Telos NX12 user manual Circuit Switched Data- See CSD

guide to telephone terminology  105

your station runs a contest) the other choke exchange users will be blocked because all trunks into the choke exchange will be busy.

In the modern network, using CCIS signaling such as SS7, actual trunks are not used to convey “busy” or “all trunks busy” conditions. Thus blocking due to a station contest should not occur as the busy status in response to a call attempt is conveyed over the separate SS7 network. Therefore, the need for choke exchanges has pretty much disappeared. Nonetheless, many Telcos still insist that Broad- casters use special choke lines for call-in lines.

See blocking and concentration.

Circuit- A physical path through which electrical signals can pass. It consists of a network of conductors and other components, sepa- rated by insulators. Technically this term cannot be applied to fiber optic or other “non-metallic” paths. See also channel.

Circuit Switched Data- See CSD

Circuit Switching- A system where a dedicated channel is allocated to the users of that call for the duration of that call. That channel is allocated for the duration of the call regardless if information is being transmitted at any given moment. Bandwidth through the channel is fixed, at no time may this bandwidth be exceeded. If this bandwidth is not used it is wasted. While inherently inefficient, the dependable and reliable nature of circuit switching makes it ideally suited to real-time voice and audio/video conferencing applications. When over loaded Circuit Switched networks will respond “all circuits are busy… try again later”. This is in stark contrast packet switched networks or to systems where statistical multiplexing is used. See Statistical Multiplexing.

CLASS- Custom Local Area Signaling Services. A variety of en- hanced features (usually on analog lines) that take advantage of the ability of modern SS7 technology’s ability to transmit information about the calling party. CLASS includes such features as Caller ID, Automatic Callback, Call Trace (initiated by subscriber), Selective Call Screening, etc.

Clear Cause. See Cause Code

CLEC- Competitive Local Exchange Carrier. Your local telephone service provider who is one of the new-generation providers rather than a RBOC or Independent. A CLEC is really just an indepen- dent, albeit one formed after the divestiture of AT&T. See LEC and Independent.

CLI - Calling Line Identity. European term for CLID. See CLID.

CLID- Calling Line Identification. This is the ISDN and SS7 equivalent of Caller ID; I.E. the number of the calling party. See also Caller ID and ANI.

CO- Central Office. The Telco facility to which your local tele- phone circuit lead. Contains “Switches” and “Trunks” as well as the local telephone circuits.

Codec- COder/DECoder. A device which takes digitized audio

and “codes” it in order to reduce the transmission bit rate and which can also simultaneously “decode” such coded audio. Strictly speak- ing, a codec does not include an ISDN terminal adapter and related equipment.

Simple codecs are also used in digital telephony. These use a simple companding scheme to reduce channel noise.

COL - COnnected Line number. European Term. The number to which you have connected. This may not be the number you dialed if call forwarding is used.

Combination Trunk- A trunk (channel) which can both make and receive calls. This generally refers to analog ground start or loop start trunks, although the term can be applied to ISDN BRI or PRI channels as well. Each combination trunk normally has a telephone number, although they are frequently part of a hunt group and only one number may be published for that group. Also called a Both Way Trunk. This is not the same as a Two-way DID trunk. See DID trunk, Hunt Group and Trunk.

Common Channel Interoffice Signaling- See CCIS Competitive Local Exchange Carrier- See CLEC.

Concentration- The basic premise is to share facilities wherever possible. For instance, while there may be thousands of custom- ers served by a given Central Office, there will be substantially less than that number of calls which can be handled simultaneously. And, even fewer long distance calls can be made simultaneously. The art of Traffic Engineering is to have enough capacity that calls are rarely blocked, but no more than that number. See also Choke Exchange and Blocking.

CPC- Calling Party Control. Sometimes referred to as “CPC Wink” or “disconnect supervision”. A call supervision feature on an analog loop start line that provides the ability for a CO (Central Office) to signal the called party when the calling party hangs up. CPC allows the PBX, key system, or telephone answering device to reset the line so that it is ready to accept or initiate another call. CPC is accomplished by either a loop current drop or reversal. With some CO equipment, it is also provided if the called party drops the call. See also MCLD.

CPE- Customer Premise Equipment- Customer owned equipment located at his/her facility, such as a CSU or terminal. In the USA and Canada, the NT1 is part of the CPE.

CPN - Called Party Number - European Term. The number that has been dialed. See Called Party Address.

CSD- Circuit Switched Data- A dial-up data communications channel which, once established, looks like a transparent data pipe. Also, the type of ISDN service required to utilize this capability of an ISDN circuit. In contrast to CSV.

CSU- Channel Service Unit. The NCTE used in the USA & Canada to terminate a T1 line. Typically the CSU must be provided