C

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 network 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 established 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 networks, determining the possibility of a path containing adequate bandwidth for a requested VCC.

call center

The call center concept is based on the ACD (automatic call distribution) system. Incoming calls are accepted; if no agents are available, the incoming calls are “buffered” and music is played while you wait for an agent.

call priority

In circuit-switched systems, the defining priority given to each originating port; it specifies in which order calls will be reconnected. Additionally, 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.

CAS

(Channel Associated Signaling) used in voice signaling to carry the signal from the trunk to the PBX.

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.

CBWFQ

(Class Based Weighted Fair Queuing) extends the standard WFQ functionality to provide support for user-defined traffic classes. For CBWFQ, you define traffic classes based on match criteria including protocols, access control lists (ACLs), and input interfaces. Packets satisfying the match criteria for a class constitute the traffic for that class. A queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class.

CCS

A common link is used to carry voice signaling information for a number of trunks to the PBX switch. This form of signaling is cheaper, supports faster connect times, and is more flexible than CAS.

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 management in ATM networks specified when a connection is established. The allowable fluctuation levels for data samples taken by the PCR in CBR transmissions are determined by the CDVT. See also: CBR and PCR.

cell

In ATM networking, the basic unit of data for switching and multiplexing. 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 circuits). Cell payload scrambling rearranges the data portion of a cell to maintain 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 hardware at high speeds, making this technology the foundation for ATM and other high-speed network protocols. See also: cell.

cell switching

A term that describes how a cellular telephone system switches from one cell to the next as the signal strength fades. The switch takes about 300 milliseconds to complete and is not noticeable by the user.

Centrex

A local exchange carrier service, providing local switching that resembles that of an on-site PBX. Centrex has no on-site switching capability. 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 channels or groups of channels connect to various destinations, supporting DDR, Frame Relay, and X.25. Also known as fractional T1. 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 mathematical functions, typically placed at the end of the data from which it is calculated, 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 transmitter that it should decrease its sending rate.

CIDR

Classless Interdomain Routing: A method supported by BGP4 and based on route aggregation that enables 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 networks 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 a two-digit subnet mask. 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 normal rate of transferring information.

Cisco CNS Network Registrar

Network Registrar automates enterprise IP address management. It includes Domain Name System (DNS), Dynamic Host Configuration Protocol (DHCP), and Trivial File Transfer Protocol (TFTP) servers. You can control these servers using the Network Registrar graphical user interface (GUI) or command-line interface (CLI). These user interfaces can control server clusters that run on different platforms.

Cisco FRAD

Cisco Frame-Relay Access Device: A Cisco product that supports 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.

Cisco Express Forwarding (CEF)

CEF evolved to best accommodate the changing network dynamics and traffic characteristics resulting from increasing numbers of short duration flows typically associated with web- based applications and interactive type sessions.

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 Internetwork Operating System software. The kernel of the Cisco line of routers and switches that supplies shared functionality, 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 configuration 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.

CiscoWorks2000

CiscoWorks2000 Server provides a unified desktop environment for managing a network of Cisco network devices.

classfull

Each of the commercial IP address classes has a classfull network mask. The network mask defines which bits out of the 32 bits of the address are defined as the network portion and which are the host portion.

classless

Some routing protocols do not assume the network mask as with classfull routing protocols and hence the network mask is always sent with the network number in routing updates.

classical IP over ATM

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

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 segments 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.

compression

Often used with data. Data compression eliminates unnecessary data and uses markers to compress the data.

committed information rate (CIR)

Frame Relay providers allow customers to buy a lower amount of bandwidth than what they really might need. This is called the committed information rate (CIR). This means that the customer can buy bandwidth of, for example, 256k, but it is possible to burst up to T1 speeds.

community string

Used with SNMP, a community string defines the community an SNMP management station and agent reside within.

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 specifying 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.

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.

control direct VCC

One of three control connections defined by Phase I LAN Emulation—a bidirectional virtual control connection (VCC) established 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

A measurement of the time required for all routers in an internetwork to update their routing tables. No data is passed during a convergence time.

Core layer

The layer at which high-speed routing and switching occur in the Cisco three-layer hierarchical design model.

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, the network administrator fixes an arbitrary hop count limit.

CPCS

Common Part Convergence Sublayer: One of two AAL sublayers that is service-dependent, it is further segmented into the CS and SAR sublayers. 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

Cyclic 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, they 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.

custom queuing

For networks that need to provide a guaranteed level of service for all traffic, Cisco offers custom queuing. Custom queuing allows a customer to reserve a percentage of bandwidth for specified protocols.

cut-through packet switching

A packet-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. Packets will be read, processed, and forwarded by devices that use cut-through packet switching as soon as the destination address is confirmed and the outgoing port is identified.