GLOSSARY

A&B bit signaling Using in T1 transmission facilities and sometimes calls "24th channel signaling." Each of the 24 T1 subchannels in this procedure uses one bit of every sixth frame to send supervisory signaling information.

AAL ATM Adaptation Layer: A service-dependent sublayer of the Data Link layer, which accepts data from other applications and brings it to the ATM layer in 48-byte, ATM payload segments. CS and SAR are the two sublayers that form AALs. Currently, the four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, & AAL5. AALs are differentiated by the source-destination timing they use, whether they are CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transmission. See also AAL1, AAL2, AAL3/4, AAL5, ATM, & ATM layer.

AAL1 ATM Adaptation Layer 1: One of four AALs recommended by the ITU-T, it is used for connection-oriented, time-sensitive services that need constant bit-rates, such as isochronous traffic and uncompressed video.

AAL2 ATM Adaptation Layer 2: One of four AALs recommended by the ITU-T, it is used for connection-oriented services that support a variable bit rate, such as voice traffic.

AAL3/4 ATM Adaptation Layer 3/4: One of four AALs (a product of two initially distinct layers) recommended by the ITU-T, supporting both connectionless and connection-oriented links. Its primary use is in sending SMDS packets of ATM networks.

AAL5 ATM Adaptation Layer 5: One of four AALs recommended by the ITU-T, it is used to support connection-oriented VBR services primarily to transfer classical IP over ATM and LANE traffic. This least complex of the AAL recommendations uses SEAL, offering lower bandwidth costs and simpler processing requirements, but also providing reduced bandwidth and error-recover capacities.

AARP AppleTalk Address Resolution Protocol: The protocol in an Apple-Talk stack that maps data-link addresses to network addresses.

AARP probe packets Packets sent by the AARP to determine whether a given node ID is being used by another node in a nonextended AppleTalk network. If the node ID is not in use, then sending node appropriates that node's ID. If the node ID is in use, the sending node will select a different ID and then send out more AARP probe packets.

ABM Asynchronous Balanced Mode: When two stations can initiate a transmission, ABM is an HDLC (or one of it's derived protocols) communication technology that supports peer-oriented, point-to-point communications between both stations.

ABR Area Border Router: An OSPF router that is located on the border of one or more OSPF areas. ABRs are used to connect OSPF areas to the OSPF backbone area.

access layer One of the layers in Cisco's three-layer hierarchical model. The access layer provides users with access to the internetwork.

access link Is a link used with switches and is only part of one Virtual AN (VLAN). Trunk links carry information from multiple VLANs.

access list A set of test conditions kept by routers that determines "interesting traffic" to and from the router for various services on the network.

access method The manner in which network devices approach gaining access to the network itself.

access server Also known as a "network access server," it is a communications process connecting asynchronous devices to a LAN or WAN through network and terminal emulation software, providing synchronous or asynchronous routing of supported protocols.

acknowledgment Verification sent from one network device to another signifying that an event has occurred. May be abbreviated as ACK. Contrast with: NAK

ACR Allowed cell rate: A designation defined by the ATM Forum for managing ATM traffic. Dynamically controlled using congestion control measures, the ACR varies between the minimum cell rate (MCR) and the peak cell rate (PCR). see also MCR and PCR

active monitor The mechanism used to manage a Token Ring. The network node with the highest MAC address on the ring becomes the active monitor and is responsible for management tasks such as preventing loops and ensuring tokens are not lost.

address learning Used with transparent bridges to learn the hardware addresses of all devices on an internetwork. The switch then filters the network with the known hardware (MAC) addresses.

address mapping By translating network addresses from one format to another, this methodology permits different protocols to operate interchangeably.

address mask A bit combination descriptor identifying which portion of an address refers to the network or subnet and which part refers to the host. Sometimes simply called the mask. see also: subnet mask

address resolution The process used for resolving differences between computer addressing schemes. Address resolution typically defines a method for tracing Network layer (Layer 3) addresses to Data-Link layer (Layer 2) addresses, See also: address mapping

adjacency The relationship made between defined neighboring routers and end nodes, using a common media segment, to exchange routing information.

administrative distance A number between 0 and 225 that expresses the value of trustworthiness of a routing information source. The lower the number, the higher the integrity rating

administrative weight A value designated by a network administrator to rate the preference given to a network link. It is one of four link metrics exchanged by PTSPs to test ATM network resource availability.

ADSU ATM data Service Unit: The terminal adapter used to connect to an ATM network through an HSSI-compatible mechanism. See also: DSU

advertising The process whereby routing or service updates are transmitted at given intervals, allowing other routers on the network to maintain a record of viable routes.

AEP AppleTalk Echo Protocol: A test for connectivity between two Apple-Talk nodes where one node sends a packet to another and receives an echo, or copy, in response.

AFI Authority and Format Identifier: The part of an NSAP ATM address that delineates the type and format of the IDI section of an ATM address.

AFP AppleTalk Filing Protocol: A Presentation-layer protocol, supporting AppleShare and Mac OS File Sharing, that permits users to share files and applications on a server.

AIP ATM Interface Processor: Supporting AAL3/4 and ALL5, this interface for Cisco 7000 series routers minimizes performance bottlenecks at the UNI. See also: AAL3/4 and AAL5

algorithm A set of rules or process used to solve a problem. In networking, algorithms are typically used for finding the best route for traffic from a source to its destination.

alignment error An error occurring in Ethernet networks in which a received frame has extra bits; that is, a number not divisible by eight. Alignment errors are generally the result of frame damage caused by collisions.

all-routes explorer packet An explorer packet that can move across an entire SRB network, tracing all possible paths to a given destination. Also known as an all-rings explorer packet. See also: explorer packet, local explorer packet, and spanning explorer packet.

AM Amplitude Modulation: A modulation method that represents information by varying the the amplitude of the carrier signal. See also: modulation.

AMI Alternate Mark Inversion: A line-code type on T1 and E1 circuits that shows zeros as "01" during each bit cell, and ones as "11" or "00", alternately, during each bit cell. The sending device must maintain ones density in AMI, but not independently of the data stream. Also known as binary-coded, alternate mark inversion Contrast with: B8ZS. See also: ones density

amplitude An analog or digital waveform's highest value.

analog transmission Signal messaging whereby information is represented by various combination of signal amplitude, frequency, and phase.

ANSI American National Standards Institute: The organization of corporate, government, and other volunteer members that coordinates standards-related activities, approves U.S. National standards, and develops US positions in international standard organizations. ANSI assist in the creation of international and US standards in disciplines such as communications, networking, and a variety of technical fields. It publishes over 13,000 standards, for engineered products and technologies ranging from screw threads to networking protocols. ANSI is a member of the IEC and ISO.

anycast An ATM address that can be shared by more than one end system, allowing requests to be routed to a node that provides a particular service.

AppleTalk Currently in two version, the group of communication protocols designed by Apple Computer for use in Macintosh environments. The earlier Phase1 protocol supports one physical network with only one network number that resides in one zone. The later Phase 2 protocols support more than one logical network on a single physical network, allowing networks to exist in more than one zone. See also: zone.

Application Layer Layer 7 of the OSI reference network model, supplying services to application procedures (Such as electronic mail or file transfer) that are outside the OSI model. This layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinate partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery.

ARA AppleTalk remote Access: A protocol for Macintosh users establishing their access to resources and data from a remote AppleTalk location.

area A logical, rather than physical, set of segments (based on either CLNS, DECnet, or OSPF) along with their attached devices. Areas are commonly connected to others using routers to create a single autonomous system. See also: Autonomous system

ARM Asynchronous Response Mode: An HDLC communication mode using one primary station and at least one additional station, in which transmission can be initiated from either the primary or one of the secondary units.

ARP Address Resolution Protocol: Defined in RFC 826, the protocol that traces IP addresses to MAC addresses. See also RARP

ASBR Autonomous System Boundary Router: An area border router placed between an OSPF autonomous system and a non-OSPF network that operates both OSPF and an additional routing protocol, such as RIP. ASBRs must be located in a non-stub OSPF area. See also: ABR, non-stub area, and OSPF

ASCII American Standard Code for Information Interchange: An 8-bit code for representing characters, consisting of seven data bits plus one parity bit.

ASICs Application-Specific Integrated Circuits: Used in layer-2 switches to make filtering decisions. The ASIC looks in the filter table of MAC addresses and determines which port the destination hardware address of a received hardware address is destined for. The frame will be allowed to traverse only that one segment. If the hardware address is unknown, the frame is forwarded out all ports.

ASN.1 Abstract Syntax Notation One: An OSI language used to describe types of data that is independent of computer structures and depicting methods. Described by ISO International Standard 8824

ASP AppleTalk Session Protocol: A protocol employing ATP to establish, maintain, and tear down sessions, as well as sequence requests. See Also: ATP

AST Automatic Spanning Tree: A function that supplies one path for spanning explorer frames traveling from one node in the network to another, supporting the automatic resolution of spanning trees in SRB networks. AST is based on the IEEE 802.1 standard. See Also: IEEE 802.1 and SRB

asynchronous transmission Digital signals sent without precise timing, usually with different frequencies and phase relationships. Asynchronous transmissions generally enclose individual characters in control bits (called start and stop bits) that show the beginning and end of each character, Contrast with: isochronous transmission and synchronous transmission

ATCP AppleTalk Control Program: The protocol for establishing and configuring AppleTalk over PPP, defined in RFC 1378. See also: PPP

ATDM Asynchronous Time-Division Multiplexing: A technique for sending information, it differs from normal TDM in that the time slots are assigned when necessary rather than pre-assigned to certain transmitters. Contrast with: FDM, Statistical multiplexing, and TDM

ATG Address Translation Gateway: The mechanism within Cisco DECnet routing software that enables routers to route multiple, independent DECnet networks and to establish a user-designated address translation for chosen nodes between networks.

ATM Asynchronous Transfer Mode: The international Standard, identifies by fixed-length 53 byte cells, for transmitting cells in multiple service systems, such as voice, video, or data. Transit deltas are reduced because the fixed-length cells permit processing to occur in the hardware. ATM is designed to maximize the benefits of high-speed transmission media, such as SONET, E3, and T3.

ATM ARP server A device that supplies logical subnets running classical IP over ATM with address-resolution services

ATM endpoint The initiating or terminating connection in an ATM network. ATM endpoints include servers, workstations, ATM-to-LAN switches, and ATM routers.

ATM Forum The international Organization founded jointly by Northern Telecom, Sprint, Cisco Systems, and NET/ADAPTIVE in 1991 to develop and promote standards-based implementation agreements for ATM technology. The ATM forum broadens official developed by ANSI and ITU-T and creates implementation agreements before official standards are published.

ATM layer A sublayer of the Data Link layer in an ATM network that is service independent. To create standard 53-byte ATM cells, the ATM layer receives 48-byte segments from the AAL and attaches a 5-byte header to each. These cells are then sent to the Physical layer for transmission across the physical medium. See Also: AAL

ATMM ATM Management: A procedure that runs on ATM switches, managing rate of enforcement and VIC translation. See also: ATM

ATM user-user connection A connection made by the ATM layer to supply communication between at least two ATM service users, such as ATMM processes. These communications can be uni- or bi-directional, using one or two VCCs, respectively. See also: ATM layer, and ATMM

ATP AppleTalk Transaction Protocol: A transport-level protocol that enables reliable transactions between two sockets, where one requests the other to perform a given task, and to report the results. ATP fastens the request and response together, assuring a loss-free exchange of request-response pairs.

attenuation In communication, weakening or loss of signal energy, typically caused by distance

AUI Attachment Unit Interface: The basic physical connection media for 10Base2 and 10Base5 ethernet networks.

AURP AppleTalk Update-based Routing Protocol: A technique for encapsulating AppleTalk traffic in the header of a foreign protocol that allows the connection of at least two noncontiguous AppleTalk internetworks through a foreign network (such as TCP/IP) to create an AppleTalk WAN. The connection made is called an AURP tunnel. By exchanging routing information between exterior routers, the AURP maintains routing tables for the complete AppleTalk WAN. See Also: AURP tunnel.

AURP tunnel A connection made in an AURP WAN that acts as a single, virtual link between AppleTalk internetworks separated physically by a foreign network such as a TCP/IP network. See also: AURP

authority zone A portion of the domain-name tree associated with DNS for which one name server is the authority: See also: DNS.

auto duplex A setting on layer-1 and -2 devices that sets the duplex of a switch or hub port automatically.

automatic call reconnect A function that enables automatic call rerouting away from a failed trunk line

autonomous confederation A collection of self-governed systems that depend more on their own network accessibility and routing information than on information received from other systems or groups

autonomous switching The ability of Cisco routers to process packets more quickly by using the ciscoBus to switch packets independently of the system processor

autonomous system(AS) A group of networks under mutual administration that share the same routing methodology. Autonomous systems are subdivided by areas and must be assigned and individual 16-bit number buy the IANA See also: area

autoreconfiguration A procedure executed by nodes within the failure domain of a Token Ring, wherein nodes automatically perform diagnostics, trying to reconfigure the network around failed areas.

auxiliary port The console port on the back of Cisco routers that allows you to dial the router and make console configuration settings.

B8ZS Binary 8-Zero Substitution: A line-code type, interpreted at the remote end of the connection, that uses a special code substitution whever eight consecutive zeros are transmitted over the link on T1 and E1 circuits. This technique assures ones density independent of the data stream. Also known as bipolar 8-zero substitution. Contrast with: AMI. See also: ones density.

backbone The basic portion of the network that provides the primary path for traffic sent to and initiated from other networks.

back end a node or software program supplying services to a front end. See also: server

bandwidth The gap between the highest and lowest frequencies employed by network signals. More commonly, it refers to the rated throughput capacity of a network protocol or medium.

baseband A feature of a netowkr technology that uses only one carrier frequency, for example, Ethernet. Also named "narrowband". Compare with: broadband.

Basic Management Setup Used with Cisco routers when in setup mode. Only provides enough management and configuration to get the router working so someone can telnet into the router and configure it.

baud Synonymous with bits per second (bps), if each signal element represents one bit. It is a unit of signaling speed equivalent to the number of separate signal elements transmitted per second.

B channel Bearer channel: A full-duplex, 64 Kbps channel in ISDN that transmits user data. Compare with: D channel, E channel, and H channel.

beacon An FDDI device or Token Ring frame that points to a serious problem with the ring, such as a broken cable. The beacon frame carries the address of the station thought to be down. See also: failure domain.

BECN Backward Explicit Congestion Notification: BECN is the bit set by a Frame Realy network in frames moving away from frames headed into a congested path. a DTE that recieves frames with the BECN may ask higher-level protocols to take necessary flow control measures. Compare with: FECN

BGP4 BGP Version 4: Version 4 of the interdomain routing protocol most commonly used on the Internet. BGP4 supports CIDR and uses route-counting mechanisms to decrease the size of routing tables. See also: CIDR

binary A two-character numbering method that uses ones and zeros. The binary numbering system underlies all digital representation of information.

BIP Bit Interleaved Parity: A method used in ATM to monitor errors on a link, sending a check bit or word in the link overhead for the previous block or frame. This allows bit errors in transmissions to be founf and delivered as maintenance information.

BISDN Broadband ISDN: ITU-T standards created to manage high-bandwidth technologies such as video. BISDN presently employs ATM technologu along SONET-based transmission circuits, supplying data rates between 155 Mbps and 622 Mbps and beyond. Contrast with N-ISDN. See also: BRI, ISDN, and PRI.

bit-oriented protocol Regardless of frame content, the class of Data-Link layer communication protocols that transmits frames. Bit-oriented protocols, as compared with byte-oriented, supply more efficient and trustworthy, full-duplex operation. Compare with: byte-oriented protocol.

Boot ROM Used in routers to put the router into bootstrap mode. Bootstrap mode then boots the device with an operating system. The ROM can also hold a small Cisco IOS.

border gateway A router that facilitates communications with routers in different autonomous systems.

BPDU Bridge Protocol Data Unit: A Spanning-Tree Protocol initializing packet that is sent at definable intervals for the purpose of exchanging information among bridges in networks.

BRI Basic Rate Interface: The ISDN interface that facilitates circuit switched communication between video, data, and voice; it is made up of two B channels (64Kbps each) and one D channel (16Kbps). Compare with: PRI. See also: BISDN.

bridge A device for connecting two segments of a network and transmitting packets between them. Both segments must use identical protocols to communicate. Bridges function at the Data Link layer, Layer 2 of the OSI reference model. The purpose of a bridge is to filter, send, or flood any incoming frame, based on the MAC address of that particular frame.

broadband A transmission methodology for multiplexing several independent signals onto one cable. In telecommunications, broadband is classified as any channel with a bandwidth greater than 4kHz (typical voice grade). In LAN terminology, it is classified as a coaxial cable on which analog signaling is employed. Also known as wideband. Contrast with: baseband.

broadcast A data frame or packet that is transmitted to every node on the local network segment (as defined by the broadcast domain). Broadcasts are known by their broadcast address, which is a destination network and host address with all the bits turned on. Also called "local broadcast." Compare with: directed broadcast

broadcast domain A group of devices receiving broadcast frames initi-ating from any device within the group. Because they do not forward broad-cast frames, broadcast domains are generally surrounded by routers.

broadcast storm An undesired event on the network caused by the simul-taneous transmission of any number of broadcasts across the network seg-ment. Such an occurrence can overwhelm network bandwidth, resulting in time-outs.

buffer A storage area dedicated to handling data while in transit. Buffers are used to receive/store sporadic deliveries of data bursts, usually received from faster devices, compensating for the variations in processing speed. Incoming information is stored until everything is received prior to sending data on. Also known as an information buffer.

bus topology A linear LAN architecture in which transmissions from var-ious stations on the network are reproduced over the length of the medium and are accepted by all other stations. Compare with: ring and star.

bus Any physical path, typically wires or copper, through which a digital signal can be used to send data from one part of a computer to another.

BUS broadcast and unknown servers: In LAN emulation, the hardware or software responsible for resolving all broadcasts and packets with unknown (unregistered) addresses into the point-to-point virtual circuits required by ATM. See also: LANE, LEC, LECS, and LES.

BX.25 AT&T’s use of X.25. See also: X.25.

bypass mode An FDDI and Token Ring network operation that deletes an interface.

bypass relay A device that enables a particular interface in the Token Ring to be closed down and effectively taken off the ring.

byte-oriented protocol Any type of data-link communication protocolthat, in order to mark the boundaries of frames, uses a specific character from the user character set. These protocols have generally been superseded by bit-oriented protocols. Compare with: bit-oriented protocol.

cable range In an extended AppleTalk network, the range of numbers allotted for use by existing nodes on the network. The value of the cable range can be anywhere from a single to a sequence of several touching net-work numbers. Node addresses are determined by their cable range value.

CAC Connection Admission Control: The sequence of actions executed by every ATM switch while connection setup is performed in order to determine if a request for connection is violating the guarantees of QoS for estab-lished connections. Also, CAC is used to route a connection request through an ATM network.

call admission control A device for managing of traffic in ATM net-works, determining the possibility of a path containing adequate bandwidth for a requested VCC.

call priority In circuit-switched systems, the defining priority given to each originating port; it specifies in which order calls will be reconnected. Addi-tionally, call priority identifies which calls are allowed during a bandwidth reservation.

call set-up time The length of time necessary to effect a switched call between DTE devices.

CBR Constant Bit Rate: An ATM Forum QoS class created for use in ATM networks. CBR is used for connections that rely on precision clocking to guarantee trustworthy delivery. Compare with: ABR and VBR.

CD Carrier Detect: A signal indicating that an interface is active or that a connection generated by a modem has been established.

CDP Cisco Discovery Protocol: Cisco’s proprietary protocol that is used to tell a neighbor Cisco device about the type of hardware, software version, and active interfaces that the Cisco device is using. It uses a SNAP frame between devices and is not routable.

CDVT Cell Delay Variation Tolerance: A QoS parameter for traffic man-agement in ATM networks specified when a connection is established. The allowable fluctuation levels for data samples taken by the PCR in CBR trans-missions are determined by the CDVT. See also: CBR and PCR.

cell In ATM networking, the basic unit of data for switching and multi-plexing. Cells have a defined length of 53 bytes, including a 5-byte header that identifies the cell’s data stream and 48 bytes of payload. See also: cell relay.

cell payload scrambling The method by which an ATM switch maintains framing on some medium-speed edge and trunk interfaces (T3 or E3 cir-cuits). Cell payload scrambling rearranges the data portion of a cell to main-tain the line synchronization with certain common bit patterns.

cell relay A technology that uses small packets of fixed size, known as cells. Their fixed length enables cells to be processed and switched in hard-ware at high speeds, making this technology the foundation for ATM and other high-speed network protocols. See also: cell.

Centrex A local exchange carrier service, providing local switching that resembles that of an on-site PBX. Centrex has no on-site switching capa-bility. Therefore, all customer connections return to the CO. See also: CO.

CER Cell Error Ratio: The ratio in ATM of transmitted cells having errors to the total number of cells sent in a transmission within a certain span of time.

channelized E1 Operating at 2.048Mpbs, an access link that is sectioned into 29 B-channels and one D-channel, supporting DDR, Frame Relay, and X.25. Compare with: channelized T1.

channelized T1 Operating at 1.544Mbps, an access link that is sectioned into 23 B-channels and 1 D-channel of 64Kbps each, where individual chan-nels or groups of channels connect to various destinations, supporting DDR, Frame Relay, and X.25. Compare with: channelized E1.

CHAP Challenge Handshake Authentication Protocol: Supported on lines using PPP encapsulation, it is a security feature that identifies the remote end, helping keep out unauthorized users. After CHAP is performed, the router or access server determines whether a given user is permitted access. It is a newer, more secure protocol than PAP. Compare with: PAP.

checksum A test for ensuring the integrity of sent data. It is a number calculated from a series of values taken through a sequence of mathemat-ical functions, typically placed at the end of the data from which it is calcu-lated, and then recalculated at the receiving end for verification. Compare with: CRC.

choke packet When congestion exists, it is a packet sent to inform a trans-mitter that it should decrease its sending rate.

CIDR Classless Interdomain Routing: A method supported by classless routing protocols, such as OSPF and BGP4, based on the concept of ignoring the IP class of address, permitting route aggregation and VLSM that enable routers to combine routes in order to minimize the routing information that needs to be conveyed by the primary routers. It allows a group of IP net-works to appear to other networks as a unified, larger entity. In CIDR, IP addresses and their subnet masks are written as four dotted octets, followed by a forward slash and the numbering of masking bits (a form of subnet notation shorthand). See also: BGP4.

CIP Channel Interface Processor: A channel attachment interface for use in Cisco 7000 series routers that connects a host mainframe to a control unit. This device eliminates the need for an FBP to attach channels.

CIR Committed Information Rate: Averaged over a minimum span of time and measured in bps, a Frame Relay network’s agreed-upon minimum rate of transferring information.

circuit switching Used with dial-up networks such as PPP and ISDN. Passes data, but needs to set up the connection first—just like making a phone call.

Cisco FRAD Cisco Frame-Relay Access Device: A Cisco product that sup-ports Cisco IPS Frame Relay SNA services, connecting SDLC devices to Frame Relay without requiring an existing LAN. May be upgraded to a fully functioning multiprotocol router. Can activate conversion from SDLC to Ethernet and Token Ring, but does not support attached LANs. See also: FRAD.

CiscoFusion Cisco’s name for the internetworking architecture under which its Cisco IOS operates. It is designed to "fuse" together the capabilities of its disparate collection of acquired routers and switches.

Cisco IOS software Cisco Internet Operating System software. The kernel of the Cisco line of routers and switches that supplies shared func-tionality, scalability, and security for all products under its CiscoFusion architecture. See also: CiscoFusion.

CiscoView GUI-based management software for Cisco networking devices, enabling dynamic status, statistics, and comprehensive configura-tion information. Displays a physical view of the Cisco device chassis and provides device-monitoring functions and fundamental troubleshooting capabilities. May be integrated with a number of SNMP-based network management platforms.

Class A network Part of the Internet Protocol hierarchical addressing scheme. Class A networks have only 8 bits for defining networks and 24 bits for defining hosts on each network.

Class B network Part of the Internet Protocol hierarchical addressing scheme. Class B networks have 16 bits for defining networks and 16 bits for defining hosts on each network.

Class C network Part of the Internet Protocol hierarchical addressing scheme. Class C networks have 24 bits for defining networks and only 8 bits for defining hosts on each network.

classical IP over ATM Defined in RFC 1577, the specification for running IP over ATM that maximizes ATM features. Also known as CIA.

classless routing Routing that sends subnet mask information in the routing updates. Classless routing allows Variable-Length Subnet Mask (VLSM) and supernetting. Routing protocols that support classless routing are RIP version 2, EIGRP, and OSPF.

CLI Command-Line Interface: Allows you to configure Cisco routers and switches with maximum flexibility.

CLP Cell Loss Priority: The area in the ATM cell header that determines the likelihood of a cell being dropped during network congestion. Cells with CLP = 0 are considered insured traffic and are not apt to be dropped. Cells with CLP = 1 are considered best-effort traffic that may be dropped during congested episodes, delivering more resources to handle insured traffic.

CLR Cell Loss Ratio: The ratio of discarded cells to successfully delivered cells in ATM. CLR can be designated a QoS parameter when establishing a connection.

CO Central Office: The local telephone company office where all loops in a certain area connect and where circuit switching of subscriber lines occurs.

collapsed backbone A nondistributed backbone where all network seg-ments are connected to each other through an internetworking device. A collapsed backbone can be a virtual network segment at work in a device such as a router, hub, or switch.

collision The effect of two nodes sending transmissions simultaneously in Ethernet. When they meet on the physical media, the frames from each node collide and are damaged. See also: collision domain.

collision domain The network area in Ethernet over which frames that have collided will spread. Collisions are propagated by hubs and repeaters, but not by LAN switches, routers, or bridges. See also: collision.

composite metric Used with routing protocols, such as IGRP and EIGRP, that use more than one metric to find the best path to a remote network. IGRP and EIGRP both use bandwidth and delay of the line by default. How-ever, Maximum Transmission Unit (MTU), load, and reliability of a link can be used as well.

configuration register A 16-bit configurable value stored in hardware or software that determines how Cisco routers function during initialization. In hardware, the bit position is set using a jumper. In software, it is set by spec-ifying specific bit patterns used to set startup options, configured using a hexadecimal value with configuration commands.

congestion Traffic that exceeds the network’s ability to handle it.

congestion avoidance To minimize delays, the method an ATM network uses to control traffic entering the system. Lower-priority traffic is discarded at the edge of the network when indicators signal it cannot be delivered, thus using resources efficiently.

congestion collapse The situation that results from the retransmission of packets in ATM networks where little or no traffic successfully arrives at destination points. It usually happens in networks made of switches with ineffective or inadequate buffering capabilities combined with poor packet discard or ABR congestion feedback mechanisms.

connection ID Identifications given to each Telnet session into a router. The show sessions command will give you the connections a local router will have to a remote router. The show users command will show the con-nection IDs of users telnetted into your local router.

connectionless Data transfer that occurs without the creating of a virtual circuit. No overhead, best-effort delivery, not reliable. Contrast with: connection-oriented. See also: virtual circuit.

connection-oriented Data transfer method that sets up a virtual circuit before any data is transferred. Uses acknowledgments and flow control for reliable data transfer. Contrast with: connectionless. See also: virtual circuit.

console port Typically an RJ-45 port on a Cisco router and switch that allows Command-Line Interface capability.

control direct VCC One of three control connections defined by Phase I LAN Emulation; a bi-directional virtual control connection (VCC) estab-lished in ATM by an LEC to an LES. See also: control distribute VCC.

control distribute VCC One of three control connections defined by Phase 1 LAN Emulation; a unidirectional virtual control connection (VCC) set up in ATM from an LES to an LEC. Usually, the VCC is a point-to-multipoint connection. See also: control direct VCC.

convergence The process required for all routers in an internetwork to update their routing tables and create a consistent view of the network, using the best possible paths. No user data is passed during a convergence time.

core layer Top layer in the Cisco three-layer hierarchical model, which helps you design, build, and maintain Cisco hierarchical networks. The core layer passes packets quickly to distribution-layer devices only. No packet filtering should take place at this layer.

cost Also known as path cost, an arbitrary value, based on hop count, bandwidth, or other calculation, that is typically assigned by a network administrator and used by the routing protocol to compare different routes through an internetwork. Routing protocols use cost values to select the best path to a certain destination: the lowest cost identifies the best path. Also known as path cost. See also: routing metric.

count to infinity A problem occurring in routing algorithms that are slow to converge where routers keep increasing the hop count to particular networks. To avoid this problem, various solutions have been implemented into each of the different routing protocols. Some of those solutions include defining a maximum hop count (defining infinity), route poising, poison reverse, and split horizon.

CPCS Common Part Convergence Sublayer: One of two AAL sublayers that is service-dependent, it is further segmented into the CS and SAR sub-layers. The CPCS prepares data for transmission across the ATM network; it creates the 48-byte payload cells that are sent to the ATM layer. See also: AAL and ATM layer.

CPE Customer Premises Equipment: Items such as telephones, modems, and terminals installed at customer locations and connected to the telephone company network.

crankback In ATM, a correction technique used when a node somewhere on a chosen path cannot accept a connection setup request, blocking the request. The path is rolled back to an intermediate node, which then uses GCAC to attempt to find an alternate path to the final destination.

CRC Cyclical Redundancy Check: A methodology that detects errors, whereby the frame recipient makes a calculation by dividing frame contents with a prime binary divisor and compares the remainder to a value stored in the frame by the sending node. Contrast with: checksum.

CSMA/CD Carrier Sense Multiple Access Collision Detect: A technology defined by the Ethernet IEEE 802.3 committee. Each device senses the cable for a digital signal before transmitting. Also, CSMA/CD allows all devices on the network to share the same cable, but one at a time. If two devices transmit at the same time, a frame collision will occur and a jamming pattern will be sent; the devices will stop transmitting, wait a predetermined amount of time, and then try to transmit again.

CSU Channel Service Unit: A digital mechanism that connects end-user equipment to the local digital telephone loop. Frequently referred to along with the data service unit as CSU/DSU. See also: DSU.

CTD Cell Transfer Delay: For a given connection in ATM, the time period between a cell exit event at the source user-network interface (UNI) and the corresponding cell entry event at the destination. The CTD between these points is the sum of the total inter-ATM transmission delay and the total ATM processing delay.

cut-through frame switching A frame-switching technique that flows data through a switch so that the leading edge exits the switch at the output port before the packet finishes entering the input port. Frames will be read, processed, and forwarded by devices that use cut-through switching as soon as the destination address of the frame is confirmed and the outgoing port is identified.

data direct VCC A bidirectional point-to-point virtual control connection (VCC) set up between two LECs in ATM and one of three data connections defined by Phase 1 LAN Emulation. Because data direct VCCs do not guarantee QoS, they are generally reserved for UBR and ABR connections. Com-pare with: control distribute VCC and control direct VCC.

flash memory Developed by Intel and licensed to other semi-conductor manufacturers, it is nonvolatile storage that can be erased electronically and reprogrammed, physically located on an EEPROM chip. Flash memeory permits software images to be stored, booted, and rewritten as needed. Cisco routers and switches use flash memory to hols the IOS by default. See also: EPROM, EEPROM

HDLC High-Level Data Link Control: using frame characters, including checksums, HDLC designates a method for data encapsulation on synchronous serial links and is the default encapsulation for Cisco routers. HDLC is a bit-oriented synchronous Data-Link layer protocol created by ISO and derived from SDLC. However, most HDLC vendor implementations (Including Cisco's) are proprietary.

PDU Protocol Data Unit: Is the name of the processes at each layer of the OSI model. PDUs at the Transport layer are called segments; PDU's at the Network layer are called packets or datagrams; and PDUs at the Data Link layer are called frames. The Physical layer uses bits.

PPP Point-to-Point Protocol: The protocol most commonly used for dial-up Internet access, superseding the earlier SLIP. Its features include address notification, authentication via CHAP or PAP, support for multiple protocols, and link monitoring. PPP has two layers: the Link Control Protocol (LCP) establishes, configures, and tests a linkl and then any of various Network Control Programs (NCPs) transport traffic for a specific protocol suite, such as IPX. See also: CHAP, PAP, and SLIP

RARP Reverse Address Resolution Protocol: The protocol within the TCP/IP stack that maps MAC addresses to IP addresses. See also: ARP.

ROM Read Only Memory: Chip used in computers to help boot the device. Cisco routers use a ROM chip to load the bootstrap, which runs a power-on self test, and then find and load the IOS in flash memory by default.

SAP 1) Service Access Point: a field psecified by IEEE 802.2 that is part of an address specification. 2) Service Advertising Protocol: The Novell NetWare protocol that supplies a way to inform network clients of resources and services availability on a network, using routers and servers (See also IPX)

synchronous transmission Signals transmitted digitally with precision clocking. These signals have identical frequencies and contain individual characters encapsualter in control bits (called start/stop bits) that designate the beginning and ending of each character. See also: asynchronous transmission and isochronous transmission

TE1 A device with a four-wire, twisted-pair digital interface is referred to as terminal equipment type 1. Most modern ISDN devices are of this type.

TE2 Devices known as terminal equipment type 2 do not understand ISDN signaling techniques, and a terminal adapter must be used to convert the signaling.

TFTP Conceptually, a stripped down version of FTP, it's the protocol of choice if you know exactly what you want and where it's to be found. TFTP doesn't provide the abundance of functions that FTP does. In particular, it has no directory browsing abilities; it can do nothing but send and receive files.