Glossary of
Telecommunications Terms

0 1 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

For terms used after divestiture of the Bell System (AT&T), click HERE.

- 0 - 
"0" or "0-" (AOS)
 Zero minus dialing. Allows a caller to dial zero and
 nothing else to get the Operator.

"0+" (AOS)
 Zero plus dialing. An operator assisted long distance call
 which is charged to the calling party.

"00+" or "00-" (OCC) (AOS)
 Double zero dialing. Allows a caller to get an AT&T;
 Operator in areas in which dialing only one zero would
 connect the caller with the local Operator because AT&T has
 given Operators back to the local telephone company.

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- 1 -
"1+" DIALING (IXC) (OCC) (AOS)
 The capability to dial "1" plus the long distance number for
 calls within the North American Numbering Plan area.
 Intra-LATA calls are carried by the local telephone company.
 Inter-LATA calls are carried by the caller's primary
 carrier, or by AT&T if equal access has not come to the
 caller's area yet.

"101-XXXX" DIALING (OCC) (AOS) (IXC)
 The ability to send calls over a carrier other than a
 caller's primary carrier by dialing "101-XXXX" then "1+" the
 long distance number, where "XXXX" is the 3-digit Carrier
 Code of the alternative long distance company (also called a
 secondary carrier). Available only to Equal Access
 customers.

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- 8 -
800 SERVICE (800)
 The ability of a caller to dial a long distance telephone
 number without incurring a charge for the call, which is
 paid for by the party offering the 800 number.
 Synonym: Inward WATS service.


- 9 -
900 SERVICE (900) (976)
 Allows callers to receive information from the service
 provider via a recorded audio message, which can range from
 60 seconds to a continuous live hookup, by calling a 900
 number. This service can also be used to enable callers to
 vote or "make a choice" by dialing one of two 900 numbers.
 900 calls are typically billed to the caller at much
 higher rates than regular calls.

976 NUMBERS (900) (976)
 Service which allows callers to listen to recorded messages
 such as horoscopes, 'adult' dialogue, stock market or sports
 reports by calling 976-xxxx. The local telephone company
 charges callers a fee which is split between the local
 telephone company and the service provider.

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- A -
A & B LEADS
 Designation of leads derived from the midpoints of the two
 pairs comprising a 4-wire circuit.

ABBREVIATED DIALING (AD)
 Preprogramming of a caller's phone system or long distance
 company's switch to recognize a 2- to 4-digit number as an
 abbreviation for a frequently dialed phone number, and
 automatically dial the whole number.
 Synonym: Speed Dialing.

ACCESS CHARGE (AC)
 Monies collected by local phone companies for use of their
 circuits to originate and terminate long distance calls.
 Can be per minute fees levied on long distance companies,
 Subscriber Line Charges (SLCs) levied directly on regular
 local lines, fixed monthly fees for special telco circuits
 (i.e. WAL, DAL,T-1), or Special Access Surcharge (SAS) on
 special access circuits.

ACCESS LINE (AL)
 A telephone circuit which connects a customer location to a
 network switching center.

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AIRLINE MILEAGE (AM)
 Calculated point-to-point mileage between terminal
 facilities.

ALL TRUNKS BUSY (ATB)
 A single tone interrupted at a 120 ipm
 (impulses per minute) rate to indicate all lines or trunks
 in a routing group are busy.

ALTERNATE ROUTE (AR)
 A secondary communications path used to reach a destination
 if the primary path is unavailable.

ALTERNATE USE (AU)
 The ability to switch communications facilities from one
 type to another, i.e. , voice to data, etc.

ALTERNATE VOICE DATA (AVD)
 A single transmission facility which can be used for either
 voice or data.

ANALOG SIGNAL (AS)
 A signal in the form of a continuous varying physical
 quantity, e.g. , voltage which reflects variations in some
 quantity, e. g. , loudness in the human voice.

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ANNUNCIATOR (AN)
 An audible intercept device that states the condition or
 restrictions associated with circuits or procedures.

ANSWER BACK (AB)
 An electrical and/or visual indication to the calling or
 sending end that the called or received station is on the
 line.

ANSWER SUPERVISION (AS)
 An electrical signal fed back up the line by the local
 telephone company at the distant end of a long distance call
 to indicate positively the call has been answered by the
 called party. Tells billing equipment to start timing the
 call.

AREA CODE (AC)
 A three digit number identifying more than 150 geographic
 areas of the United States and Canada which permits direct
 distance dialing on the telephone system. A similar global
 numbering plan has been established for international
 subscriber dialing.
 Synonym: Numbering Plan Area (NPA).

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ATTENDANT POSITION (AP)
 A telephone switchboard operator's position. It provides
 either automatic (cordless) or manual (plug and jack)
 operator controls for incoming and/or outgoing telephone
 calls.

ATTENUATION (AT)
 A general term used to denote the decrease in power between
 that transmitted and that received due to loss through
 equipment, lines, or other transmission devices. It is
 usually expressed as a ratio in dB (decibel).

AUDIBLE RINGING TONE (ART)
 An audible signal heard by the calling party during the
 ringing-interval.

AUTHORIZATION CODE (AC)
 A 5- to 14-digit number entered using a touch-tone phone to
 identify the caller as a customer of the long distance
 service. Used primarily before Equal Access as a way to
 verify the caller as a customer and bill calls.

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AUTO ANSWER (AA)
 A machine feature that allows a transmission control unit or
 station to automatically respond to a call that it receives.

AUTOMATIC CALL DISTRIBUTOR (ACD)
 A switching system designed to queue and/or distribute a
 large volume of incoming calls to a group of attendants to
 the next available "answering" position.

AUTOMATIC DIALING UNIT (ADU)
 A device which is programmed with frequently called numbers.
 The caller presses one to three digits and the preprogrammed
 number is automatically dialed into the phone circuit.

AUTOMATIC IDENTIFICATION OF OUTWARD DIALING (AIOD)
 The ability of some centrex units to provide an itemized
 breakdown of charges (including individual charges for toll
 calls) for calls made by each telephone extension.

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AUTOMATIC NUMBER IDENTIFICATION (ANI)
 On long distance calls, the process by which the local phone
 company passes a caller's local billing phone number to
 his/her long distance company when a "1+" or "10-XXX" call
 is made. With ANI a caller's long distance carrier knows
 who (what phone number) to bill without requiring the caller
 to enter any extra digits to be identified.

AUTOMATIC ROUTE SELECTION (ARS)

 Synonym: Least Cost Routing

- B -
BAND
 (1) The range of frequencies between two defined limits.
 (2) In reference to WATS, one of the five specific
 geographic areas as defined by the carrier.
 Synonym: Bandwidth.

BANDWIDTH
 see BAND.

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BASEBAND
 The total frequency band occupied by the aggregate of
 all the voice and data signals used to modulate a radio
 carrier.

BAUD
 A unit of signaling speed. The speed in Baud is the number
 of discrete conditions or signal elements per second. If
 each signal event represents only one bit condition, then
 Baud is the same as bits per second. Baud does not equal
 bits per second.

BLOCKED CALLS
 Attempted calls that are not connected because (1) all lines
 to the central offices are in use; or (2) all connecting
 paths through the PBX/switch are in use.

BREAK
 A means of interrupting transmission, a momentary
 interruption of a circuit.

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BREAKEVEN POINT (BP)
 Level of usage at which the total cost of a service with a
 high fixed up-front monthly fee but low minute costs becomes
 equal to the total cost of another service with low (or
 zero) monthly fee but relatively high per minute cost. At
 usage levels higher than breakeven, the service with the
 high monthly fee is cheaper.

BROADBAND
 A transmission facility having a bandwidth of greater than
 20 kHz.

BUS
 A heavy conductor, or group of conductors, to which several
 units of the same type of equipment may be connected.

BUSY (BY)
 The condition in which facilities over which a call is to be
 transmitted are already in use.

BUSY HOUR (BH)
 The time of day when phone lines are most in demand.

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BUSY TONE (BT)
 A single tone that is interrupted at 60 ipm (impulses per
 minute) to indicate that the terminal point of a call is
 already in use.

BYPASS (BP)
 The direct connection to customer-premises equipment by an
 IC. This occurs when an IC connects its own facilities (or
 facilities leased from a non-BOC entity) directly to an end
 user's premises, circumventing the use of the BOC network.

- C -
CARRIER
 A long distance company which uses primarily its own
 transmission facilities, as opposed to resellers which lease
 or buy most or all transmission facilities from carriers.
 Many people refer to any type of long distance company,
 whether it has its own network or not, as a carrier, so the
 term is not as restrictive as it used to be.

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CARRIER ACCESS CODE (CAC)
 The sequence an end user dials to obtain access to the
 switched services of a carrier. Carrier Access Codes for
 Feature Group D are composed of five digits, in the form
 10XXX, where XXX is the Carrier Identification Code.

CARRIER COMMON LINE CHARGE (CCLC)
 A per minute charge paid by long distance companies to local
 phone companies for the use of local public switched
 networks at either or both ends of a long distance call.
 This charge goes to pay part of the cost of telephone poles,
 wires, etc.

CARRIER IDENTIFICATION CODE (CIC)
 The three-digit number that uniquely identifies a carrier.
 The Carrier Identification Code is indicated by XXX in the
 Carrier Access Code. The same code applies to an individual
 carrier throughout the area served by the North American
 Numbering Plan.

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CARRIER SYSTEM (CS)
 A system for providing several communications channels over
 a single path.

CELLULAR MOBILE RADIO (CMR)
 A high capacity land mobile radio system in which an
 assigned frequency spectrum is divided into discrete
 channels that are assigned to a cellular geographic serving
 area.

CENTRAL OFFICE (CO)
 With local telephone companies, the nearby building
 containing the local telco switch which provides local
 telephone service. Also the physical point where calls
 enter the long distance network. Sometimes referred to as
 Class 5 office, end office, or Local Dial Office.

CENTREX, CO
 PBX Service provided by a switch located at the telephone
 company central office.

CENTREX, CU
 A variation on Centrex CO provided by a telephone company
 maintained "Central Office" type switch located at the
 customer's premises.

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CHANNEL
 A communications path via a carrier or microwave radio.

CIRCUIT
 A path for the transmission of electromagnetic signals to
 include all conditioning and signaling equipment.
 Synonym: Facility.

CIRCUIT SWITCHING
 A switching system that completes a dedicated transmission
 path from sender to receiver at the time of transmission.

CLASS OF SERVICE/CLASS MARK (COS)
 A subgrouping of telephone customers or users for the sake
 of rate distinction or limitation of service.

COAXIAL CABLE (COAX)
 A cable with a solid outer shield, a space and then a solid
 inner conductor. The electromagnetic wave travels between
 the outer shield and the conductor. It can carry a much
 higher band width than a wire pair.

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CODEC
 Coder-Decoder. Used to convert analog signals to digital
 form for transmission over a digital median and back again
 to the original analog form.

COMMON CARRIER (CC)
 A government regulated private company that provides the
 general public with telecommunications services and
 facilities.

COMMON CHANNEL INTEROFFICE SIGNALING (CCIS)
 A digital technology used by AT&T to enhance their
 Integrated Services Digital Network. It uses a separate
 data line to route interoffice signals to provide faster
 call set-up and more efficient use of trunks.

COMMON CONTROL SWITCHING ARRANGEMENT (CCSA)
 The use of carrier switches under a carrier's control as
 part of a customer's private network. The carrier's
 software controls and switches the customer's calls over
 private lines the customer has rented. Control of the
 switch and switching functions is done in common for all
 users using the software and switching system.

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CONDITIONING EQUIPMENT (CE)
 Equipment modifications or adjustments necessary to match
 transmission levels and impedances and which equalize
 transmission and delay to bring circuit losses, levels, and
 distortion within established standards.

CONFIGURATION
 The combination of long-distance services and/or equipment
 that make up a communications system.

CONTROL UNIT (CU)
 The central processor of a telephone switching device.

COST COMPONENT (CC)
 The price of each type of long distance service and/or
 equipment that constitutes a configuration.

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CROSS CONNECTION (CC)
 The wire connections running between terminals on the two
 sides of a distribution frame, or between binding posts in a
 terminal.

CROSS TALK (CT)
 The unwanted energy (speech or tone) transferred from one
 circuit to another circuit.

CUSTOMER ACCESS LINE CHARGE (CALC)
 The FCC-imposed monthly surcharge added to all local lines
 to recover a portion of the cost of telephone poles, wires,
 etc. from end users. Before deregulation, a large part of
 these costs were financed by long distance users in the form
 of higher charges.

CUSTOMER OWNED AND MAINTAINED (COAM)
 Customer provided communications apparatus and associated
 wiring.

CUSTOMER PREMISE EQUIPMENT (CPE)
 Telephone equipment, usually including wiring located within
 the customer's part of a building.

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CUT
 To transfer a service from one facility to another.

CUT THROUGH
 The establishment of a complete path for signaling and/or
 audio communications.

- D -
DATA SET (DS)
 A device which converts data into signals suitable for
 transmission over communications lines.

DATA TERMINAL (DT)
 A station in a system capable of sending and/or receiving
 data signals.

DECIBEL (DB)
 A unit measurement represented as a ratio of two voltages,
 currents or powers and is used to measure transmission loss
 or gain.

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DEDICATED ACCESS LINE (DAL)
 An analog special access line going from a caller's own
 equipment directly to a long distance company's switch or
 POP. Usually provided by a local telephone company. The
 line may go through the local telco Central Office, but the
 local telco does not switch calls on this line.

DELAY DIAL
 A dialing configuration whereby local dial equipment will
 wait until it receives the entire telephone number before
 seizing a circuit to transmit the call.

DELTA MODULATION (DM)
 A variant of pulse code modulation whereby a code
 representing the difference between the amplitude of a
 sample and the amplitude of the previous one is sent.
 Operates well in the presence of noise, but requires a wide
 frequency band.

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DEMODULATION (MOD)
 The process of retrieving data from a modulated signal.

DIAL LEVEL (DL)
 The selection of stations or services associated with a PBX
 using a one to four digit code (e.g., dialing 9 for access
 to outside dial tone).

DIAL PULSING (DP)
 The transmitting of telephone address signals by momentarily
 opening a DC circuit a number of times corresponding to the
 decimal digit which is dialed.

DIAL REPEATING TIE LINE/DIAL REPEATING TIE TRUNK (TT)
 A tie line arrangement which permits direct trunk to trunk
 connections without use of the attendant.

DIAL SELECTIVE SIGNALING (DSS)
 A multipoint network in which the called party is selected
 by a prearranged dialing code.

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DIAL TONE (DT)
 A tone indicating that automatic switching equipment is
 ready to receive dial signals.

DIALING PLAN (DP)
 A description of the dialing arrangements for customer use
 on a network.

DIRECT DISTANCE DIALING (DDD)
 A basic toll service that permits customers to dial their
 own long distance call without the aid of an operator.

DIRECT INWARD DIALING (DID)
 A PBX or CENTREX feature that allows a customer outside the
 system to directly dial a station within the system.

DIRECT OUTWARD DIALING (DOD)
 A PBX or CENTREX feature that allows a station user to gain
 direct access to an exchange network.

DROP
 The portion of outside telephone plant which extends from
 the telephone distribution cable to the subscriber's
 premises.

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DRY CIRCUIT (DC)
 A circuit which transmits voice signals and carries no
 direct current.

DUAL TONE MULTI-FREQUENCY (DTMF)
 Also known as Touch-Tone. A type of signaling which emits
 two distinct frequencies for each indicated digit.

DUPLEX
 Simultaneous two-way independent transmission.

DUPLEX SIGNALING (DS)
 A long-range bi-directional signaling method using paths
 derived from transmission cable pairs. It is based on a
 balanced and symmetrical circuit that is identical at both
 ends. This circuit presents an E&M lead interface to
 connecting circuits.

- E -
ECHO
 A signal that has been reflected or otherwise returned with
 sufficient magnitude and delay to be perceived by the
 speaker.

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ECHO RETURN LOSS (ERL)
 The loss which must be in the echo path to reduce echo to a
 tolerable amount.

ECHO SUPPRESSOR
 A device which detects speech signals transmitted in either
 direction on a four-wire circuit, and introduces loss in the
 direction of transmission.

EITHER END HOP OFF (EEHO)
 In private networks, a switch program that allows a call
 destined for an off-net location to be placed into the
 public network at either the closest switch to the
 origination or to the destination. The choice is usually by
 time of day. Uses either Head End Hop Off or Tail End Hop
 Off.

ELECTRONIC KEY TELEPHONE SETS (EKTS)
 A generic term indicating key telephones with built-in
 microprocessors which allow access to PBX-like features as
 well as access to multiple CO lines, using 2 to 4 pair
 wiring.

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ELECTRONIC SWITCH (ESS)
 Modern programmable switch (often denoted ESS, for
 Electronic Switching System) used in most BOC telephone
 companies, many independent telephone companies, and by
 virtually all new long distance companies. Completely solid
 state electronics, as opposed to older electro-mechanical
 switches.

ELECTRONIC SWITCHING SYSTEM (ESS)
 Used as a station instrument on a PBX. Also a Bell System
 term for electronic exchange switching equipment.

ELECTRONIC TANDEM NETWORK (ETN)
 (1) A private network automatically and electronically
 connecting the calling office to the called office through
 Tandem-Tie Trunks. The network switches also function as
 PBXs. (2) An AT&T product name. (3) Used as a generic term
 for a PBX base network.

ENHANCED PRIVATE SWITCHED COMMUNICATIONS SERVICE (EPSCS)
 A private network utilizing Bell provided equipment located
 in the central office and dedicated to a specific customer.

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E&M LEADS
 A pair of leads which carry signals between trunk equipment
 and separate signaling equipment unit. The M lead transmits
 battery or ground signals to the signaling equipment, and
 the E lead receives open or ground signals from the
 signaling equipment.

E&M SIGNALING
 An arrangement whereby signaling between a trunk circuit and
 an associated signaling unit is effected over two leads
 providing full-time, 2-way, 2-level supervision.

ENTERPRISE NUMBER
 A unique telephone exchange number that permits the called
 party to be automatically billed for incoming calls.

EQUAL ACCESS
 Reprogramming of Local Exchange Company (LEC) switches to
 allow other long distance companies besides AT&T to be the
 "1+" primary long distance company for users of long
 distance (by creating a new type of Feature Group access
 circuit, FGD). Also provides "10-XXX" dialing for secondary
 and casual calling, generates true hardware Answer
 Supervision when calls are terminated over FGD circuits, and
 provides ANI (Automatic Number Identification) on
 originating calls.

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EQUALIZATION
 The procedure of compensating for fluctuation in circuit
 amplitude, delay, or distortion.

ERLANG
 A unit of traffic intensity. One Erlang is the intensity at
 which one traffic path would be continuously occupied, e. g.
 one call per hour.

ERLANG B TABLE
 A widely used table derived from a mathematical formula
 which allows the determination of the traffic capacity of a
 given group of circuits.

EXCHANGE
 A telephone switching center.

EXCHANGE NETWORK FACILITIES FOR INTERSTATE ACCESS (ENFIA)
 AT&T's pricing arrangement for local loops offered to OCCs
 for connecting the OCC's network to the local telephone
 company's central office.

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EXTENDED AREA SERVICE (EAS)
 Adding expanded local calling areas to a caller's basic
 local calling area for a (generally) small additional
 monthly fee. The EAS local calls can be either free (after
 a small additional monthly fee is paid) or at a cost of
 reduced per call charges.

- F -
FACILITIES
 Typically refers to transmission lines or circuits, or long
 distance services. A caller's facilities are the circuits
 available to make calls.

FACSIMILE (FAX)
 The transmission of pictures, maps or other documents via
 communications circuits using a device which scans the
 original document, transforms the image into coded signals
 and reproduces the original document at a distant point.

FEATURE GROUP A (FGA)
 Line-side originating and terminating LATA access for which
 an originating subscriber dials an assigned telephone number
 that connects to a specific IC. The IC returns a tone to
 signal the caller to input additional tone-generated digits
 of the called number.

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FEATURE GROUP B (FGB)
 Trunk-side originating and terminating LATA access for which
 an originating subscriber dials a 950-WXXX number (where
 W=0,1 and XXX is the Carrier Access Code), which is
 translated to a specified XXX carrier trunk group. Optional
 rotary dial service and ANI may be available.

FEATURE GROUP C (FGC)
 Trunk-side LATA access for AT&T, generally, on a direct
 basis between each EO and an AT&T switching system.

FEATURE GROUP D (FGD)
 Also referred to as "Equal Access," Feature Group D is
 trunk-side LATA access affording call supervision to an IC,
 a uniform access code (10XXX), optional calling-party
 identification, recording of access-charge billing details,
 and presubscription to a customer-specified IC.

FEDERAL COMMUNICATIONS COMMISSION (FCC)
 The government agency established by the Communications Act
 of 1934 which regulates the interstate communications
 industry.

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FIBER OPTICS
 High speed transmission using light to send images (in
 telecommunications: voice or data) through a flexible
 bundle of glass fibers.

FOUR WIRE CIRCUITS
 Circuits which use two separate one-way transmission paths
 of two wires each, as opposed to regular local lines which
 usually only have two wires to carry conversations in both
 directions. One set of wires carries conversation in one
 direction, the other in the opposite direction.

FREQUENCY
 The number of complete cycles per unit of time.

FREQUENCY DIVISION MULTIPLEXING (FDM)
 The division of an available frequency range (bandwidth)
 into various subdivisions, each having enough bandwidth
 to carry one voice or data channel.

FREQUENCY RESPONSE (FR)
 The reaction of frequencies to the circuit components.

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FULL DUPLEX (FD)
 A circuit which allows transmission of a message in both
 directions at the same time.
 Synonym: 4-wire.

FULL PERIOD (FP)
 Relates to private line service, which is rented for the
 exclusive use of a single customer for an entire month.

FX (FOREIGN EXCHANGE) SERVICE (FX)
 A service which allows a customer to appear to have a local
 presence in a distant part of town or, a different town
 altogether, by connecting his/her phone directly to a local
 business line in a part of town with a different exchange
 than his/her local calling area over a leased private line,
 or to a local telco in a distant town through long haul
 private lines purchased from a long distance carrier.

- G -
GRADE OF SERVICE (GS)
 The probability of a call being blocked by busy trunks,
 expressed as a decimal fraction, and usually meaning the
 busy-hour probability.

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GROUP
 12 circuits processed as a unit in a carrier system.

- H -
HALF DUPLEX (HD)
 A circuit for transmitting or receiving signals in one
 direction at a time.

HARDWIRE
 To wire or cable directly between units of equipment.

HARMONIC
 The full multiple of a base frequency.

HARMONIC DISTORTION (HD)
 The ratio, expressed in decibels, of the power at the
 fundamental frequency, to the power of a harmonic of that
 fundamental.

HEAD END HOP OFF (HEHO)
 A method of traffic engineering whereby calls are completed
 by using long distance facilities directly off the switch
 that serves that location.

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HERTZ (HZ)
 International standard unit of frequency. Replaces, and is
 identical to, the order unit "Cycles-per-second. "

HOMING
 Returning to the starting position, as in a rotary stepping
 switch.

HOOKSWITCH
 The device on which the telephone receiver hangs or on which
 a telephone handset hangs or rests when not in use. The
 weight of the receiver or handset operates a switch which
 opens the telephone circuit, leaving only the bell connected
 to the line.

HOT-CUT
 Virtually instantaneous replacement of one line with
 another.

HYBRID
 An electronic circuit which performs the wire conversions
 necessary for the connection of a local loop with a long-
 haul facility.


- I -
INTERCEPT
 To stop a telephone call directed to an improper telephone
 number, and redirect that call to an operator or a
 recording.

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INTERCONNECT (IC)
 (1) The arrangement that permits the connection of
 customer's telecommunications equipment to a communications
 common carrier network. (2) The industry name for
 manufacturers, excluding the Bell system, which provide
 telephone equipment for the customer premises.

INTER-EXCHANGE MILEAGE (IXC)
 The airline mileage between two cities.
 Synonym: Long Haul Mileage.

INTEREXCHANGE PLANT (IP)
 The facilities between the subscriber switching center and
 another switching center.

INTERFACE
 The junction or point of interconnection between two systems
 or equipment having different characteristics.

INTERFERENCE
 Any unwanted noise or crosstalk on a communications circuit
 which acts to reduce the intelligibility of the desired
 signal or speech.

INTER-MACHINE TRUNK (IMT)
 A circuit which connects two automatic switching centers.

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INTER-OFFICE TRUNK (IOT)
 A direct trunk between local exchange offices.

INTERNATIONAL RECORD CARRIER (IRC)
 Carriers providing international telecommunications
 services, including voice, telex, and data communications.

INTERSTATE
 Any connection made between two states.

INTRASTATE
 Any connection made that remains within the boundaries of a
 single state.

- J -
JITTER
 Short term instability of the amplitude and/or phase of a
 signal. Commonly called PHASE JITTER.

- K -
KEYSET
 A telephone instrument having an appearance of two or more
 telephone lines which can be accessed by depressing a button
 (key) on the face of the set.

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KEY SYSTEM
 The equipment utilized to provide the features associated
 with key sets, including keysets, multipair cable, key
 service unit, distribution frames.

- L -
LEASED LINES (LL)
 Any circuit or combination of circuits designated to be at
 the exclusive disposal of a given subscriber.
 Synonym: Private line; Full Period Line.

LEAST COST ROUTING (LCR)
 A method of automatically selecting the least costly
 facility for transmission of a call.
 Synonym: Most Economical Route Selection (MERS); Automatic
 Route Selection; Flexible Route Selection.

LEVEL
 An expression of the relative signal strength at a point in
 a communications circuit compared to a standard.

LOADING
 A system for adding regularly spaced inductance units to a
 circuit to improve its transmission characteristics.

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LOCAL ACCESS AND TRANSPORT AREA (LATA)
 A geographic area (called "exchange" or "exchange area" in
 the MFJ) within each BOC's franchised area that has been
 established by a BOC in accordance with the provisions of
 the MFJ for the purpose of defining the territory within
 which a BOC may offer its telecommunications services.

LOCAL AREA NETWORK (LAN)
 Intraoffice communication system usually used to provide
 data transmission in addition to voice transmission.

LOCAL EXCHANGE CARRIER (LEC)
 A local telephone company, either one of the Bell Operating
 Companies or one of the 1400+ independent local telephone
 companies.

LOCAL LOOP
 The local connection between the end user and the Class 5
 central office.

LONG HAUL
 Circuits spanning considerable distances.

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LOOP BACK
 A method of performing transmission tests on a circuit not
 requiring the assistance of personnel at the distant end.

LOOP SIGNALING
 Any of the three signaling methods which use the metallic
 loop formed by the trunk conductors and the terminating
 equipment bridges.

- M -
MAIN DISTRIBUTION FRAME (MDF)
 The point where outside plant cables terminate and from
 which they cross connect to terminal or central office line
 equipment.

MAIN PBX (PBX)
 A PBX directly connected to a tandem switch via an access
 trunk group.

MANUAL TIE LINE (TIE LINE)
 A tie line which requires the assistance of an attendant at
 both ends of the circuit in order to complete a call.

MASTER GROUP (MG)
 240 circuits processed as a unit in a carrier system.

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MESSAGE TELEPHONE SERVICE (MTS)
 AT&T's tariffed pricing name for long distance telephone
 calls.

MESSAGE UNIT (MU)
 A local toll rate calling plan which is time and distance
 sensitive.

MICROWAVE (M/W)
 Radio transmission using very short lengths, corresponding
 to a frequency of 1,000 megahertz or greater.
 Synonym: Microwave Radio.

MICROWAVE RADIO
 Synonym: Microwave.

MODEM
 A device which modulates and demodulates signals on a
 carrier frequency and allows the interface of digital
 terminals with analog carrier systems.

MODIFIED FINAL JUDGEMENT (MFJ)
 The agreement between the U.S. Department of Justice and
 AT&T governing the breakup of the pre-Divestiture Bell
 System into AT&T and 22 Bell Operating Companies and other
 entities. On August 26, 1982, U. S. District Court Judge
 Harold Greene accepted, with modifications, an AT&T/Justice
 Department settlement terminating the government's 1974
 antitrust suit against AT&T. Judge Greene's decree did away
 with the provisions of the 1956 consent decree that had kept
 AT&T out of competitive, unregulated ventures.

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MODULATION
 Alterations in the characteristics of carrier waves. Usually
 impressed on the amplitude and/or the frequency.

MONITORING DEVICE
 Records data on calls placed through a company's telephone
 system: number called, length of calls, calling location.

MOST ECONOMICAL ROUTE SELECTION (MERS)
 Synonym: Least Cost Routing.

MULTIPLEXING (MP)
 The act of combining a number of individual message circuits
 for transmission over a common path. Two methods are used:
 (1) frequency division, and (2) time division.

- N -
NETWORK
 A collection of switches connected to one another by
 transmission facilities.

NETWORK NUMBERING EXCHANGE (NXX)
 The three digit location code representing the central
 office. "N" may be any number between "2" and "9" and "X"
 may be any number.

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NETWORK TRUNKS
 Circuits connecting switching centers.

NNX CODES
 The 3-digit code used historically for local Exchange Codes.
 "N" can be any number from 0 to 2, "X" can be any digit.
 The current numbering plan allows for more variation in
 assigning Exchange Codes, and under it Exchange Codes are
 commonly referred to as "NXXs."

NODE
 A major switching center of a network.

NON-BLOCKING
 A switching network having a sufficient number of paths such
 that a subscriber originating a call can always reach any
 other idle subscriber without encountering a busy.

NUMBERING PLAN AREA (NPA) (NANP) (AREA)
 A geographical division within which no two telephones will
 have the same 7 digit number. "N" is any number between "2"
 and "9"; "P" is always "1" or "0"; and "A" is any number
 excluding "0". Commonly referred to as "area code."

NXX CODES (NNX)
 The current general configuration for Exchange Codes within
 each Area Code. See also: "NNX Codes"

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- O -
OFFERED TRAFFIC
 The number of call attempts in any specified period of time.

OFF HOOK
 The condition which results when a telephone is lifted from
 its mounting, allowing the hookswitch to operate.

OFF-NETWORK ACCESS LINE (ONAL)
 A local exchange (Feature Group access), Foreign Exchange,
 or WATS line connecting both incoming and outgoing traffic
 from a long distance company's network to the public
 switched network. Generally a circuit leased by a long
 distance carrier to be used by many customers not hooked
 directly into the long distance carrier's network.

OFF NETWORK CALLING
 Telephone calls through a private switching system and
 transmission network which extend to the public telephone
 system.

OFF PREMISES EXTENSION (OPX)
 An extension telephone or keyset that is geographically
 separated from its associated PBX.

ON HOOK
 The condition which results when a telephone handset is
 placed on its mounting, which causes the hook-switch to open
 its contacts.

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ON NETWORK CALLING
 A term used to describe a call that originates and
 terminates on a private network.

OPERATOR ASSISTED CALLS (OAC) (AOS)
 Non-DDD calls requiring manual intervention.

ORIGINATING OFFICE
 The central office that serves the calling party.

OTHER COMMON CARRIER (OCC) (AOS)
 A long distance company other than AT&T having many of its
 own long distance circuits, either owned or leased. Some
 people use OCC to refer to all AT&T long distance
 competitors, including resellers, but this is not
 technically correct.

OUT-OF-BAND
 Any frequency outside the band used for voice frequencies.

OUT-OF-BAND SIGNALING
 Use of narrow band filters to place the voice signal on a
 carrier channel below 3,400 CPS, reserving the 3,400 - 3,700
 CPS band for supervisory signals.

OVERBUILD
 Adding radio capacity to a telecommunications network.

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OVERFLOW
 Switching equipment which operates when the traffic load
 exceeds the capacity of the regular equipment.

- P -
PAD
 A non-adjustable resistance network used to insert
 transmission loss into a circuit.

PHASE JITTER
 SEE Jitter

POINT OF PRESENCE (POP)
 A physical location within a LATA at which an IC establishes
 itself for the purpose of obtaining LATA access and to which
 the BOC provides access services.

POINT-TO-POINT
 A communications circuit between two terminations which does
 not connect with a public telephone system.

PORT
 Entrance or access point to a computer, multiplexor device
 or network where signals may be supplied, extracted or
 observed.

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POSTAL TELEPHONE AND TELEPGRAPH (PTT)
 Foreign government agencies responsible for regulating
 communications.

PRIMARY AREA
 A customer's local telephone calling area.

PRIMARY INTEREXCHANGE CARRIER (PIC)
 The IC designated by a customer to provide inter-LATA
 service automatically without requiring the customer to dial
 an access code for that carrier.

PRIMARY ROUTING POINT (PRP)
 The switch designated as the control point for a longhaul
 telephone call.

PRIVATE BRANCH EXCHANGE (PBX)
 A private phone system (switch) used by medium and large
 companies which is connected to the public telephone network
 (local telco) and performs a variety of in-house routing and
 switching. User usually dial "9" to get outside system to
 the local lines.

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PRIVATE LINE (PL)
 A full-time leased line directly connecting two points, used
 solely by purchaser. The most common form is a tie line
 connecting two pieces of a user's own phone equipment - flat
 rate billing, not usage sensitive.

PRIVATE USE NETWORK
 Two or more private line channels contracted for by a
 customer and restricted for use by that customer only.

PUBLIC SWITCHED NETWORK (PSN)
 The pre-Divestiture nationwide network maintained by AT&T; and the
 independent telephone companies which provides
 nationwide, unrestricted telephone service.

PUBLIC UTILITY COMMISSION (PUC)/PUBLIC SERVICE COMMISSION (PSC)
 The state commissions regulating intrastate communications.

PULSE CODE MODULATION (PCM)
 The form of modulation in which the information signals are
 sampled at regular intervals and a series of pulses in coded
 form are transmitted representing the amplitude of the
 information signal at that time.

PULSE-LINK REPEATER
 Connects one E&M signaling circuit directly to another.

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PULSE MODULATION (PAM) (PWM) (PPM) (PCM)
 The modulation of a series of pulses which
 represents information-bearing signals. Typical methods
 involve modifying the amplitude (PAM), width or duration
 (PWM) or position (PPM). Pulse Code Modulation (PCM) is the
 most common modulation technique involved in telephone work.

PUSH BUTTON DIALING
 Synonym: Dual Tone Multi-Frequency.

- Q -
QUEUE
 A temporary delay in providing service caused by the
 inability of the system provided to handle the number of
 messages or calls attempted.

- R -
RADIO COMMON CARRIER (RCC)
 A communications common carrier that provides radio paging
 and mobile telephone services to the public.

RATE CENTER (RC)
 A specified geographic location used by the telephone
 company to determine interchange mileage for rate
 determination purposes.

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REDUNDANCY
 Duplicate equipment that is provided to minimize the effect
 of failures or equipment breakdowns.

REGENERATION
 The process of receiving distorted signal pulses and from
 them recreating new pulses at the correct repetition rate,
 pulse amplitude, and pulse width.

RE-HOMING
 A major network change which involves moving customer
 services from one switching center to another and
 establishing the necessary trunking facilities to do so.

REMOTE ACCESS (RA)
 The ability of transmission points to gain access to a
 computer which is at a different location.

REPEATER
 An electronic device used to amplify signals which have
 become too weak.

REPEATING COIL (RC)
 The telephone industry's term for a voice-frequency
 transformer.

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RESELLER (AOS) (OCC)
 A long distance company that purchases large amounts of
 transmission capacity or calls from other carriers and
 resells it to smaller users.

RESTORATION
 The re-establishment of service by rerouting, substitution
 of component parts, or as otherwise determined.

RETARD COIL
 A coil having a large inductance which retards sudden
 changes of the current flowing through its winding.

RINGBACK TONE (RT}
 Synonym: Audible Ringing Tone.

RINGDOWN
 A circuit or method of signaling where the incoming signal
 is actuated by alternating current over the circuit.

ROUTE DIVERSITY
 Two (or more) private line channels (circuits) furnished
 partially or entirely over two physically separate routes.
 Serves to prevent total loss of service if one cable gets
 cut or goes out.

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ROUTE OPTIMIZATION
 Synonym: Least Cost Routing.

ROTARY HUNT
 An arrangement which allows calls placed to seek out an idle
 circuit in a prearranged multi-circuit group and find the
 next open line to establish a through circuit.

- S -
SATELLITE RELAY
 An active or passive repeater in geosynchronous orbit around
 the Earth which amplifies the signal it receives before
 transmitting it back to earth.

SELECTIVE CALLING
 The ability of a transmitting station to specify by the use
 of assigned codes which of several stations is to receive a
 message.

SERVICE AND EQUIPMENT RECORD (SAER)
 A list of equipment billed to customer by type, quantity,
 monthly charge, location and billing dates.

SF SIGNALING (SINGLE-FREQUENCY) (SFS)
 A signaling system which uses a 2,600 Hz in-band signal on
 the voice path. The tone is on in the idle condition,
 pulsed for dialing, and off when the circuit is in use.

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SHORT HAUL
 Circuits designed for use over distances of 10-200 miles.

SIGNALING
 The process of transferring information between two parts of
 a telephone network to control the establishment of
 communications between long distance carrier terminal
 points, and customer equipment required for voice grade
 dedicated circuits.

SIGNALING CONVERTER (SC)
 A device with input and output signals that contain the same
 information but employ different electrical systems for
 transmitting that information. Used at the terminal of a
 trunk to convert the equipment signals to the system used on
 the trunk. Examples are: (1) ring down to SF, (2) E&M to
 SF.

SIGNALING, IN-BAND
 A type of signaling using an AC signal (usually 2,600 Hz)
 within the normal voice band. This signal can be
 transmitted from end to end of a long voice circuit without
 an intermediate signaling equipment. Since the signaling is
 audible, the signaling equipment must be arranged for "tone
 on when idle" operation.

SIGNAL TO NOISE RATIO (SNR)
 Ratio of the signal power to the noise power in a specified
 bandwidth, usually expressed in db.

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SIMPLEX (SX) SIGNALING
 A signaling path over a dry talking
 circuit which uses the two sides of the circuit in parallel,
 derived by connecting the midpoints of repeating coils or
 retardation coils which are across the circuit.

SINGLE SIDEBAND RADIO (SSB)
 A form of amplitude modulation of a radio signal in which
 only one of the two sidebands is transmitted. Either of the
 two sidebands may be transmitted, and the carrier may be
 transmitted, reduced or suppressed.

SINGING
 A continued whistle or howl in an amplified telephone
 circuit. It occurs when the sum of the repeater gains
 exceeds the sum of the circuit losses.

SOFTWARE DEFINED NETWORK (SDN)
 A switched long distance service for very large users with
 multiple locations. Instead of putting together their own
 network, large users can get special usage rates for calls
 made on regular long distance company switched long distance
 services.
 Synonym: Virtual Private Network.

SPECIAL GRADE NETWORK TRUNK (SGNT)
 A trunk specially conditioned by providing amplitude and
 delay equalization for the purpose of handling special
 services such as medium-speed data (600 to 2400 BPS).

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SPECIALIZED COMMON CARRIER (SCC)
 Synonym: Other Common Carrier.

SPEED NUMBER
 A one, three, or four digit number that replaces a seven or
 ten digit telephone number. These numbers are programmed
 into the switch in the carrier's office or in a PBX.

STATION
 Any customer location on a network capable of sending or
 receiving messages or calls.

STATION MESSAGE DETAIL RECORDING (SMDR)
 A computer generated report showing internal usage on a
 telephone system. Usually including extension number, trunk
 number used, phone number dialed, time of call, duration and
 operator involvement.

STORE-AND-FORWARD
 A technique in which a message is received from the
 originator and held in storage until a circuit to the
 addressee becomes available.

STORED PROGRAM CONTROL (SPC)
 A system whereby the instructions are placed in the memory
 of a common-controlled switching unit and to which it refers
 while processing a call for instructions regarding class
 marks, code conversions, routing, as well as for trouble
 analysis.

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SUPERGROUP (SG)
 60 circuits processed as a unit in a carrier system.

SUPERMASTERGROUP (SMG)
 600 circuits processed as a unit in a carrier system.

SUPERVISION (AS)
 Synonym: Answer Supervision.

SUPERVISORY SIGNALS (SS) (AS)
 A signal, such as "on-hook" or "off-hook," which indicates
 whether a circuit or line is in use.

SWITCH
 Equipment used to interconnect lines and trunks.

SWITCHED ACCESS (SA)
 Connection between caller's phone system and switch of
 chosen long distance carrier when a regular long distance
 call using regular local lines is made. Also the connection
 between the switch of caller's long distance carrier in the
 distant city and the phone being called.

SWITCH HOOK
 Synonym: Hookswitch.

SWITCHING
 The operations involved in interconnecting circuits in order
 to establish communications.

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SWITCHING CENTER (SC) (CO)
 A location at which telephone traffic, either local or toll,
 is switched or connected from one circuit or line to
 another.

SWITCHING OFFICE (SO) (CO)
 A telephone company office which contains a switch.

- T -
T-1(T-1) (T1)
 24 voice channels digitized at 64,000 bps, combined into a
 single 1. 544 Mbps digital stream (8,000 bps signaling), and
 carried over two pairs of regular copper telephone wires.
 Used primarily by telephone companies until 1983. Now used
 for dedicated local access to long distance facilities,
 long-haul private lines, and for regular local service.
 Today, most any 1.544 Mbps digital stream is called T-1,
 regardless of its makeup or what the transmission medium is.

T-CARRIER (T-1)
 A time-division, pulse-code modulation, voice carrier used
 on exchange cable to provide short-haul trunks.

TAIL END HOP OFF (TEHO)
 In a private network, a call which is carried over flat rate
 facilities (Intermachine Trunks or IMT) to the closest
 switch node to the destination of the call, and then
 connected into the public network as a local call.

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TANDEM
 A switching arrangement in which the trunk from the calling
 office is connected to a trunk to the called office through
 an intermediate point.

TANDEM SWITCHING SYSTEM (TTTN) (TSS)
 Synonym: Tandem Tie Trunk Network.

TANDEM TIE TRUNK NETWORK (TTTN)
 A serving arrangement which permits sequential connection of
 tie trunks between PBX/CENTREX locations by utilizing tandem
 operation.

TANDEM TRUNKING
 Trunks which connect two or more switches together.

TARIFF
 The published rates, regulations, and descriptions governing
 the provisions of communications service.

TELCO (BOC)
 Local telephone company.

TELECOMMUNICATIONS
 The transmission of voice and/or data through a medium by
 means of electrical impulses and includes all aspects of
 transmitting information.

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TELEGRAPH (TELEX) (TWX)
 A system employing the interruption of, or change in, the
 polarity of DC current signaling to convey coded
 information.

TELEPHONE
 A device which converts acoustical (sound) energy into
 electrical energy for transmission to a distant point.

TELETYPEWRITER (TTY) (TWX) (TELEX)
 A machine used to transmit and/or receive communications on
 printed page and/or tape.

TERMINAL
 A point at which information can enter or leave a
 communications network.

TERMINAL EQUIPMENT (TE)
 Devices, apparatus and their associated interfaces used to
 forward information to a local customer or distant terminal.

TERMINATION
 (1) An item that is connected to the terminal of a circuit
 or equipment. (2) An impedance connected to the end of a
 circuit being tested. (3) The points on a switching network
 to which a trunk or line may be attached.

TIE-LINE
 A private leased line linking two phones or phone systems
 directly. Can ring distant phone automatically when
 telephone is lifted from its mounting, or when a short code
 is dialed.

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TIME DIVISION MULTIPLEXING (TDM)
 Equipment which enables the transmitting of a number of
 signals over a single common path by transmitting them
 sequentially at different instants of time.

TOLL CALL
 Any call to a point outside the local service area.

TOLL CENTER (TC) (CO)
 A central office where operators (human or mechanical) are
 present to assist in completing incoming toll calls.

TOLL OFFICE (TO) (TC)
 A center for the switching of toll calls.

TOLL PLANT (TP) (TO)
 The facilities that connect toll offices throughout the
 country.

TOLL RESTRICTION (TR)
 A restriction in outgoing trunks which counts the first
 three digits dialed and diverts calls to forbidden codes
 either to a busy tone, to the operator, or to a recorded
 announcement.

TOUCH-TONE ADAPTER (TT)
 A device that can be connected to a rotary dial telephone to
 allow for DTMF signaling.

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TRAFFIC
 Calls being sent and received over a communications network.

TRAFFIC MEASUREMENT AND RECORDING SYSTEMS (TMRS)
 A computer generated report showing usage information of
 telephone systems. Usually this includes trunk utilization,
 outages, queuing time, and the need for additional common
 equipment.

TRAFFIC SERVICE POSITION SYSTEM (TSPS)
 A toll switchboard position configured as a push button
 console.

TRANSMISSION (XMISSION) (XMIT)
 The electrical transfer of a signal, message or other form
 of data from one location to another without unacceptable
 loss of information content due to attenuation, distortion,
 or noise.

TRANSMISSION LEVEL (TL)
 The level of power of a signal, normally 1,000 Hz, which
 should be measured at a particular reference point.

TRANSMISSION SPEED (TS) (WPM) (BAUD) (BPS)
 Number of pulses or bits transmitted in a given period of
 time, usually expressed as Bits Per Second (BPS) or Words
 Per Minute (WPM).

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TRUNK
 A telephone circuit or path between two switches, at least
 one of which is usually a telephone company Central Office
 or switching center. Regular local CO circuits are called
 PBX trunks, because there is a switch at both ends of the
 circuit.

TRUNK GROUP (TG)
 An arrangement of communications channels into an identical
 group.

TRUNK TYPE (TT)
 Trunks that use the same type of equipment going to the same
 terminating location.

TRUNK UTILIZATION REPORT (TUR)
 A computer printout detailing the traffic use of a trunk.

TWO-WIRE CIRCUIT
 (1) A channel for transmitting data in one direction at a
 time. (2) A short distance channel using a single
 send/receive pathway, usually 2 copper wires, connecting a
 telephone to a switch.

TELETYPEWRITER EXCHANGE SERVICE (TWX) (TELEX) (TTY)
 A service whereby a customer's leased teletypewriter is
 connected to a "TWX" switchboard and from there connected
 over regular toll circuits to a teletypewriter of any U.S.
 customer who subscribes to a similar service.

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- U -
UNIFORM CALL DISTRIBUTOR (UCD) (ACD)
 A device located at the telephone office or in a PABX which
 distributes incoming calls evenly among individuals.

UNIFORM SERVICE ORDER CODE (USOC)
 The information in coded form for billing purposes by the
 local telephone company pertaining to information on service
 orders and service equipment records.

- V -
VALUE-ADDED NETWORK SERVICE (VANS)
 A data transmission network which routes messages according
 to available paths, assures that the message will be
 received as it was sent, provides for user security, high
 speed transmission and conferencing among terminals.

VIA NET LOSS (VNL)
 The lowest loss in dB at which a trunk facility can be
 operated considering limitations of echo, crosstalk, noise
 and singing.

VOICE CONNECTING ARRANGEMENT (VCA)
 An interface arrangement provided by the telephone company
 to accommodate the connections of non-carrier provided voice
 terminal equipment to the public switched telephone network.

VOICE FREQUENCY (VF)
 Any of the frequencies in the band 300-3,400 Hz which must
 be transmitted to reproduce the voice with reasonable
 fidelity.

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VOICE GRADE (VG)
 An access line suitable for voice, low-speed data,
 facsimile, or telegraph service. Generally, it has a
 frequency range of about 300-3000 Hz.

VOICE GRADE FACILITY (VGF)
 A circuit designed to DDD network standards which is
 suitable for voice, low-speed data, facsimile, or telegraph
 service.

- W -
WIDE AREA TELECOMMUNICATIONS SERVICE (WATS)
 WATS permits customers to make (OUTWATS) or receive
 (INWATS) long-distance calls and to have them billed on
 a bulk rather than individual call basis. The service
 is provided within selected service areas, or bands,
 by means of special private access lines connected to
 the pubic telephone network via WATS-equipped central
 offices. A single access line permits inward or outward
 service, but not both.

WIDEBAND (WB)
 A term applied to facilities or circuits where bandwidths
 are greater than that required for one voice channel.

WIRE CENTER (WC)
 The physical structure that houses one or more central
 office switching systems.

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Glossary of post-divestiture telecommunications terms

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