Table of Contents

C

C

CA

See congestion avoidance.

cable

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

cable range

Range of network numbers that is valid for use by nodes on an extended AppleTalk network. The cable range value can be a single network number or a contiguous sequence of several network numbers. Node addresses are assigned based on the cable range value.

cable television

See CATV.

California Education and Research Federation Network

See CERFnet.

call admission control

Traffic management mechanism used in ATM networks that determines whether the network can offer a path with sufficient bandwidth for a requested VCC.

call priority

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

call setup time

The time required to establish a switched call between DTE devices.

CAM

content-addressable memory. See associative memory.

Canadian Standards Association

See CSA.

carrier

Electromagnetic wave or alternating current of a single frequency, suitable for modulation by another, data-bearing signal. See also modulation.

Carrier Detect

See CD.

carrier sense multiple access collision detect

See CSMA/CD.

Category 1 cabling

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

Category 2 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 2 cabling is capable of transmitting data at speeds up to 4 Mbps. Compare with Category 1 cabling, Category 3 cabling, Category 4 cabling, and Category 5 cabling. See also EIA/TIA-586 and UTP.

Category 3 cabling

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

Category 4 cabling

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

Category 5 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 5 cabling is used for running CDDI and can transmit data at speeds up to 100 Mbps. Compare with Category 1 cabling, Category 2 cabling, Category 3 cabling, and Category 4 cabling. See also EIA/TIA-586 and UTP.

catenet

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

CATV

cable television. Communication system where multiple channels of programming material are transmitted to homes using broadband coaxial cable. Formerly called Community Antenna Television.

CBDS

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

CBR

constant bit rate. QOS class defined by the ATM Forum for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery. Compare with ABR (available bit rate), UBR, and VBR.

CCITT

Consultative Committee for International Telegraph and Telephone. International organization responsible for the development of communications standards. Now called the ITU-T. See ITU-T.

* RFC 1983 This organization is now part of the International Telecommunications Union and is responsible for making technical recommendations about telephone and data communications systems. Every four years CCITT holds plenary sessions where they adopt new standards; the most recent was in 1992. Recently, the ITU reorganized and CCITT was renamed the ITU-TSS. See also: International Telecommunications Union - Telecommunications Standards Sector.

CCS

common channel signaling. Signaling system used in telephone networks that separates signaling information from user data. A specified channel is exclusively designated to carry signaling information for all other channels in the system. See also SS7.

CD

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

CDDI

Copper Distributed Data Interface. Implementation of FDDI protocols over STP and UTP cabling. CDDI transmits over relatively short distances (about 100 meters), providing data rates of 100 Mbps using a dual-ring architecture to provide redundancy. Based on the ANSI Twisted-Pair Physical Medium Dependent (TPPMD) standard. Compare with FDDI.

CDP

Cisco Discovery Protocol. Media- and protocol-independent device-discovery protocol that 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 of a WAN. Runs on all media that support SNAP, including LANs, Frame Relay, and ATM media.

CDPD

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

CDVT

cell delay variation tolerance. Parameter defined by the ATM Forum for ATM traffic management. In CBR transmissions, determines the level of jitter that is tolerable for the data samples taken by the PCR. See also CBR and PCR.

cell

The basic unit for ATM switching and multiplexing. Cells contain identifiers that specify the data stream to which they belong. Each cell consists of a 5-byte header and 48 bytes of payload. See also cell relay.

cell delay variation tolerance

See CDVT.

cell line card

See CLC.

cell loss priority

See CLP.

cell payload scrambling

Technique used on the LightStream 2020 ATM switch to maintain framing on some medium-speed edge and trunk interfaces.

cell relay

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

cells per second

Abbreviated cps.

Cellular Digital Packet Data

See CDPD.

cellular radio

Technology that uses radio transmissions to access telephonecompany networks. Service is provided in a particular area by a low-power transmitter.

CEMAC

circuit emulation access card. T1 or E1 circuit emulation card in the LightStream 2020 ATM switch. See also access card.

central office

See CO.

Centrex

AT&T PBX that provides direct inward dialing and automatic number identification of the calling PBX.

CEPT

Conférence Européenne des Postes et des Télécommunications. Association of the 26 European PTTs that recommends communication specifications to the ITU-T.

CERFnet

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

CGS

Compact Gateway Server. Cisco midrange multiprotocol router designed for medium to small regional and district environments. The CGS is a 2-slot router that supports up to four interfaces (all of the same type).

chaining

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

Challenge Handshake Authentication Protocol

See CHAP.

channel

1. A communication path. Multiple channels can be multiplexed over a single cable in certain environments.
2. In IBM, the specific path between large computers (such as mainframes) and attached peripheral devices.

channel-attached

Pertaining to attachment of devices directly by data channels (input/output channels) to a computer.

Channel Interface Processor

See CIP.

channelized E1

Access link operating at 2.048 Mbps that is subdivided into 30 B-channels and 1 D-channel. Supports DDR, Frame Relay, and X.25. Compare with channelized T1.

channelized T1

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

channel service unit

See CSU.

CHAP

Challenge Handshake Authentication Protocol. Security feature supported on lines using PPP encapsulation that prevents unauthorized access. CHAP does not itself prevent unauthorized access, it merely identifies the remote end. The router or access server then determines whether that user is allowed access. Compare to PAP.

* RFC 1334 PPP Authentication Protocols and
* RFC 1994 PPP Challenge Handshake Authetication Protocol (CHAP)

chat script

String 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 to receive from the remote system and what the local system should send as a reply.

Cheapernet

Industry term used to refer to the IEEE 802.3 10Base2 standard or the cable specified in that standard. Compare with Thinnet. See also 10Base2, Ethernet, and IEEE 802.3.

checksum

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

* RFC 1983 A computed value which is dependent upon the contents of a packet. This value is sent along with the packet when it is transmitted. The receiving system computes a new checksum based upon the received data and compares this value with the one sent with the packet. If the two values are the same, the receiver has a high degree of confidence that the data was received correctly. See also: Cyclic Redundancy Check.
[Source: NNSC]

choke packet

Packet sent to a transmitter to tell it that congestion exists and that it should reduce its sending rate.

CIA

See classical IP over ATM.

CICNet

Regional network that connects academic, research, nonprofit, and commercial organizations in the Midwestern United States. Founded in 1988, CICNet was a part of the NSFNET and was funded by the NSF until the NSFNET dissolved in 1995. See also NSFNET.

CICS

Customer Information Control System. IBM application subsystem allowing transactions entered at remote terminals to be processed concurrently by user applications.

CIDR

classless interdomain routing. Technique supported by BGP4 and based on route aggregation. CIDR allows routers to group routes together in order to cut down on the quantity of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity. See also BGP4.

CIO

Cisco Information Online. Online service available to Cisco customers that provides electronic services and online information relating to Cisco products. CIO services include product information, software updates, release notes, technical tips, configuration notes, brochures, and download offerings.

CIP

Channel Interface Processor. Channel attachment interface for Cisco 7000 series routers. The CIP is used to connect a host mainframe to a control unit, eliminating the need for an FEP for channel attachment.

CIR

committed information rate. The rate at which a Frame Relay network agrees to transfer information 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 also Bc.

circuit

Communications path between two or more points.

circuit emulation access card

See CEMAC.

circuit group

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

circuit switching

Switching system in which a dedicated physical circuit path must exist between sender and receiver for the duration of the "call." Used heavily in the telephone company network. Circuit switching can be contrasted with contention and token passing as a channel-access method, and with message switching and packet switching as a switching technique.

Class A station

See DAS.

Class B station

See SAS.

classical IP over ATM

Specification for running IP over ATM in a manner that takes full advantage of the features of ATM. Defined in RFC 1577. Sometimes called CIA.

classless interdomain routing

See CIDR.

class of service

See COS.

CLAW

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

CLC

cell line card. Card on the LightStream 2020 ATM switch that, in conjunction with an access card, supports up to two OC-3c edge ports or one OC-3c trunk port. A CLC can be configured as an edge card or a trunk card.

Clear To Send

See CTS.

CLI

command line interface. The command-line interface on the LightStream 2020 that runs on NPs and Sun SPARCstations and is used to monitor and control an ATM network.

client

Node or software program (front-end device) that requests services from a server. See also back end, front end, and server.

* RFC 1983 A computer system or process that requests a service of another computer system or process. A workstation requesting the contents of a file from a file server is a client of the file server. See also: client-server model, server.
[Source: NNSC]

client-server computing

Term used to describe distributed computing (processing) network systems in which transaction responsibilities are divided into two parts: client (front end) and server (back end). Both terms (client and server) can be applied to software programs or actual computing devices. Also called distributed computing (processing). Compare with peer-to-peer computing. See also RPC.

CLNP

Connectionless Network Protocol. OSI network layer protocol that does not require a circuit to be established before data is transmitted. See also CLNS.

CLNS

Connectionless Network Service. OSI network layer service that does not require a circuit to be established before data is transmitted. CLNS routes messages to their destinations independently of any other messages. See also CLNP.

CLP

cell loss priority. Field in the ATM cell header that determines the probability of a cell being 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 might be dropped in congested conditions in order to free up resources to handle insured traffic.

cluster controller

1. Generally, an intelligent device that provides the connections for a cluster of terminals to a data link.
2. In SNA, a programmable device that controls the input/output operations of attached devices. Typically, an IBM 3174 or 3274 device.

CMI

coded mark inversion. ITU-T line coding technique specified for STS-3c transmissions. Also used in DS-1 systems. See also DS-1 and STS-3c.

CMIP

Common Management Information Protocol. OSI network management protocol created and standardized by ISO for the monitoring and control of heterogeneous networks. See also CMIS.

CMIS

Common Management Information Services. OSI network management service interface created and standardized by ISO for the monitoring and control of heterogeneous networks. See also CMIP.

CMNS

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

CMT

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

CO

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

coaxial cable

Cable consisting of a hollow outer cylindrical conductor that surrounds a single inner wire 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 signaling and high-speed digital signaling.

CODEC

coder-decoder. Device that typically uses PCM to transform analog signals into a digital bit stream, and digital signals back into analog.

coded mark inversion

See CMI.

coder-decoder

See CODEC.

coding

Electrical techniques used to convey binary signals.

collapsed backbone

Nondistributed backbone in which all network segments are interconnected by way of an internetworking device. A collapsed backbone might be a virtual network segment existing in a device such as a hub, a router, or a switch.

collision

In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device impact and are damaged when they meet on the physical media. See also collision domain.

collision detection

See CSMA/CD.

collision domain

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

command line interface

See CLI.

Committed Burst

See Bc.

committed information rate

See CIR.

common carrier

Licensed, private utility company that supplies communication services to the public at regulated prices.

common channel signaling

See CCS.

Common Link Access for Workstations

See CLAW.

Common Management Information Protocol

See CMIP.

Common Management Information Services

See CMIS.

common part convergence sublayer

See CPCS.

Common Programming Interface for Communications

See CPI-C.

common transport semantic

See CTS.

communication

Transmission of information.

communication controller

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

communication server

Communications processor that connects asynchronous devices to a LAN or WAN through network and terminal emulation software. Performs only asynchronous routing of IP and IPX. Compare with access server.

communications line

The physical link (such as wire or a telephone circuit) that connects one or more devices to one or more other devices.

community

In SNMP, a logical group of managed devices and NMSs in the same administrative domain.

Community Antenna Television

Now known as CATV. See CATV.

community string

Text string that acts as a password and is used to authenticate messages sent between a management station and a router containing an SNMP agent. The community string is sent in every packet between the manager and the agent.

Compact Gateway Server

See CGS.

companding

Contraction derived from the opposite processes of compression and expansion. Part of the PCM process whereby analog signal values are logically rounded to discrete scale-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 terminal using the same nonlinear scale. Compare with compression and expansion. See also a-law and mu-law.

complete sequence number PDU

See CSNP.

Compressed Serial Link Internet Protocol

See CSLIP.

compression

The running of a data set through an algorithm that reduces the space required to store or the bandwidth required to transmit the data set. Compare with companding and expansion.

Computer Science Network

See CSNET.

concentrator

See hub.

Conférence Européenne des Postes et des Télécommunications

See CEPT.

Configuration Builder

Cisco software application that lets you create configuration files for multiple routers without knowing the router command-line syntax. Configuration Builder is a Microsoft Windows-based application that enables you to configure multiple routers simultaneously; automatically detect the model, software version, image type, and the number and type of installed interfaces on the router you are configuring; and quickly import predefined priority queuing lists, access lists, and filters into multiple configuration files.

configuration database

File of attribute settings created using the Cisco LightStream configurator. A global database holds configuration information for the entire LightStream-based ATM backbone and is stored on the NMS. A local database, stored in each LightStream 2020 ATM switch, contains just the configuration information for that switch. Configuration data includes definitions of chassis, cards, ports, VCs, and the attributes that describe them. See also configurator.

configuration management

One of five categories of network management defined by ISO for management of OSI networks. Configuration management subsystems are responsible for detecting and determining the state of a network. See also accounting management, fault management, performance management, and security management.

configuration register

In Cisco routers, a 16-bit, user-configurable value that determines how the router functions during initialization. The configuration register can be stored in hardware or software. In hardware, the bit position is set using a jumper. In software, the bit position is set by specifying a hexadecimal value using configuration commands.

configurator

Management tool used with the LightStream 2020 ATM switch that is used to create configuration database files for the nodes in an ATM network. The configurator is an HP OpenView-based application that runs on an NMS. See also configuration database.

congestion

Traffic in excess of network capacity.

congestion avoidance

The mechanism by which a LightStream-based ATM network controls traffic entering the network to minimize delays. In order to use resources most efficiently, lower-priority traffic is discarded at the edge of the network if conditions indicate that it cannot be delivered. Sometimes abbreviated CA.

connectionless

Term used to describe data transfer without the existence of a virtual circuit. Compare with connection-oriented. See also virtual circuit.

* RFC1983 The data communication method in which communication occurs between hosts with no previous setup. Packets between two hosts may take different routes, as each is independent of the other. UDP is a connectionless protocol. See also: circuit switching, connection-oriented, packet switching, User Datagram Protocol.

Connectionless Broadband Data Service

See CBDS.

Connectionless Network Protocol

See CLNP.

Connectionless Network Service

See CLNS.

connection management

See CMT.

Connection-Mode Network Service

See CMNS.

connection-oriented

Term used to describe data transfer that requires the establishment of a virtual circuit. See also connectionless. See also virtual circuit.

* RFC 1983 The data communication method in which communication proceeds through three well-defined phases: connection establishment, data transfer, connection release. TCP is a connection-oriented protocol. See also: circuit switching, connectionless, packet switching, Transmission Control Protocol.

Connection-Oriented Network Protocol

See CONP.

CONP

Connection-Oriented Network Protocol. OSI protocol providing connection-oriented operation to upper-layer protocols. See also CMNS.

console

DTE through which commands are entered into a host.

constant bit rate

See CBR.

Consultative Committee for International Telegraph and Telephone

See CCITT.

content-addressable memory

See associative memory.

contention

Access method in which network devices compete for permission to access the physical medium. Contrast with circuit switching and token passing.

control point

See CP.

ControlStream traffic management

Traffic management scheme used by the LightStream 2020 ATM switch. Includes congestion avoidance, traffic shaping, and traffic policing, and allows links to operate at high levels of utilization by scaling back lower-priority, delay-tolerant traffic at the edge of the network when congestion begins to occur.

convergence

The speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology.

convergence sublayer

See CS.

conversation

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

Cooperation for Open Systems Interconnection Networking in Europe

See COSINE.

Copper Distributed Data Interface

See CDDI.

core gateway

The primary routers in the Internet.

core router

In a packet-switched star topology, a router that is part of the backbone and that serves as the single pipe through which all traffic from peripheral networks must pass on its way to other peripheral networks.

Corporation for Open Systems

See COS.

Corporation for Research and Educational Networking

See CREN.

COS

1. class of service. Indication of how an upper-layer protocol requires that a lower-layer protocol treat its messages. In SNA subarea routing, COS definitions are used by subarea nodes to determine the optimal route to establish a given session. A COS definition comprises a virtual route number and a transmission priority field. Also called TOS (type of service).
2. Corporation for Open Systems. Organization that promulgates the use of OSI protocols through conformance testing, certification, and related activities.

COSINE

Cooperation for Open Systems Interconnection Networking in Europe. European project financed by the European Community (EC) to build a communication network between scientific and industrial entities in Europe. The project ended in 1994.

cost

Arbitrary value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment. Cost values are used by routing protocols to determine the most favorable path to a particular destination: the lower the cost, the better the path. Sometimes called path cost. See also routing metric.

count to infinity

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

CP

control point. In SNA networks, element that identifies the APPN networking components of a PU 2.1 node, manages device resources, and can provide services to other devices. In APPN, CPs are able to communicate with logically adjacent CPs by way of CP-to-CP sessions. See also EN and NN.

CPCS

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

CPE

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

CPI-C

Common Programming Interface for Communications. Platform-independent API developed by IBM and used to provide portability in APPC applications. See also APPC.

cps

cells per second.

CRC

cyclic redundancy check. Error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calculated remainder to a value stored in the frame by the sending node.

CREN

Corporation for Research and Educational Networking. The result of a merger of BITNET and CSNET. CREN is devoted to providing Internet connectivity to its members, which include the alumni, students, faculty, and other affiliates of participating educational and research institutions, via BITNET III. See also BITNET, BITNET III, and CSNET.

cross talk

Interfering energy transferred from one circuit to another.

CS

convergence sublayer. One of the two sublayers of the AAL CPCS, responsible for padding and error checking. PDUs passed from the SSCS are appended with an 8-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 are then passed to the SAR sublayer of the CPCS for further processing. See also AAL, CPCS, SAR, and SSCS.

CSA

Canadian Standards Association. Agency within Canada that certifies products that conform to Canadian national safety standards.

CSLIP

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

CSMA/CD

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

CSNET

Computer Science Network. Large internetwork consisting primarily of universities, research institutions, and commercial concerns. CSNET merged with BITNET to form CREN. See also BITNET and CREN.

CSNP

complete sequence number PDU. PDU sent by the designated router in an OSPF network to maintain database synchronization.

CSU

channel service unit. Digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to together with DSU, as CSU/DSU. See also DSU.

csumon

Tool available on the LightStream 2020 ATM switch, accessible from the bash shell. Csumon allows connection to an external CSU/DSU on a low-speed line for monitoring and control purposes, and can display statistics on the internal CSU/DSU of a medium-speed line.

CTS

1. Clear To Send. Circuit in the EIA/TIA-232 specification that is activated when DCE is ready to accept data from DTE.
2. common transport semantic. Cornerstone of the IBM strategy to reduce the number of protocols on networks. CTS provides a single API for developers of network software and enables applications to run over APPN, OSI, or TCP/IP.

Customer Information Control System

See CICS.

customer premises equipment

See CPE.

cut-through packet switching

Packet switching approach that streams data through a switch so that the leading edge of a packet exits the switch at the output port before the packet finishes entering the input port. A device using cut-through packet switching reads, processes, and forwards packets as soon as the destination address is looked up, and the outgoing port determined. Also known as on-the-fly packet switching. Contrast with store and forward packet switching.

CxBus

Cisco Extended Bus. Data bus for interface processors on Cisco 7000 series routers that operates at 533 Mbps. See also Switch Processor.

CyBus

1.067-Gbps data bus for interface processors. Used in the Cisco 7500 series routers. See also Cisco 7500.

cycles per second

See hertz.

cyclic redundancy check

See CRC.