Glossary - C



Carrier Access Business

Cable capacity

The capacity an international cable has for carrying calls. Cable capacity can be expressed in terms of design capacity (the amount of capacity a cable is technically designed to carry) and notional capacity (the amount of capacity that has been subscribed for by cable consortia members).

Call barring

Specified numbers or ranges which cannot be dialled.

Call completion

A directory enquiry service which directly connects the enquirer to the number they seek. A call completion service without number disclosure directly connects the enquirer without revealing the number.

Call diversion

This when a call is re-routed to another number.

Call Divert Service

A network service that allows a customer to have a network redirect incoming calls to another number.


Ensures efficient routing of calls to ported numbers. Is an additional loop which continues for duration of call, within donor network in a call path to a ported number. Called tromboning also.

Call Minder

A network based call answering service provided by BT. Incoming messages can be left if the customer's line is busy or not answering.

Call origination

See originating operator.

Call return

A user calling an engaged number can request that the call is returned when the number is free.

Call routing apparatus

Any equipment capable of switching two-way live speech telephone calls between two or more exchange lines and two or more extensions, such as PBXs, Automatic Call Distribution equipment and key telephone systems.

Call termination

See terminating operator.

Call transit

See transit segment.

Call waiting

Caller to an engaged number is placed on hold whilst the called line is notified of a call waiting to be answered.

Caller display

A service based on calling line identity that allows customers to see the number from which someone is calling them before they answer the telephone.

Calling card

A service that allows calls made from a telephone to be billed to a different telephone or credit card account.

Calling Line Identity (CLI)

A facility that enables identification of the number from which a call is being made.


A carrier is a local (intra-LATA) or long distance (inter-LATA) telecommunications service providing organization. A waveform, pulsed or continuous, that is modulated by another information bearing waveform.

Carrier pre-selection (CPS)

The facility offered to customers which alows them to opt for certain defined classes of call to be carried by an operator selected in advance (and having a contract with the customer), without having to dial a routing prefix or follow any other different procedure to invoke such routing.

Carrier System

A carrier system is a transmission system for transmitting one or more channels of information by processing and converting to a form suitable for the transmission medium used. Carrier systems are classified as either analog carrier systems or digital carrier systems.


Continuous bit rate.


Common-channel interoffice signaling.


CCITT is the International Consultative Committee for Telephone and Telegraph, a consultative committee to the International Telecommunications Union (ITU) which recommend international standards for telephone and telegraph services and facilities to aid international connectivity and interoperability.


Common-control switching arrangement.


Centi-call seconds or common-channel signaling.


Code division multiple access.

Cell Relay

Call relay is the simply the process of transferring data which is in the form of fixed-length packets (called cells). Cell relay is used in low-delay, high-bandwidth, packet-like switching and also multiplexing techniques. Its objective is to develop a switching mechanism/single multiplexing for dividing up a usable capacity (bandwidth) in a l-manner so that it supports its allocation to both packet data communications services and isochronous (e.g., video and voice traffic) as well as . It has been debated by standards groups what the optimum cell size. Smaller cells - although they involve a higher header to user information overhead penalty than would be needed for most data applicationstend - to favor low delay for isochronous applications but . The current specification from CCITT for BISDN is for a 53-byte cell that includes a 5- byte header and also a 48-byte payload.

CENTRal EXchange (Centrex)

Centrex is an LEC-provided switching service provided for business customers. IT permits station-to-station dialing, direct inward dialing, listed directory number service and station number identification on outgoing calls. Switching functions are performed in a central office. The advanced features of fourth generation PBXs are offered by digital Centrex without needing to purchase or lease equipment. In most cases it eliminates the need for floor space, electrical prime power, heating, ventilation and also air conditioning.


A channel connects two or more points in a transmission medium. It is a single communications path in a network. Each path is separated by some means, for example by multiplex or spatial separation,maybe frequency or time division multiplexing. "Channel" is often used interchangeablywith "circuit" , however circuit can describe a physical configuration of equipment which enables the provision of a network transmission capability mainly for multiple channels. The characteristics of both channels and circuits are determined by the media and network equipment used to support them.

Calling line identity (CLID)

This is a digital network feature that enables identity information (from the device making a call) to be interpreted by the device that answers the call. It usually takes the the form of the telephone number of the person who is calling.

Cellular Radio

Cellular Radio is the technology responsible for making made wide scale mobile telephony possible. Before the existence of cellular radio the problem with the concept of the mobile phone was how to facilitate large numbers of users sharing only small amounts of the radio spectrum. Cellular radio solved this problem. It works by allowing the re-use of radio frequencies that are the same by assigning them to cells that are far enough apart thus preventing any noticeable interference. FDMA, or Frequency Division Multiple, was the basis for the first generation cellular radio systems. The second generation cellular radio systems support high bit rate voice and limited data communications and use digital techniques such as CDMA and TDMA. Third generation (3G) systems are now becoming more popular, actually support voice and high bit rate data. This allows mobile multimedia applications (please also see Narrowband, Wideband).


A service provided by a PTO. It uses the local telephone exchange and provides PABX facilities to the customer premises. (for example, BT Featureline).


Call Information Logging Equipment.


This is a means of creating telecoms connections. It works by setting up a circuit end-to-end. The circuit remains open only for the duration of the communication and until the circuit is closed a fixed share of network resources is tied up. No one else is able to make use of them. The main advantage of this circuit-switching is that it enables the offer of a guarantee of performance. Please also see Packet Switching.


A type of cable that is generally used for the conveying of television pictures.


Codec is a contraction of a coder and decoder. It is a device that takes a analog signals and encodes them into digital signals. It does this to enable them for transmission through a network in their digital format. It also decodes the received digital signals and puts them back into analog signals.

Common Channel Signaling (CCS)

CCS is a signaling system which was developed for use between a stored program control digital switching system (in which all the signaling information for one or more of the trunk groups is transmitted over a signaling channel that is dedicated) usually, but not always the case, this is done completely separate from the user traffic bearing facilities.

Common Management Information Protocol (CMIP)

Common Management Information Protocol is the OSI protocol for network management. It is a structure for firstly formatting messages and then transmitting information between data collection programs and reporting devices (agents). It was developed by the International Standards Organization and also designated ISO/IEC 9596.


Communications is the process used amongst persons, places, or machines, of representing, transferring, processing or interpreting information (data). Communications implies three things, a sender, a receiver, and a the transmission medium the information must travel over. It is essentail that the meaning assigned to the data be recoverable without degradation.

Communications provider

This is a person who provides an Electronic Communications Network or also provides an Electronic Communications Service.

Competitive service

The interconnection services that BT offered deemed to be competitive at the start of the Network Charge Controls. Also interconnection services introduced after the commencement of Network Charge Controls. BT has the liberty to set the rates for these services, though they are subject to normal competition rules.

Computer Supported Telephony (CST)

Based on that the ability of the phone and computer system to exchange information. An example of such an application might be the ability to present information on screen, such as scripts, at the same time as an incoming call. Scripts are based on information that is gathered about the caller before the telephone is answered. It is generally achieved by the menu systems or alternatively by an identification of the incoming line which the caller has dialled.


With the aim to improve Conditioning is an adjustment made to the electrical characteristics of a cable pair to improve its performance for a specific use.


A conductor is any material that permits a flow of electrons within electrical circuits. Although, optical fibers are also referred to as conductors as they conduct light waves.

Copper line (or metallic line)

In telephony networks this is the main transmission medium used to connect a telephone (or another apparatus) to the local exchange. Because of their narrrow bandwidth, copper lines have limited ability to carry broadband services, like video, unless they are combined with an enabling technology, an example of such may be ADSL.

Corporation for Open Systems (COS)

COS is composed of service, manufacturing, and user organizations in the computer communications arena. It is a nonprofit organization. It seeks to facilitate the development of the multi-vendor, international marketplace through development, introduction, and verification of ISDN and OSI standards. It does this also by ensuring vendor equipment interoperability.

Class of Service (COS)

The combination of PABX features allowed for a particular extension or group of extensions.


See Calling Line Identition.


Central processing unit.

Cross Connect

1.within a premises distribution system it is equipment used to administe and terminate communications circuits. In a wire cross connect, patch cords or jumper wires are used to make the circuit connections, between backbone and horizontal wiring segments. 2. in transmission systems it is a patch panel for connecting circuits.


Circuit-switched digital capability.


Carrier sense multiple access/collision detection. This is a local area network contention that is based on access control protocol techniques. Through this all devices that are attached to the network "listen" for transmissions that are in progress before they attempt to transmit themselves. If two or more begin transmission at the same time CSMA/CD can detect the "collision". If this is the case each defers (backs off) for a variable period before again attempting to transmit. This is determined by a preset algorithm. (Defined by the IEEE 802.3 standard).

Customer Premises Equipment (CPE)

Customer Premises Equipment is all of the telecommunications terminal equipment that is located on the customer premises. This is with the exception of coin operated telephones. It is sometimes referred to as customer apparatus or consumer equipment. it must be noted that as it is equipment on the consumers’ premises it is not part of the public telecommunications network and is therefore not directly or indirectly attached to it.


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