stations and performs other managementfunctions such as error control through polling or other means. Primary stations sendcommands to secondary stations and receive responses. Also called, simply, a primary.See alsosecondary station.

print serverNetworked computer system that fields, manages, and executes (or sends for execution)print requests from other network devices.

priority queuingRouting feature in which frames in an interface output queue are prioritized based onvarious characteristics such as packet size and interface type.

Privacy Enhanced MailSeePEM.

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private branch exchangeSeePBX.

Private Network-Network InterfaceSeePNNI.

Private Network Node InterfaceSeePNNI.

PRMDPrivate Management Domain. X.400 Message Handling System private organizationmail system (for example, NASAmail).

process switchingSeeprocess switching in the “Cisco Systems Terms and Acronyms” section.

programmable read-only memorySeePROM.

PROMprogrammable read-only memory. ROM that can be programmed using specialequipment. PROMs can be programmed only once. Compare withEPROM.

propagation delayTime required for data to travel over a network, from its source to its ultimatedestination.

protocolFormal description of a set of rules and conventions that govern how devices on anetwork exchange information.

protocol addressSeenetwork address.

protocol control informationSeePCI.


protocol converterEnables equipment with different data formats to communicate by translating the datatransmission code of one device to the data transmission code of another device.

protocol data unitSeePDU.

Protocol Independent MulticastSeePIM.

protocol stackSet of related communications protocols that operate together and, as a group, addresscommunication at some or all of the seven layers of the OSI reference model. Not everyprotocol stack covers each layer of the model, and often a single protocol in the stackwill address a number of layers at once. TCP/IP is a typical protocol stack.

protocol translatorNetwork device or software that converts one protocol into another similar protocol.

proxy1. Entity that, in the interest of efficiency, essentially stands in for another entity.

2. Special gateways that relay one H.323 session to another.

proxy Address Resolution ProtocolSeeproxy ARP.

proxy ARPproxy Address Resolution Protocol. Variation of the ARP protocol in which anintermediate device (for example, a router) sends an ARP response on behalf of an endnode to the requesting host. Proxy ARP can lessen bandwidth use on slow-speed WANlinks. See alsoARP.

proxy explorerTechnique that minimizes exploding explorer packet traffic propagating through anSRB network by creating an explorer packet reply cache, the entries of which are reusedwhen subsequent explorer packets need to find the same host.

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proxy pollingSeeproxy polling in the “Cisco Systems Terms and Acronyms” section.

PSDNpacket-switched data network. SeePSN.

PSEpacket switch exchange. Essentially, a switch. The term PSE is generally used inreference to a switch in an X.25 packet-switch. See alsoswitch.

PSNpacket-switched network. Network that uses packet-switching technology for datatransfer. Sometimes called a PSDN. Seepacket switching.

PSTNPublic Switched Telephone Network. General term referring to the variety of telephonenetworks and services in place worldwide. Sometimes calledPOTS.

PTIpayload type identifier. 3-bit descriptor in the ATM cell header indicating the type ofpayload that the cell contains. Payload types include user and management cells; onecombination indicates that the cell is the last cell of an AAL5 frame.

PTSEPNNI topology state element. Collection of PNNI information that is flooded among alllogical nodes within a peer group. See alsopeer group andPNNI.

PTSPPNNI topology state packet. Type of PNNI routing packet used to exchange reachabilityand resource information among ATM switches to ensure that a connection request isrouted to the destination along a path that has a high probability of meeting therequested QoS. Typically, PTSPs include bidirectional information about the transitbehavior of particular nodes (based on entry and exit ports) and current internal state.See alsoPNNI andQoS.


PTTPost, Telephone, and Telegraph. Government agency that provides telephone services.PTTs exist in most areas outside North America and provide both local andlong-distance telephone services.

PUphysical unit. SNA component that manages and monitors the resources of a node, asrequested by an SSCP. There is one PU per node.

PU 2Physical Unit 2. SNA peripheral node that can support only DLUs that require servicesfrom a VTAM host and that are only capable of performing the secondary LU role inSNA sessions.

PU 2.1Physical Unit type 2.1. SNA network node used for connecting peer nodes in apeer-oriented network. PU 2.1 sessions do not require that one node reside on VTAM.APPN is based upon PU 2.1 nodes, which can also be connected to a traditionalhierarchical SNA network.

PU 4Physical Unit 4. Component of an IBM FEP capable of full-duplex data transfer. Eachsuch SNA device employs a separate data and control path into the transmit and receivebuffers of the control program.

PU 5Physical Unit 5. Component of an IBM mainframe or host computer that manages anSNA network. PU 5 nodes are involved in routing within the SNA path control layer.

public data networkSeePDN.

Public Switched Telephone NetworkSeePSTN.

pulse amplitude modulationSeePAM.

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pulse code modulationSeePCM.

pulse densitySeeones density.

PUPPARC Universal Protocol. Protocol similar to IP developed at PARC.

PVCpermanent virtual circuit or connection. Virtual circuit that is permanently established.PVCs save bandwidth associated with circuit establishment and tear down in situationswhere certain virtual circuits must exist all the time. In ATM terminology, called apermanent virtual connection. Compare withSVC. See alsovirtual circuit.

PVPpermanent virtual path. Virtual path that consists of PVCs. See alsoPVC andvirtualpath.

PVP tunnelingpermanent virtual path tunneling. Method of linking two private ATM networks acrossa public network using a virtual path. The public network transparently trunks the entirecollection of virtual channels in the virtual path between the two private networks.


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QQ.2931

ITU-T specification, based on Q.931, for establishing, maintaining, and clearingnetwork connections at the B-ISDN user-network interface. The UNI 3.1 specificationis based on Q.2931. See alsoQ.931 andUNI.

Q.920/Q.921ITU-T specifications for the ISDN UNI data link layer. See alsoUNI.

Q.922AITU-T specification for Frame Relay encapsulation.

Q.931ITU-T specification for signaling to establish, maintain, and clear ISDN networkconnections. See alsoQ.93B.

Q.931ITU standard that describes ISDN signaling. The H.225.0 standard uses a variant ofQ.931 to establish and disconnect H.323 sessions.

Q.93BITU-T specification for signaling to establish, maintain, and clear BISDN networkconnections. An evolution of ITU-T recommendation Q.931. See alsoQ.931.

QAMquadrature amplitude modulation. Method of modulating digital signals onto aradio-frequency carrier signal involving both amplitude and phase coding. QAM is amodulation scheme mostly used in the downstream direction (QAM-64, QAM-256).


QAM-16 is expected to be usable in the upstream direction. Numbers indicate numberof code points per symbol. The QAM rate or the number of points in the QAMconstellation can be computed by 2 raised to the power of <number of bits/symbol>.

QLLCQualified Logical Link Control. Data link layer protocol defined by IBM that allowsSNA data to be transported across X.25 networks.

QoSquality of service. Measure of performance for a transmission system that reflects itstransmission quality and service availability.

QoS parametersquality of service parameters. Parameters that control the amount of traffic the source inan ATM network sends over an SVC. If any switch along the path cannot accommodatethe requested QoS parameters, the request is rejected, and a rejection message isforwarded back to the originator of the request.

QPSKQuadrature Phase-Shift Keying: a method of modulating digital signals onto aradio-frequency carrier signal using four phase states to code two digital bits.

QRSSquasi-random signal sequence: a test pattern widely used to simulate voice signals.

QSIGQ (point of the ISDN model) Signaling. Signaling standard. Common channel signalingprotocol based on ISDN Q.931 standards and used by many digital PBXs.

Qualified Logical Link ControlSeeQLLC.

quality of serviceSeeQoS.

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quartet signalingSignaling technique used in 100VG-AnyLAN networks that allows data transmission at100 Mbps over four pairs of UTP cable at the same frequencies used in 10BaseTnetworks. See also100VG-AnyLAN.

queryMessage used to inquire about the value of some variable or set of variables.

queue1. Generally, an ordered list of elements waiting to be processed.

2. In routing, a backlog of packets waiting to be forwarded over a router interface.

queuing delayAmount of time that data must wait before it can be transmitted onto a statisticallymultiplexed physical circuit.

queuing theoryScientific principles governing the formation or lack of formation of congestion on anetwork or at an interface.

QUIPUPioneering software package developed to study the OSI Directory and provideextensive pilot capabilities.

QWPquery with permission.


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RRACE

Research on Advanced Communications in Europe. Project sponsored by the EC for thedevelopment of broadband networking capabilities.

race condition rangingThe process of acquiring the correct timing offset such that the transmissions of a cablemodem are aligned with the correct mini-slot boundary.

radio frequencySeeRequest For Comments. Document series used as the primary means forcommunicating information about the Internet. Some RFCs are designated by the IABas Internet standards. Most RFCs document protocol specifications such as Telnet andFTP, but some are humorous or historical. RFCs are available online from numeroussources..

radio frequency interferenceSeeRFI.

RASregistration, admission, and status protocol. Protocol used in the H.323 protocol suitefor discovering and interacting with a Gatekeeper.

RDIremote defect identification. In ATM, when the physical layer detects loss of signal orcell synchronization, RDI cells are used to report a VPC/VCC failure. RDI cells are sentupstream by a VPC/VCC endpoint to notify the source VPC/VCC endpoint of thedownstream failure.


RADIUSRemote Dial-In User Service. Database for authenticating modem and ISDNconnections and for tracking connection time.

RAMrandom-access memory. Volatile memory that can be read and written by amicroprocessor.

random-access memorySeeRAM.

Rapid Transport ProtocolSeeRTP.

RARERéseaux Associés pour la Recherche Européenne. Association of European universitiesand research centers designed to promote an advanced telecommunicationsinfrastructure in the European scientific community. RARE merged with EARN to formTERENA. See alsoEARN andTERENA.

RARPReverse Address Resolution Protocol. Protocol in the TCP/IP stack that provides amethod for finding IP addresses based on MAC addresses. Compare withARP.

rate enforcementSeetraffic policing.

rate queueIn ATM, a value associated with one or more virtual circuits that defines the speed atwhich an individual virtual circuit transmits data to the remote end. Each rate queuerepresents a portion of the overall bandwidth available on an ATM link. The combinedbandwidth of all configured rate queues should not exceed the total available bandwidth.

RBHCregional Bell holding company. One of seven regional telephone companies formed bythe breakup of AT&T. RBHCs differ from RBOCs in that RBHCs cross stateboundaries.

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RBOCregional Bell operating company. Seven regional telephone companies formed by thebreakup of AT&T. RBOCs differ from RBHCs in that RBOCs do not cross stateboundaries.

rcpremote copy protocol. Protocol that allows users to copy files to and from a file systemresiding on a remote host or server on the network. The rcp protocol uses TCP to ensurethe reliable delivery of data.

rcp serverRouter or other device that acts as a server for rcp. See alsorcp.

RDRequest Disconnect

read-only memorySeeROM.

Real Time Streaming ProtocolSeeRTSP.

Real-time Transport ProtocolSeeRTP.

reassemblyThe putting back together of an IP datagram at the destination after it has beenfragmented either at the source or at an intermediate node. See alsofragmentation.

RedialerInterface hardware device that interconnects between a fax device and a PublicSwitched Telephone Network (PSTN) network. A redialer is used to forward a dialednumber to another destination. Redialers contain a database of referral telephonenumbers. When the user dials a specific number, the redialer collects the dialed digitsand matches them to a listing in its database. If there is a match, the redialer dials thereferral number (transparent to the user) and forwards the call to the referral number.


redirectPart of the ICMP and ES-IS protocols that allows a router to tell a host that using anotherrouter would be more effective.

redirectorSoftware that intercepts requests for resources within a computer and analyzes them forremote access requirements. If remote access is required to satisfy the request, theredirector forms an RPC and sends the RPC to lower-layer protocol software fortransmission through the network to the node that can satisfy the request.

redistributionAllowing routing information discovered through one routing protocol to be distributedin the update messages of another routing protocol. Sometimes called routeredistribution.

redundancy1. In internetworking, the duplication of devices, services, or connections so that, in theevent of a failure, the redundant devices, services, or connections can perform the workof those that failed. See alsoredundant system.

2. In telephony, the portion of the total information contained in a message that can beeliminated without loss of essential information or meaning.

redundant systemComputer, router, switch, or other system that contains two or more of each of the mostimportant subsystems, such as two disk drives, two CPUs, or two power supplies.

regional Bell holding companySeeRBHC.

regional Bell operating companySeeRBOC.

Registration, Admission, and Status (RAS) protocolThis is the protocol used between endpoints and the gatekeeper.

registered jack connectorSeeRJ connector.

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relayOSI terminology for a device that connects two or more networks or network systems.A data link layer (Layer 2) relay is a bridge; a network layer (Layer 3) relay is a router.See alsobridge androuter.

relative rateSeeRR.

Reliable SAP Update ProtocolSeeRSUP in the “Cisco Systems Terms and Acronyms” section.

reliabilityRatio of expected to received keepalives from a link. If the ratio is high, the line isreliable. Used as a routing metric.

reloadThe event of a Cisco router rebooting, or the command that causes the router to reboot.

remote bridgeBridge that connects physically disparate network segments via WAN links.

remote copy protocolSeercp.

remote defect identificationSeeRDI.

remote job entrySeeRJE.

remote loginSeerlogin.

Remote MonitoringSeeRMON.

Remote Operations Service ElementSeeROSE.


remote-procedure callSeeRPC.

remote shell protocolSeersh.

remote source-route bridgingSeeRSRB.

rendezvous pointRouter specified in PIM sparse mode implementations to track membership in multicastgroups and to forward messages to known multicast group addresses. See alsoPIMsparse mode.

repeaterDevice that regenerates and propagates electrical signals between two networksegments. See alsosegment.

replicationProcess of keeping a copy of data, either through shadowing or caching. Seecachingandshadowing.

Request For CommentsSeeRFC.

Request To SendSeeRTS.

request/response unitSeeRU.

required visual inspectionSeeRVI.

Research on Advanced Communications in EuropeSeeRACE.

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Réseaux Associés pour la Recherche EuropéenneSeeRARE.

Resource Reservation ProtocolSeeRSVP.

Reverse Address Resolution ProtocolSeeRARP.

Reverse Path ForwardingSeeRPF.

RFradio frequency. Generic term referring to frequencies that correspond to radiotransmissions. Cable TV and broadband networks use RF technology.

RFCRequest For Comments. Document series used as the primary means forcommunicating information about the Internet. Some RFCs are designated by the IABas Internet standards. Most RFCs document protocol specifications such as Telnet andFTP, but some are humorous or historical. RFCs are available online from numeroussources.

RFIradio frequency interference. Radio frequencies that create noise that interferes withinformation being transmitted across unshielded copper cable.

RFSRemote File System. Distributed file system, similar to NFS, developed by AT&T anddistributed with their UNIX System V operating system.

RFPrequest for proposal.

RHCregional holding company.


RIFRouting Information Field. Field in the IEEE 802.5 header that is used by a source-routebridge to determine through which Token Ring network segments a packet must transit.A RIF is made up of ring and bridge numbers as well as other information.

RIIRouting Information Identifier. Bit used by SRT bridges to distinguish between framesthat should be transparently bridged and frames that should be passed to the SRBmodule for handling.

RIMRequest Initialization Mode

ringConnection of two or more stations in a logically circular topology. Information ispassed sequentially between active stations. Token Ring, FDDI, and CDDI are based onthis topology.

ring groupCollection of Token Ring interfaces on one or more routers that is part of a one-bridgeToken Ring network.

ring latencyTime required for a signal to propagate once around a ring in a Token Ring or IEEE802.5 network.

ring monitorCentralized management tool for Token Ring networks based on the IEEE 802.5specification. See alsoactive monitor andstandby monitor.

ring topologyNetwork topology that consists of a series of repeaters connected to one another byunidirectional transmission links to form a single closed loop. Each station on thenetwork connects to the network at a repeater. While logically a ring, ring topologies aremost often organized in a closed-loop star. Compare withbus topology, star topology,andtree topology.

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RIPRouting Information Protocol. IGP supplied with UNIX BSD systems. The mostcommon IGP in the Internet. RIP uses hop count as a routing metric. See alsohop count,IGP, andOSPF. See alsoEnhanced IGRP andIGRP (Interior Gateway RoutingProtocol) in the “Cisco Systems Terms and Acronyms” section.

RIPERéseaux IP Européennes. Group formed to coordinate and promote TCP/IP-basednetworks in Europe.

RISCreduced instruction set computing

RJ connectorregistered jack connector. Standard connectors originally used to connect telephonelines. RJ connectors are now used for telephone connections and for 10BaseT and othertypes of network connections. RJ-11, RJ-12, and RJ-45 are popular types of RJconnectors.

RJEremote job entry. Application that is batch-oriented, as opposed to interactive. In RJEenvironments, jobs are submitted to a computing facility, and output is received later.

rloginremote login. Terminal emulation program, similar to Telnet, offered in most UNIXimplementations.

RLMRedundant Link Manager.

RMresource management. Management of critical resources in an ATM network. Twocritical resources are buffer space and trunk bandwidth. Provisioning can be used toallocate network resources in order to separate traffic flows according to servicecharacteristics.


RMONremote monitoring. MIB agent specification described in RFC 1271 that definesfunctions for the remote monitoring of networked devices. The RMON specificationprovides numerous monitoring, problem detection, and reporting capabilities.

ROLCrouting over large clouds. Working group in IETF created to analyze and proposesolutions to problems that arise when performing IP routing over large, shared medianetworks such as ATM, Frame Relay, SMDS, and X.25.

ROMread-only memory. Nonvolatile memory that can be read, but not written, by themicroprocessor.

root accountPrivileged account on UNIX systems used exclusively by network or systemadministrators.

root bridgeExchanges topology information with designated bridges in a spanning-treeimplementation in order to notify all other bridges in the network when topologychanges are required. This prevents loops and provides a measure of defense againstlink failure.

ROSERemote Operations Service Element. OSI RPC mechanism used by various OSInetwork application protocols.

round-trip timeSeeRTT.

routePath through an internetwork.

Route ProcessorSeeRP in the “Cisco Systems Terms and Acronyms” section.

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routed protocolProtocol that can be routed by a router. A router must be able to interpret the logicalinternetwork as specified by that routed protocol. Examples of routed protocols includeAppleTalk, DECnet, and IP.

route extensionIn SNA, a path from the destination subarea node through peripheral equipment to aNAU.

route mapMethod of controlling the redistribution of routes between routing domains.

route summarizationConsolidation of advertised addresses in OSPF and IS-IS. In OSPF, this causes a singlesummary route to be advertised to other areas by an area border router.

routerNetwork layer device that uses one or more metrics to determine the optimal path alongwhich network traffic should be forwarded. Routers forward packets from one networkto another based on network layer information. Occasionally called a gateway (althoughthis definition of gateway is becoming increasingly outdated). Compare withgateway.See alsorelay.

route redistributionSeeredistribution.

Route/Switch ProcessorSeeRSP in the “Cisco Systems Terms and Acronyms” section.

routingProcess of finding a path to a destination host. Routing is very complex in largenetworks because of the many potential intermediate destinations a packet mighttraverse before reaching its destination host.

routing domainGroup of end systems and intermediate systems operating under the same set ofadministrative rules. Within each routing domain is one or more areas, each uniquelyidentified by an area address.


Routing Information FieldSeeRIF.

Routing Information IdentifierSeeRII.

Routing Information ProtocolSeeRIP.

routing metricMethod by which a routing algorithm determines that one route is better than another.This information is stored in routing tables. Metrics include bandwidth, communicationcost, delay, hop count, load, MTU, path cost, and reliability. Sometimes referred tosimply as ametric. See alsocost.

routing over large cloudsSeeROLC.

routing protocolProtocol that accomplishes routing through the implementation of a specific routingalgorithm. Examples of routing protocols include IGRP, OSPF, and RIP.

routing tableTable stored in a router or some other internetworking device that keeps track of routesto particular network destinations and, in some cases, metrics associated with thoseroutes.

Routing Table Maintenance ProtocolSeeRTMP.

Routing Table ProtocolSeeRTP.

routing updateMessage sent from a router to indicate network reachability and associated costinformation. Routing updates are typically sent at regular intervals and after a change innetwork topology. Compare withflash update.

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RPSeeRP (Route Processor) in the “Cisco Systems Terms and Acronyms” section.

RPCremote-procedure call. Technological foundation of client-server computing. RPCs areprocedure calls that are built or specified by clients and executed on servers, with theresults returned over the network to the clients. See alsoclient/server computing.

RPFReverse Path Forwarding. Multicasting technique in which a multicast datagram isforwarded out of all but the receiving interface if the receiving interface is the one usedto forward unicast datagrams to the source of the multicast datagram.

RRrelative rate. In ATM, one of the congestion feedback modes provided by ABR service.In RR mode, switches set a bit in forward and backward RM cells to indicatecongestion. See alsoABR andRLM.

RS-232Popular physical layer interface. Now known as EIA/TIA-232. SeeEIA/TIA-232.

RS-422Balanced electrical implementation of EIA/TIA-449 for high-speed data transmission.Now referred to collectively with RS-423 as EIA-530. See alsoEIA-530 andRS-423.

RS-423Unbalanced electrical implementation of EIA/TIA-449 for EIA/TIA-232 compatibility.Now referred to collectively with RS-422 as EIA-530. See alsoEIA-530 andRS-422.

RS-449Popular physical layer interface. Now known asEIA/TIA-449. SeeEIA/TIA-449.

RSAAcronym stands for Rivest, Shamir, and Adelman, the inventors of the technique.Public-key cryptographic system which may be used for encryption and authentication.


rshremote shell protocol. Protocol that allows a user to execute commands on a remotesystem without having to log in to the system. For example, rsh can be used to remotelyexamine the status of a number of access servers without connecting to eachcommunication server, executing the command, and then disconnecting from thecommunication server.

RSMRoute Switch Module

RSPSeeRSP (Route/Switch Processor) in the “Cisco Systems Terms and Acronyms”section.

RSRBremote source-route bridging. SRB over WAN links. See alsoSRB.

RSUPSeeRSUP (Reliable SAP Update Protocol) in the “Cisco Systems Terms andAcronyms” section.

RSVPResource Reservation Protocol. Protocol that supports the reservation of resourcesacross an IP network. Applications running on IP end systems can use RSVP to indicateto other nodes the nature (bandwidth, jitter, maximum burst, and so forth) of the packetstreams they want to receive. RSVP depends on IPv6. Also known as ResourceReservation Setup Protocol. See alsoIPv6.

RTCPRTP Control Protocol. Protocol that monitors the QOS of an IPv6 RTP connection andconveys information about the on-going session. See alsoRTP (Real-Time TransportProtocol).

RTFMread the fantastic manual. Acronym often used when someone asks a simple or commonquestion.

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RTMPRouting Table Maintenance Protocol. Apple Computer’s proprietary routing protocol.RTMP establishes and maintains the routing information that is required to routedatagrams from any source socket to any destination socket in an AppleTalk network.Using RTMP, routers dynamically maintain routing tables to reflect changes intopology. RTMP was derived from RIP. See alsoRIP (Routing Table Protocol).

RTP1. Routing Table Protocol. VINES routing protocol based on RIP. Distributes networktopology information and aids VINES servers in finding neighboring clients, servers,and routers. Uses delay as a routing metric. See alsoSRTP.

2. Rapid Transport Protocol. Provides pacing and error recovery for APPN data as itcrosses the APPN network. With RTP, error recovery and flow control are doneend-to-end rather than at every node. RTP prevents congestion rather than reacts to it.

3. Real-Time Transport Protocol. One of the IPv6 protocols. RTP is designed to provideend-to-end network transport functions for applications transmitting real-time data,such as audio, video, or simulation data, over multicast or unicast network services. RTPprovides services such as payload type identification, sequence numbering,timestamping, and delivery monitoring to real-time applications.

RTP Control ProtocolSeeRTCP.

RTSRequest To Send. EIA/TIA-232 control signal that requests a data transmission on acommunications line.

RTSCread the source code.

RTSPReal Time Streaming Protocol. Enables the controlled delivery of real-time data, suchas audio and video. Sources of data can include both live data feeds, such live audio andvideo, and stored content, such as pre-recorded events. RTSP is designed to work withestablished protocols, such as RTP and HTTP.


RTTround-trip time. Time required for a network communication to travel from the sourceto the destination and back. RTT includes the time required for the destination to processthe message from the source and generate a reply. RTT is used by some routingalgorithms to aid in calculating optimal routes.

RUrequest/response unit. Request and response messages exchanged between NAUs in anSNA network.

RUDPReliable User Data Protocol

run-time memoryMemory accessed while a program runs.

RVIrequired visual inspection.

C H A P T E R

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2 0

SSAC

single-attached concentrator. FDDI or CDDI concentrator that connects to the networkby being cascaded from the master port of another FDDI or CDDI concentrator.

sampling rateRate at which samples of a particular waveform amplitude are taken.

SAP1. service access point. Field defined by the IEEE 802.2 specification that is part of anaddress specification. Thus, the destination plus the DSAP define the recipient of apacket. The same applies to the SSAP. See alsoDSAP andSSAP.

2. Service Advertising Protocol. IPX protocol that provides a means of informingnetwork clients, via routers and servers, of available network resources and services.See alsoIPX.

SARsegmentation and reassembly. One of the two sublayers of the AAL CPCS, responsiblefor dividing (at the source) and reassembling (at the destination) the PDUs passed fromthe CS. The SAR sublayer takes the PDUs processed by the CS and, after dividing theminto 48-byte pieces of payload data, passes them to the ATM layer for furtherprocessing. See alsoAAL, ATM layer, CPCS, CS, andSSCS.

SAS1. single attachment station. Device attached only to the primary ring of an FDDI ring.Also known as a Class B station. Compare withDAS. See alsoFDDI.


2. statically assigned socket. Socket that is permanently reserved for use by a designatedprocess. In an AppleTalk network, SASs are numbered 1 to 127; they are reserved foruse by specific socket clients and for low-level built-in network services.

satellite communicationUse of orbiting satellites to relay data between multiple earth-based stations. Satellitecommunications offer high bandwidth and a cost that is not related to distance betweenearth stations, long propagation delays, or broadcast capability.

SBusBus technology used in Sun SPARC-based workstations and servers. The SBusspecification was adopted by the IEEE as a new bus standard.

SCCPSignaling Connection Control Part. Trillium software that supports routing andtranslation and management functions and data transfer without logical signalingconnections.

SCPService Control Point. An element of an SS7-based Intelligent Network which performsvarious service functions, such as number translation, call setup and teardown, etc.

SCRsustainable cell rate. Parameter defined by the ATM Forum for ATM trafficmanagement. For VBR connections, SCR determines the long-term average cell ratethat can be transmitted. See alsoVBR.

SCTEserial clock transmit external. Timing signal that DTE echoes to DCE to maintainclocking. SCTE is designed to compensate for clock phase shift on long cables. Whenthe DCE device uses SCTE instead of its internal clock to sample data from the DTE, itis better able to sample the data without error even if there is a phase shift in the cable.See alsophase shift.

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SDHSynchronous Digital Hierarchy. European standard that defines a set of rate and formatstandards that are transmitted using optical signals over fiber. SDH is similar to SONET,with a basic SDH rate of 155.52 Mbps, designated at STM-1. See alsoSONET andSTM-1.

SDLCSynchronous Data Link Control. SNA data link layer communications protocol. SDLCis a bit-oriented, full-duplex serial protocol that has spawned numerous similarprotocols, including HDLC and LAPB. See alsoHDLC andLAPB.

SDLC broadcastSeeSDLC broadcast in the “Cisco Systems Terms and Acronyms” section.

SDLC TransportSeeSDLC Transport in the “Cisco Systems Terms and Acronyms” section.

SDLLCSeeSDLLC in the “Cisco Systems Terms and Acronyms” section.

SDSLsingle-line digital subscriber line. One of four DSL technologies. SDSLdelivers1.544 Mbps both downstream and upstream over a single copper twisted pair.The use of a single twisted pair limits the operating range of SDSL to 10,000 feet(3048.8 meters). Compare withADSL, HDSL, andVDSL.

SDSUSMDS DSU. DSU for access to SMDS via HSSIs and other serial interfaces.

SDUservice data unit. Unit of information from an upper-layer protocol that defines a servicerequest to a lower-layer protocol.

SEALsimple and efficient AAL. Scheme used by AAL5 in which the SAR sublayer segmentsCS PDUs without adding additional fields. See alsoAAL, AAL5, CS, andSAR.


secondarySeesecondary station.

secondary ringOne of the two rings making up an FDDI or CDDI ring. The secondary ring is usuallyreserved for use in the event of a failure of the primary ring. Compare withprimary ring.

secondary stationIn bit-synchronous data link layer protocols such as HDLC, a station that responds tocommands from a primary station. Sometimes referred to simply as asecondary. Seealsoprimary station.

Section DCCSection Data Communications Channel. In OSS, a 192-kbps data communicationschannel embedded in the section overhead for OAM&P traffic between two SONETnetwork elements. See alsoOAM&P andSONET.

security managementOne of five categories of network management defined by ISO for management of OSInetworks. Security management subsystems are responsible for controlling access tonetwork resources. See alsoaccounting management, configuration management, faultmanagement, andperformance management.

seed routerRouter in an AppleTalk network that has the network number or cable range built in toits port descriptor. The seed router defines the network number or cable range for otherrouters in that network segment and responds to configuration queries from nonseedrouters on its connected AppleTalk network, allowing those routers to confirm ormodify their configurations accordingly. Each AppleTalk network must have at leastone seed router. See alsononseed router.

segment1. Section of a network that is bounded by bridges, routers, or switches.

2. In a LAN using a bus topology, a segment is a continuous electrical circuit that isoften connected to other such segments with repeaters.

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3. Term used in the TCP specification to describe a single transport layer unit ofinformation. The termsdatagram, frame, message, andpacketare also used to describelogical information groupings at various layers of the OSI reference model and invarious technology circles.

segmentation and reassemblySeeSAR.

selectorIdentifier (octet string) used by an OSI entity to distinguish among multiple SAPs atwhich it provides services to the layer above.

sequence number protectionSeeSNP.

Sequenced Packet ExchangeSeeSPX.

Sequenced Packet ProtocolSeeSPP.

Sequenced Routing Update ProtocolSeeSRTP.

serial clock transmit externalSeeSCTE.

Serial Line Internet ProtocolSeeSLIP.

serial transmissionMethod of data transmission in which the bits of a data character are transmittedsequentially over a single channel. Compare withparallel transmission.

serial tunnelSeeSTUN in the “Cisco Systems Terms and Acronyms” section.


serverNode or software program that provides services to clients. See alsoback end, client,andFRF.11.

Server Message BlockSeeSMB.

service access pointSeeSAP.

Service Advertising ProtocolSeeSAP.

service data unitSeeSDU.

service pointInterface between non-SNA devices and NetView that sends alerts from equipmentunknown to the SNA environment.

service profile identifierSeeSPID.

Service Specific Connection Oriented ProtocolSeeSSCOP.

service specific convergence sublayerSeeSSCS.

session1. Related set of communications transactions between two or more network devices.

2. In SNA, a logical connection enabling two NAUs to communicate.

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session layerLayer 5 of the OSI reference model. This layer establishes, manages, and terminatessessions between applications and manages data exchange between presentation layerentities. Corresponds to thedata flow control layer of the SNA model. See alsoapplication layer, data-link layer, network layer, physical layer, PQ, andtransportlayer.

SETSecure Electronic Transactions. SET specification developed to allow for secure creditcard and off-line debit card (check card) transactions over the World-Wide Web.

SFSuper Frame. Common framing type used on T1 circuits. SF consists of 12 frames of192 bits each, with the 193rd bit providing error checking and other functions. SF issuperseded by ESF, but is still widely used. Also called D4 framing. See alsoESF.

S-frameSupervisory frame. One of three SDLC frame formats. See alsoI-frame andU-frame.

SGCPSimple Gateway Control Protocol. Controls Voice over IP gateways by an external callcontrol element (called a call-agent). This has been adapted to allow SGCP to controlswitch ATM Circuit Emulation Service circuits (called endpoints in SGCP). Theresulting system (call-agents and gateways) allows for the call-agent to engage inCommon Channel Signalling (CCS) over a 64-Kbps CES circuit, governing theinterconnection of bearer channels on the CES interface.

SGMLStandardized Generalized Markup Language. International standard for the definition ofsystem-independent, device-independent methods of representing text in electronicform.

SGMPSimple Gateway Monitoring Protocol. Network management protocol that wasconsidered for Internet standardization and later evolved into SNMP. Documented inRFC 1028. See alsoSNMP.


shadowingForm of replication in which well-defined units of information are copied to severalDSAs.

shapingSeetraffic shaping.

shielded cableCable that has a layer of shielded insulation to reduce EMI.

shielded twisted-pairSeeSTP.

shortest path first algorithmSeeSPF.

shortest-path routingRouting that minimizes distance or path cost through application of an algorithm.

SIDService ID: a number that defines (at the MAC sublayer) a particular mapping betweena cable modem (CM) and the CMTS. The SID is used for the purpose of upstreambandwidth allocation and class-of-service management.

Signal pathRoute of a signal channel that carries signaling data.

signalingProcess of sending a transmission signal over a physical medium for purposes ofcommunication.

signaling packetGenerated by an ATM-connected device that wants to establish a connection withanother such device. The signaling packet contains the ATM NSAP address of thedesired ATM endpoint, as well as any QoS parameters required for the connection. Ifthe endpoint can support the desired QoS, it responds with an accept message, and theconnection is opened. See alsoQoS.

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Signaling System 7SeeSS7.

signal quality errorSeeSQE.

silicon switchingSeesilicon switching in the “Cisco Systems Terms and Acronyms” section.

silicon switching engineSeeSSE in the “Cisco Systems Terms and Acronyms” section.

Silicon Switch ProcessorSeeSSP in the “Cisco Systems Terms and Acronyms” section.

SIMSet Initialization Mode

simple and efficient AALSeeSEAL.

Simple Gateway Monitoring ProtocolSeeSGMP.

Simple Mail Transfer ProtocolSeeSMTP.

Simple Multicast Routing ProtocolSeeSMRP.

Simple Network Management ProtocolSeeSNMP.

simplexCapability for transmission in only one direction between a sending station and areceiving station. Broadcast television is an example of a simplex technology. Comparewith full duplex andhalf duplex.


single-attached concentratorSeeSAC.

single attachment stationSeeSAS.

single-mode fiberFiber-optic cabling with a narrow core that allows light to enter only at a single angle.Such cabling has higher bandwidth than multimode fiber, but requires a light sourcewith a narrow spectral width (for example, a laser). Also called monomode fiber. Seealsomultimode fiber.

single-route explorer packetSeespanning explorer packet.

single-vendor networkNetwork using equipment from only one vendor. Single-vendor networks rarely suffercompatibility problems. See alsomultivendor network.

SIP1. SMDS Interface Protocol. Used in communications between CPE and SMDSnetwork equipment. Allows the CPE to use SMDS service for high-speed WANinternetworking. Based on the IEEE 802.6 DQDB standard. See alsoDQDB.

2. serial interface processor.

3. session initiation protocol.

SiteGroup of closely related configuration data. It can be the name of a physical location orit can be a name you choose to give to one segment of your overall system.

SLACStanford Linear Accelerator Center.

SLCSignaling link code. Code that identifies a linkset.

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sliding window flow controlMethod of flow control in which a receiver gives transmitter permission to transmit datauntil a window is full. When the window is full, the transmitter must stop transmittinguntil the receiver advertises a larger window. TCP, other transport protocols, and severaldata link layer protocols use this method of flow control.

SLIPSerial Line Internet Protocol. Standard protocol for point-to-point serial connectionsusing a variation of TCP/IP. Predecessor of PPP. See alsoCSI andPPP.

slotted ringLAN architecture based on a ring topology in which the ring is divided into slots thatcirculate continuously. Slots can be either empty or full, and transmissions must start atthe beginning of a slot.

SMACsource MAC. MAC address specified in the Source Address field of a packet. Comparewith DMAC. See alsoMAC address.

SMBServer Message Block. File-system protocol used in LAN manager and similar NOSsto package data and exchange information with other systems.

SMDSSwitched Multimegabit Data Service. High-speed, packet-switched, datagram-basedWAN networking technology offered by the telephone companies. See alsoCBDS.

SMDS Interface ProtocolSeeSIP.

SMFsingle-mode fiber.

SMIStructure of Management Information. Document (RFC 1155) specifying rules used todefine managed objects in the MIB. See alsoMIB.


SMOstate machine object.

smoothingSeetraffic shaping.

SMRPSimple Multicast Routing Protocol. Specialized multicast network protocol for routingmultimedia data streams on enterprise networks. SMRP works in conjunction withmulticast extensions to the AppleTalk protocol.

SMTStation Management. ANSI FDDI specification that defines how ring stations aremanaged.

SMTPSimple Mail Transfer Protocol. Internet protocol providing e-mail services.

SNASystems Network Architecture. Large, complex, feature-rich network architecturedeveloped in the 1970s by IBM. Similar in some respects to the OSI reference model,but with a number of differences. SNA is essentially composed of seven layers. Seedataflow control layer, data-link control layer, path control layer, physical control layer,presentation services layer, transaction services layer, andtransmission control layer.

SNA Distribution ServicesSeeSNADS.

SNA Network InterconnectionSeeSNI.

SNADSSNA Distribution Services. Consists of a set of SNA transaction programs thatinterconnect and cooperate to provide asynchronous distribution of informationbetween end users. One of three SNA transaction services. See alsoDDM andDIA.

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SNAPSubnetwork Access Protocol. Internet protocol that operates between a network entityin the subnetwork and a network entity in the end system. SNAP specifies a standardmethod of encapsulating IP datagrams and ARP messages on IEEE networks. TheSNAP entity in the end system makes use of the services of the subnetwork andperforms three key functions: data transfer, connection management, and QoS selection.

SNI1. Subscriber Network Interface. Interface for SMDS-based networks that connectsCPE and an SMDS switch. See alsoUNI.

2. SNA Network Interconnection. IBM gateway connecting multiple SNA networks.

SNMPSimple Network Management Protocol. Network management protocol used almostexclusively in TCP/IP networks. SNMP provides a means to monitor and controlnetwork devices, and to manage configurations, statistics collection, performance, andsecurity. See alsoSGMP andSNMP2.

SNMP communitiesAuthentication scheme that enables an intelligent network device to validate SNMPrequests.

SNMP2SNMP Version 2. Version 2 of the popular network management protocol. SNMP2supports centralized as well as distributed network management strategies, and includesimprovements in the SMI, protocol operations, management architecture, and security.See alsoSNMP.

SNPsequence number protection.

SNPAsubnetwork point of attachment. Data link layer address (such as an Ethernet address,X.25 address, or Frame Relay DLCI address). SNPA addresses are used to configure aCLNS route for an interface.


SNRMSet Normal Response.

SNRMESet Normal Response. Mode Exchange.

socket1. Software structure operating as a communications end point within a network device.

2. Addressable entity within a node connected to an AppleTalk network; sockets areowned by software processes known as socket clients. AppleTalk sockets are dividedinto two groups: SASs, which are reserved for clients such as AppleTalk core protocols,and DASs, which are assigned dynamically by DDP upon request from clients in thenode. An AppleTalk socket is similar in concept to a TCP/IP port.

socket clientSoftware process or function implemented in an AppleTalk network node.

socket listenerSoftware provided by a socket client to receive datagrams addressed to the socket. Seealsosocket client.

socket number8-bit number that identifies a socket. A maximum of 254 different socket numbers canbe assigned in an AppleTalk node.

SOHOsmall office, home office. Networking solutions and access technologies for offices thatare not directly connected to large corporate networks.

SONETSynchronous Optical Network. High-speed (up to 2.5 Gbps) synchronous networkspecification developed by Bellcore and designed to run on optical fiber. STS-1 is thebasic building block of SONET. Approved as an international standard in 1988. See alsoSDH, STS-1, andSTS-3c.

source addressAddress of a network device that is sending data. See alsodestination address.

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source MACSeeSMAC.

source-route bridgingSeeSRB.

source-route translational bridgingSeeSR/TLB.

source-route transparent bridgingSeeSRT.

source service access pointSeeSSAP.

Southeastern Universities Research Association NetworkSeeSURAnet.

SP1. Signaling Processor, Signaling Point.

2. SeeSP (Switch Processor) in the “Cisco Systems Terms and Acronyms” section.

SPAGStandards Promotion and Application Group. Group of European OSI manufacturersthat chooses option subsets and publishes these in the “Guide to the Use of Standards”(GUS).

spamTerm used to describe unsolicited e-mail or newsgroup posts, often in the form ofcommercial announcements. The act of sending a spam is called, naturally spamming.

spanFull-duplex digital transmission line between two digital facilities.

SPANSeeSPAN (Switched Port Analyzer) in the “Cisco Systems Terms and Acronyms”section.


spanning explorer packetFollows a statically configured spanning tree when looking for paths in an SRBnetwork. Also known as a limited-route explorer packet or a single-route explorerpacket. See alsoall-routes explorer packet, explorer packet, andlocal explorer packet.

spanning treeLoop-free subset of a network topology. See alsospanning-tree algorithm andSpanning-Tree Protocol.

spanning-tree algorithmAlgorithm used by the Spanning-Tree Protocol to create a spanning tree. Sometimesabbreviated as STA. See alsospanning tree andSpanning-Tree Protocol.

Spanning-Tree ProtocolBridge protocol that uses the spanning-tree algorithm, enabling a learning bridge todynamically work around loops in a network topology by creating a spanning tree.Bridges exchange BPDU messages with other bridges to detect loops, and then removethe loops by shutting down selected bridge interfaces. Refers to both the IEEE 802.1Spanning-Tree Protocol standard and the earlier Digital Equipment CorporationSpanning-Tree Protocol upon which it is based. The IEEE version supports bridgedomains and allows the bridge to construct a loop-free topology across an extendedLAN. The IEEE version is generally preferred over the Digital version. Sometimesabbreviated as STP. See alsoBPDU, learning bridge, MAC address learning, spanningtree, andspanning-tree algorithm.

sparse mode PIMSeePIM sparse mode.

speed matchingFeature that provides sufficient buffering capability in a destination device to allow ahigh-speed source to transmit data at its maximum rate, even if the destination device isa lower-speed device.

SPFshortest path first algorithm. Routing algorithm that iterates on length of path todetermine a shortest-path spanning tree. Commonly used in link-state routingalgorithms. Sometimes called Dijkstra’s algorithm. See alsolink-state routingalgorithm.

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SPIDservice profile identifier. Number that some service providers use to define the servicesto which an ISDN device subscribes. The ISDN device uses the SPID when accessingthe switch that initializes the connection to a service provider.

split-horizon updatesRouting technique in which information about routes is prevented from exiting therouter interface through which that information was received. Split-horizon updates areuseful in preventing routing loops.

SPNNI connectionSeeSPNNI connection in the “Cisco Systems Terms and Acronyms” section.

spoofing1. Scheme used by routers to cause a host to treat an interface as if it were up andsupporting a session. The router spoofs replies to keepalive messages from the host inorder to convince that host that the session still exists. Spoofing is useful in routingenvironments such as DDR, in which a circuit-switched link is taken down when thereis no traffic to be sent across it in order to save toll charges. See alsoDDR.

2. The act of a packet illegally claiming to be from an address from which it was notactually sent. Spoofing is designed to foil network security mechanisms such as filtersand access lists.

spoolerApplication that manages requests or jobs submitted to it for execution. Spoolersprocess the submitted requests in an orderly fashion from a queue. A print spooler is acommon example of a spooler.

SPPSequenced Packet Protocol. Provides reliable, connection-based, flow-controlled packettransmission on behalf of client processes. Part of the XNS protocol suite.

SPXSequenced Packet Exchange. Reliable, connection-oriented protocol that supplementsthe datagram service provided by network layer (Layer 3) protocols. Novell derived thiscommonly used NetWare transport protocol from the SPP of the XNS protocol suite.


SQEsignal quality error. Transmission sent by a transceiver back to the controller to let thecontroller know whether the collision circuitry is functional. Also calledheartbeat.

SQLStructured Query Language. International standard language for defining and accessingrelational databases.

SRAMType of RAM that retains its contents for as long as power is supplied. SRAM does notrequire constant refreshing, like DRAM. Compare withDRAM.

SRBsource-route bridging. Method of bridging originated by IBM and popular in TokenRing networks. In an SRB network, the entire route to a destination is predetermined,in real time, prior to the sending of data to the destination. Contrast withtransparentbridging.

SRPspatial reuse protocol

SRTsource-route transparent bridging. IBM bridging scheme that merges the two mostprevalent bridging strategies: SRB and transparent bridging. SRT employs bothtechnologies in one device to satisfy the needs of all ENs. No translation betweenbridging protocols is necessary. Compare withSR/TLB.

SR/TLBsource-route translational bridging. Method of bridging where source-route stations cancommunicate with transparent bridge stations with the help of an intermediate bridgethat translates between the two bridge protocols. Compare withSRT.

SRTPSequenced Routing Update Protocol. Protocol that assists VINES servers in findingneighboring clients, servers, and routers. See alsoRTP (Routing Table Protocol).

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SS7Signaling System 7. Standard CCS system used with BISDN and ISDN. Developed byBellcore. See alsoCCS.

SSAPsource service access point. SAP of the network node designated in the Source field ofa packet. Compare toDSAP. See alsoSAP (service access point).

SSCPsystem services control points. Focal points within an SNA network for managingnetwork configuration, coordinating network operator and problem determinationrequests, and providing directory services and other session services for network endusers.

SSCP-PU sessionSession used by SNA to allow an SSCP to manage the resources of a node through thePU. SSCPs can send requests to, and receive replies from, individual nodes in order tocontrol the network configuration.

SSCOPService Specific Connection Oriented Protocol. Data link protocol that guaranteesdelivery of ATM signaling packets.

SSCSservice specific convergence sublayer. One of the two sublayers of any AAL. SSCS,which is service dependent, offers assured data transmission. The SSCS can be null aswell, in classical IP over ATM or LAN emulation implementations. See alsoAAL, ATMlayer, CPCS, CS, andSAR.

SSESeeSSE (silicon switching engine) in the “Cisco Systems Terms and Acronyms”section.

SSLSecure Socket Layer. Encryption technology for the Web used to provide securetransactions such as the transmission of credit card numbers for e-commerce.


SSNSubsystem number

SSP1. Switch-to-Switch Protocol. Protocol specified in the DLSw standard that routers useto establish DLSw connections, locate resources, forward data, and handle flow controland error recovery. See alsoDLSw.

2. Silicon Switch Processor. SeeSSP in the “Cisco Systems Terms and Acronyms”section.

statically assigned socketSeeSAS.

STASeespanning-tree algorithm.

stackSeeprotocol stack.

standardSet of rules or procedures that are either widely used or officially specified. See alsodefacto standardandde jure standard.

standby monitorDevice placed in standby mode on a Token Ring network in case an active monitor fails.See alsoactive monitor andring monitor.

StarLANCSMA/CD LAN, based on IEEE 802.3, developed by AT&T.

star topologyLAN topology in which end points on a network are connected to a common centralswitch by point-to-point links. A ring topology that is organized as a star implements aunidirectional closed-loop star, instead of point-to-point links. Compare withbustopology, ring topology, andtree topology.

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start-stop transmissionSeeasynchronous transmission.

startup rangeRange of values (from 65280 to 65534) from which an AppleTalk node selects thenetwork number part of its provisional address if it has not saved another networknumber.

static routeRoute that is explicitly configured and entered into the routing table. Static routes takeprecedence over routes chosen by dynamic routing protocols.

Station ManagementSeeSMT.

statistical multiplexingTechnique whereby information from multiple logical channels can be transmittedacross a single physical channel. Statistical multiplexing dynamically allocatesbandwidth only to active input channels, making better use of available bandwidth andallowing more devices to be connected than with other multiplexing techniques. Alsoreferred to asstatistical time-division multiplexingor stat mux. Compare withATDM,FDM, andTDM.

statistical time-division multiplexingSeestatistical multiplexing.

stat muxSeestatistical multiplexing.

STDSubseries of RFCs that specify Internet standards. The official list of Internet standardsis in STD 1.

STM-1Synchronous Transport Module level 1. One of a number of SDH formats that specifiesthe frame structure for the 155.52-Mbps lines used to carry ATM cells. See alsoSDH.


store and forwardFunction whereby a message is transmitted to some intermediate relay point andtemporarily stored before forwarding to the next relay point.

store and forward packet switchingPacket-switching technique in which frames are completely processed before beingforwarded out the appropriate port. This processing includes calculating the CRC andchecking the destination address. In addition, frames must be temporarily stored untilnetwork resources (such as an unused link) are available to forward the message.Contrast withcut-through packet switching.

STP1. shielded twisted-pair. Two-pair wiring medium used in a variety of networkimplementations. STP cabling has a layer of shielded insulation to reduce EMI.Compare withUTP. See alsotwisted pair.

2. SeeSpanning-Tree Protocol.

stream-orientedType of transport service that allows its client to send data in a continuous stream. Thetransport service will guarantee that all data will be delivered to the other end in thesame order as sent and without duplicates.

Structure of Management InformationSeeSMI.

STS-1Synchronous Transport Signal level 1. Basic building block signal of SONET, operatingat 51.84 Mbps. Faster SONET rates are defined as STS-n, wheren is a multiple of51.84 Mbps. See alsoSONET.

STS-3cSynchronous Transport Signal level 3, concatenated. SONET format that specifies theframe structure for the 155.52-Mbps lines used to carry ATM cells. See alsoSONET.

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stub areaOSPF area that carries a default route, intra-area routes, and interarea routes, but doesnot carry external routes. Virtual links cannot be configured across a stub area, and theycannot contain an ASBR. Compare withnon-stub area. See alsoASAM andOSPF.

stub networkNetwork that has only a single connection to a router.

subareaPortion of an SNA network that consists of a subarea node and any attached links andperipheral nodes.

subarea nodeSNA communication controller or host that handles complete network addresses.

STUNSeeSTUN (serial tunnel) in the “Cisco Systems Terms and Acronyms” section.

SU1. signaling unit. Another name for the TransPath product.

2. service unit or signaling unit.

subchannelIn broadband terminology, a frequency-based subdivision creating a separatecommunications channel.

subinterfaceOne of a number of virtual interfaces on a single physical interface.

subnetSeesubnetwork.

subnet addressPortion of an IP address that is specified as the subnetwork by the subnet mask. See alsoIP address, subnet mask, andsubnetwork.


subnet mask32-bit address mask used in IP to indicate the bits of an IP address that are being usedfor the subnet address. Sometimes referred to simply as mask. See also address maskandIP address.

subnetwork1. In IP networks, a network sharing a particular subnet address. Subnetworks arenetworks arbitrarily segmented by a network administrator in order to provide amultilevel, hierarchical routing structure while shielding the subnetwork from theaddressing complexity of attached networks. Sometimes called a subnet. See alsoIPaddress, subnet address, andsubnet mask.

2. In OSI networks, a collection of ESs and ISs under the control of a singleadministrative domain and using a single network access protocol.

Subnetwork Access ProtocolSeeSNAP.

subnetwork point of attachmentSeeSNPA.

Subscriber Network InterfaceSeeSNI.

subvectorData segment of a vector in an SNA message. A subvector consists of a length field, akey that describes the subvector type, and subvector specific data.

Super FrameSeeSF.

Super-JANETLatest phase in the development of JANET, the UK educational and research networkrun by UKERNA. It uses SMDS and ATM to provide multiservice network facilities formany new applications including multimedia conferencing.

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supernetAggregation of IP network addresses advertised as a single classless network address.For example, given four Class C IP networks—192.0.8.0, 192.0.9.0, 192.0.10.0 and192.0.11.0—each having the intrinsic network mask of 255.255.255.0, one canadvertise the address 192.0.8.0 with a subnet mask of 255.255.252.0.

supervisory processorSeeRP in the “Cisco Systems Terms and Acronyms” section.

SURAnetSoutheastern Universities Research Association Network. Network connectinguniversities and other organizations in the Southeastern United States. SURAnet,originally funded by the NSF and a part of the NSFNET, is now part of BBN Planet. SeealsoBBN Planet, NSF, andNSFNET.

sustainable cell rateSeeSCP.

SVCswitched virtual circuit. Virtual circuit that is dynamically established on demand andis torn down when transmission is complete. SVCs are used in situations where datatransmission is sporadic. Called a switched virtual connection in ATM terminology.Compare withPVC.

switch1. Network device that filters, forwards, and floods frames based on the destinationaddress of each frame. The switch operates at the data link layer of the OSI model.

2. General term applied to an electronic or mechanical device that allows a connectionto be established as necessary and terminated when there is no longer a session tosupport.

switched callsNormal telephone calls in which a user picks up a phone, hears dial tone, enters thedestination phone number to reach the other phone. Switched calls can also be privateline auto-ringdown (PLAR) calls, or tie-line calls for fixed point-to-point connections.See alsoPLAR.


switched LANLAN implemented with LAN switches. SeeLAN switch.

Switched Multimegabit Data ServiceSeeSMDS.

Switched Port AnalyzerSeespan.

switched virtual circuitSeeSVC.

switched virtual connectionSeeSVC.

Switch ProcessorSeeSwitch Processor in the “Cisco Systems Terms and Acronyms” section.

Switch-to-Switch ProtocolSeeSSP.

synchronizationEstablishment of common timing between sender and receiver.

Synchronous Data Link ControlSeeSDLC.

Synchronous Digital HierarchySeeSDH.

Synchronous Optical NetworkSeeSONET.

S 20-27

synchronous transmissionTerm describing digital signals that are transmitted with precise clocking. Such signalshave the same frequency, with individual characters encapsulated in control bits (calledstart bits and stop bits) that designate the beginning and end of each character. Comparewith asynchronous transmission, isochronous transmission, andplesiochronoustransmission.

Synchronous Transport Module level 1SeeSTM-1.

Synchronous Transport Signal level 1SeeSTS-1.

Synchronous Transport Signal level 3, concatenatedSeeSTS-3c.

sysgensystem generation. Process of defining network resources in a network.

system generationSeesysgen.

system services control pointsSeeSSCP.

Systems Network ArchitectureSeeSNA.


C H A P T E R

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2 1

TT1

Digital WAN carrier facility. T1 transmits DS-1-formatted data at 1.544 Mbps throughthe telephone-switching network, using AMI or B8ZS coding. Compare withE1. SeealsoAMI, B8ZS, andDS-1.

T.120ITU standard that describes data conferencing. H.323 provides for the ability toestablish T.120 data sessions inside of an existing H.323 session.

T3Digital WAN carrier facility. T3 transmits DS-3-formatted data at 44.736 Mbps throughthe telephone switching network. Compare withE3. See alsoDS-3.

TABSTelemetry Asynchronous Block Serial. AT&T polled point-to-point or multipointcommunication protocol that supports moderate data transfer rates over intra-office wirepairs.

TAC1. Terminal Access Controller. Internet host that accepts terminal connections fromdial-up lines.

2. Cisco Technical Assistance Center. SeeTAC andTACACS+ in theCisco SystemsTerms and Acronyms section.


TACACSTerminal Access Controller Access Control System. Authentication protocol,developed by the DDN community, that provides remote access authentication andrelated services, such as event logging. User passwords are administered in a centraldatabase rather than in individual routers, providing an easily scalable network securitysolution. See alsoTACACS+ in the “Cisco Systems Terms and Acronyms” section.

TACACS+SeeTACACS+(Terminal Access Controller Access Control System Plus) in the “CiscoSystems Terms and Acronyms” section.

tagIdentification information, including a number plus other information.

tag switchingHigh-performance, packet-forwarding technology that integrates network layer (Layer3) routing and data link layer (Layer 2) switching and provides scalable, high-speedswitching in the network core. Tag switching is based on the concept of label swapping,in which packets or cells are assigned short, fixed-length labels that tell switching nodeshow data should be forwarded.

tagged trafficATM cells that have their CLP bit set to 1. If the network is congested, tagged traffic canbe dropped to ensure delivery of higher-priority traffic. Sometimes called DE traffic. SeealsoCLP.

Tandem switchingDynamic switching of voice calls between VoFR, VoATM, or VoHDLC PVCs andsubchannels; also called tandeming. Tandem switching is often encountered inmulti-hop VoFR call connection paths.

TARPTID Address Resolution Protocol. In OSS, a protocol that resolves a TL-1 TID to aCLNP address (NSAP).

T 21-3

TAXI 4B/5BTransparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byte. Encodingscheme used for FDDI LANs as well as for ATM. Supports speeds of up to 100 Mbpsover multimode fiber. TAXI is the chipset that generates 4B/5B encoding on multimodefiber. See also4B/5B local fiber.

TBOS protocolTelemetry Byte Oriented Serial protocol. Protocol that transmits alarm, status, andcontrol points between NE and OSS. TBOS defines one physical interface for directconnection between the telemetry equipment and the monitored equipment.

TCtransmission convergence. Sublayer of the ATM physical layer that transforms the flowof cells into a steady flow of bits for transmission over the physical medium. Whentransmitting, the TC sublayer maps the cells into the frame format, generates the HEC,and sends idle cells when there is nothing to send. When receiving, the TC sublayerdelineates individual cells in the received bit stream and uses HEC to detect and correcterrors. See alsoHEC andPHY.

T-carrierTDM transmission method usually referring to a line or cable carrying a DS-1 signal.

TCAPtransaction capabilities application part.

TCCterminating call control.

TCL Interfacetool command line interface.

TCPTransmission Control Protocol. Connection-oriented transport layer protocol thatprovides reliable full-duplex data transmission. TCP is part of the TCP/IP protocolstack. See alsoTCP/IP.

TCP and UDP over Lightweight IPSeeTULIP.


TCP and UDP over Nonexistent IPSeeTUNIP.

TCP/IPTransmission Control Protocol/Internet Protocol. Common name for the suite ofprotocols developed by the U.S. DoD in the 1970s to support the construction ofworldwide internetworks. TCP and IP are the two best-known protocols in the suite. SeealsoIP andTCAP.

TCUtrunk coupling unit. In Token Ring networks, a physical device that enables a station toconnect to the trunk cable.

TDMtime-division multiplexing. Technique in which information from multiple channels canbe allocated bandwidth on a single wire based on preassigned time slots. Bandwidth isallocated to each channel regardless of whether the station has data to transmit.Compare withATDM, FDM, andstatistical multiplexing.

TDRtime domain reflectometer. Device capable of sending signals through a networkmedium to check cable continuity and other attributes. TDRs are used to find physicallayer network problems.

Technical Assistance CenterSeeTAC.

Technical Office ProtocolSeeTOP.

Technology prefixDiscriminators used to distinguish between gateways having specific capabilities withina given zone. In the exchange between the gateway and the gatekeeper, the technologyprefix is used to select a gateway after the zone has been selected. Technology prefixescan be used to tell the gatekeeper that a certain technology is associated with a particularcall (for example, 15# could mean a fax transmission), or it can be used like an area code

T 21-5

for more generic routing. No standard defines what the numbers in a technology prefixmean; by convention, technology prefixes are designated by a pound (#) symbol as thelast character.

TEIterminal endpoint identifier. Field in the LAPD address that identifies a device on anISDN interface. See alsoTE.

TEterminal equipment. Any ISDN-compatible device that can be attached to the network,such as a telephone, fax, or computer.

telcoAbbreviation for telephone company.

Telecommunication Management NetworkSeeTMN.

telecommunicationsTerm referring to communications (usually involving computer systems) over thetelephone network.

Telecommunications Industry AssociationSeeTIA.

telephonyScience of converting sound to electrical signals and transmitting it between widelyremoved points.

Telemetry Asynchronous Block SerialSeeTABS.

telexTeletypewriter service allowing subscribers to send messages over the PSTN.

telemetryCapability of transmitting or retrieving data over long distance communication links,such as satellite or telephone.


TelnetStandard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used forremote terminal connection, enabling users to log in to remote systems and useresources as if they were connected to a local system. Telnet is defined in RFC 854.

TempestU.S. military standard. Electronic products adhering to the Tempest specification aredesigned to withstand EMP. See alsoEMP.

TERENATrans-European Research and Education Networking Association. Organization thatpromotes information and telecommunications technologies development in Europe.Formed by the merger of EARN and RARE. See alsoEARN andRARE.

termidSNA cluster controller identification for switched lines only. Also calledXid.

terminalSimple device at which data can be entered or retrieved from a network. Generally,terminals have a monitor and a keyboard, but no processor or local disk drive.

Terminal Access ControllerSeeTAC.

Terminal Access Controller Access SystemSeeTACACS.

terminal adapterDevice used to connect ISDN BRI connections to existing interfaces such asEIA/TIA-232. Essentially, an ISDN modem.

terminal emulationNetwork application in which a computer runs software that makes it appear to a remotehost as a directly attached terminal.

terminal endpoint identifierSeeTEI.

T 21-7

terminal equipmentSeeTE.

terminal serverCommunications processor that connects asynchronous devices such as terminals,printers, hosts, and modems to any LAN or WAN that uses TCP/IP, X.25, or LATprotocols. Terminal servers provide the internetwork intelligence that is not available inthe connected devices.

terminatorDevice that provides electrical resistance at the end of a transmission line to absorbsignals on the line, thereby keeping them from bouncing back and being received againby network stations.

TESTtest.

Texas Higher Education NetworkSeeTHEnet.

TFTPTrivial File Transfer Protocol. Simplified version of FTP that allows files to betransferred from one computer to another over a network.

THtransmission header. SNA header that is appended to the SNA basic information unit(BIU). The TH uses one of a number of available SNA header formats. See alsoFID0,FID1, FID2, FID3, andFID4.

THC over X.25SeeTHC over X.25 in the “Cisco Systems Terms and Acronyms” section.

THEnetTexas Higher Education Network. Regional network comprising over 60 academic andresearch institutions in the Texas (United States) area.


ThinnetTerm used to define a thinner, less expensive version of the cable specified in theIEEE 802.3 10Base2 standard. Compare withCheapernet. See also10Base2, Ethernet,andIEEE 802.3.

three-way handshakeProcess whereby two protocol entities synchronize during connection establishment.

throughputRate of information arriving at, and possibly passing through, a particular point in anetwork system.

TIATelecommunications Industry Association. Organization that develops standardsrelating to telecommunications technologies. Together, the TIA and the EIA haveformalized standards, such as EIA/TIA-232, for the electrical characteristics of datatransmission. See alsoEIA.

TICToken Ring interface coupler. Controller through which an FEP connects to a TokenRing.

TIDTerminal Identifier.

TINA-CTelecommunications Information Networking Architecture. Services applications builtin C and corresponding to TINA guidelines.

TIOSTranspath Input Output Subsystem

TIRKSTrunk Information Record Keeping System. Bellcore OSS that provides record keepingfor interoffice trunk facilities. See alsoOSS.

time-division multiplexingSeeTDM.

T 21-9

time domain reflectometerSeeTDR.

Time NotifySeeTNotify.

timeoutEvent that occurs when one network device expects to hear from another network devicewithin a specified period of time, but does not. The resulting timeout usually results ina retransmission of information or the dissolving of the session between the two devices.

Time To LiveSeeTR VLAN.

TL-1Transaction Language One. Bellcore term for intelligent network elements.

TLAPTokenTalk Link Access Protocol. Link-access protocol used in a TokenTalk network.TLAP is built on top of the standard Token Ring data-link layer.

TMtraffic management.

TMNTelecommunication Management Network. ITU-T generic model for transporting andprocessing OAM&P information for a telecommunications network. See alsoOAM&P.

TN3270Terminal emulation software that allows a terminal to appear to an IBM host as a 3278Model 2 terminal.

TNotifyTime Notify. Specifies how often SMT initiates neighbor notification broadcasts. SeealsoSMT.


tokenFrame that contains control information. Possession of the token allows a networkdevice to transmit data onto the network. See alsotoken passing.

token busLAN architecture using token passing access over a bus topology. This LANarchitecture is the basis for the IEEE 802.4 LAN specification. See alsoIEEE 802.4.

token passingAccess method by which network devices access the physical medium in an orderlyfashion based on possession of a small frame called a token. Contrast withcircuitswitching andcontention. See alsotoken.

Token RingToken-passing LAN developed and supported by IBM. Token Ring runs at 4 or16 Mbps over a ring topology. Similar to IEEE 802.5. See alsoIEEE 802.5, ringtopology, andtoken passing.

Token Ring interface couplerSeeTIC.

TokenTalk Link Access ProtocolSeeTLAP.

TokenTalkApple Computer’s data-link product that allows an AppleTalk network to be connectedby Token Ring cables.

TOPTechnical Office Protocol. OSI-based architecture developed for officecommunications.

topologyPhysical arrangement of network nodes and media within an enterprise networkingstructure.

T 21-11

ToStype of service. SeeCoS.

TP0Transport Protocol Class 0. OSI connectionless transport protocol for use over reliablesubnetworks. Defined by ISO 8073.

TP4Transport Protocol Class 4. OSI connection-based transport protocol. Defined byISO 8073.

TPDMechanism used by some ATM switches that allows the remaining cells supporting anAAL5 frame to be discarded when one or more cells of that AAL5 frame are dropped.This avoids sending partial AAL5 frames through the ATM network when they willhave to be retransmitted by the sender. Compare withEPD.

TPPMDtwisted-pair physical medium dependent.

tracerouteProgram available on many systems that traces the path a packet takes to a destination.It is mostly used to debug routing problems between hosts. There is also a tracerouteprotocol defined in RFC 1393.

traffic managementTechniques for avoiding congestion and shaping and policing traffic, Allows links tooperate at high levels of utilization by scaling back lower-priority, delay-tolerant trafficat the edge of the network when congestion begins to occur.

Traffic pathRoute of a bearer channel that carries voice traffic.

traffic policingProcess used to measure the actual traffic flow across a given connection and compareit to the total admissible traffic flow for that connection. Traffic outside of the agreedupon flow can be tagged (where the CLP bit is set to 1) and can be discarded en route if


congestion develops. Traffic policing is used in ATM, Frame Relay, and other types ofnetworks. Also know as admission control, permit processing, rate enforcement, andUPC. See alsotagged traffic.

traffic profileSet of CoS attribute values assigned to a given port on an ATM switch. The profileaffects numerous parameters for data transmitted from the port including rate, cell dropeligibility, transmit priority, and inactivity timer. See alsoCoS.

traffic shapingUse of queues to limit surges that can congest a network. Data is buffered and then sentinto the network in regulated amounts to ensure that the traffic will fit within thepromised traffic envelope for the particular connection. Traffic shaping is used in ATM,Frame Relay, and other types of networks. Also known as metering, shaping, andsmoothing.

trailing packet discardSeeTPD.

trailerControl information appended to data when encapsulating the data for networktransmission. Compare withheader.

transactionResult-oriented unit of communication processing.

transaction services layerLayer 7 in the SNA architectural model. Represents user application functions, such asspreadsheets, word-processing, or e-mail, by which users interact with the network.Corresponds roughly with theapplication layer of the OSI reference model. See alsodata flow control layer, data-link control layer, path control layer, physical controllayer, presentation services layer, andtransaction services layer.

transceiverSeeMAU.

transceiver cableSeeAUI.

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Trans-European Research and Education Networking AssociationSeeTERENA.

transfer syntaxDescription on an instance of a data type that is expressed as a string of bits.

transit bridgingBridging that uses encapsulation to send a frame between two similar networks over adissimilar network.

translational bridgingBridging between networks with dissimilar MAC sublayer protocols. MAC informationis translated into the format of the destination network at the bridge. Contrast withencapsulation bridging.

transmission control layerLayer 4 in the SNA architectural model. This layer is responsible for establishing,maintaining, and terminating SNA sessions, sequencing data messages, and controllingsession level flow. Corresponds to thetransport layer of the OSI model. See alsodataflow control layer, data-link control layer, path control layer, physical control layer,presentation services layer, andtransaction services layer.

Transmission Control ProtocolSeeTCAP.

Transmission Control Protocol/Internet ProtocolSeeTCP/IP.

transmission convergenceSeeTC.

transmission groupIn SNA routing, one or more parallel communications links treated as onecommunications facility.

transmission headerSeeTH.


transmission linkSeelink.

TRANSPACMajor packet data network run by France Telecom.

Transparent Asynchronous Transmitter/Receiver Interface 4-byte/5-byteSeeTAXI 4B/5B.

transparent bridgingBridging scheme often used in Ethernet and IEEE 802.3 networks in which bridges passframes along one hop at a time based on tables associating end nodes with bridge ports.Transparent bridging is so named because the presence of bridges is transparent tonetwork end nodes. Contrast withSRB.

TransPath componentThe part of your signaling controller system where signals are identified, converted, androuted.

transport layerLayer 4 of the OSI reference model. This layer is responsible for reliable networkcommunication between end nodes. The transport layer provides mechanisms for theestablishment, maintenance, and termination of virtual circuits, transport fault detectionand recovery, and information flow control. Corresponds to thetransmission controllayer of the SNA model. See alsoapplication layer, data-link layer, network layer,physical layer, PQ, andsession layer.

Transport Protocol Class 0SeeTP0.

Transport Protocol Class 4SeeTP4.

trapMessage sent by an SNMP agent to an NMS, console, or terminal to indicate theoccurrence of a significant event, such as a specifically defined condition or a thresholdthat was reached. See alsoalarm andevent.

T 21-15

tree topologyLAN topology similar to a bus topology, except that tree networks can contain brancheswith multiple nodes. Transmissions from a station propagate the length of the mediumand are received by all other stations. Compare withbus topology, ring topology, andstar topology.

TRIPSeeTRIP (Token Ring Interface Processor) in the “Cisco Systems Terms andAcronyms” section.

TRISLToken Ring Inter-Switch Link

Trivial File Transfer ProtocolSeeTFTP.

trunkPhysical and logical connection between two switches across which network traffictravels. A backbone is composed of a number of trunks.

trunk coupling unitSeeTCU.

trunk up-downSeeTUD.

TR VLANToken Ring virtual LAN

TSItransmitting subscriber information. Frame that can be sent by the caller with thecaller’s telephone number that can be used to screen calls.

TSPtag-switched path.

TTLTime To Live. Field in an IP header that indicates how long a packet is considered valid.


tunnelingArchitecture that is designed to provide the services necessary to implement anystandard point-to-point encapsulation scheme. See alsoencapsulation.

TUDtrunk up-down. Protocol used in ATM networks that monitors trunks and detects whenone goes down or comes up. ATM switches send regular test messages from each trunkport to test trunk line quality. If a trunk misses a given number of these messages, TUDdeclares the trunk down. When a trunk comes back up, TUD recognizes that the trunkis up, declares the trunk up, and returns it to service. See alsotrunk.

TULIPTCP and UDP over Lightweight IP. Proposed protocol for running TCP and UDPapplications over ATM.

TUNIPTCP and UDP over Nonexistent IP. Proposed protocol for running TCP and UPDapplications over ATM.

TUVGerman test agency that certifies products to European safety standards.

two-way simultaneousSeeTWS in the “Cisco Systems Terms and Acronyms” section.

twisted pairRelatively low-speed transmission medium consisting of two insulated wires arrangedin a regular spiral pattern. The wires can be shielded or unshielded. Twisted pair iscommon in telephony applications and is increasingly common in data networks. SeealsoSTP andUTP.

TYMNETSeeXStream.

Type 1 operationIEEE 802.2 (LLC) connectionless operation.

T 21-17

Type 2 operationIEEE 802.2 (LLC) connection-oriented operation.

type of serviceSeeToS.


C H A P T E R

U 22-1

2 2

UUA

unnumbered acknowledgement.

UARTUniversal Asynchronous Receiver/Transmitter. Integrated circuit, attached to theparallel bus of a computer, used for serial communications. The UART translatesbetween serial and parallel signals, provides transmission clocking, and buffers datasent to or from the computer.

UB Net/OneUngermann-Bass Net/One. Routing protocol, developed by UB Networks, that useshello packets and a path-delay metric, with end nodes communicating using the XNSprotocol. There are a number of differences between the manner in which Net/One usesthe XNS protocol and the usage common among other XNS nodes.

UBRunspecified bit rate. QoS class defined by the ATM Forum for ATM networks. UBRallows any amount of data up to a specified maximum to be sent across the network, butthere are no guarantees in terms of cell loss rate and delay. Compare withABR, CBR,andVBR.

UBR+unspecified bit rate plus. UBR service complemented by ATM switches that useintelligent packet discard mechanisms such as EPD or TPD. See alsoEPD andTPD.

UCMuniversal call model.


UDLPUniDirectional Link Protocol. Protocol used by inexpensive, receive-only antennas toreceive data via satellite.

UDPUser Datagram Protocol. Connectionless transport layer protocol in the TCP/IPprotocol stack. UDP is a simple protocol that exchanges datagrams withoutacknowledgments or guaranteed delivery, requiring that error processing andretransmission be handled by other protocols. UDP is defined in RFC 768.

U-frameUnnumbered frame. One of three SDLC frame formats. See alsoI-frame andS-frame.

UIunnumbered information.

UIOUniversal I/O serial port (Cisco router).

UKERNAUK Education and Research Networking Association.

ULUnderwriters Laboratories. Independent agency within the United States that testsproduct safety.

U-lawCompanding technique commonly used in North America. U-law is standardized as a64-kbps CODEC in ITU-T G.711.

ULPupper-layer protocol. Protocol that operates at a higher layer in the OSI reference model,relative to other layers. ULP is sometimes used to refer to the next-highest protocol(relative to a particular protocol) in a protocol stack.

unbalanced configurationHDLC configuration with one primary station and multiple secondary stations.

U 22-3

Underwriters LaboratoriesSeeUL.

Ungermann-Bass Net/OneSeeUB Net/One.

UNIUser-Network Interface. ATM Forum specification that defines an interoperabilitystandard for the interface between ATM-based products (a router or an ATM switch)located in a private network and the ATM switches located within the public carriernetworks. Also used to describe similar connections in Frame Relay networks. See alsoNNI, Q.920/Q.921, andSNI (Subscriber Network Interface).

unicastMessage sent to a single network destination. Compare withbroadcast andmulticast.

unicast addressAddress specifying a single network device. Compare withbroadcast address andmulticast address. See alsounicast.

UniDirectional Link ProtocolSeeUCM.

uninsured trafficTraffic within the excess rate (the difference between the insured rate and maximumrate) for an ATM VCC. This traffic can be dropped by the network if congestion occurs.See alsoCLP, insured rate, andmaximum rate.

unipolarLiterally meaning one polarity, the fundamental electrical characteristic of internalsignals in digital communications equipment. Contrast withbipolar.

unity gainIn broadband networks, the balance between signal loss and signal gain throughamplifiers.

Universal Asynchronous Receiver/TransmitterSeeUCM.


Universal Resource LocatorSeeURL.

UNIXOperating system developed in 1969 at Bell Laboratories. UNIX has gone throughseveral iterations since its inception. These include UNIX 4.3 BSD (Berkeley StandardDistribution), developed at the University of California at Berkeley, and UNIX SystemV, Release 4.0, developed by AT&T.

UNIX-to-UNIX Copy ProgramSeeUUCP.

unnumbered framesHDLC frames used for various control and management purposes, including linkstartup and shutdown, and mode specification.

unshielded twisted-pairSeeUTP.

unspecified bit rateSeeUBR.

UPCusage parameter control. Seetraffic policing.

upper-layer protocolSeeU-law.

upstreamSet of frequencies used to send data from a subscriber to the headend.

urban legendStory, which may start with a grain of truth, that has been retold and end up on theInternet. Some legends that periodically make their rounds include “The InfamousModem Tax,” “Craig Shergold/Brain Tumor/Get Well Cards,” and “The $250 CookieRecipe.” Urban Legends are conceptually similar to space junk that stays in orbit foryears.

U 22-5

URLUniversal Resource Locator. Standardized addressing scheme for accessing hypertextdocuments and other services using a browser. See alsobrowser.

usage parameter controlSeetraffic policing.

USENETInitiated in 1979, one of the oldest and largest cooperative networks, with over 10,000hosts and a quarter of a million users. Its primary service is a distributed conferencingservice called news.

User Datagram ProtocolSeeUDP.

User-Network InterfaceSeeUNI.

UTCCoordinated Universal Time. Time zone at zero degrees longitude. Formerly calledGreenwich Mean Time (GMT) and Zulu time.

UTPunshielded twisted-pair. Four-pair wire medium used in a variety of networks. UTP doesnot require the fixed spacing between connections that is necessary with coaxial-typeconnections. There are five types of UTP cabling commonly used:Category 1 cabling,Category 2 cabling, Category 3 cabling, Category 4 cabling, andCategory 5 cabling.Compare withSTP. See alsoEIA/TIA-586 andtwisted pair.

UTSP1024C data-link layer protocol.

UUCPUNIX-to-UNIX Copy Program. Protocol stack used for point-to-point communicationbetween UNIX systems.


uuencodeUNIX-to-UNIX encoding. Method of converting binary files to ASCII so that they canbe sent over the Internet via e-mail. The name comes from its use by the UNIXoperating system’s uuencode command. See alsouudecode.

uudecodeUNIX-to-UNIX decode. Method of decoding ASCII files that were encoded usinguuencode. See alsouuencode.

UVMUniversal Voice Module.

UVM-CUniversal Voice Module-Channelized.

UVM-UUniversal Voice Module-Unchannelized.

C H A P T E R

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2 3

VV.24

ITU-T standard for a physical layer interface between DTE and DCE. V.24 is essentiallythe same as the EIA/TIA-232 standard. See alsoEIA/TIA-232.

V.25bisITU-T specification describing procedures for call setup and tear down over theDTE-DCE interface in a PSDN.

V.32ITU-T standard serial line protocol for bidirectional data transmissions at speeds of4.8 or 9.6 Kbps. See alsoV.32bis.

V.32bisITU-T standard that extends V.32 to speeds up to 14.4 Kbps. See alsoV.32.

V.34ITU-T standard that specifies a serial line protocol. V.34 offers improvements to theV.32 standard, including higher transmission rates (28.8 Kbps) and enhanced datacompression. Compare withV.32.

V.35ITU-T standard describing a synchronous, physical layer protocol used forcommunications between a network access device and a packet network. V.35 is mostcommonly used in the United States and in Europe, and is recommended for speeds upto 48 Kbps.

V.42ITU-T standard protocol for error correction using LAPM. See alsoLAPM.


VACvolts alternating current.

VADvoice activity detection. When enabled on voice port or a dial peer, silence is nottransmitted over the network, only audible speech. When VAD is enabled, the soundquality is slightly degraded, but the connection monopolizes much less bandwidth.

variable bit rateSeeVBR.

variable-length subnet maskSeeVLSM.

VBRvariable bit rate. QoS class defined by the ATM Forum for ATM networks. VBR issubdivided into a real time (RT) class and non-real time (NRT) class. VBR (RT) is usedfor connections in which there is a fixed timing relationship between samples. VBR(NRT) is used for connections in which there is no fixed timing relationship betweensamples, but that still need a guaranteed QoS. Compare withABR, CBR, andUBR.

VCSeevirtual circuit.

VCCvirtual channel connection. Logical circuit, made up of VCLs, that carries data betweentwo end points in an ATM network. Sometimes called avirtual circuit connection. SeealsoVCD, VCL, andVPI.

VCDvirtual circuit descriptor.

VCIvirtual channel identifier. 16-bit field in the header of an ATM cell. The VCI, togetherwith the VPI, is used to identify the next destination of a cell as it passes through a seriesof ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to

V 23-3

identify the next network VCL that a cell needs to transit on its way to its finaldestination. The function of the VCI is similar to that of the DLCI in Frame Relay.Compare withDLCI. See alsoVCL andVPI.

VCLvirtual channel link. Connection between two ATM devices. A VCC is made up of oneor more VCLs. See alsoVCC.

VCNvirtual circuit number. 12-bit field in an X.25 PLP header that identifies an X.25 virtualcircuit. Allows DCE to determine how to route a packet through the X.25 network. SeealsoLCI andLCN.

VDCvolts direct current.

VDSLvery-high-data-rate digital subscriber line. One of four DSL technologies. VDSLdelivers 13 to 52 Mbps downstream and 1.5 to 2.3 Mbps upstream over a single twistedcopper pair. The operating range of VDSL is limited to 1,000 to 4,500 feet (304.8 to1,372 meters). Compare withADSL, HDSL, andSDSL.

vectorData segment of an SNA message. A vector consists of a length field, a key thatdescribes the vector type, and vector-specific data.

Veronicavery easy rodent oriented netwide index to computer archives. Gopher utility thateffectively searches Gopher servers based on a user’s list of keywords.

Versatile Interface ProcessorSeeVIP in the “Cisco Systems Terms and Acronyms” section.

VFvariance factor. One of three link attributes exchanged using PTSPs to determine theavailable resources of an ATM network. VF is a relative measure of CRM normalizedby the variance of the aggregate cell rate on the link.


VINESVirtual Integrated Network Service. NOS developed and marketed by Banyan Systems.

VIPSeeVIP (Versatile Interface Processor) in the “Cisco Systems Terms and Acronyms”section.

virtual addressSeenetwork address.

virtual channelSeevirtual circuit.

virtual channel connectionSeeVCC.

virtual channel identifierSeeVCD.

virtual channel linkSeeVCL.

virtual circuitLogical circuit created to ensure reliable communication between two network devices.A virtual circuit is defined by a VPI/VCI pair, and can be either permanent (PVC) orswitched (SVC). Virtual circuits are used in Frame Relay and X.25. In ATM, a virtualcircuit is called avirtual channel. Sometimes abbreviatedVC. See alsoPVC, SVC, VCD,virtual route, andVPI.

virtual circuit connectionSeeVCC.

virtual circuit numberSeeVCN.

virtual connectionIn ATM, a connection between end users that has a defined route and endpoints. See alsoPVC andSVC.

V 23-5

Virtual Integrated Network ServiceSeeVINES.

virtual IPSeeVIP in the “Cisco Systems Terms and Acronyms” section.

virtualizationProcess of implementing a network based on virtual network segments. Devices areconnected to virtual segments independent of their physical location and their physicalconnection to the network.

virtual LANSeeVLAN.

virtual LAN internetworkSeeVLI.

Virtual Networking ServicesSeeVirtual Networking Services in the “Cisco Systems Terms and Acronyms” section.

virtual pathLogical grouping of virtual circuits that connect two sites. See alsovirtual circuit.

virtual path connectionSeeVPC.

virtual path identifierSeeVPI.

virtual path identifier/virtual channel identifierSeeVPI/VCI.

virtual path linkSeeVPL.


virtual ringEntity in an SRB network that logically connects two or more physical rings togethereither locally or remotely. The concept of virtual rings can be expanded across routerboundaries.

virtual routeIn SNA, a logical connection between subarea nodes that is physically realized as aparticular explicit route. SNA terminology forvirtual circuit. See alsovirtual circuit.

virtual subnetLogical grouping of devices that share a common Layer 3 subnet.

virtual telecommunications access methodSeeVTAM.

Virtual Terminal ProtocolSeeVTP.

VLANvirtual LAN. Group of devices on one or more LANs that are configured (usingmanagement software) so that they can communicate as if they were attached to thesame wire, when in fact they are located on a number of different LAN segments.Because VLANs are based on logical instead of physical connections, they areextremely flexible.

VLIvirtual LAN internetwork. Internetwork composed of VLANs. See alsoVLAN.

VLSMvariable-length subnet mask. Ability to specify a different subnet mask for the samenetwork number on different subnets. VLSM can help optimize available address space.

VMACVirtual Media Access Control.

VNSSeeVirtual Networking Services in the “Cisco Systems Terms and Acronyms” section.

V 23-7

VoATMVoice Over ATM. Voice over ATM enables a router to carry voice traffic (for example,telephone calls and faxes) over an ATM network. When sending voice traffic over ATM,the voice traffic is encapsulated using a special AAL5 encapsulation for multiplexedvoice.

VoATM dial peerDial peer connected via an ATM network. VoATM peers point to specific VoATMdevices.

VoFRVoice Over Frame Relay. Voice over Frame Relay enables a router to carry voice traffic(for example, telephone calls and faxes) over a Frame Relay network. When sendingvoice traffic over Frame Relay, the voice traffic is segmented and encapsulated fortransit across the Frame Relay network using FRF.12 encapsulation.

VoFR dial peerDial peer connected via a Frame Relay network. VoFR peers point to specific VoFRdevices.

VoHDLCVoice Over HDLC. Voice over HDLC enables a router to carry live voice traffic (forexample, telephone calls and faxes) back-to-back to a second router over a serial line.

VoHDLC dial peerDial peer connected via an HDLC network. VoHDLC peers point to specific VoHDLCdevices.

VoIPVoice over IP. The ability to carry normal telephony-style voice over an IP-basedinternet with POTS-like functionality, reliability, and voice quality

Voice over IPVoice over IP enables a router to carry voice traffic (for example, telephone calls andfaxes) over an IP network. In Voice over IP, the DSP segments the voice signal intoframes, which are then coupled in groups of two and stored in voice packets. Thesevoice packets are transported using IP in compliance with ITU-T specification H.323.


VPvirtual path. One of two types of ATM circuits identified by a VPI. A virtual path is abundle of virtual channels, all of which are switched transparently across an ATMnetwork based on a common VPI. See alsoVPI.

VPCvirtual path connection. Grouping of VCCs that share one or more contiguous VPL. SeealsoVCC andVPL.

VPDNvirtual private dial-up network. See alsoVPN.

VPIvirtual path identifier. 8-bit field in the header of an ATM cell. The VPI, together withthe VCI, is used to identify the next destination of a cell as it passes through a series ofATM switches on its way to its destination. ATM switches use the VPI/VCI fields toidentify the next VCL that a cell needs to transit on its way to its final destination. Thefunction of the VPI is similar to that of the DLCI in Frame Relay. Compare withDLCI.See alsoVCD andVCL.

VPI/VCISeeVCD andVPI.

VPLvirtual path link. Within a virtual path, a group of unidirectional VCLs with the sameend points. Grouping VCLs into VPLs reduces the number of connections to bemanaged, thereby decreasing network control overhead and cost. A VPC is made up ofone or more VPLs.

VPNVirtual Private Network, which Enables IP traffic to travel securely over a public TCP/IPnetwork by encrypting all traffic from one network to another. A VPN uses “tunneling”to encrypt all information at the IP level.

VRMLVirtual Reality Modeling Language. Specification for displaying 3-dimensional objectson the World Wide Web. Think of it as the 3-D equivalent of HTML.

V 23-9

VSCSeeVSC in theCisco Systems Terms and Acronyms section.

VSIVirtual Switch Interface

VS/VDvirtual source/virtual destination.

VT-nVirtual Tributary leveln. SONET format for mapping a lower-rate signal into a SONETpayload. For example, VT-1.5 is used to transport a DS-1 signal. See alsoDS-1 andSONET.

VTAMvirtual telecommunications access method. Set of programs that control communicationbetween LUs. VTAM controls data transmission between channel-attached devices andperforms routing functions. See alsoLU.

VTPVirtual Terminal Protocol. ISO application for establishing a virtual terminalconnection across a network.

vtyvirtual type terminal, but commonly used as virtual terminal lines.


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WWAIS

Wide Area Information Server. Distributed database protocol developed to search forinformation over a network. WAIS supports full-text databases, which allow an entiredocument to be searched for a match (as opposed to other technologies that only allowan index of key words to be searched).

WANwide-area network. Data communications network that serves users across a broadgeographic area and often uses transmission devices provided by common carriers.Frame Relay, SMDS, and X.25 are examples of WANs. Compare withLAN andMAN.

watchdog packetUsed to ensure that a client is still connected to a NetWare server. If the server has notreceived a packet from a client for a certain period of time, it sends that client a seriesof watchdog packets. If the station fails to respond to a predefined number of watchdogpackets, the server concludes that the station is no longer connected and clears theconnection for that station.

watchdog spoofingSubset of spoofing that refers specifically to a router acting for a NetWare client bysending watchdog packets to a NetWare server to keep the session between client andserver active. See alsospoofing.

watchdog timer1. Hardware or software mechanism that is used to trigger an event or an escape from aprocess unless the timer is periodically reset.


2. In NetWare, a timer that indicates the maximum period of time that a server will waitfor a client to respond to a watchdog packet. If the timer expires, the server sendsanother watchdog packet (up to a set maximum). See alsowatchdog packet.

waveform codingElectrical techniques used to convey binary signals.

Web browserSeebrowser.

WDMwave division multiplexing

W-DCSWideband Digital Crossconnect System. SONET DCS capable of crossconnectingDS-1 and VT1.5 signals. See alsoDCS, DS-1, SONET, andVT-n.

WFQweighted fair queuing. Congestion management algorithm that identifies conversations(in the form of traffic streams), separates packets that belong to each conversation, andensures that capacity is shared fairly between these individual conversations. WFQ is anautomatic way of stabilizing network behavior during congestion and results inincreased performance and reduced retransmission.

wide-area networkSeeWAN.

widebandSeebroadband.

wildcard mask32-bit quantity used in conjunction with an IP address to determine which bits in an IPaddress should be ignored when comparing that address with another IP address. Awildcard mask is specified when setting up access lists.

W 24-3

WinSockWindows Socket Interface. Software interface that allows a wide variety of applicationsto use and share an Internet connection. WinSock is implemented asdynamic linklibrary (DLL) with some supporting programs, such as a dialer program that initiates theconnection.

wiring closetSpecially designed room used for wiring a data or voice network. Wiring closets serveas a central junction point for the wiring and wiring equipment that is used forinterconnecting devices.

WISCNETTCP/IP network in Wisconsin (United States) connecting University of Wisconsincampuses and a number of private colleges. Links are 56 Kbps and T1.

workgroupCollection of workstations and servers on a LAN that are designed to communicate andexchange data with one another.

Workgroup DirectorSeeVSC in the “Cisco Systems Terms and Acronyms” section.

workgroup switchingMethod of switching that provides high-speed (100-Mbps) transparent bridgingbetween Ethernet networks, and high-speed translational bridging between Ethernet andCDDI or FDDI.

World Wide WebSeeWWW.

wrapAction taken by an FDDI or CDDI network to recover in the event of a failure. Thestations on each side of the failure reconfigure themselves, creating a single logical ringout of the primary and secondary rings.

WWWWorld Wide Web. Large network of Internet servers providing hypertext and otherservices to terminals running client applications such as a browser. See alsobrowser.


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XX.121

ITU-T standard describing an addressing scheme used in X.25 networks. X.121addresses are sometimes called IDNs.

X.21ITU-T standard for serial communications over synchronous digital lines. The X.21protocol is used primarily in Europe and Japan.

X.21bisITU-T standard that defines the physical layer protocol for communication betweenDCE and DTE in an X.25 network. Virtually equivalent toEIA/TIA-232. See alsoEIA/TIA-232 andX.25.

X.25ITU-T standard that defines how connections between DTE and DCE are maintained forremote terminal access and computer communications in PDNs. X.25 specifies LAPB,a data link layer protocol, and PLP, a network layer protocol. Frame Relay has to somedegree superseded X.25. See alsoFrame Relay, LAPB, andPLP.

X.25 Level 3SeePLP.

X.25 ProtocolSeePLP.

X.28ITU-T recommendation that defines the terminal-to-PAD interface in X.25 networks.See alsoPAD andX.25.


X.29ITU-T recommendation that defines the form for control information in theterminal-to-PAD interface used in X.25 networks. See alsoPAD andX.25.

X.3ITU-T recommendation that defines various PAD parameters used in X.25 networks.See alsoPAD andX.25.

X3T9.5Number assigned to the ANSI Task Group of Accredited Standards Committee for theirinternal, working document describing FDDI.

X.400ITU-T recommendation specifying a standard for e-mail transfer.

X.500ITU-T recommendation specifying a standard for distributed maintenance of files anddirectories.

X.75ITU-T specification that defines the signaling system between two PDNs. X.75 isessentially anNNI. See alsoNNI.

X Display Manager Control ProtocolSeeXDMCP.

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

XDReXternal Data Representation. Standard for machine-independent data structuresdeveloped by Sun Microsystems. Similar toBER.

xDSLGroup term used to refer toADSL, HDSL, SDSL andVDSL. All are emerging digitaltechnologies using the existing copper infrastructure provided by the telephonecompanies. xDSL is a high-speed alternative to ISDN.

X 25-3

XETransPath Execution Environment. Layer of software providing shared services for allapplication software on the TransPath and isolating higher-level software fromoperating system dependencies.

Xerox Network SystemsSeeXNS.

XIDexchange identification. Request and response packets exchanged prior to a sessionbetween a router and a Token Ring host. If the parameters of the serial device containedin the XID packet do not match the configuration of the host, the session is dropped.

XidSeetermid.

XMLeXtensible Markup Language. Text markup language designed to enable the use ofSGML on the World-Wide Web. XML allows you to define your own customizedmarkup language.

XNSXerox Network Systems. Protocol suite originally designed by PARC. Many PCnetworking companies, such as 3Com, Banyan, Novell, and UB Networks used orcurrently use a variation of XNS as their primary transport protocol. See alsoX WindowSystem.

XOTX.25 over TCP.

X RecommendationsCCITT documents that describe data communication network standards. Well-knownones include X.25 Packet Switching standard, X.400 Message Handling System, andX.500 Directory Services.

XRemoteProtocol developed specifically to optimize support for the X Window System over aserial communications link.


XStreamMajor public PSN in the United States operated by MCI. Formerly called TYMNET.

XTagATMextended tag ATM.

X terminalTerminal that allows a user simultaneous access to several different applications andresources in a multivendor environment through implementation of X Windows. SeealsoX Window System.

X Window SystemDistributed, network-transparent, device-independent, multitasking windowing andgraphics system originally developed by MIT for communication between X terminalsand UNIX workstations. See alsoX terminal.

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Zzero code suppression

Line coding scheme used for transmission clocking. Zero line suppression substitutes a1 in the 7th bit of a string of 8 consecutive zeros. See alsoones density.

ZIPZone Information Protocol. AppleTalk session layer protocol that maps networknumbers to zone names. ZIP is used by NBP to determine which networks contain nodesthat belong to a zone. See alsoZIP storm andzone.

ZIP stormBroadcast storm that occurs when a router running AppleTalk propagates a route forwhich it currently has no corresponding zone name. The route is then forwarded bydownstream routers, and a ZIP storm ensues. See alsoZIP.

zone1. collection of all terminals, gateways, and multipoint control units (MCUs) managedby a single gatekeeper. A zone includes at least one terminal, and can include gatewaysor MCUs. A zone has only one gatekeeper. A zone can be independent of LAN topologyand can be comprised of multiple LAN segments connected using routers or otherdevices.

2. In AppleTalk, a logical group of network devices. See alsoZIP.

Zone Information ProtocolSeeZIP.


zone multicast addressData-link-dependent multicast address at which a node receives the NBP broadcastsdirected to its zone. See alsoNBNS.

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Cisco SystemsTerms and Acronymsaddress translation gateway

SeeATG.

AIPATM Interface Processor. ATM network interface for Cisco 7000 series routersdesigned to minimize performance bottlenecks at the UNI. The AIP supports AAL3/4and AAL5. See alsoAAL3/4 andAAL5.

APaRTautomated packet recognition/translation. Technology that allows a server to beattached to CDDI or FDDI without requiring the reconfiguration of applications ornetwork protocols. APaRT recognizes specific data link layer encapsulation packettypes and, when these packet types are transferred from one medium to another,translates them into the native format of the destination device.

ATGaddress translation gateway. Cisco DECnet routing software function that allows arouter to route multiple, independent DECnet networks and to establish a user-specifiedaddress translation for selected nodes between networks.

ATM Interface ProcessorSeeAIP.

ATM networkTraditional Cisco ATM network built around BPX switches.

ATM network interface cardESP card that is used as the OC-3 interface to the BPX’s BXM.


Automated Packet Recognition/TranslationSeeAPaRT.

autonomous switchingFeature on Cisco routers that provides faster packet processing by allowing the ciscoBusto switch packets independently without interrupting the system processor.

BIGABus Interface Gate Array. Technology that allows the Catalyst 5000 to receive andtransmit frames from its packet-switching memory to its MAC local buffer memorywithout the intervention of the host processor.

BOBIbreak-out/break-in. VNS feature that allows interworking between Euro-ISDN (ETSI)and other VNS-supported signaling variants, such as DPNSS and QSIG.

BPX Service NodeClosely integrated BPX switch, AXIS interface shelf, and extended services processordesigned to support ATM and Frame Relay switched virtual circuits, as well astraditional PVCs.

break-out/break-inSeeBOBI.

Bus Interface Gate ArraySeeBIGA.

Call Detail RecordSeeCDR.

CAMCisco Access Manager.

CDPCisco Discovery Protocol. Media- and protocol-independent device-discovery protocolthat runs on all Cisco-manufactured equipment including routers, access servers,bridges, and switches. Using CDP, a device can advertise its existence to other devices


and receive information about other devices on the same LAN or on the remote side ofa WAN. Runs on all media that support SNAP, including LANs, Frame Relay, and ATMmedia.

CDRCall Detail Record. VNS record of voice or data SVCs, which includes calling andcalled numbers, local and remote node names, data and timestamp, elapsed time, andCall Failure Class fields.

CEFCisco express forwarding.

CFRADSeeCisco FRAD.

Channel Interface ProcessorSeeCIP.

CIPChannel Interface Processor. Channel attachment interface for Cisco 7000 seriesrouters. The CIP is used to connect a host mainframe to a control unit, eliminating theneed for an FEP for channel attachment.

C-ISUPProprietary Cisco protocol based on ISUP.

ciscoBus controllerSeeSP.

Cisco Discovery ProtocolSeeCDP.

Cisco FRADCisco Frame Relay access device. Cisco product that supports Cisco IOS Frame RelaySNA services and can be upgraded to be a full-function multiprotocol router. The CiscoFRAD connects SDLC devices to Frame Relay without requiring an existing LAN.However, the Cisco FRAD does support attached LANs and can perform conversionfrom SDLC to Ethernet and Token Ring. See alsoFRAD.


Cisco Frame Relay access deviceSeeCisco FRAD.

CiscoFusionCisco internetworking architecture that “fuses” together the scalability, stability, andsecurity advantages of the latest routing technologies with the performance benefits ofATM and LAN switching, and the management benefits of VLANs. See alsoCisco IOS.

Cisco Internet Operating SystemSeeCisco IOS.

Cisco IOSCisco system software that provides common functionality, scalability, and security forall products under the CiscoFusion architecture. Cisco IOS allows centralized,integrated, and automated installation and management of internetworks, whileensuring support for a wide variety of protocols, media, services, and platforms. SeealsoCiscoFusion.

Cisco Link ServicesSeeCLS.

Cisco Link Services InterfaceSeeCLSI.

Cisco-trunk (private line) callA Cisco-trunk (private line) call is established by the forced connection of a dynamicswitched call. A Cisco-trunk call is established during configuration of the trunk andstays up for the duration of the configuration. It optionally provides a pass-throughconnection path to pass signaling information between the two telephony interfaces ateither end of the connection.

CiscoViewGUI-based device-management software application that provides dynamic status,statistics, and comprehensive configuration information for Cisco internetworkingdevices. In addition to displaying a physical view of Cisco device chassis, CiscoViewalso provides device monitoring functions and basic troubleshooting capabilities, andcan be integrated with several leading SNMP-based network management platforms.


CLSCisco link services. A front-end for a variety of data-link control services.

CLSICisco Link Services Interface. Messages that are exchanged between CLS and data-linkusers such as APPN, SNA service point, and DLSw+.

configuration registerIn Cisco routers, a 16-bit, user-configurable value that determines how the routerfunctions during initialization. The configuration register can be stored in hardware orsoftware. In hardware, the bit position is set using a jumper. In software, the bit positionis set by specifying a hexadecimal value using configuration commands.

CPPCombinet Proprietary Protocol.

CxBusCisco Extended Bus. Data bus for interface processors on Cisco 7000 series routers. SeealsoSP.

Data movement processorSeeDMP.

Diffusing update algorithmSeeDUAL.

DistributedDirectorMethod of distributing Web traffic by taking into account Web server availability andrelative client-to-server topological distances in order to determine the optimal Webserver for a client. DistributedDirector uses the Director Response Protocol to queryDRP server agents for BGP and IGP routing table metrics.

DLSw+data-link switching plus. Cisco implementation of the DLSw standard for SNA andNetBIOS traffic forwarding. DLSw+ goes beyond the standard to include the advancedfeatures of the current Cisco RSRB implementation, and provides additionalfunctionality to increase the overall scalability of data-link switching. See alsoDLSwinthe main glossary.


DMPData Movement Processor. Processor on the Catalyst 5000 that, along with themultiport packet buffer memory interface, performs the frame-switching function forthe switch. The DMP also handles translational bridging between the Ethernet andFDDI interfaces, IP segmentation, and intelligent bridging with protocol-based filtering.

DRPDirector Response Protocol. Protocol used by the DistributedDirector feature in IProuting.

DSPU concentrationCisco IOS feature that enables a router to function as a PU concentrator for SNA PU 2nodes. PU concentration at the router simplifies the task of PU definition at the upstreamhost while providing additional flexibility and mobility for downstream PU devices.

DUALDiffusing Update Algorithm. Convergence algorithm used in Enhanced IGRP thatprovides loop-free operation at every instant throughout a route computation. Allowsrouters involved in a topology change to synchronize at the same time, while notinvolving routers that are unaffected by the change. See alsoEnhanced IGRP.

EIGRPSeeEnhanced IGRP.

EIPEthernet Interface Processor. Interface processor card on the Cisco 7000 series routers.The EIP provides high-speed (10-Mbps) AUI ports that support Ethernet Version 1 andEthernet Version 2 or IEEE 802.3 interfaces, and a high-speed data path to otherinterface processors.

Enhanced IGRPEnhanced Interior Gateway Routing Protocol. Advanced version of IGRP developed byCisco. Provides superior convergence properties and operating efficiency, and combinesthe advantages of link state protocols with those of distance vector protocols. Comparewith IGRP. See alsoIGP, OSPF, andRIP.

Enhanced Interior Gateway Routing ProtocolSeeEnhanced IGRP.


Enhanced Monitoring ServicesSet of analysis tools on the Catalyst 5000 switch, consisting of an integrated RMONagent and the SPAN. These tools provide traffic monitoring, and network segmentanalysis and management. See alsoRMON andspan.

ESPExtended Services Processor. Rack-mounted adjunct processor that is co-located with aCisco BPX/AXIS (all three units comprise a BPX service node) and has IP connectivityto a StrataView Plus Workstation.

Ethernet Interface ProcessorSeeEIP.

EXECInteractive command processor of Cisco IOS.

Extended Services ProcessorSeeESP.

Fast Ethernet Interface ProcessorSeeFEIP.

Fast Sequenced TransportSeeFST.

Fast Serial Interface ProcessorSeeFSIP.

fast switchingCisco feature whereby a route cache is used to expedite packet switching through arouter. Contrast withprocess switching.

FDDI Interface ProcessorSeeFIP.

FEIPFast Ethernet Interface Processor. Interface processor on the Cisco 7000 series routers.The FEIP supports up to two 100-Mbps 100BaseT ports.


FIPFDDI Interface Processor. Interface processor on the Cisco 7000 series routers. The FIPsupports SASs, DASs, dual homing, and optical bypass, and contains a 16-mipsprocessor for high-speed (100-Mbps) interface rates. The FIP complies with ANSI andISO FDDI standards.

FRASFrame Relay access support. Cisco IOS feature that allows SDLC, Token Ring,Ethernet, and Frame Relay-attached IBM devices to connect to other IBM devicesacross a Frame Relay network. See alsoFRAD.

FSIPFast Serial Interface Processor. Default serial interface processor for Cisco 7000 seriesrouters. The FSIP provides four or eight high-speed serial ports.

FSTFast Sequenced Transport. Connectionless, sequenced transport protocol that runs ontop of the IP protocol. SRB traffic is encapsulated inside of IP datagrams and is passedover an FST connection between two network devices (such as routers). Speeds up datadelivery, reduces overhead, and improves the response time of SRB traffic.

Gateway Discovery ProtocolSeeGDP.

GDPGateway Discovery Protocol. Cisco protocol that allows hosts to dynamically detect thearrival of new routers as well as determine when a router goes down. Based on UDP.See alsoUDP in the main glossary.

generic routing encapsulationSeeGRE.

GREgeneric routing encapsulation. Tunneling protocol developed by Cisco that canencapsulate a wide variety of protocol packet types inside IP tunnels, creating a virtualpoint-to-point link to Cisco routers at remote points over an IP internetwork. By


connecting multiprotocol subnetworks in a single-protocol backbone environment, IPtunneling using GRE allows network expansion across a single-protocol backboneenvironment.

helper addressAddress configured on an interface to which broadcasts received on that interface willbe sent.

High-Speed Communications InterfaceSeeHSCI.

HIPHSSI Interface Processor. Interface processor on the Cisco 7000 series routers. The HIPprovides one HSSI port that supports connections to ATM, SMDS, Frame Relay, orprivate lines at speeds up to T3 or E3.

Hot Standby Router ProtocolSeeHSRP.

HSCIHigh-Speed Communications Interface. Single-port interface, developed by Cisco,providing full-duplex synchronous serial communications capability at speeds up to52 Mbps.

HSRPHot Standby Router Protocol. Provides high network availability and transparentnetwork topology changes. HSRP creates a Hot Standby router group with a lead routerthat services all packets sent to the Hot Standby address. The lead router is monitoredby other routers in the group, and if it fails, one of these standby routers inherits the leadposition and the Hot Standby group address.

HSSI Interface ProcessorSeeHIP.

IGRPInterior Gateway Routing Protocol. IGP developed by Cisco to address the issuesassociated with routing in large, heterogeneous networks. Compare withEnhancedIGRP. See alsoIGP, OSPF, andRIP.


interface processorAny of a number of processor modules used in the Cisco 7000 series routers. SeeAIP,CIP, EIP, FEIP, FIP, FSIP, HIP, MIP, SIP (serial interface processor), andTRIP.

Interior Gateway Routing ProtocolSeeIGRP.

Inter-Switch LinkSeeISL.

IOSSeeCisco IOS.

ISLInter-Switch Link. Cisco-proprietary protocol that maintains VLAN information astraffic flows between switches and routers.

local adjacencyTwo VNSs that control different VSN areas, but communicate with one another througha Frame Relay PVC, are considered to be locally adjacent.

MICAMultiservice IOS Channel Aggregation. Technology that enables the simultaneoussupport of remote-access users through both analog modems and ISDN devices.

MIPMultiChannel Interface Processor. Interface processor on the Cisco 7000 series routersthat provides up to two channelized T1 or E1 connections via serial cables to a CSU.The two controllers on the MIP can each provide up to 24 T1 or 30 E1 channel-groups,with each channel-group presented to the system as a serial interface that can beconfigured individually.

MultiChannel Interface ProcessorSeeMIP.

native client interface architectureSeeNCIA.


NCIAnative client interface architecture. SNA applications-access architecture, developed byCisco, that combines the full functionality of native SNA interfaces at both the host andclient with the flexibility of leveraging TCP/IP backbones. NCIA encapsulates SNAtraffic on a client PC or workstation, thereby providing direct TCP/IP access whilepreserving the native SNA interface at the end-user level. In many networks, thiscapability obviates the need for a standalone gateway and can provide flexible TCP/IPaccess while preserving the native SNA interface to the host.

NetFlowNetwork flow is defined as a unidirectional sequence of packets between given sourceand destination endpoints. Network flows are highly granular: flow endpoints areidentified both by IP address as well as by transport layer application port numbers.(NetFlow also uses IP Protocol, ToS and the input interface port to uniquely identifyflows.) Conventional network layer switching handles incoming packets independently,with separate serial tasks for switching, security, services and traffic measurementsapplied to each packet. With NetFlow switching, this process is applied only to the firstpacket of a flow. Information from the first packet is used to build an entry in theNetFlow cache. Subsequent packets in the flow are handled via a single streamlined taskthat handles switching, services, and data collection concurrently.

NETscoutCisco network management application that provides an easy-to-use GUI formonitoring RMON statistics and protocol analysis information. NETscout also providesextensive tools that simplify data collection, analysis, and reporting. These tools allowsystem administrators to monitor traffic, set thresholds, and capture data on any set ofnetwork traffic for any segment.

NMPNetwork Management Processor. Processor module on the Catalyst 5000 switch used tocontrol and monitor the switch.

NSPNetwork Service Point.

physical layer interface moduleSeePLIM.


PLIMphysical layer interface module. Interface that allows the AIP (ATM InterfaceProcessor) to a variety of physical layers, including TAXI and SONET multimodefiber-optic cable, SDH/SONET single-mode fiber cable, and E3 coaxial cable.

process switchingOperation that provides full route evaluation and per-packet load balancing acrossparallel WAN links. Involves the transmission of entire frames to the router CPU, wherethey are repackaged for delivery to or from a WAN interface, with the router making aroute selection for each packet. Process switching is the most resource-intensiveswitching operation that the CPU can perform. Contrast withfast switching.

proxy pollingTechnique that alleviates the load across an SDLC network by allowing routers to act asproxies for primary and secondary nodes, thus keeping polling traffic off of the sharedlinks. Proxy polling has been replaced by SDLC Transport. SeeSDLC Transport.

Reliable SAP Update ProtocolSeeRSUP.

Route ProcessorSeeRP.

Route/Switch ProcessorSeeRSP.

RPRoute Processor. Processor module in the Cisco 7000 series routers that contains theCPU, system software, and most of the memory components that are used in the router.Sometimes called asupervisory processor.

RSPRoute/Switch Processor. Processor module in the Cisco 7500 series routers thatintegrates the functions of the RP and the SP. See alsoRPandSP.


RSUPReliable SAP Update Protocol. Bandwidth-saving protocol developed by Cisco forpropagating services information. RSUP allows routers to reliably send standard NovellSAP packets only when the routers detect a change in advertised services. RSUP cantransport network information either in conjunction with or independently of theEnhanced IGRP routing function for IPX.

SDLC broadcastFeature that allows a Cisco router that receives an all-stations broadcast on a virtualmultidrop line to propagate the broadcast to each SDLC line that is a member of thevirtual multidrop line.

SDLC TransportCisco router feature with which disparate environments can be integrated into a single,high-speed, enterprise-wide network. Native SDLC traffic can be passed throughpoint-to-point serial links with other protocol traffic multiplexed over the same links.Cisco routers can also encapsulate SDLC frames inside IP datagrams for transport overarbitrary (non-SDLC) networks. Replaces proxy polling. See alsoproxy polling.

SDLLCSDLC Logical Link Control. Cisco IOS feature that performs translation betweenSDLC and IEEE 802.2 type 2.

serial tunnelSeeSTUN.

silicon switchingSwitching based on the SSE, which allows the processing of packets independent of theSSP (Silicon Switch Processor) system processor. Silicon switching provideshigh-speed, dedicated packet switching. See alsoSSE andSSP (Silicon SwitchProcessor).

silicon switching engineSeeSSE.

Silicon Switch ProcessorSeeSSP.


SIP1. SMDS Interface Protocol. Used in communications between CPE and SMDSnetwork equipment. Allows the CPE to use SMDS service for high-speed WANinternetworking. Based on the IEEE 802.6 DQDB standard. See alsoDQDB.

2. serial interface processor.

SPSwitch Processor. Cisco 7000-series processor module that acts as the administrator forall CxBus activities. Sometimes called ciscoBus controller. See alsoCxBus.

SPANSwitched Port Analyzer. Feature of the Catalyst 5000 switch that extends themonitoring abilities of existing network analyzers into a switched Ethernetenvironment. SPAN mirrors the traffic at one switched segment onto a predefined SPANport. A network analyzer attached to the SPAN port can monitor traffic from any of theother Catalyst switched ports.

SPNNI connectionFrame Relay connection between two VNSs in different areas or domains. The SPNNIconnection gets its name from the proprietary Network-to-Network Interface protocolthat operates over this connection.

SSEsilicon switching engine. Routing and switching mechanism that compares the data linkor network layer header of an incoming packet to a silicon-switching cache, determinesthe appropriate action (routing or bridging), and forwards the packet to the properinterface. The SSE is directly encoded in the hardware of the SSP (Silicon SwitchProcessor) of a Cisco 7000 series router. It can therefore perform switchingindependently of the system processor, making the execution of routing decisions muchquicker than if they were encoded in software. See alsosilicon switching andSSP.

SSPSilicon Switch Processor. High-performance silicon switch for Cisco 7000 seriesrouters that provides distributed processing and control for interface processors. TheSSP leverages the high-speed switching and routing capabilities of the SSE to


dramatically increase aggregate router performance, minimizing performancebottlenecks at the interface points between the router and a high-speed backbone. Seealsosilicon switching andSSE.

STUNserial tunnel. Router feature allowing two SDLC- or HDLC-compliant devices toconnect to one another through an arbitrary multiprotocol topology (using Ciscorouters) rather than through a direct serial link.

supervisory processorSeeRP.

Switch ProcessorSeeSP.

TACA Cisco Technical Assistance Center. There are 4 TACs worldwide.

TACACS+Terminal Access Controller Access Control System Plus. Proprietary Ciscoenhancement to Terminal Access Controller Access Control System (TACACS).Provides additional support for authentication, authorization, and accounting. See alsoTACACS in main glossary.

THC over X.25Feature providing TCP/IP header compression over X.25 links, for purposes of linkefficiency.

TRIPToken Ring Interface Processor. High-speed interface processor on the Cisco 7000series routers. The TRIP provides two or four Token Ring ports for interconnection withIEEE 802.5 and IBM Token Ring media with ports independently set to speeds of either4 or 16 Mbps.

two-way simultaneousSeeTWS.


TWStwo-way simultaneous. Mode that allows a router configured as a primary SDLC stationto achieve better utilization of a full-duplex serial line. When TWS is enabled in amultidrop environment, the router can poll a secondary station and receive data fromthat station while it sends data to or receives data from a different secondary station onthe same serial line.

Versatile Interface ProcessorSeeVIP.

VIP1. Versatile Interface Processor. Interface card used in Cisco 7000 and Cisco 7500 seriesrouters. The VIP provides multilayer switching and runs Cisco IOS. The most recentversion of the VIP is VIP2.2. virtual IP. Function that enables the creation of logically separated switched IPworkgroups across the switch ports of a Catalyst 5000 running Virtual NetworkingServices software. See alsoVirtual Networking Services.

virtual IPSeeVIP.

Virtual Networking ServicesSoftware on some Catalyst 5000 switches that enables multiple workgroups to bedefined across switches and offers traffic segmentation and access control.

VNSSeeVirtual Networking Services.

VSCCisco’s virtual switch controller.

WorkGroup DirectorCisco SNMP-based network-management software tool. Workgroup Director runs onUNIX workstations either as a standalone application or integrated with anotherSNMP-based network management platform, providing a seamless, powerfulmanagement system for Cisco workgroup products. See alsoSNMP.



Documents

Prefacedbillings.com/networking/Routing_Terms.pdf · Supports speeds of up to 100 Mbps over multimode fiber. See also TAXI 4B/5B. 4-byte/5-byte local fiber See 4B/5B local fiber