Telecom Terms

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ACD - Automated Call Distributor: A special facility of a PBX or centrol office switch that automatically routes incoming calls to the next available or longest idle agent or attendant in a line hunt group.

AGC - Automatic Gain Control: Electronic circuit which automatically increases the volum when someone is speaking quietly and rops the volume when someone is speaking loudly, to keep the transmitted signal constant.

Analog Single Line Station: An extension prot on a PBX/KSU that will allow a standard household type phone to interface with the system (POTS)


BOM - Beginning of a Message: A short electric pulse provided by a digital annoucer when used in certain 4 wire E & M modes.  The pulse alerts the PBX that the beginning of the message is coming.


Carbon Transmitter (Handset): The microphone of a telephone handset which uses carbon graules and a diaphragm. The diaphragm responds to voice and varies the pressure on the granules and hence, their resistance.

C.O./CO - Central Office: A main telephone office where switching equipment is housed and customer lines originate.  Also contains trunk line connections to other Central Offices.  A CO line is a phone line coming from the Central Office and terminating at the user's premise.

Centrex: A name for an umbrella service offering of the local telephone company. Customers lease a portion of the Central Office switch to create a centralized point of control and routing. In concept, Centrex is providing similar features to a PBX.

CNG - Tone Detection (known as the CallNG party tone): The ability to recognize the tone emitted from an auto-dialing faxcimile machine, then automcatically transfer the call to the fax machine.

Contact Closure: Contacts that when "closed" connect a pair of wires together and disconnects the wires when "open". A doorbell button is a simple example of a momentary contact closure.

CPC - Calling Party Control (Disconnect Supervisor): A short breat in the line current in the called party's phone line when the calling party hangs up. Not available in PBX's.


Distinctive Ringing (also known as ringmate, custon ring, identifying ring master): Different patterns of ringing that allow a user to identify the source of the call, outside or inside (intercom) or who the call is for. Usually two burst or three burst with different durations.

DTMF - Dual Tone Multi-Frequency (Touch Tone): Audible tones which conform to North American Telephone Standards.  These are generated by push-button telephones to dial telephone numbers, as well as to provide control or data input to voice processing systems. 


E & M - Control (4 wire E & M), (The receive and transmit leads of the signaling system): An equipment signaling system usually used in PBX applications with a digital announcer.

Earth Ground: A wire conductor that terminates in the earth for electrical purposes.


F.C.C. The Federal Communications Commission


Ground Start Line: A type of call signaling, in this scheme, when a call is routed to a subscriber, the phone company will apply a ground signal on the TIP side of the line even before a ring signal is sent.  By detecting the signal, phone systems can immediately determine that the line is about to be in use and avoid call crashing.


Hook Switch Flash or Switch Hook Flash: A signal originated by momentarily depressing the switch hook on a single line phone.  Typically used for accessing CO or PBX features.

Hz - Hertz: Indicates the frequency (number of cycles per second) of alternating current such as ringing voltage or Touch Tone signals.


Intercept: To stop a telephone call driected to an improper telephone number and redirect that call to an operator or recording.

ISDN - Integrated Services Digital Network - an emerging international standard that defines interfaces for simultaneous voice and data transmission through a single channel over the public network.

ISDN Terminal Adapter: An interface between an ISDN line and a computer.  Often these provide 1 or 2 analog POTS output ports



KSU - Key Service Unit - A central control cabinet serving many multi-button key telephones. contains equipment which enables the user to pick up and hold PBX or Central Office lines and provide flashing line, steady, busy and wink hold signals. Dial intercom service is also on option.


LED - Light emitting diode

Loop Current - current that flows from the CO when a telecom device goes off hook.

Loop Start Line - A type of calling signal that relies on a telecom device to allow DC current to flow when it goes into an off hook condition.  To CO or PBX senses the current an dintercepts it as a request for service. Also known as POTS.


MOH - Music On Hold


No KSU - a phone containing all control functions normally associated with a KSU, thus not requiring a KSU

Non-Volatile E Memory: Memory that will not be lost from a power failure. Does not need battery backup.



OPX - Off Premise Extension - a telephone line from the phone system terminated in a different building than the phone system.


PABX - Private Automated Branch Exchange - see PBX

PBX - Private Branch Exchange - Phone system used to switch telephones between extensions and to outside lines.  For incoming and outgoing calls.

POTS - Plain Old Telephone Service - See CO, Analog Single Line Station, Loop Start Line



Rack Mount - Equipment which is designed to mount in a standard steel framework consisting of parallel pairs of stell channel verticals spaced 19" and drilled for mounting screws.

RAD/RAN Device - a Record/ANnouce device such as a digital announcer.

RAN Trunk/RAN Circuit - A Record/ANnouce trunk or circuit that connects the telephone system to a reconrd/announce device.

REN - Ringer Equivalency Number = 1 REN = the energy to ring 1 POTS.  The REN number can be found on the FCC label on the device. The total ringer load on a line is equal to the sum of all the REN numbers of all telephone devices connected to the line.

Ring Back Tone - The sound you hear when the phone you call is ringing.

Ringdown Circuit - A circuit connecting 2 devices so that when one device goes off hook it automatically rings the other device.

Ring Generator - a circuit producing AC voltage that is higher than 50 VRMS and has a frequency of 20 Hz

Ring Signal - same as ringback

Ring Trip - The process of stopping the AC ring signal anc connecting the voice path at the CO when the ringing telephone is answered

RJ-11, RJ-14, RJ-21X, RJ-25 - Request Fax-Back Document No 839

RS232 - An Electronic Industry Association (EIA) standard that defines a computer interface for connecting serial communication devices such as printers and modems


66 Block - A terminal block where up to 4 sets of upt to 25 pairs of wire can be interconnected. The installer uses a special "punch down" tools to connect the wires.

SLT - Single Line Telephone

SOHO - Small Office/Home Office - an acronym for a new calls of equipment where the employee may be performing part or all of their work in a small office set up.

Subscriber Line - the telephone line connecting the local telco CO to the customer's telephone instrument or telephone system.


Talk Battery - DC voltage suppplied by the CO to the subscriber's loop to operate the telephone equipment

TelCo - Local Telephone Company

Trunk - a telephone communication path or channel between 2 points, one usually being a CO

Trunk Port - An input on a PBX or KSU that is used to interface the system with CO lines


UCD - Uniform Call Distributor - An ACD programmed to distribute calls to agents or representatives on a basis other than the next available.  Each person in the group receives the same number of calls.

UPS - Uninterruptable Power Supply - Provides power to your phone equipment, computers, etc., in the event of a commercial power failure.







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Date Last Modified: 03/28/06