A & B LEADS - Designation of leads derived from the midpoints of the two pairs comprising a 4-wire circuit.
ABBREVIATED DIALING - 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. Know as Terminal Speed Calling in some LEC tariffs.
ABSOLUTE DELAY - The actual time taken for a signal to transit a telecommunications circuit from end to end; affected by the actual circuit length and the "propagation constants" of the type of medium in use.
ACCEPTANCE TEST - Test operation of a new or modified device or system before usage by customers; ascertaining performance is to specifications. FCC equivalent - "Proof of Performance Testing."
ACCESS CHARGE - Cost assessed to users for connection ability to the interexchange, interstate message toll telephone networks of IEC's by the user's LEC, to send and receive calls beyond the immediate local exchange area. May 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 CODE - In PBXs, a digit or digits dialed prior to dialing an outside call (most typically "9", or a digit entered on a feature phone set to activate functions like Call Forwarding or Conference Add-on (most typically "*"or "#."
ACCESS LINE - A telephone circuit connecting a customer location to a public network switching center.
ACCESS METHOD - 1.) In data processing, the software environment encountered by data communications entering and leaving the machine, as in IBM's VTAM or TCAM; 2.) In LANs, the technique used to arbitrate use of the physical transmission medium, as in CSMA/CD or Token Passing.
ACCOUNT CODE - In PBXs, a code digit or digits a user must enter before or after a call is dialed, to establish accounting for the charges for the call. (Compare to "Authorization Code" as used in public networks.)
ACCOUNTING RATE - The charge per unit of traffic or usage, which can be a unit of time or transmission capacity or actual information volume between territories controlled by different telecommunications authorities; used to establish international tariffs.
ACOUSTIC COUPLER - Means of connecting external devices to a telephone handset avoiding direct electrical connection; most commonly used for low-speed data terminals.
ACF - Advanced Communications Function in IBM data networks; a series of software products providing sophisticated data networking functions.
ACK - The "acknowledge" character in many data codes; used most commonly for an affirmative response of correct receipt. (Compare to NACK).
ADAPTIVE EQUALIZATION - Equalization of received digital signals capable of adjustment during actual transmission.
ADAPTIVE ROUTING - In data networks, routing algorithms capable of adjusting message routes in response to changes in traffic patterns or transmission channel failures.
ADCCP - Advanced Data Communications Control Procedure, a bit-oriented data link protocol developed as an ANSI standard, adopted in whole or in part by various computer makers.
ADDRESS - 1.) In software, a location that can be specifically located in a program; 2.) In communications networks, the distinct identifier of a place, service or function to be reached.
ADPCM - Adaptive Differential Pulse Code Modulation; A digital encoding technique that transmits only the changed information between successive samples of the signal; reduces the number of bits per sample from 8 to 4, permitting doubling of transmission capacity of facilities; used primarily for international facilities; common carriers recommend data rates not exceeding 4800 bps on ADPCM facilities.
AGGREGATOR - A wholesale buyer of (typically) dial network capacity who acts as the billing and collection agent for a common carrier, without needing a switch or switches for access or billing of its clients; a relatively new approach as of late 1989.
AIOD - See "Automatic Identification of Outward Dialing."
AIRLINE MILEAGE - Calculated point-to-point mileage between two connected locations in a network.
ALARM DISPLAY - An indicator or set of indicators of the operational status of a system or device; in telecommunications, usually grouped into "Major Alarms" indicating an entire system down or affected, and "Minor Alarms" indicating an impairment to partial operation.
ALGORITHM - a prescribed set of steps for accomplishment of a function or task; equivalent to a mathematical solution.
ALL TRUNKS BUSY SIGNAL ( ATB ) - In North America, a single tone interrupted at 120 impulses per minute rate to indicate all lines or trunks in a routing group are busy, and no alternate route has been found. Internationally, any of several signals are recognized by the CCITT, most being variations on a steady tone.
ALTERNATE ROUTE - A secondary communications path used to reach a destination if the primary path is unavailable.
ALTERNATE USE - The ability to use communications transmission facilities for multiple applications - voice or data being the most common example.
ALTERNATE VOICE DATA ( AVD ) - A single transmission facility usable for either voice or data. Most commonly applied to transoceanic private line circuits.
AMBIENT NOISE - Constant noise present in all forms of telecommunications paths; a given that communications networks must perform with.
AMERICAN NATIONAL STANDARDS INSTITUTE ( ANSI ) - Official repository of standards for the United States of America. Compare: British Standards Institute (BSI) for the United Kingdom. Example: ASCII data communications code is ANSI C.64 and CCITT International Telegraph Alphabet Number 5, with only the smallest variation in a few definitions.
AMERICAN WIRE GAUGE ( AWG ) - Descriptive of the diameter of wire conductors, particularly in twisted pair cabling; has effect on the transmission capacity and distance a given wire can offer. Internationally, wire size is described in millimetric diameter, except in England, where the weight in pounds per mile is used.
AMPLIFIER ( REPEATER ) - Electronic device to boost the strength of a signal, usually analog when an "amplifier."
AMPLITUDE - Magnitude or size; voltage or power of an electronic signal.
AMPLITUDE MODULATION (AM) - Modifying a "carrier" signal by varying its instantaneous power to represent the information it carries; most commonly called "AM."
ANALOG LOOPBACK ( ALB ) - Connecting a received analog signal to the return transmitting path; a common test method for locating transmission problems in data transmission systems.
ANALOG SIGNAL - A signal in the form of a continuous varying in step with the actual transmitted information; attempts to transmit an exact replica of the inputted signal down a communications channel.
ANALOG TRANSMISSION - Communications by transmission of continuously varying representations of the input signal, as compared to coded words in digital transmission.
ANGLE MODULATION - Refers to the change of angle of a signal that occurs in Frequency Modulation or Phase Modulation communications systems; usually analog.
ANNUNCIATOR - (1) An audible intercept device that states the condition or restrictions associated with circuits or procedures. See also "Recorded Announcement Trunk." (2) - A buzzer, lamp, or other device to signal a line requiring operator attention on a switchboard.
ANSWER BACK - An electrical and/or visual indication to the call originating end that the call terminating end is on the line. First associated with the International Telex network, answerbacks are also recommended in CCITT-standard fax machines and are provided in most PC data communications software packages.
ANSWER SUPERVISION - An electrical signal fed in the reverse direction of a telephone circuit indicating the called location has responded to the connection request; starts billing of a call.
APPLICATION LAYER - The topmost, visible to the user, presentation of a communications network; the user interface point in network architectures.
ARC - "Attached Resource Computer," the root name of the LAN developed by Datapoint Corporation called "Arcnet" (which see elsewhere).
ARCNET - A LAN architecture developed by Datapoint Corporation featuring low cost for connection of groups of (Async) terminals to a (mini) computer within a premises.
AREA CODE - Colloquial term for a three digit number identifying one of more than 150 geographic areas of North America providing for Direct Distance Dialing on the public telephone network system. This entire system of numbers is under the umbrella of Country Code 01 in the World Numbering Plan of the CCITT. - Synonym: Numbering Plan Area (NPA).
ARQ - Telegraphic code signal for "Automatic Repeat Request," a time-honored method of telegraphic error correction upon which most data transmission error correction is based. ARQ receivers check for errors and initiate an order to retransmit data blocks determined to be corrupted in transmission.
ASCII - AMERICAN (NATIONAL) STANDARD CODE FOR INFORMATION INTERCHANGE; The most common code used for asynchronous data transmission by minicomputers and personal computers; derived from the TWX code of the Bell Model 28 teleprinter, expanded to use all possible character combinations; consists of 7 information bits with an 8th parity bit for error checking; numerous variations exist, for example use of the 8th bit in personal computers to extend the code with a number of graphics, special language characters and diacritical marks. Many common carrier data services cannot transmit the 8th bit needed to use that common PC extension of ASCII's alphabet.
ASR - Telegraphic name for an "Automatic Send/Receive" terminal station, typically one that has storage for outbound messages and holds them until called upon by the communications network to send. Compare to "KSR."
ASYNCHRONOUS - Occurring without central control or in an unpredictable time interval between successive elements; the typical mode of telegraphy, minicomputers and personal computers; requires s transmission of "start" and "stop" bits to provide decoding synchronization at the receiver.
ATTENDANT POSITION - 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. Archaic: Turret.
ATTENUATION - Term denoting a decrease in power between that transmitted and that received due to loss through equipment, lines, or other transmission devices. Usually expressed as a ratio in dB (decibels). - Synonym: Loss; Antonym: Gain
AUDIBLE RINGING TONE - An audible signal heard by the calling party while a called station is being signaled of the call attempt. In North America, a unique signal of two tones on for two seconds and off for four seconds. Worldwide, variations abound in both tone character and interruption rate. - Synonym: Audible Ringback; Colloquialism: "Pacifier Ring."
AUTHORIZATION CODE - A 5 to 14-digit number entered on a dual-tone keypad that identifies the caller as a customer of the long distance service. Used primarily prior to Equal Access as a way to verify the caller and bill calls; continues today as a means to enter Calling Card numbers manually or from card readers on payphones. (Compare to "Accounting Code" in PBXs.)
AUTOMATIC ALTERNATE ROUTING (AAR) - Provision of function in a network to obtain a connection via secondary routes between two locations without need for user intervention.
AUTOMATIC ANSWER (AUTOANSWER; AUTO ANSWER) - A function providing for a transmission control unit or station to automatically detect being called and respond to an inbound call. Most commonly associated with telephone answering sets, voice mail, automated attendants and data sets (modems) for computer terminals.
AUTOMATIC BAUD RATE DETECTION (AUTOBAUD) - A function of data TDM's (See "Time Division Multiplex") to sense and self-adjust to the data rate of connections made to one of the TDM's ports. Typically used when connecting a dial-up modem to a port of a remote TDM to provide data dial-in service at a distant location.
AUTOMATIC CALL DISTRIBUTOR (ACD) - Often abbreviated "ACD;" provided as a feature on a PBX or Centrex, or as a stand-alone unit on user premises to distribute large amounts of incoming calls to a group of attendants or agents on a controlled and managed basis. Many management options exist to control and oversee the flow of call traffic to the agents. A major tool of inbound telemarketing centers.
AUTOMATIC CALLING UNIT (ACU) - Often abbreviated "ACU;" devices that dial a telephone call automatically on receipt of a demand for a connection and desired network number information from a Data Terminal Equipment (DTE). Bell 801 and CCITT V.25 units are the generic domestic and international equivalents.
AUTOMATIC DATA SPEED IDENTIFICATION - A function of integrated voice/data PBX switching systems to permit data devices and terminals of various data speeds to be connected in the PBX. Typically identified as "data classes of service" to sense and accommodate the data speed of a calling data terminal in order to route data calls. Frequently associated with an "automatic modem select" function in "modem pooling" arrangements or with "format and protocol conversion" capability to provide internal data calls between dissimilar devices and terminals.
AUTOMATIC DIALING UNIT (AUTODIALER) - A device programmed with frequently called numbers. The caller presses one to three digits and the preprogrammed number is automatically dialed into the phone circuit.
AUTOMATIC EXCLUSION (PRIVACY) - A telephone terminal systems function in which the first station on a line prevents access by all others on that line; identified with "bridge lifters" in local wire telephone plant; can be duplicated in digital local exchanges with software.
AUTOMATIC IDENTIFICATION OF OUTWARD DIALING (AIOD) - The function in some centrex units providing an itemized breakdown of charges (including individual charges for toll calls) for calls made by each telephone extension. Equivalent to Call Detail Recording ( CDR ) in electronic PBXs.
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 "10XXX" call is made. With ANI a caller's long distance carrier knows who (what phone number) to bill without requiring the caller to enter an Authorization Code to be identified and billed.
AUTOMATIC REDIAL (AUTOMATIC RECALL) - A terminal function capable of detecting a busy signal and redialing the call until a connection is obtained; often called "Demon Dialer" after a consumer unit made for telephones.
AUTOMATIC ROUTE OPTIMIZATION - 1.) Synonym for "Least Cost Routing" (which see elsewhere); 2.) In some systems, may denote added ability to adjust routing depending on traffic load or time of day.
AUTOMATIC RINGDOWN (ARD) - A dedicated circuit on which no dialing is needed; simply seizing one end starts ringing at the other. Colloquially called a " hotline ."
AUTOMATIC ROUTE SELECTION (ARS) - See: Least Cost Routing.
AUTOMATIC TRUNK - A dial connection whose destination is predetermined, thus a request for service (called a seizure) initiates dialing to the programmed destination.
BACKBONE FACILITIES - A transmission facility designed to interconnect tributary facilities from clusters of dispersed users or devices; viewpoint of what is a "backbone" can range from a single building's wiring to an intercontinental network.
BALUN - A form of impedance-matching transformer adapted by datacomm from radio antennas to connect unbalanced coaxially - interfaced data terminals to balanced twisted pair wiring in buildings.
BAND - (1) In analog transmission, the range of frequencies between two defined limits. - Synonym: Bandwidth . (2) In reference to WATS, one of five or more specific geographic areas as defined by the carrier.
BANDWIDTH - see BAND.
BASEBAND 1.) In radio and analog communications. the total frequency band occupied by the aggregate of all the voice and data signals used to modulate a carrier. 2.) In LANs, a physical transmission path using direct digital signaling, usually at a rate stated in megabits per second.
BASIC TELECOMMUNICATIONS ACCESS METHOD (BTAM) - IBM's earliest I/O method for communications and computers; a series of macro - routines becoming extremely burdensome on the computer if more than about 15 remote terminals become active; replaced by VTAM at most users.
BATCH - 1.) In data processing, the processing of data accumulated over a period of time; 2.) In telecommunications, the accumulation of messages for transmission in a single group.
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.
BELL OPERATING COMPANY (BOC) - Any of 22 LEC's spun off from AT&T as a result of the 1984 consent decree settlement; now grouped into 7 regional Bell holding companies (see also RBHC).
BINARY CODED DECIMAL (BCD) - A binary-coded notation in which each decimal digit of a number is expressed in binary form; Example: 23 decimal is 10111 in binary, and 0010 0011 in BCD.
BINARY CODED DECIMAL INTERCHANGE CODE (BCDIC) - IBM's earlier 7-bit implementation of a code for synchronous data communications. Virtually completely replaced by EBCDIC at this point in time.
BINARY SYNCHRONOUS COMMUNICATIONS (BISYNC, BSC) - IBM's half-duplex, character-oriented data protocol dating to 1964; more than 300 varieties of Bisync exist in IBM books alone with variations using BCDIC, EBCDIC, ASCII and Transcode and various error-checking schemes plus special characters as well as numerous non-IBM interpretations by other vendors. Still widely used as IBM attempts to unify all these in SDLC but most require extensive new hardware purchase.
BIPOLAR - 1.) In communications, the predominant line signaling method used for baseband digital transmission on local wire links for DDS and T-1; zero and one values are represented by positive and negative voltages respectively, as in " polar " telegraph circuits. Compare with AMI. 2.) In high-speed integrated circuit electronics, a similar technique used for transmission within a chip; typified by the name Bipolar CMOS .
BIT - The smallest unit of information in data processing; a contraction of the words," binary digit ."
BIT DURATION - The time taken for one bit to pass a point on a transmission link. Also called: bit length , bit time , bit interval , bit period , bit interval .
BIT ERROR RATE (BER) - The ratio between the number of bits transmitted and the number received in error on a transmission link; often used as a measure of quality for data links.
BIT ERROR RATE TEST (BERT) - Test method for digital links using a psuedorandom repeating sequence of (typically) 63, 511 or 2047 bits, comparing the received pattern with the known transmitted pattern for errors, computing the gross errors over a fixed test interval, typically 15 minutes. Error rate is stated in scientific notation as the number of errored bits per hundred thousand, million ten million or hundred million. Minimum acceptable error rate for voice channel data operations is one errored bit per 100,000 transmitted; computer users often find BERT testing insufficient for their needs. (see also BLERT ).
BIT ORIENTED - Descriptive of data communications protocols using a control byte rather than embedded control characters or control messages as well as having a high degree of transparency to codes used for messages. Typified by IBM's SDLC and the CCITT's HDLC of X.25 packet networks.
BITS PER SECOND (BPS) - basic unit of measurement for raw transmission throughput on a link. Often stated in kilobits , megabits , or gigabits .
BLOCK - In data communications, a string of data set into an "envelope" of synchronizing, addressing, control and error-checking characters transmitted as an entity. Synonomous with "packet" in X.25 packet networks. Equivalent to "frames" in digital communications systems.
BLOCK CHECK CHARACTER (BCC) - An error-checking character appended to a block of data representing the resulting parity check of all the characters in the block; associated technique known as Longitudinal Redundancy Check, or LRC .
BLOCK ERROR RATE (BLER) - The ratio of number of blocks transmitted to the number of blocks containing errors; a user-oriented measure of data transmission quality. Synonomous with "frame error rate" in digital communications line systems.
BLOCK ERROR RATE TEST (BLERT) - Data transmission testing in which the error rate counted is the number of blocks containing errored bits instead of the raw number of errored bits. Many users claim this is more representative of the real throughput quality of a circuit than simple raw BER testing.
BLOCK LENGTH - Measure of the size of a transmission block in data communications; stated in characters, records, language words, computer words, characters; but rarely bits.
BLOCKED CALLS (BLOCKING, CALL BLOCKING) - 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.
BOOTSTRAP LOADER - A computer input routine in which preset operations are placed into a computer that enable it to get into operation whenever a reset condition occurs; in electronic PBXs this may be called Automatic Program Loading or a similar term; in personal computers it is the sequence that searches predetermined disks for a Command Interpreter program, then a Configure System file; finally an Autoexecution Batch file.
BREAK - 1.) In the sense of a transmission impairment, a momentary interruption of a circuit. 2.) In data transmission systems, a timed interruption of about 300 milliseconds, often intended to interrupt a distant transmitting station.
BREAKEVEN POINT - 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.
BREAKOUT BOX - A very common and low-cost data interface test adapter permitting physical access to the interface pins to view the status; sometimes force conditions on some pins or even connect the pins differently. When mounted in a larger device, often called a breakout panel.
BRIDGE - 1.) In WANs, a device to connect transmission circuits in parallel; 2.) In LANs, an electronic device providing a logical connection path between two LAN physical segments.
BROADBAND - 1.) In analog transmission plant, a channel facility having a bandwidth of greater than nominal voice grade of about 3 khz. 2.) In LANs, an analog physical transport medium of very wide bandwidth, typically 300 megahertz or more.
BROADCAST - In telecommunications, a transmission mode in which every message is transmitted to all stations; only those authorized or ad or addressed will record, read or display it, however.
BUFFER - A temporary storage medium to permit some difference in the capacity of two data devices to emit and accept data from each other.
BUG - Computer term for an error or mistake causing a processing delay or stoppage.
BULLETIN BOARD SYSTEM (BBS, EBBS) - A communicating computer equipped so as to provide informational messages, file storage and transfer and a degree of message exchange to dial-up data terminal or personal computer users.
BURIED CABLE - Cable installed underground in such a way that digging is required to expose it; also known as underground cable.
BURROUGHS DATA LINK CONTROL (BDLC) - Burroughs' proprietary bit-oriented protocol similar to IBM's SDLC.
BURST - 1.) In data communications, an event containing a number of elements, as in a burst of errors; 2.) in other communications areas, a term descriptive of the intermittent occurrence of errors, demands for service or similar events.
BUS - A common physical conductor, to which several units of compatible type are connected in parallel, sharing use of the bus.
BUS INTERFACE UNIT (BIU) - IN LANs, the device furnishing direct connection of a DTE to the LAN bus.
BUSY - The condition in which facilities over which a call is to be transmitted are already in use. Synonym: "off hook." British: "engaged." French: "occupe," reported as "occ" in Telex networks.
BUSY HOUR - The time of day when connections are most in demand in a switched network.
BUSY TONE - A single tone interrupted at 60 impulses per minute in North America, indicating that the called point is already in use. Internationally, variations of both tone character and interruption rate abound. - Synonym: "station busy signal." British: "number engaged tone."
BYPASS - The connection of two customer locations circumventing use of the Public Switched Network, particularly locally. When transmission facilities of the LEC are rented, but "bypass" the LEC switching machines, a "service bypass" is claimed. If the LEC's physical transmission plant is avoided (as with private microwave), a "facility bypass" is claimed.
BYTE - In computers, a very specifically-sized unit containing 8 bits for the computer to operate on; frequently called a "word" in computer systems.
C BAND - A portion of the electromagnetic spectrum used heavily microwave and satellite transmission; frequencies in the region of 4 Gigahertz to 6 Gigahertz.
C CONDITIONING - A type of line conditioning that puts specified limits on amplitude and delay distortion to keep them within tolerable limits for certain data transmissions.
CABLE -An assembly of one or more wire conductors or optical fibers within an envelope (commonly called a sheath ) so as to permit use of the conductors singly or in groups.
CARRIER - 1.) a signal of known characteristics that is modified ( modulated )so it carries information. The receiver, knowing the expected characteristics of the carrier, can extract the information from it. However, noise or unintended changes to the carrier will, of course be also interpreted as part of the information. 2.) See "Common Carrier."
CARRIER ACCESS CODE (CAC) - The sequence an end user dials to select 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 ACCESS LINE CHARGE (CALC) - 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 local 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,dialed by customers in the form 10XXX. The same code applies to an individual carrier throughout the area served by the North American Numbering Plan.
CARRIER SENSE MULTIPLE ACCESS (CSMA) - A LAN access method in which stations listen to see if the transmission path is clear before starting to transmit.
CARRIER SENSE MULTIPLE ACCESS/COLLISION DETECTION (CSMA/CD) - A refinement of CSMA in which stations can identify not only an idle channel, but if a collision has just occurred, in which case they wait additional time to give the preceding stations access priority to clear their traffic.
CARRIER SYSTEM - A transmission system capable of providing multiple communications channels over a single physical path.
CABLE TELEVISION ; COMMUNITY ANTENNA TELEVISION (CATV) - Telecommunications based on guided radio frequency ( RF ) transmissions using 75 - Ohm coaxial cable as the physical medium. Cable TV offers multiple frequency - divided channels, permitting mixed transmissions to be carried simultaneously. Broadband LANs are a variant of Cable TV technology.
CATHODE RAY TUBE (CRT) - A common form of visual display for data terminals, similar to a television picture tube.
CCIR - Comite Consultatif Internationale des Radio, a major constituent of the International Telecommunications Union ( See "ITU", issuing both Radio Regulations and Recommendations for all uses of radio transmission.
CCITT - Comite Consultatif Internationale des Telephones et Telegraphes, a major constituent of the International Telecommunications Union ( ITU ) that sets standards for the operation of telecommunications services across international boundaries. Many CCITT standards are adopted for use domestically.
CCITT SIGNALING SYSTEM NUMBER 1 (SS1) - A tone supervision system using a 500 Hertz tone modulated at a 20 Hertz rate to signal call requests between switchboards. International equivalent of Bell's 1000/20 manual ringdown signaling.
CCITT SIGNALING SYSTEM NUMBER 2 (SS2) - A two-tone (600/750 Hertz) system for dial-pulsing selection information; never used internationally, closely akin to early Bell mobile radiotelephone dialing systems.
CCITT SIGNALING SYSTEM NUMBER 3 (SS3) - A single-frequency (2280 Hertz) tone system used on one-way circuits only; not intended for transit connections involving a third nation; a prime method through the late 1970's.
CCITT SIGNALING SYSTEM NUMBER 4 (SS4) - A two-tone (2040 and 2400 Hertz) system for international transit and terminal traffic; the first truly global "direct dialing" signaling system.
CCITT SIGNALING SYSTEM NUMBER 5 (SS5) - A two-tone (2400 and 2600 Hertz) system combined with multifrequency inter-register signaling for both terminal and transit traffic; closest international equivalent to North American Bell "DDD trunks using SF supervision."
CCITT SIGNALING SYSTEM NUMBER 6 (SS6) - A common digital data path between two switching machines to negotiate and oversee connection control on transmission facility trunks between the machines; international equivalent of Bell CCIS; typically a 2400bps data circuit.
CCITT SIGNALING SYSTEM NUMBER 7 (SS7) - Sends packetized machine-language messages about each call connection between and among machines of a network to achieve connection control; international equivalent of Bell DNHR; permits many ISDN services such as CNID or Random RCF; typically a 64 kilobit data circuit.
CCITT SIGNALING SYSTEM R1 - International equivalent of Bell 2600 Hertz "SF signaling" used in North American "DDD trunks." Largely used only in Third World nations.
CCITT SIGNALING SYSTEM R2 - International equivalent of Bell "out of band" analog carrier system signaling; uses a tone of 3825 Hertz placed between voice channels of a carrier system; used primarily in Europe.
CELLULAR MOBILE RADIO - A high capacity land mobile radio system in which its radio channels are divided into clustered groups covering limited areas, with the radio units automatically switching channels as the user moves from cell to cell.
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.
CHANNEL - A communications transmission path via any sort of transmission medium - wire, radio, optical fiber or otherwise.
CHANNEL BANK - Hardware typically used in a common carrier office that performs the end point multiplexing and demultiplexing of individual channels to and from a telephone-type carrier system; including provision for supervisory signaling used by telephone switching systems.
CHANNEL SERVICE UNIT (CSU) - A part of CPE terminating a physical digital circuit such as a DDS or T-1 facility; forms up the digital stream into bipolar signals suitable for line transmission, performs loopback functions, and more recent versions provide overall error measurement in operation. The central office equivalent is an Office Channel Unit, or OCU.
CHANNELIZATION - Division of a larger capacity channel into a number of smaller channels for use by multiple functions.
CHARACTER - a unit of typographic information, usually variable as part of a language. Because data is handled and transferred as a series of characters,the term also can mean one bit pattern in a specific data code; frequently called a "word" in the computer programming sense of the term. In digital ( T carrier ) communications lines, " octects " are for all practical purposes synonomous with data communications characters, in their nature as 8-bit "words" conveying the instantaneous amplitude level of their transported signal.
CHARACTER ORIENTED - Descriptive of a communications protocol or transmission procedure that has control information transmitted in the form of special bytes called control characters. Implies by its nature limitation to a particular character code. Compare to " bit oriented ".
CIRCUIT - A path including any needed supervisory and signaling or conditioning equipment for the transmission of electromagnetic signals. - Synonym: Facility, Transmission Facility.
CIRCUIT CONDITIONING - Modification of (most typically) analog data circuits to bring transmission parameters of the channel into narrower limits than provided by randomly-selected voice channels; conditioning is also utilized to a lesser extent in certain other services.
CIRCUIT SWITCHING - A switching system that completes a dedicated transmission path from sender to receiver at the time of transmission, then releases that path for another user when the transmission is completed. Closely akin to "session" in data systems. Compare to Message Switching and Packet Switching .
CLADDING - In fiber optic cable, a low refractive index material surrounding the core of each optical fiber, providing guidance to the lightwaves within the fiber.
CLASS OF SERVICE/CLASS MARK (COS) - An identifier to group customers or users for the purpose of rate distinction or setting of service functions and features.
CLOCKING - Repetitive, regularly timed signals used to control synchronous transmissions.
CLUSTER CONTROLLER - A device that handles remote communications control (and sometimes processing) for multiple data terminals or workstations.
COAXIAL CABLE - A cable in which one conductor surrounds the other. The electromagnetic wave travels between the grounded outer shield and the central conductor. Coaxials can carry much wider bandwidth and higher frequencies than twisted wire pair, while suffering less interference problems due to the grounded outer conductor. Where the maximum frequency capable on twisted pair wiring is about 16 megahertz and then only for short distances, coaxial cable readily carries several hundred megahertz for a thousand feet.
CODEC - Contraction of Coder-Decoder. Used to convert analog signals to digital pulses for transmission and back again to the original analog form for reception. Originally located in digital channel banks, codecs are now located within digital telephone sets.
COLLISION - Simultaneous transmissions on a LAN that interfere with each other in a contention-based access scheme. Occurs when two or more terminal devices attempt to transmit at nearly the same instant; token-passing schemes avoid this, while well-planned contention schemes keep it to a tolerable level.
COMMON CARRIER - A business enterprise offering telecommunications services and facilities to the general public.
COMMON CHANNEL INTEROFFICE SIGNALING (CCIS) - A digital technology used by AT&T; uses a separate data line to route interoffice signals providing faster call set-up and more efficient use of trunks than can trunk groups with discrete signaling; closely equivalent to CCITT Signaling System Number 6 (which see elsewhere).
COMMON CONTROL SWITCHING ARRANGEMENT (CCSA) - The use of common-carrier owned switching machines 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; primary function is for intercity use as compared to the primarily local function of centrex (which see elsewhere).
COMMON CONTROL SYSTEM - An automatic switching system that makes use of common equipment to establish a connection. Once connection is made, the common control equipment is available to establish another connection.
COMPANDOR - Contraction of "Compressor/Expandor." 1.) In analog operations, used to transmit signals higher above noise with a restricted dynamic range, then restore that range by expansion at the receiver; 2.) In digital systems, used to restrict the dynamic range of speech signals prior to digital transmission encoding, then expand them at the receiving terminal.
COMPATIBILITY - A property in data processing and telecommunications systems permitting exchange of information directly and in usable form; implies identical or interchangeable signals and methodologies. Achieving compatibility is a major function of standards organizations.
COMPRESSION - 1.) In analog communications, restricting the range of volume levels of signals in order to transmit them at a higher average power above noise on the channel; 2.) In data communications, compacting the number of bits used to represent the information, losing the character structure while reducing the circuit time or capacity needed to transfer the data; decompression is then needed at the receiving end to again render the data useful.
CONCATENATION - Linking of transmission media by looping through devices; used in various ways by many data terminals, this term most closely associated with Burroughs data terminals. Other names: Daisy Chaining ( NCR ), Multiple Wiring ( telephony ), Paralleling , Looping .
CONCENTRATOR - Any communications device that allows a shared transmission medium to accommodate more terminals than channels available on the medium. Other names: Contention switches , Channel Contenders .
CONDITIONED CIRCUIT - A circuit with added equipment to obtain desired characteristics for transmission; most commonly associated with analog data transmission circuits, correcting amplitude and envelope delay characteristics that give rise to timing jitter in demodulated data. The addition of such equipment is called " conditioning ".
CONDITIONING EQUIPMENT - Equipment added to a circuit for the express purposes of matching transmission levels and impedances or equalizing transmission and delay to bring circuit losses, levels, and distortion within specified limits of CCITT standards, or in U.S. practice, common carrier tariffs.
CONFIGURATION - The combination of long-distance services and/or equipment that make up a communications system.
CONTENTION - A method of line or resource control in which multiple terminals request use of a line or resource at random from the remote end to transmit. Compare with "polling ".
CONTINUITY CHECK - 1.) In wiring, verifying that tandem wires have in fact a continuous path, an inadequate test of transmission viability for sophisticated applications; 2.) In common-channel signaling arrangements, a short test performed to verify that a path has been established just before cutting it through to the user.
CONTROL CHARACTER - A character inserted into a data stream to notify the receiver of a special function to perform or to identify the purpose of the data or message associated with it. While permitting simpler terminal equipments, control characters add a throughput burden to transmission links and limit transparency sufficiently that modern protocols seek to eliminate their use. The " framing bits " of ESF digital carrier are synonomous with control characters in data transmission.
CONTROL MODE - The state data terminals must be in to permit line control actions to occur. A telephone equivalent is muting of the transmitter when a keypad button is depressed to prevent extraneous noises at the instant of dialing tone transmission.
CONTROL SIGNALS - Signals passed between parts of a communications system to oversee operation and configuration of the system. In telephony, supervision signals are a form of control signal.
CONTROL UNIT (CU) - The central processor of a telephone switching device; may be mechanical or electronic.
CONVERSATIONAL MODE (CHAT MODE) - Interactive data communications carried on between data terminals in a fashion emulating speech conversation.
COS - 1.) In the telephone switching sense, "Class of Service" (which see elsewhere); 2.) In the data processing sense, "Corporation for Open Systems."
COST COMPONENT - The price of each element of telecommunications service and/or equipment that comprises a configuration (which see elsewhere).
CR (CARRIAGE RETURN) - A control character causing the print or display position to move to the first position on the line, drawn from the typewriter and teleprinter function with similar action.
CROSS CONNECTION (X-CONNECT) - 1.) In the physical wire sense, wire connections running between two points on a distribution frame, or between binding posts in a terminal wiring box; 2.) In digital communications systems, a bit stream addressed between two ports of a switching or routing unit. See also "digital cross-connect."
CROSS TALK - Unwanted energy (speech, tone or digital pulses) transferred from one transmission path to another. Comprises part of the "noise" observed on analog communications circuits.
CURRENT LOOP - A digital transmission method that recognizes current flows as compared to voltage changes; long used in telegraphy, some use is found in vendor proprietary terminal interfaces, most notably those of Digital Equipment Corporation.
CUSTOMER ACCESS LINE CHARGE (CALC) - The FCC-authorized 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 INFORMATION CONTROL SYSTEM (CICS) - An IBM computer system function to oversee and control the flow of information by means of software interfaces between application programs and the computer's operating system.
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.
CUT - To transfer a service from one facility to another.
CUTOVER - The function of transferring a service from one facility to another; most often in the sense of changing switching machines, accomplished during a night or weekend.
CUT THROUGH - Connecting of a path for signaling and/or communications between two points through an intermediary or "transit" point; known in telegraphic communications as "DF."
CYCLIC REDUNDANCY CHECK (CRC) - A powerful error checking method for data and digital communications. The transmitting terminal computes a numeric value representative of the number of marking bits in the associated block of data and sends that value to the receiver, where the number is recomputed to compare against the block as received. Depending on the number of bits in the CRC numeric value the error trapping efficiency ranges from about 97 % at CRC-6 to 99.997% at CRC-32. Values of CRC-8 and CRC-16 are adequate for most data message block sizes, while CRC-32 is needed mainly for very long blocks of tens of thousands of characters.
D CONDITIONING - A type of channel conditioning setting limits on harmonic distortion and signal-to-noise ratio of analog signals. Largely archaic in that it was developed as a result of flaws in the design of T carrier channel banks long since removed from the plant.
DATA - Multiple units of information. Singular is " datum ".
DATA ACCESS ARRANGEMENT (DAA) - Once a mandated form of Network Protective Device ( NPD ) for COAM data devices, some still require a DAA for physical interfacing.
DATA CIRCUIT - Communications channels provided specifically for the exchange of data as compared to voice or other information forms. Modern data terminals often operate effectively on channels not so designated, offering users economy and flexibility of operation.
DATA COMMUNICATIONS EQUIPMENT (DCE) - Standards body term for devices that perform signal conversion at the extremities of a data circuit. A data set ( modem ) or a CSU are common examples of a DCE. Compare to DTE.
DATA ENCRYPTION STANDARD (DES) - A cryptographic standard defined by the former National Bureau of Standards ( NBS ), now the National Institute of Science and Technology ( NIST ) for the general public to encrypt and decrypt digital and data transmissions.
DATA LINE INTERFACE - PBX term for a point at which a data station can be connected to wiring.
DATA LINE PRIVACY - A line exclusion function provided on data and fax lines to prevent noises from other users picking up the line during facsimile or data transmission use.
DATA LINK - A serial data communications pathway, generally between two adjacent nodes without an intervening node.
DATA LINK CONTROL - 1.) Synonomous with link layer data protocols 2.) The second layer of the ISO Reference Model for Open Systems Interconnection ( OSI ).
DATA OVER VOICE (DOV) - Frequency Division Multiplexing of local physical wire within a premises to provide simultaneous and independent data transmission capability on twisted pair telephone wiring; capable of providing a fairly efficient form of LAN, largely for lower-speed async terminals; the basis of AT&T's Datakit product sold by LECs under various names largely to Centrex customers.
DATA PBX - A specialized switching unit operating on switching DCE - DTE interfaces, most typically RS-232 type.
DATA SERVICE UNIT (DSU) - DTE interface unit used in combination with a CSU to connect data terminals to transmission facilities. Contains interface control circuitry not located in the CSU. (Note that some CSU's also contain DSU functions.)
DATA SET - A device converting data into signals suitable for transmission over telecommunications lines; the term "modem" commonly used by computer communications personnel has a much broader sense and range of application in general telecommunications.
DATA TERMINAL - A station in a system capable of sending and/or receiving data signals.
DATA TERMINAL EQUIPMENT - The standards-body term for a computer, user terminal, workstation or personal computer used for data communications; abbreviated DTE . Compare to DCE .
DATA UNDER VOICE (DUV) - AT&T's earliest means of intercity and interstate transmission of DDS by Frequency Division Multiplexing a 1.544 megabit four-phase modem in otherwise unoccupied spectrum space of its analog microwave radios, below the voice channels.
DATABASE - An organized compilation of computerized data. Formalized rules ( CODASYL ) and standardized terminology have the objective of avoiding duplication, controlling access and maintaining currency of contents.
DATAPHONE - Registered trademark of AT&T for both data transmission services and terminals using public networks and channel facilities.
DATAPHONE DIGITAL SERVICE (DDS) - AT&T's registered trademark name for a series of digital transmission services at transmission rates ranging from 2400 bps to 1.544 megabits. Being supplanted in the AT&T product line by products named Accunet.
dBm - Identifier meaning "decibels referred to one milliwatt," the common reference point for power levels in telecommunications circuits.
dBm0 - Identifier meaning "decibels referred to one milliwatt and corrected to a Zero dBm effective power level;" used to state the relation of a signal level on a transmission line at other than a one-milliwatt point. Example: Throughout an analog system, a data set signal is to be kept 13 dB below that for a single test tone, stated as "minus 13 dBm0;" at a carrier modulator input where test tone level is -16 dBm, a data signal should then be 13 dB lower, or -19 dBm.
dBmp - Identifier meaning "decibels below reference tone using psophometric (filter) weighting," the CCITT method for noise measurements; has about 2 dB variance from Bell methods.
dBm0p - Identifier for CCITT psophometric-weighted noise measurements adjusted to a relative 0 dBm transmission level point. Example: An absolute measurement of minus 40 dBmp noise at a carrier channel output point would mean a signal-to-noise ratio of about 47 dBm0p exists at that point on the circuit.
dBrn - Identifier meaning "decibels above reference noise," the reference commonly used being 90 decibels below one milliwatt in North American telecommunications work.
dBrnC - Identifier meaning "decibels above reference noise measured through a (filter) weighting network approximating a " type C voice message channel;" the common North American nomenclature for a DDD trunk channel; having a reference of 90 decibels below one milliwatt of power.
dBrnC0 - Commonly pronounced "de-brink-o," identifier meaning "decibels above reference noise with C message weighting adjusted for equivalence to a 0 dBm (one milliwatt) equivalent circuit point." Example: A direct measurement of 49 dBrnC0 at the nominal +7 dBm output of a carrier demodulator would mean the noise had been offset by 7 dB; thus the reading in DBrnC0 would be 42.
DECIBEL (dB) - A unit of measurement representing the logarithmic a ratio of two voltages, currents or power levels; used in telecommunications to express transmission loss or gain; defined as one-tenth of a Bel, hence the appropriate notation is dB, shown here.
DEDICATED ACCESS LINE (DAL) - An analog special access line going from a user's premises directly to an IEC's POP, bypassing the LEC's switching machine. When the circuit is provided by a local telephone company, they claim it is a "service bypass". When the physical circuit totally avoids the LEC's transmission plant (as with microwave radio or non-LEC fiber paths), the LEC claims a "facility bypass" has occurred.
DEFAULT - 1.) In data systems, the process or action programmed to occur if the user specifies no option; 2.) Regarding Equal Access, the IEC assigned to a line's "1+" dialing sequence; the IEC all calls will be directed to unless the user dials "10XXX."
DEFENSE COMMUNICATIONS AGENCY (DCA) - The U.S. Military's interservices telecommunications operating agency, procuring and operating much domestic military communications and virtually all overseas military communications; based in the Pentagon.
DEFENSE ELECTRONICS COMMUNICATIONS CONTRACTING OFFICE (DECCO) - The U.S.Military's interservices domestic telecommunications services procurement function; located at Scott AFB, IL.
DELAY - In communications technology, the waiting time between two events, such as the time from taking a handset off hook to receiving a dial tone ( Dial Tone Delay ) or the time from completing dialing until the start of ringing at the distant end ( Connection Delay ). Specific to data communications, the time from transmission of communications until a reaction is received. Compare with Propagation Delay, Response Time and Processing Delay.
DELAY DIAL - A mode of operation whereby dial equipment waits until it receives the entire desired number before seizing a circuit to initiate a connection.
DELAY DISTORTION - An unequal propagation delay for tones of different frequencies in passage through a transmission channel. Synonyms: Envelope Delay Distortion ( EDD ); Group Delay .
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. Uses simpler electronics than PCM (which see elsewhere),but offers limited speech quality for the size channel it uses; limited primarily to military tactical applications and useful for only the simplest, slowest data transmission.
DEMODULATION - The process of retrieving an original inputted waveform from a modulated signal; widely used in numerous ways by telephone carrier systems, data sets and radio receivers of all types.
DERIVED FACILITY/DERIVED FACILITIES - Transmission paths created by use of multiplexing methods that provide more than one virtual path per physical facility.
DESTINATION FIELD - Location in a message header that contains the address of the station for which the message is destined.
DIAL LEVEL - 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); alludes to the upward motion of mechanical switches in a PBX.
DIAL PULSING (DP) - The transmitting of telephone address signals by momentarily opening a DC circuit a number of times corresponding to the digit which is dialed; often called " Rotary Dialing ." Compare with " Key Pulsing ".
DIAL REPEATING TIE LINE/DIAL REPEATING TIE TRUNK (DRTL/DRTT) - A tie line arrangement permitting direct trunk to trunk connections without an attendant's intervention; capable of repeating dial pulses to connect trunks in tandem.
DIAL SELECTIVE SIGNALING (SS-1)- A multipoint network in which the called party is selected by a prearranged dialing code; typified by the Bell SS-1 Selective Signaling System.
DIAL TONE - A uniquely identifiable tone showing that automatic switching equipment is ready to receive dial selection signals; International equivalent: "Proceed to Select" signals in various form of telecommunications networks.
DIAL UP - Using facilities of the PSTN to establish a connection.
DIALING PLAN - A description of the dialing arrangements for customer use on a network; DDD is the Bell-generated plan for North America.
DIGITAL CROSS CONNECT (DSX) - A mechanical arrangement of patch jacks and cords or a digital electronic unit that semi-permanently routes digital telecommunications bit streams between its ports to establish commmunications transmission paths.
DIGITAL DATA COMMUNICATIONS MESSAGE PROTOCOL (DDCMP) - Digital Equipment Corporation's ( DEC ) synchronous data transfer protocol.
DIGITAL LOOPBACK (DLB) - Testing method in which received data is fed back to the transmitting source without processing; used to isolate circuit problems.
DIGITAL NETWORK ARCHITECTURE (DNA) - The data network architecture of Digital Equipment Corporation ( DEC ).
DIGITAL SIGNALING - Using techniques that transmit information as a series of discontinuous pulsed signals in a pattern representative of the inputted signals; requiring reconstitution at the receiver; capable of being regenerated to minimize noise contribution in transmission. Compare to Analog.
DIGITAL SWITCHING - Establishing and maintaining a connection under stored program control, when information passed through the switching matrix is in the form of binary encoded information. The transmission medium through the matrix is assigned "time slots" on a digital bus. Analog circuits may be switched digitally by providing codecs in the physical ports of the switch.
DIRECT DISTANCE DIALING (DDD) - The Bell-generated plan and service for public calls in North America; permits users to dial their own long distance calls without operator intervention. International equivalent: "Subscriber Trunk Dialing - STD ."
DIRECT INWARD DIALING (DID) - A PBX or CENTREX feature providing for users outside the system to dial directly to a station within the system; often delineated by the last four digits of an exchange number assigned to a specific desktop.
DIRECT INWARD LINE (DIL) - A PBX feature set to dial within the PBX when a designated outside line rings. Similar to a "night connection" of an outside line to a designated station for after hours answering, except electronically reprogrammable.
DIRECT INWARD SWITCH ACCESS (DISA) - A PBX feature that permits outside callers to obtain a PBX dial tone and dial within the PBX If so programmed, outside callers can dial into the DISA, then dial back out again, even on WATS lines; a common source of telephone budget abuse.
DIRECT OUTWARD DIALING - A PBX or CENTREX feature that allows a station user to obtain direct access to an exchange network.
DISTORTION - Generally an unwanted, often unpredicted change in a transmission signal that renders the resulting output less than optimum.
DISTRIBUTED COMMUNICATIONS ARCHITECTURE (DCA) - Data network architecture established in Sperry Univac ( UNISYS ) computer networks.
DISTRIBUTED DATA PROCESSING (DDP) - Using multiple data processing machines or intelligent terminals at locations geographically separated with a shared peer relation rather than a hierarchical relation as has been classic in mainframe data networks; adds a dimension of data communications issues to a network in which all locations are now of equal need to the network's operation.
DISTRIBUTED SYSTEMS ARCHITECTURE (DSA) - The data network architecture of Honeywell Corporation.
DISTRIBUTED SYSTEMS NETWORKING (DSN) - The data network architecure of Hewlett-Packard Corporation.
DIVESTITURE - That portion of the antitrust settlement with AT&T that removed the 22 Bell LEC's from AT&T ownership to public ownership on January 1, 1984.
DROP - The portion of outside telephone plant which extends from the telephone distribution cable to the subscriber's premises.
DRY CIRCUIT - A circuit which transmits voice signals and carries no direct current.
DIGITAL CROSS-CONNECT 1 (DSX-1) - Office twisted pair facilities for interconnecting 1.544 megabit digital streams.
DIGITAL CROSS-CONNECT 2 (DSX-2) - Office coaxial cable facilities for interconnecting 6.312 megabit digital streams.
DIGITAL CROSS-CONNECT 3 (DSX-3) - Office fiber optic facilities for interconnecting 44.736 megabit digital streams.
DIGITAL STREAM 1 (DS-1) - A digital transmission rate of 1.544 megabits in North America; 2.048 megabits in much of the rest of the world; often miscalled "T-1," when it is simply the transmission rate of the LEC's T-1 digital carrier system.
DIGITAL STREAM 1C (DS-1C) - A digital transmission rate of 3.152 megabits per second in North America, containing 2 DS-1's.
DIGITAL STREAM 2 (DS-2) - A digital transmission rate of 6.312 megabits in North America, containing 4 DS-1's. Frequently miscalled "T-2," even though there is no such digital carrier system; it is merely a digital transmission rate.
DIGITAL STREAM 3 (DS-3) - A digital transmission rate of 44.736 megabits, containing 28 DS-1's or one network video signal. Frequently miscalled "T-3," even though there is no such digital carrier system. Carried locally on AT&T fiber optic transmission systems called FT3 or short-haul microwave radio.
DIGITAL STREAM 4 (DS-4) - A digital transmission rate of 274.016 megabits, containing 672 DS-1's. Carried via digital microwave radio (typical of AT&T DR-18) or multiplexed with other DS-4's for carriage via 565 megabit (or even larger) fiber optic lines interstate.
DUAL TONE MULTI-FREQUENCY (DTMF) - Also known as Touch-Tone. A type of signaling which emits two distinct frequencies for each indicated digit. Also called " Key Pulsing " at operator positions in toll networks.
DUMB TERMINAL - A data communications euphemism indicating a DTE with no processing capability. The data equivalent of a KSR teleprinter.
DUPLEX (DX) - Two-way transmission.
DUPLEX SIGNALING (DX SIGNALING; DX-1; DX-2)- A long-range bidirectional signaling method for local cable facilities 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 at each end.
EARTH STATION (GROUND STATION) - A microwave radio transmitting and receiving station working with communications satellites. This term most closely associated with large common carrier operations, particularly international.
EARTH TERMINAL - Microwave radio equipment used to communicate with communications satellites. This term most commonly associated with VSATs, TVROs, portable and mobile satellite operations, particularly domestic.
ECHO - A signal that has been reflected or otherwise returned with sufficient magnitude and delay to be perceived at the far end of the circuit. Both " talker echo " and " listener echo " are defined terms. " Echo suppressors ," now largely being replaced by " echo cancelers ," are designed into circuits requiring treatment for echoes.
ECHO CANCELER - A echo removal device that operates by generating an exact opposite of any echo signal and injecting it into the transmission path to cancel echoes. Used in both speech telephony and in some higher-speed dial-up data modems, notably those compliant with CCITT V.32.
ECHO CHECK - One method of verifying accuracy of data transmission; functions by returning (echoing) received data back to the transmitter as verification; primarily used on async data links.
ECHOPLEX - An echo checking data accuracy method used rather commonly and specifically for async keyboard terminals; the received data at the computer port is echoed back to be used as the terminal operator's CRT screen copy; the operator is the error checker.
ECHO RETURN LOSS (ERL) - The loss in the echo path; as a specification, the amount of loss required in the return path to reduce echo to a tolerable level.
ECHO SUPPRESSOR - A speech-activated switch that detects echo conditions and cuts off the echo path momentarily when echo conditions exist; largely obsoleted by echo cancelers.
EDIT - Preparation of data for a later operation; may include rearrangement or addition of data, deletion of unwanted data, format changes, code conversion, or data compression.
EIA RS-232 - The most common DTE serial interface by far, in use for almost 30 years, with several revisions and additions; international equivalent: The suite of CCITT V.24 and V.28 combined with ISO 2110.
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. Capable of either Head End Hop Off ( HEHO ) or Tail End Hop Off ( TEHO ) as required.
ELECTROMAGNETIC SPECTRUM - The entire range of wavelengths ( the inverse of frequency ) of electromagnetic waves extending from cosmic and Gamma rays down through visible light and heat to every form of radio communications signal.
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.
ELECTRONIC MAIL - 1.) A common-carrier data communications service to transmit computer-generated messages between locations or cities; 2.) A feature of LANs for transmission of computer-generated messages within a closed community of users on the LAN. Note that the two forms of electronic mail may be used independently or they may be interfaced.
ELECTRONIC SWITCH - A programmable switching system (often denoted ESS, for Electronic Switching System) used in most LEC telephone companies, by all long distance companies, and most end users of PBXs. Completely solid state electronics, as opposed to older electro-mechanical switches.
ELECTRONIC SWITCHING SYSTEM (ESS) - A Bell term for electronic telephone exchange switching equipment; the largest PBXs of thousands of station lines may be Bell ESS machines modified for use as a PBX.
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-based private network by some parties.
EMULATE/EMULATION - Imitating a system or device such that a connected device accepts the same information, executes the same computer programs and achieves the same results as if the emulator were one of its own kind. Most often, emulation is a downward step in capability of the device being used, as when a personal computer is used to emulate a mechanical teleprinter or a "dumb" terminal on a computer network. While some degree of upward emulation is possible, it is less prevalent in the broad view of computer communications.
ENCRYPTION - The systematic encoding of a message or bit stream before transmission to prevent unauthorized recipients from understanding it. The process of again rendering the information readable is DECRYPTION.
ENHANCED PRIVATE SWITCHED COMMUNICATIONS SERVICE (EPSCS) - A private network utilizing Bell provided equipment located in the central office and dedicated to a specific customer. In one sense, EPSCS is electronic Centrexes combined with tielines.
E&M LEADS (E & M LEADS, E and M LEADS) - A pair of signaling leads for telephone trunklines carrying signals between trunk equipment or to separate local loop signaling equipment units. The M lead transmits battery or ground signals toward the line signaling equipment, and the E lead receives open or ground signals from the line signaling equipment.
E&M SIGNALING (E & M SIGNALING, E and M SIGNALING) - An arrangement for supervisory (and sometimes dial pulse) signaling between a line circuit and an associated signaling unit effected over two leads providing full-time, 2-way, supervision. Originally used widely only within toll offices for 75 feet maximum, several variations now permit use over much longer distances.
ENTERPRISE NUMBER - An telephone numbering service on which the LEC will supply its operator services to accept charges for all inbound calls without an acceptance check from the customer. Numbers often take the form of "ENterprise NNNN," or the operator term, "WX NNNN," where NNNN is the assigned number for the account. Largely automated today by the "976" offerings of LECs.
EQUAL ACCESS - Provision by Local Exchange Carrier (LEC) switches of ability for other long distance companies than AT&T to be the "1+" primary long distance company for users of long distance (by providing a new type of " Feature Group D " access connection arrangement, 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, so calls can be billed automatically on any IEC the user selects. Some 350 IECs have obtained Bellcore-issued Equal Access code numbers; for example, dialing 10222 nationwide will route a call via MCI, no matter what IEC the line has been presubscribed to.
EQUALIZATION - The procedure of compensating for fluctuation in circuit amplitude, or envelope delay distortion.
ERGONOMICS - a discipline treating the consideration of human factors in design of the working environment and its components; intended to promote productivity and safety in the tools people work with.
ERLANG - A widely-used unit of telecommunications traffic intensity, named after work of the Danish statistician, D. K. Erlang. One Erlang is the intensity at which one traffic path would be continuously occupied, e.g. one call per hour; equivalent to 3600 call-seconds or 36 ccs in early Bell traffic engineering systems.
ERLANG B TABLE - A widely used table derived from D.K. Erlang's statistical work; provides determination of the traffic capacity of a given group of circuits, or inversely permits estimation of the number of circuits needed for a given user group. Later work has resulted in so-called " Erlang C tables ."
ERROR - 1.) In data communications, any unwanted change in the contents of a transmission; 2.) In data processing, incorrect performance of a program routine, data entry, or unwanted stoppage by software or hardware flaws in the computer.
ERROR BURST - The result of an event that causes a stream of consecutively transmitted bits to be corrupted. Engineering studies of more than 5 decades consistently show data transmission errors to have a "bursty" character; this has been widely documented.
ERROR CONTROL - In data communications, methods used to detect and correct transmission channel errors.
ERROR RATE - 1.) In data communications, the ratio of bits, characters, elements, blocks, messages or files incorrectly received to the total number transmitted during a specified time interval; 2.) In business operations or factory production, a similar measure on human performance of transactions or tasks.
ETHERNET - Originally the trade name for a LAN developed by Xerox Corporation; later supported by Digital Equipment Corporation, Intel Corporation and Hewlett-Packard; now standardized as IEEE specification 802.3 .
EXCHANGE - A local telephone switching center or " Central Office "; in some LEC parlances, the group of local switching centers for a community.
EXCHANGE NETWORK FACILITIES FOR INTERSTATE ACCESS (ENFIA) - LEC pricing arrangements for local loops offered to IECs for connecting IEC interstate and interLATA networks to LEC central offices.
EXCLUSION - A line privacy arrangement in telephone station equipment to keep other extensions or pick ups off the line when the primary phone is off hook. Some arrangements are automatic, others manual; use has expanded to provide avoidance of interruption to dial-up data positions.
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.
EXTENDED BINARY CODED DECIMAL INTERCHANGE CODE (EBCDIC) - IBM's proprietary 8-bit code for synchronous data communications. Has numerous variations of control character meanings; is undergoing some standardization as IBM moves SNA and SAA forward.
FACILITY/FACILITIES (TRANSMISSION FACILITY/TRANSMISSION FACILITIES) - Typically refers to transmission lines or circuits. A user's facilities are the physical circuits or circuit paths available to make calls.
FACSIMILE (FAX) - Graphic transmission of pictures, maps or documents via communications circuits using terminal devices that scan documents, transforming scanned images into coded data-like signals and reproduce likenesses of original documents at a distant point. Has a long history dating to the 1840's as a method of telegraphy; archaic press implementations of this century used the names Wirephoto and Telephoto for early, slow photographic transmissions.
FCC (FEDERAL COMMUNICATIONS COMMISSION) - A board of Presidentially-appointed commissioners empowered to regulate interstate and international communications and all uses of radio in the United States. Operates under the Communications Act of 1934 and several more recent laws; promulgates its own regulations interpreting those laws, as Title 47 of the Code of Federal Regulations ( CFR 47 , 47CFR ).
FEATURE GROUP A (FGA,FG A) - Subscriber-line-side originating and terminating LEC switched access for which an originating subscriber dials a 7-digit telephone number that essentially connects to another subscriber-type 2-wire line to an IEC. The IEC must operate like a subscriber, interpreting ringing signal as a demand for a call, returning a distinctive tone to signal the caller for input additional tone-generated digits of an account code and the called number; does not provide the function of supervision signaling to tell the IEC when to start billing on inward calls.
FEATURE GROUP B (FGB,FG B) - 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 its corresponding IEC's trunk group. Optional rotary dial service and ANI may be available; permits IECs to offer coin-free local access and credit card calling. For example, dialing 950-1022 at any phone nationwide should place the caller on MCI's network, returning a tone ready for the caller's desired number and account code; this is the number and sequence dialed by card-reading payphones.
FEATURE GROUP C (FGC,FG C) - Trunk-side local switching office access for AT&T; generally on a direct basis between each LEC switching machine and AT&T's nearest switching machine.
FEATURE GROUP D (FGD,FG D) - Also referred to as "Equal Access," Feature Group D is trunk-side local exchange switching machine access affording call supervision to IECs, alternative access code use (10XXX), optional calling-party identification, recording of access-charge billing details, and presubscription to a customer-specified IEC, for convenient use of 1+ dialing to the user's choice of IEC's.
FIBER OPTICS - a technology using light as a digital information bearer. Fiber optic cables ( light guides ) are a direct replacement for conventional wire, coaxial cable and many forms of radio, including microwave. Fiber optic lines actually cost less, occupy less space and provided far more transmission capacity than earlier methods, while providing superior quality due to virtual immunity to electrical interference.
FILE SERVER (SERVER) - In LANs, a station, often microcomputer-based, that provides the mass storage and file access to users on the LAN. Server capabilities vary widely; some even include ability to " gateway " to other communications means or " bridge " to a similar nearby LAN, or even determine for users if those functions are needed and decide the route to the user's destination, in which case they are called " routers ".
FIRMWARE - Permanent or semipermanent control coding built into a software-operated computer device that operates an application program, instruction set, operating routine or other user-oriented instructions to a computer; often resident in a ROM ( Read Only Memory ) chip to simplify installation.
FLAG - In data transmission, an indicator of an expected event like the beginning or end of a block of data. In CCITT standards for X.25 networks, the 8-bit character 01111110 has been uniquely established with the name "Flag" to be used at the beginning and end of a block; many other proprietary protocols have adopting it, even if the rest of their data block is not CCITT-compliant.
FLOW CONTROL - In data communications, the use of buffering and other mechanisms that operate to avoid data loss in case the receiver cannot keep up with the transmitter. The ASCII control characters X-ON and X-OFF are frequently-used examples; they are returned in the reverse direction as an instruction for the sender to hold or restart held transmission.
FOOTPRINT - 1.) In physical hardware planning, the floor space or desktop space a unit requires; 2.) In satellite communications, the area on the surface of the earth that a given satellite covers; this technology has reached a high state of development, with satellite coverages capable of being quite shaped and tailored to the purpose of the satellite.
FORWARD CHANNEL - The communications path from the originator of information toward the recipient; a path in the return direction (perhaps established for error checking and/or flow control ) is called the " reverse channel ."
FORWARD ERROR CORRECTION (FEC) - An error-correcting technique that avoids the need for any reverse channel by enabling self-correction of errors at the receiver; operates by adding information (at the expense of throughput) to enable the receiver to determine what the error was and the correct information to substitute for the error. Operates by using "convolutional coding" which means adding a repeating sequence to the information that is known to the receiver. By subtracting the convolutional code and performing other tests, the receiver can insert what should have been received with a high degree of accuracy; now extends to making error-correcting data modems called Trellis-coded (trellis coded) modems.
FOUR WIRE CIRCUITS (FOUR WIRE CHANNELS) - Circuits which using two separate transmission paths of two wires each or their virtual equivalent, for simultaneous two-way ( Full Duplex ) transmission, as opposed to regular local lines which usually only have two wires to carry conversations in both directions. One set of wires carries information in one direction; the other in the opposite direction. Contrast with Two wire circuits ; two wire channels .
FRAME - 1.) In digital communications, a group of bits or characters sent serially over a channel; generally a logical unit of information between data link layer entities that contains its own control information for addressing and error checking. Synonomous with " block ." 2.) In communications wiring, an openwork structure upon which terminal connections for wires can be run to interconnect different devices; typically called a " distributing frame " or " distribution frame ."
FRAMING - A control procedure used with digital transmission to permit the receiver to synchronize itself with the transmitter's sequence of bits or characters; framing bits in more recent systems also carry alarm and control messages in entities like T-1 carrier systems. This makes framing bits in communications carrier systems closely akin to control characters in data communications.
FREQUENCY - The number of complete cycles of an event (in communications typically an alternating current signal) per unit of time; usually expressed by means of the unit ' Hertz ," named after Heinrich Hertz an early German investigator of the properties of high-frequency alternating current waves.
FREQUENCY DIVISION MULTIPLEXING (FDM) The analog method of deriving multiple transmission channels from a single physical facility; divides of an available frequency range (bandwidth) into various subchannels. Widely used in many formats by virtually every kind of telecommunications technology; the " carrier systems " of telephony. Also called Frequency Division Multiplex ; Frequency Division Mux .
FREQUENCY RESPONSE - The measurement of how well a transmission channel or device transports all the frequencies sent into it; a measure of how faithfully signals are transported or reproduced. In Bell terms, the arithmetic is reversed and the name " amplitude response " or " amplitude variation " is used for the same expression.
FREQUENCY SHIFT KEYING (FSK) - One of the more basic and durable forms of transmitting binary information; in FSK, one of the binary states is represented by one known frequency and the other by another known frequency. The receiver produces outputs only when one of the two known frequencies is received in the absence of the other. Applications of FSK abound in every form of telecommunications; computer data modems used FSK exclusively for speeds up to 120 bps for years, and the classic FDM of telegraphers was 24 FSK channels on a voice channel to produce what was called " carrier telegraph " and variations on that name.
FRESNEL ZONE - The area in open space that must be practically free of obstructions for a microwave radio path to function properly; some degree of Fresnel consideration is required in the immediate vicinity of the form of microwave radio used on satellite links.
FULL DUPLEX (FDX) - A circuit which allows independent transmission information in both directions simultaneously. - Synonym: In wire telephony, 4 wire circuit.
FULL PERIOD - In private line telephone and telegraph service, a circuit rented for the exclusive use of a single customer on a month-by-month basis. Archaic in the sense that part-time circuits have not been rented for years.
FUNCTIONAL TEST - Operation carried out under normal working conditions (but not necessarily full operating load) to verify that a circuit or device operates correctly. Compare to Acceptance test .
FX (FOREIGN EXCHANGE) SERVICE - A telephone service allowing a customer the appearance of a local presence in a distant city, by connecting a local phone line into a distant local telephone exchange switching machine. Accomplished over a leased private line circuit purchased from either an LEC or IEC as appropriate to the distance being covered. In certain
LEC territories, local distances are sufficient or tariff limits make appropriate connection to a Central Office within the same " exchange area " of central offices, and the distinctive name " Foreign Central Office " ( FCO ) is applied to these short-haul "FX" types of circuit; the term " FEX " is used for FX in some non-Bell LECs.
GAIN - Denotes an increase in signal power in transmission from one point to another; usually expressed in decibels. Antonym: " Loss ," a reduction in signal power.
GATEWAY - A conceptual, logical or physical connection between two different networks; the term implies a need for conversion of some aspect of the information or communication in order to operate through a function named a gateway. Compare to " port ," which implies a point not requiring significant conversion of the message or information.
GEOSYNCHRONOUS ORBIT - a position at an approximate altitude of 23,000 miles above the Equator, where a velocity of about 1,000 miles per hour in the same direction as Earth's rotation makes a satellite appear stationary over the Earth's surface. At such a point, ground-based microwave antennae can remain fixed and achieve linkage with transponders on board the satellite to produce a microwave relay between points as much as one-third of the way around the globe, or about 8,000 miles; this concept first proposed by British physicist and science fiction writer Arthur C. Clarke in a 1947 publication.
GRADE OF SERVICE - The probability of a call being blocked by busy trunks, expressed as a decimal fraction, and usually meaning the busy-hour probability. A common "good" grade of service figure is stated as P=.01, indicating that 99.99% of attempts will be serviced, and only 1 of 1,000 will fail. Most traffic designs are made to such a level; poor plant installation or maintenance rapidly degrades such performance.
GROUP - In analog (FDM) carrier systems, 12 telephone circuits transported as a unit. Larger units are supergroups, mastergroups, supermastergroups and jumbogroups.
HALF DUPLEX (HDX) - A circuit capable of transmitting or receiving information both ways, but only one direction at a time; a function of both computer protocols and transmission channels. Many computers operate only half duplex on transmission channels capable of full duplex operation.
HAMMING CODE - In data transmission, a code with added redundant bits for error detection purposes.
HANDOFF - In cellular radiotelephony, the protocol by which a call in progress is transferred from one cell to another as the mobile unit moves.
HANDSHAKE - The exchange of control sequences between two locations to set up needed conditions for communications. In the strictest sense, even the steps of dialing a telephone call can be thought of as a "handshake sequence."
HARDWIRE - Using physical wire or cable directly between units of hardware equipment.
HARMONIC - An exact multiple of a base, or " fundamental " frequency. Examples: Second harmonic = double the fundamental frequency; third harmonic = three times, and 12th harmonic = one dozen times and so on. Sometimes an expression of the required bandwidth for transmission, as in the case of binary transmission where it is well established that a bandwidth equal to the tenth odd harmonic is needed for accurate reception of the pulses; this means the fundamental, third, fifth, seventh, ninth, eleventh, thirteenth, fifteenth, seventeenth, nineteenth and twenty-first harmonics must be passed; thus a 50 bps binary pulse requires a bandwidth of 50 x 21 or 1050 Hertz for accurate capture of the pulses ... unless it is somehow encoded for transmission.
HARMONIC DISTORTION - The ratio, expressed in decibels, of the power at the fundamental frequency, to the power of a harmonic of that fundamental. Odd harmonics, particularly third harmonics, are especially harmful to many forms of telecommunications by both wire and radio; as well, they are annoyingly difficult to design out of electronic circuitry. Some forms of telecommunications require measurement of a parameter called " total harmonic distortion ," meaning the sum of all harmonics reaching the receiver along with the base signal.
HDLC (HIGH LEVEL DATA LINK CONTROL) - A bit-oriented international standard data link protocol used in CCITT X.25 packet network links and influencing many others such as SDLC, BDLC and DDCMP; also Honeywell Data Link Control, Honeywell Corporation's bit-oriented protocol for its computer networks.
HEAD END - A broadband LAN term from its progenitor, Cable TV, for a location on the LAN providing remodulation, retiming, message accountability, contention control and diagnostic control.
HEADER - That portion of a message containing information for routing, handling and delivering a message, such as address, size, priority, intermediate routing and synchronization signals.
HEAD-ON COLLISION - Descriptive of a condition in message telephony when two switching exchanges seize a bothways trunk at the same instant and attempt to send outbound call instructions to each other.
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.
HERTZ (Hz) - International standard unit of frequency. Replaces, and is identical to, the older unit "Cycles-per-second."
HEXADECIMAL - A number system based on 16, providing convenient notation of the 16 possible combinations of half an 8-bit data processing byte; uses digits 0 through 9 followed by letters A through F to count to 16, thus two " hex " digits can describe one byte in software. Example: ASCII letter capital "A" has the decimal value 65 but is written as 41 in hex software code, while small "z" has the decimal value 122 but is noted as 7A in hex, still requiring only two digits instead of three. Using only two hex digits from 00 to FF, a code of 256 different characters can be described as is done with the adaptation of ASCII used by personal computers; the added characters beyond 128 are often called "Extended ASCII," or "IBM graphics characters."
HOMING - Returning to the starting position, as in a rotary stepping switch when its connection is released.
HOOKSWITCH/SWITCH HOOK (HOOK SWITCH) - 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 that disconnects the telephone circuit, and transfers the line connection to the telephone's bell or " ringer " circuit.
HORIZONTAL - Descriptive of the side of a North American wire Distributing Frame on which terminal blocks are mounted horizontally; this is the side equipment is terminated on, as opposed to the "vertical side," on which cables terminate in vertically mounted blocks. In some locations, frames of horizontal blocks only are called Horizontal Intermediate Distributing Frame, or HIDF. Compare to: VIDF.
HOT STANDBY/HOT SPARE - Alternate equipment ready for immediate use if the normal equipment fails.
HOT CUT - Virtually instantaneous replacement of one line with another; when done in multiples to change major facilities, sometimes called a " Flash Cut " or " Flash Cutover ."
HOUSEKEEPING OPERATION - 1.) In data processing, an operation contributing nothing to the throughput or output, but necessary to maintaining operation; 2.) In communications, more commonly called " overhead ."
HYBRID - A very common communications electronics circuit performs the wire conversions necessary for the connection of a 2 wire local loop with a 4 wire long-haul facility; sometimes called a " two wire / four wire converter ." The classic source of echo problems in long-distance telephony, hybrids were once made of expensive, bulky, specially-built transformers; today they are an IC chip full of microelectronics that is even self-adjusting.
IEC - 1.) In telecommunications, see "Interexchange Carrier." 2.) In electrical power safety, the International Electrotechnical Commission, promulgators of power safety standards for the world.
IMS/VS (INFORMATION MANAGEMENT SYSTEM/VIRTUAL SYSTEM) - A common IBM large computer operating arrangement, usually using the MVS ( Multiple Virtual System ) operating system oriented toward batch data processing and data communications-based transaction applications.
INFRARED - That portion of the electromagnetic spectrum located at frequencies just below visible light; in fact, the lasers used for so-called " long-wavelength " fiber optics communications at 1300 nanometers are really in the " near-infrared " region, producing an extremely deep ruby red color of such purity and strength as to damage human eye retinae.
INITIALIZE - Setting all counters, switches, addresses or contents of storage to zero at the beginning of, or at prescribed points in the operation of a computer routine or a communications transfer; a major function of " rebooting " a computer, giving everything a " reset ." Also: Initialize ; re-initialize .
INPUT/OUTPUT (I/O) CHANNEL/PORT - 1.) In computers, the hardware function, usually a bus of parallel wires, hence "channel," that transports data in parallel form between the CPU and peripherals like storage, printers or communications; 2.) In communications, usually a serial data port to a computer, then often with controls per RS-232.
INTELLIGENT TERMINAL - A terminal containing a programmable processor capable of some degree of local processing; the range of functions and degree of capability is not standardized and subject to wide discussion of what constitutes "intelligence."
INTERACTIVE - Involving a duplex, "conversational" exchange between a user and a computer.
INTERCEPT - 1.) In telephony, redirecting a telephone call improper or invalid telephone number to an operator or a recording; 2.) In espionage, receiving telecommunications by wire or radio intended for another party; especially surreptitiously.
INTERCONNECT - 1.) Any of a variety of hardware arrangements that permit the connection of customer's telecommunications equipment to a communications common carrier network. 2.) A portal or communications path or gateway between two common carrier networks. 3.) Industry jargon for non-telcos that provide telephone equipment for customer premises; often called " Interconnect Companies ."
INTER-EXCHANGE MILEAGE (IXC) - The airline mileage between two cities. - Synonym: Long Haul Mileage.
INTEREXCHANGE CARRIER (IEC) - A company providing telecommunications facilities and services principally between the local exchange area LECs, typically identifiable as between two cities or metropolitan areas; particularly interstate and internationally.
INTEREXCHANGE PLANT - The facilities between one switching center and another switching center; sometimes including line-to-line, no-user switching centers called "tandem exchanges ."
INTERFACE - The junction or point of interconnection between two systems or equipment having different characteristics; has both hardware and software implications.
INTERFERENCE - Any unwanted noise, crosstalk or spurious signals on a communications circuit that acts to reduce the intelligibility of the desired information signal or speech.
INTERLEAVING (INTERLEAVE) - 1.) Alternate or sequential transmission of signals from two or more sources in time division multiplexing of a single path; in data communications the interleaving may be character or bit interleaved, with optimum applications for each; 2.) In conjunction with error-correcting codes, a technique to minimize effects of error bursts on message throughput by interleaving groups of characters or blocks that approximate the burst length, so errors tend to affect only one multiplexed channel during an event.
INTER-MACHINE TRUNK (IMT) - A circuit connected between two automatic switching machines; sometimes called a "tandem trunk".
INTERMEDIATE DISTRIBUTING FRAME (IDF) - Descriptive of a wiring termination and rearrangement point located between two main distributing frames (MDF) or between and MDF and an end point in a building.
INTERNATIONAL RECORD CARRIER (IRC) - Carriers providing international telecommunications services, including voice, telex, and data communications. For example, MCI purchased the IRC known as Western Union International and operates it today as MCI International.
INTERNATIONAL STANDARDS ORGANIZATION (ISO) - A world standards body that generally sets standards for any fabricated or manufactured; thus ISO standards apply to physical aspects of the " 25-pin connector " of CCITT V.24/28 data interfaces ( RS-232 ), definitions of the several layers of data networks for which different hardware units may be required, and such diverse things as the exposure speeds of photographic film and definitions of units of measurement. ISO is an Anglicized form of the proper French name of l'Organisme pour l'Standardization Internationale, resident in Geneva, Switzerland.
INTERNATIONAL TELECOMMUNICATIONS UNION (ITU) - Anglicization of the proper French name of the Union Internationale des Telecommunications (UIT), resident in Geneva, Switzerland. ITU is the treaty-established world center for agreements on telecommunications technical and operating standards and is a constituent body of the United Nations, engaging also in international development and education concerning telecommunications.
INTERNATIONAL TELEGRAPH ALPHABET #1 (ITA 1) - World-standard CCITT version of the manual telegrapher's code. Colloquial name: International Morse Code.
INTERNATIONAL TELEGRAPH ALPHABET #2 (ITA 2) - World-standard CCITT version of the 5-unit (also called 7.5 unit) teleprinter code used for Telex, international telegrams and most general telegraphy by wirelines; Colloquial name: Baudot code.
INTERNATIONAL TELEGRAPH ALPHABET #3 (ITA 3) - World-standard CCITT version of a 6-unit extended set of ITA 2 to include characters needed for automatic typesetting directly from telegraph circuits. Colloquial names: Teletypesetter code, press code, extended Baudot code, and others.
INTERNATIONAL TELEGRAPH ALPHABET #4 (ITA 4) - World-standard CCITT version of a 7-unit code in which only the combinations using 4 marking bits are valid; receiving any character with more or less than 4 marking bits is its error checking feature. Colloquial names: 7-unit ARQ Code, Moore ARQ Code, Moore Code, RCA Code, and others.
INTERNATIONAL TELEGRAPH ALPHABET #5 (ITA 5) - World-standard CCITT version of a 7-unit teleprinter code with an 8th parity bit also used for asynchronous data terminals such as minicomputers or PCs. Colloquial name: ASCII code.
INTER-OFFICE TRUNK (IOT) - A direct trunk between local exchange offices.
INTERRUPT - Data processing term for a processing stoppage made in such a way as to be resumable. Compare to a " halt ," typically meaning a stoppage that requires initializing the machine to restart processing.
INTERSTATE - A connection or transaction made across the boundaries of two or more states.
INTRASTATE - Any connection or transaction made within the boundaries of a single state.
ISDN (INTEGRATED SERVICES DIGITAL NETWORK) - A global plan under auspices of the CCITT to provide any information service users may desire on a single worldwide public switched network; the ultimate ISDN has as its goal the elimination of need for discrete telephone, telegraph, data, packet and other networks as well as physically dedicated " special services " circuits as now required to provide broadcast, video, high-speed data and numerous functions users want. Where regulatorily permitted, telecommunications entities may also provide information processing services as well as transportation services; while the CCITT is working on defining these, U.S. common carriers are prohibited from doing so ... at least for the present.
IVDT (INTEGRATED VOICE/DATA TERMINAL) - Perhaps the Holy Grail of telecommunications terminal design, as everyone seems to have a perfect plan for what a single electronic device on a desktop ought to be; what functions it should perform when connected by very specific means to the set of resources it needs. But, like the Holy Grail, nobody can say they have really seen one yet. In fact, anyone who does see one may well go berserk, in keeping with the legend of anyone who does look inside the Holy Grail.
JACK - Any of a range of hardware devices on which the permanent wiring of telecommunications circuits or apparatus is terminated to provide a physical access point by the insertion of a plug.
JITTER - Short term instability of the amplitude and/or phase of a signal. The latter condition is commonly called PHASE JITTER.
JUMP SCROLLING - Characteristic of a terminal with vertical motions of a whole line of characters at a time in discrete steps of one line, much as a teleprinter terminal might do. Contrast with " smooth scrolling " as done by graphics terminals.
KEYSET - A telephone instrument having capacity for two or more telephone lines which are accessed by depressing a button (key) on the face of the set.
Ka BAND - Microwave radio frequencies in the region of 20 to 30 gigahertz, portions of which are used for local microwave " bypass ," notably the region near 23 gigahertz , while future satellite applications have allocations reserved as well.
KEY SYSTEM - The equipment utilized to provide features associated with key sets, including keysets, multipair cable (rapidly disappearing from the scene), key service unit, distribution blocks, and miscellaneous devices.
KSR - Telegraphic nomenclature for "Keyboard Send/Receive," denoting a terminal capable only of transmitting and receiving on line; not able to store sent or received messages. Contrast with ASR .
Ku BAND - Microwave radio frequencies in the region of 10 to 20 gigahertz, portions of which are used for satellite (notably VSAT ) operations at 12 to 14 gigahertz and local terrestrial " bypass " at 10 and 18 gigahertz.
LADT (LOCAL AREA DATA TRANSPORT) - 1.) One of a number of similar names for LEC service offerings of in-city data transport using baseband (short haul) modems rented from the LEC, with names such as LADS ( Local Area Data Service ) being typical; 2.) AT&T's product name for providing simultaneous voice and data (see SVD) by Frequency Division Multiplexing 4800 bps data above speech signals on a two-wire voice telephone exchange line; when used in combination with Centrex and a data PBX in the LEC's exchange building, the resultant data operation is called Datakit.
LASER - Acronym for: Light Amplification by Stimulated Emission of Radiation; converters of electrical energy into light of great purity and thus controllability adequate for telecommunications by guiding the energy along glass waveguides; used more commonly for high-capacity, long-haul fiberoptic links of common carriers; the fiber links of end users most commonly employ less expensive Light Emitting Diodes ( LED ) for shorter distances and lower capacities.
LATENCY - Used by some to describe the time delay from the point when a network station seeks access to a transmission channel and the time access is granted by the network; an evolving term unique to certain LANs.
LAYER; LAYERING; LAYERED ARCHITECTURE - Nomenclature of the OSI Reference Model for data transport that defines the functions in each step of a hierarchy of operations that must occur in order to transfer information between points.
LEASED LINES - Any circuit or combination of circuits designated to be at the exclusive disposal of a given user. - Synonym: Private line; Full Period Line; Dedicated Line, Tie Line (colloquial).
LEAST COST ROUTING (LCR) - The function of automatically selecting the least expensive means for connection of a given call. - Synonym: Most Economical Route Selection (MERS); Automatic Route Selection (ARS); Flexible Route Selection. Typically a feature of electronic PBXs, which vary widely in their approach and abilities to perform the task; requires custom loading of a table of the services available, class-marking of each station on the PBX and maintenance of the tables with changes.
LED (LIGHT EMITTING DIODE) - A semiconductor device that emits light under proper electrical conditions. Used both for simple indicators on electronic equipment and (with proper selection and use)as the source of signals for short-range (multimode) fiber optic transmission systems.
LEVEL - An expression of the relative signal strength at a point in a communications circuit compared to a standard; the standard is most often 1 milliwatt of power at a frequency of 1000 Hertz in a 600 Ohm circuit for 4 wire circuits and 900 Ohms for 2 wire circuits. Internationally, the frequency is 800 Hertz per CCITT standards.
LIGHTWAVE COMMUNICATIONS (LIGHTGUIDE COMMUNICATIONS, OPTICAL COMMUNICATIONS, FIBEROPTIC COMMUNICATIONS) - Descriptive of the use of frequencies in the visible light portion of the electromagnetic spectrum for the transmission of information. "Optical communications" may also mean use of lightwaves in free space.
LIMITED DISTANCE MODEM (LDM); SHORT-HAUL MODEM; SHORT-RANGE MODEM (SRM); BASEBAND MODEM - Descriptive of a wide variety of data modems that take advantage of the wide bandwidth available for, in some cases, up to 20 miles in non-loaded local telephone cable; there are no standards as to how these modems do their job, but there are LEC restrictions on use of certain types on LEC cable facilities. A class of these devices with names like "cable line driver" imply the most minimal of modulation processes; confusion abounds in the descriptive names of this class of devices.
LINE - A widely-variable term for may sorts of transmission paths in telecommunications work.
LINE CONTROL UNIT (LCU) - A data communications term used by some vendors for hardware that controls polling and access by remote terminals; most commonly found in minicomputer networks, LCU capability ranges from simple hardware to processor-based devices with a history dating to telegraph networks.
LINE HIT - Descriptive of short, random disturbances to transmission facilities that cause errors in transmission; non-specific as to the cause or remedy for them.
LINE PRINTER - Descriptive of computer printers that set and print an entire line of data at a time; often indicative of a high-speed, high volume printer.
LINE SIDE - Descriptive of observing from or sending into the transmission path. Antonym: Equipment Side; Drop Side; Terminal Side.
LINE SIGNALS - 1.) In the CCITT sense, equivalent to "supervisory signaling" in Bell network language; 2.) In North American colloquial use, any of a wide variety of communications signals as found transiting a physical transmission medium..
LINE UP - Plant technician colloquialism for testing and adjusting transmission performance of a circuit to specified levels and losses at various points along the circuit.
LINK - 1.) Non-specific term for any sort of communications facilities; 2.) A portion of a circuit intended to connect two points in tandem.
LINK LAYER - The logical second layer of the OSI Reference Model for Open Systems Interconnection, located between the Physical and Network layers.
LOAD FACTOR - Ratio of the Peak to Average ratio over a designated time period; has meaning in both traffic engineering and in transmission technology, particularly for data transmission.
LOADING - A method of adding regularly spaced inductance coils to a physical wire circuit to improve its transmission characteristics for speech; may cause adverse effects to baseband digital transmission. Internationally called "Pupinisation," honoring its Polish developer, the physicist Pupin.
LOCAL ACCESS AND TRANSPORT AREA (LATA) - A geographic area (called "exchange" or "exchange area" in the MFJ) within each LEC's franchised territory established in accordance with the provisions of the MFJ; its purpose is to define the territory within which a LEC may offer its telecommunications services. Transportation of information beyond the LATA must be accomplished by an IEC.
LOCAL AREA NETWORK (LAN) - Intraoffice communication system usually used to provide high-speed data transmission within a limited area. Definitions of the geographic scope of a LAN are fuzzy, as are definitions of its precise capability and capacity.
LOCAL EXCHANGE CARRIER (LEC) - A local telephone company, either one of the 23 Bell Operating Companies (BOC) or one of the 1400+ independent local (non-Bell) telephone companies; their scope of total service offerings is now restricted to within their LATA, with transport beyond their LATA mandated to an IEC.
LOCAL MEASURED SERVICE (LMS, MEASURED SERVICE, MESSAGE RATE SERVICE) - A pricing system used by LECs to charge users ( in the form of " message units ") for local exchange telephone usage, in addition to a basically low monthly fee for telephone service. Widely used since the 19-teens in established, dense telephone markets as a basis for local telephone service. Also rather common in European cities, where users are charged for "pulses" that represent some number of seconds of usage that vary with the distance of individual calls, even on long distance calls extending to intercontinental lengths. Alternatives are " Flat Rate " pricing and " Call Detail Billing ," both seen more frequently in North America, particularly in non-Bell areas.
LOCAL LOOP - The local circuit connection between the end user and the user's nearest telephone exchange office; notorious for being the poorest, weakest link in data circuits.
LOGICAL LINK CONTROL (LLC) - a protocol standardized by the IEEE for use in 802-type LAN, for data link level transmission control; the upper sublayer of Layer 2 of IEEE's OSI equivalent model, complementing Medium Access Control (MAC) in IEEE 802.2.
LONG HAUL (LONG-HAUL) - Descriptive of communications circuits spanning considerable distances; ranging from inter-LATA to intercontinental.
LOOP BACK (LOOPBACK; LOOP-BACK) - A transmission testing method of simply having the distant end on a circuit send back the signals it receives for evaluation at the transmitting end; subject to a variety of ambiguities, even false affirmatives in certain situations. Historical synonyms: Busback; Bus Back; Bust-back.
LOOP SIGNALING - Any of several supervisory signaling methods which use the metallic loop formed by cable wire conductors and terminating equipment; in historic toll cable working, the terms Simplex (SX), DX (DUPLEX) and EMX describe several Bell methods.
LOS (LINE OF SIGHT; LOSS OF SIGNAL) -1.) Descriptive of the use of free-space communications technologies requiring an unobstructed path between transmitter and receiver, as in microwave radio, satellite communications, and infrared optical communications links; 2.) NASA operating term for interruption of communications with a given ground station from a space vehicle as it passes rapidly over the Earth's surface.
MAIN DISTRIBUTION FRAME (MDF) - The point where outside plant cables terminate and from which they cross connect to terminal or central office line equipment. Synonym: Main Distribution Frame; Main Distributing Frame; Mainframe; Main Frame.
MAINFRAME - 1.) One of the telephony synonyms for Main Distributing Frame; 2.) Data processing term descriptive of very large computers.
MAIN PBX - A PBX directly connected to a tandem switching exchange via an access trunk group, bypassing the local end office switching machine.
MAN (METROPOLITAN AREA NETWORK) - A non-specific term arising from the Cable TV industry's description of use of its transmission facilities throughout a city area, with a nominal range of 50 miles. The term has been adopted by LECs to describe fiber-based ring structures with transmission capacity large enough to connect LANs in various parts of a city together.
MANUAL TIE LINE - A tie line which requires the assistance of an attendant at both ends of the circuit in order to complete a connection; incapable of passing dialing down the link. Synonym: Manual Trunk, Ringdown Trunk, Ringdown Tie Line.
MAP (MANUFACTURING AUTOMATION PROTOCOL) - General Motors' data protocol fitting guidelines of the OSI Reference Model, optimized for the short, random messages typical of a factory floor communications environment; most used on broadband coaxial LANs. Contrast: TOP.
MAPPING -In network operations, the logical association of one set of values, such as addresses on one network, with quantities or values of another set, such as devices on a second network; this might be name-address mapping, internetwork-route mapping, or as mundane as telephone jack location-extension number mapping, hence the telephony synonym is Assignment Sheet, Assignment Record, or perhaps even Telephone Directory, as all take the form of columns of equalities listed side by side.
MARK - The signal state on a binary channel representing a "1." Corresponds to current on, hole in paper tape, and (usually) negative voltage, as in EIA RS-232. Antonym: Space; current off; 0; no hole; positive voltage. Telegraphic circuits (with the notable exception of Telex trunks) hold a mark state when idle as a form of circuit assurance alarm technique.
MASTERGROUP (MG) - 10 supergroups each comprised of 5 groups of 12 channels summing up to 600 circuits transported as a unit in an analog FDM carrier system; first used in Bell's type L1 carrier systems on intercity coaxial cable.
MEGAHERTZ (MHZ) - a unit of a million Hertz; meaning millions of cycles per second of AC current.
MESSAGE - In telecommunications, a complete transmission of data or information; often synonomous with "block" in data communications, as 256 characters approximates the length of a world-standard 50-word telegram.
MESSAGE SWITCHING - A telegraphic/data communications technique for receiving an entire message, perhaps storing temporarily if need be, and routing the message on to another switch or a terminal. Compare to Packet Switching and Circuit Switching.
MESSAGE TELEPHONE SERVICE (MTS) - The tariff name for basic long distance telephone calling service; Telegraphic equivalent is Message Telegram Service, to draw a comparison.
MESSAGE TRANSFER PART - That part of a common channel signaling system that transfers signaling messages, error control and signaling link security related to the transfer; the "protocols" of such telephony networks. See also: CCIS, CCITT Signaling System Number 6; CCITT Signaling System Number 7.
MESSAGE UNIT (MU) - A local dial telephone charging plan that is time and distance sensitive; see LMS.
MICROWAVE (M/W) - Radio transmission using very short lengths, corresponding to a frequency of 1,000 megahertz or greater. - Synonym: Microwave Radio; Line of Sight; LOS.
MODEM - Contraction of the term Modulator/Demodulator; device that modulates and demodulates signals on and off a "carrier" frequency; not limited to computer data use, thus the telco-specific term "data set" for data modems.
MODIFIED FINAL JUDGMENT (MFJ) - The consent decree agreement between the U.S. Department of Justice and AT&T governing the breakup of the monopoly-era Bell System into AT&T, 7 Regional Bell Holding Companies (RBHC) 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, but left many details for refinement, a process that continues to this day.
MODULATION - Alterations in the characteristics of analog carrier waves, impressed on the amplitude, phase and/or the frequency of the wave.
MONITORING - Listening in on telephone calls in progress. Subject to severe restrictions as to the purpose and use of information obtained in doing so.
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.
MSU (MODEM SHARING UNIT) - A hardware device, most often a simple contention switch operating on RTS (Request to Send) leads of DTE interfaces, that permits only one terminal at a time to use the modem.
MTBF (MEAN TIME BETWEEN FAILURE) - A statistical method developed and administered by the U.S. military for purposes of estimating maintenance levels required by various devices and systems. Since accurate statistics require a basis of "failures per million hours of operation," an MTBF estimate on a single device is not very accurate; it would take 114 years to see if the device really had that many failures! Similarly, since the MTBF is an estimate of averages, half of the devices can be expected to fail before then, and half after. MTBF cannot be used as a guarantee. Telecommunications systems operate on the principle of "Availability," for which there is a body of CCITT Recommendations.
MTTR - (MEAN TIME TO REPAIR) - A U.S. military statistical method for estimating electronics repair time on hardware devices; not appropriate to telecommunications facilities "restoration," which is the appropriate term.
MULTILEAVING - An IBM Bisync-era method of interspersing message blocks for various applications on a single line. Compare: Interleaving, in its second sense.
MULTIPLE ACCESS - In satellite communications, the capability of a communications satellite to function as part of the communications linkage between more than a single pair of ground stations. When accomplished with analog methods, FDMA is the term; if digital, TDMA.
MULTIPLEX/MULTIPLEXING - Transmitting more than a single message simultaneously on a physical transmission path; if analog, the technique is FDM, but if digital it is TDM.
MULTIPLEXER/MULTIPLEXOR - The hardware device performing combination and decombination of communications signals to and from a larger-capacity "composite" signal on line between multiplexers. Note: the spelling ending "-or" is properly used only for optical units; some sales people attempt it as a stylized name for wireline data units.
MULTIPOINT/MULTIDROP - An arrangement of communications facilities ranging from across a floor to across the globe in which one common facility is shared by all; one station at a time, usually under central polling control, sometimes contention for the channel, as in many LANs, has a history running deep into telegraphic communications. Contrast: Point to Point.
MULTIPLEXING - 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.
NACK/NAK - "Negative Acknowledge" character in many data codes; typically used to indicate receipt of a corrupted message, ordering retransmission; compare to "REJect" character in IBM data systems.
NETWORK - A collection of terminals, computers, peripheral devices, and/or switches connected to one another for a defined purpose by transmission facilities.
NETWORK ARCHITECTURE - The philosophy and organizational concept for enabling communications between multiple locations and multiple organizational units. Network architecture is a structured statement of the terminal devices, switching elements and the protocols and procedures to be used for the establishment and maintenance of effective telecommunications.
NETWORK INTERFACE - 1.) The point of interconnection between the Public Switched Telephone Network (PSTN) and privately-owned terminal equipment; 2.) The point of interconnection between one network and another. Compare: Gateway.
NETWORK LAYER - 1.) Generically, any of several strata of functions in a communications network, such as Terminal, Switching an Transmission; 2.) Specifically in the OSI Reference Model, the third layer, servicing the Transport Layer, ensuring that information is properly routed throughout the network.
NETWORK MANAGEMENT/NETWORK MANAGEMENT SYSTEM (NMS)/NETWORK MANAGEMENT CENTER (NMC) - Treating of the functions of operational control of a network. Functions may include, but not be restricted to: Traffic Analysis; Configuration Control; Transmission Quality; Reliability/Availability; Maintenance Control; Fault Detection or Call Detail Recording. Definitions and strong opinions about what constitutes "Network Management" abound.
NETWORK NUMBERING EXCHANGE - See "NNX Codes."
NETWORK TOPOLOGY - Descriptive of the physical and logical relations of nodes in a network; the schematic arrangement of the links and nodes; often classed as star, ring, tree or bus topology; detail variations and hybrid combinations abound.
NETWORK VIRTUAL TERMINAL - A data communications concept in which a variety of DTE's with different data rates, codes, speeds, formats and protocols are accommodated on a single network. In such networks, Network Processors convert all into a common format for exchange between processors, then reconvert as needed to output data elsewhere.
NNX CODES - The 3-digit code used historically for local telephone exchange prefixes. In the electromechanical era, limitations applied to the range of numbers usable in each position of the prefix. The current numbering plan allows for more variation in assigning Exchange Codes, and under it Exchange Codes are commonly referred to as "NXXs."
NODE/NETWORK NODE - A termination point for communications links; in the sense of data communications, a DTE or processor point, while in the telephony sense, usually limited to a switching machine.
NOISE - Unplanned energy introduced into a communications channel, resulting in transmission errors.
NON-BLOCKING - A switching network with sufficient paths such that a caller can always reach any idle subscriber without encountering a busy; always claimed by vendors, but not always achieved.
NON-VOLATILE (NONVOLATILE) STORAGE - A storage medium that does not lose its contents when power is removed.
NUMBERING PLAN AREA (NPA)/NPA CODE - A geographical division in the North American numbering plan within which no two telephones will have the same 7 digit number. "N" originally was any number between "2" and "9"; "P" was always "1" or "0"; and "A" was any number excluding "0" in the electromechanical era of switching machines. Colloquial synonym: "area code." Internationally equivalent to "City Codes" or "Routing Codes" in other nations, where various number sequences may be used.
NXX CODES - The current general configuration for Exchange Codes within each Area Code. See also: "NNX Codes."
OBJECT PROGRAM/OBJECT CODE - A computer program in its ready to use condition; the result of processing source code through assembler or compiler operations, sometimes called "compiled code"or a "compiled program."
OFFERED TRAFFIC - The volume of communications attempts in any specified period of time.
OFF HOOK - The condition occurring when a telephone is lifted from its mounting, allowing the hookswitch to operate. Synonymous with a "busy" line. British and CCITT English: Engaged; French and CCITT French: Occupe.
OFF LINE (OFF-LINE; OFFLINE) - 1.) Pertaining to terminal devices operating without being under direct control of a central control, as in teleprinter or data terminals preparing messages for later transmission while "off line;" 2.) Descriptive of terminal equipment not connected to a transmission line in any way. Example: An off line card reader or keypunch.
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 PSTN at a distant exchange; the subject of "Leaky PBX" discussion.
OFF PREMISES EXTENSION (OPX) - An extension telephone or keyset that is separated from the premises of its associated PBX (and source of "dial tone"; Inter- or IntraLATA, Inter- or Intrastate, or even international rates and regulation may apply.
ON HOOK - The condition occurring when a telephone handset is placed on its mounting, which causes the hook-switch to open its contacts. Synonomous with an "idle" line. British and CCITT English: Available; French and CCITT French: Libere.
ON LINE SYSTEM (ONLINE SYSTEM; ON-LINE SYSTEM) - A communicating system that requires a terminal to be in communication with and usually under control of a central point ... as in the case of a telephone set.
ON NETWORK CALLING - A term used to describe a call that originates and terminates within the limits and facilities of a private network.
OPERATOR ASSISTED CALLS - PSTN calls requiring manual intervention or assistance to complete; subject to increasing surcharges for the labor cost involved with providing assistance operators.
ORIGINATING OFFICE - The central office that serves the calling party.
OTHER COMMON CARRIER (OCC) - 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. The correct nomenclature has evolved into IEC for any communications common carrier operating across LATA boundaries.
OUT-OF-BAND - Any frequency outside the band used for information frequencies.
OUT-OF-BAND SIGNALING - 1.) In analog telephony, use of narrow band filters to restrict voice signals on a carrier channel below 3,400 Hertz, placing line supervisory signals in the region just above 3,400 Hertz. Common placements for the signaling tone are at 3700 Hertz; the CCITT R2 System places it at 3825 Hertz. 2.) In data and digital communications, using bit or character times not located in the terminal information transfer time slots; rather reserving time for such supervisory information in a special field, as in the T-1 ESF scheme for sending added information while eliminating "bit robbing" as done in earlier T-1s.
OVERBUILD - Adding capacity to a telecommunications transmission network by increasing the capacity of existing facilities.
OVERFLOW - Excess traffic on a particular route that is offered to another route for completion.
OVERRIDE - 1.) In telephony, seizure of a circuit even though it is already occupied; 2.) Other disciplines: Taking control in the presence of an automatic or remote control, modifying its operational decisions if necessary.
PACKET - In the sense of communications, a structured group of binary digits in a prearranged sequence containing synchronism, address, control an error-checking data. Specialized synonym for a "block" of data in CCITT Packet Data Network standards.
PACKET SWITCHED NETWORK - A network dedicated to the routing and delivery of data put in the form of standardized "packets."
PACKET SWITCHING - The technique in which a stream of data is broken into standardized units called "packets," each of which contains address, sequence, control, size and error checking information in addition to the user data. Specialized packet switches operate on this added information to move the packets to their destination in the proper sequence and again present them in a contiguous stream. Compare: Circuit Switching; Message Switching.
PAD - 1.) An attenuator deliberately placed in a transmission channel to cause transmission loss; 2.) Contraction of the term Packet Assembler/Disassembler in Packet Switched networks, the instrumentality that converts user data between steady streams and packets.
PAD CHARACTER/FILL CHARACTER - A control character inserted into data fields when insufficient characters are supplied to fill the data block or the block is incomplete. Interesting to note that a common "pad" character is hex 7F, equating to ASCII "Delete," the same as TWX "rubout" and Baudot "Letter Shift," all essentially the same in every generation of data communications.
PARALLEL TRANSMISSION - Simultaneous transmission of all parts of a signal at one time; in data transmission, requiring a separate signal path for each of the bits of a character; internal to computers, this is called a "parallel bus."
PARITY - a constant state of equality; one of the oldest and simplest methods of error checking data transmission. Characters are forced into parity (total number of marking bits odd or even as selected by choice) by adding a one or zero bit as appropriate when transmitted; parity is then checked as odd or even at the receiver.
PARITY BIT - a check bit appended to an array of binary digits to make the sum of all the digits always odd or always even.
PARITY CHECK - a checking method that determines if the sum of all the digits in an array is odd or even.
PATCH - 1.) a temporary electrical wiring change, usually made with plugs and jacks; 2.) making an improvised modification, to change a computer software routine in a rough or expedient way.
PHYSICAL LAYER - Layer 1 within the OSI Reference Model for Open Systems Interconnection; providing electrical, mechanical and handshaking procedures for transmission. Example: RS-232 is a Layer One function.
POINT OF PRESENCE (POP) - A physical location within a LATA at which an IEC establishes itself and an end point of its network for the purpose of obtaining LATA access and to which the LEC provides access services.
POINT-TO-POINT - A communications circuit between two terminations only. Contrast with Multipoint.
POLAR KEYING/POLAR SIGNALING - A technique in current loop signaling that uses opposite polarities for the two binary states. Tends to provide longer range and suffer less interference than do simpler single-polarity methods. Synonyms: Bipolar; Double Current. Antonyms: Unipolar; Neutral; Single Current.
PORT - Entrance or access point to a computer, multiplexor device or network where signals may be supplied, extracted or observed.
POST TELEPHONE AND TELEGRAPH (PTT) - Foreign government agencies responsible for regulating communications, having the voting status of "Administrations" in the ITU charter Privately-owned entities such as AT&T or MCI are "Recognized Private Operating Agencies, but have only an influence on the vote of their "Administration;" in the U.S. structure, the "Administration" is the Department of State of the U.S.
PREGROUP - Now obsolete term used in certain non-Bell carrier terminal equipments to form up sets of 3 channels for placement into a basic carrier group of 12 channels.
PRESENTATION LAYER - In the OSI Reference Model, the next-to-topmost layer that provides services to the user-interfaced Application Layer. The Presentation Layer is where messages to be transmitted are grouped, formed, converted to and from serial to parallel and undergo conversion between computer and communications codes.
PRIMARY AREA - A customer's local telephone calling area.
PRIMARY INTEREXCHANGE CARRIER (PIC) - The IEC designated by a customer to provide inter-LATA service automatically when the customer dials 1+ an area code and number. Synonym : Default Carrier.
PRIMARY ROUTING POINT - The switch designated as the control point for a long haul telephone call.
PRIVATE BRANCH EXCHANGE (PBX) - A private phone system (switch) used by medium and large companies connected to the local public switched telephone network (LEC), performing a variety of in-house routing and switching. Users usually dial "9" to get outside the PBX to the local lines (in North America).
PRIVATE LINE (PL) - A full-time leased line directly connecting two or more 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. Synonyms: Dedicated Line; Leased Line.
PRIVATE USE NETWORK - Two or more private line channels contracted for by a customer and restricted for use by that customer only.
PROCESSING DELAY - In data communications, the time taken by a computer to operate on an inbound message and return a response; frequently not accounted for in complaints of telecommunications response time problems.
PROPAGATION DELAY/PROPAGATION TIME - The time period between injecting a signal into a communications channel and its exit at the distant end. While not generally significant to speech, propagation delay can be very restrictive to computers, particularly the long delays on satellite channels and to a lesser extent even those between continents on submarine cable; the most common cause in these cases is the "stop-and-wait" nature of many pessimistic computer protocols, which are disappearing from the scene.
PROTECTOR/PROTECTOR BLOCK - Hardware interfacing devices used in telephone cable between inside and outside wire for the purpose of limiting voltages and/or currents to values safe for personnel and equipment.
PROTOCOL - A set of procedures for establishing and controlling the transmission of information. Examples: SDLC; Bisync.
PROTOCOL CONVERSION/PROTOCOL CONVERTER - Generic name and name of the devices that perform a widely variable set of conversions of code, speed, electrical interface and/or block formatting and error checking/correction in data circuits. Example: a packet network PAD operating between an ASCII terminal and a packet network is a form of protocol converter. When the function is to make the line signals of a particular terminal appear like another, the protocol converter is a type called a Terminal Emulator.
PUBLIC SWITCHED NETWORK (PSN) - The pre-Divestiture nationwide network maintained by AT&T and the local telephone companies that provided nationwide telephone service as a monolithic entity.
PUBLIC UTILITY COMMISSION (PUC)/PUBLIC SERVICE COMMISSION (PSC) - The state commissions regulating intrastate communications; variations on the term abound.
PULSE - In communications, typically a signal characterized by a constant amplitude and duration; the line signal representation of a binary digit.
PULSE CODE MODULATION (PCM) - The form of digital transmission in which information for transmission is sampled at regular intervals and a series of pulses in coded form are transmitted representing the amplitude of the information signal at that time. The most common form of PCM in North America is one in which analog signals of less than 4 kilohertz are sampled 8,000 times per second and converted into an 8-bit code, resulting in 64 kilobits per second for transmission. Twenty-four of these streams are Time Division Multiplexed into a 1.544 megabit stream for T-1 transmission. The European equivalent is 30 channels in a 2.048 megabit stream.
PULSE-LINK REPEATER - Connects two E&M signaling circuits back to back, changing the E of one circuit to the M of the other and vice versa.
PULSE MODULATION The modulation of a series of pulses to represent information-bearing signals. Typical methods involve modifying the amplitude (PAM), width (PWM), duration (PDM) or position (PPM). Pulse Code Modulation (PCM) is the most common modulation technique involved in telephone work.
PUSH BUTTON DIALING - Synonyms: Dual Tone Multi-Frequency; Touch-Tone (an AT&T service mark); Key Pulsing (on long distance operator positions).
QUANTIZING NOISE/QUANTIZING DISTORTION - Transmission anomalies of a dynamic nature involved during the analog/digital conversion process of codecs in digital transmission systems; the dynamic nature is that these extraneous artifacts appear only when an input signal is present; they cannot be measured or observed on an idle, quiet channel.
QUEUE - A delay in providing service caused by the inability of the system provided to handle the quantity of transactions attempted; many computer and communications systems operate with deliberate queues as part of their basic economics.
RADIO COMMON CARRIER (RCC) - A communications common carrier providing radio paging, mobile telephone and occasionally microwave relay services to the public.
RADIO FREQUENCY - A broad part of the electromagnetic spectrum ranging from above audio frequencies to beneath infrared; characterized by the use of antennas to couple signals to the Earth's magnetic field.
RASTER/RASTER SCANNING - Providing a means to place information on a screen by sweeping it completely from side to side an top to bottom in a regular fashion. Both CRT tube displays and fax machines utilize forms of raster scanning.
RATE CENTER - A specified geographic location used by the telephone company to determine interexchange mileage for pricing purposes. Prices are often charged from customer to rate center to rate center to customer in LECs even if that is not the most direct route.
REALTIME - Processes that occur instantly as they occur; closely related to "online."
RECORD - In data processing, one complete group of logically related information; closely related to "block" and "packet" in communications technology.
REDUNDANCY - 1.) Duplicate equipment provided to minimize the effect of failures or equipment breakdowns; 2.) Duplicate or added information placed in communications messages to provide a cross-check for transmission errors.
REGENERATION - The process of recreating new, retimed signal pulses from distorted received pulses; being able to send forward signals free of distortion and noise; the "secret" of the clean, clear, noise-free transmission of digital transmission, whether on fiber optic lines or not.
REGIONAL BELL HOLDING COMPANY ( RBHC ) - One of 7 companies established by AT&T to own some portion of the 22 Bell LEC's divested as a result of the 1984 consent decree that took AT&T out of the ownership and operation of local telephone companies. Often inaccurately called RBOC .
RE-HOMING - A major network change involving moving customer services from one switching center to another and establishing the necessary trunking facilities to do so.
REMOTE ACCESS - The ability of transmission points to gain access to a computer at a different location.
REPEATER - 1.) Analog: An electronic device used to amplify signals which have become too weak; 2.) Digital: Electronics that receive distorted pulses and retransmit them in fresh, clean form.
REPEATING COIL/REPEAT COIL - The telephone industry's term for a voice-frequency transformer.
RESELLER - A long distance company that purchases large amounts of transmission capacity or calls from other carriers and resells it to smaller users, using its own switching system for access and billing. Compare: Aggregator.
RESIDUAL ERROR RATE/UNDETECTED ERROR RATE/LATENT ERROR RATE - The error rate at higher levels of a communications system resulting from inability of the error detection/correction methods to trap all errors.
RESPONSE TIME - 1.) A measure of the time for telephone assistance operators to respond to requests for services; sometimes measured in ringing period counts; 2.) The time from entering (typically) a data field on a remote terminal until the response is delivered to the remote by a central computer; a frequent measure of the overall business performance of an interactive computer system.
RESTORATION - The re-establishment of communications service by rerouting, substitution of component parts, or as otherwise determined; the telecommunications service equivalent of "repair" in most other disciplines.
RETARD COIL - Telephone industry term for an inductance coil of fairly large value; acts to impede sudden changes of the current flowing through its winding, hence the descriptive name.
REVERSE CHANNEL - A simultaneous, typically slow-speed data channel from the data receiver to transmitter; usually used in blocked Remote Job Entry types of data circuits to return simply error-checking ACKS and NAKS to remote devices like card readers.
RFI - 1.) REQUEST FOR INFORMATION, a formalized general notice of intent to purchase computer or communications equipment or services, sent to solicit vendor interest and general information on appropriate products; 2.) RADIO FREQUENCY INTERFERENCE - Disturbance to or from radio communications connected with electrical or electronic devices.
RFP (REQUEST FOR PROPOSAL) - A formal invitation to suppliers to provide a fixed, firm price for a specified combination of goods and services, including statements of performance of vendor services.
RFQ (REQUEST FOR QUOTATION) - A formal request for a fixed price for specified goods or services; typically used only for commodity items readily definable in the street market or clearly defined by industry standards; sometimes used for complex major purchases at the stage of setting project budgets.
RING - 1.) The second wire of a two-wire line; second contact of a telephone jack (Compare: Tip); 2.) Attention signal sent forward on a telephone line to alert the destination party of a call for them; alludes to the ringing of a telephone bell; 3.) A network topology in which circuits are connected in a logical circle and transmitted around the entire ring, offering certain advantages in operation and maintenance. Example: IBM Token-Passing Ring LAN topology.
RINGBACK TONE - - Synonym: Audible Ringing Tone.
RINGDOWN - A circuit or method of signaling where the incoming signal is actuated by alternating current over the circuit.
RO (RECEIVE ONLY) - A terminal without a keyboard or other device for information entry. Dating to teleprinters without keyboards, today's very common equivalent is the hundreds of CRT screens found in every major airport; most need only simplex communications.
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 path gets cut or goes out.
ROUTE OPTIMIZATION - - Synonym: Least Cost Routing.
ROUTINE - 1.) Scheduled preventative maintenance on telecommunications physical plant; 2.) A computer data processing program the machine uses to perform repetitive tasks.
ROUTING - The process of determining a prescribing the path a connection or message will take through a switched network.
ROUTING TABLE - A matrix associated with a network control protocol giving the preferred network link directions beyond that point.
ROTARY HUNT/HUNT GROUP/LINE HUNTING - An arrangement allowing incoming dialed calls seek out an idle circuit in a prearranged multi-circuit group or find the next open line to establish a through circuit; several variations exist.
RS-232 - The most common technical specification for interconnection of DTEs to DCEs; extremely close equivalent to the suite of CCITT V.24/V.28 and ISO-2110.
SAMPLING FREQUENCY - the rate at which an inputted information signal is sampled to determine its instantaneous amplitude for subsequent quantization, coding and modulation to be digitally transmitted. Classic theory here requires that the sampling frequency be twice the highest frequency to be transmitted; thus 8 kilohertz becomes the sampling rate for 4 kilohertz analog speech, while 30 kilohertz is needed for 15 kilohertz music.
SATELLITE RELAY - An active or passive repeater in geosynchronous Earth orbit that amplifies the signal it receives, often shifting the radio frequency as well before retransmitting it back to earth; these functions are performed aboard the satellite in a device called a transponder.
SATURATION - 1.) The absolute limit of a traffic handling system to accept and complete orders for connections; 2.) An electrical condition in electronic devices that permits no more electrical signal to pass; saturable inductors called "bridge lifters" in local wire telephony make advantageous use of this principle to provide automatic privacy.
SCATTERING - Diffusion of an electromagnetic signal as it passes through a transmission medium; the ultimate cause of signal loss with distance in fiberoptic lightguides. Used advantageously to create forward tropospheric scatter ( "over-the-horizon microwave") radio, however.
SECONDARY CHANNEL - a low-speed transmission channel provided in many dedicated line data modems to permit simultaneous control and network management transmissions to coexist with the main higher-speed data channel; actually a form of FDM of a voice channel for use by two modems in one DCE.
SEIZING/SEIZURE - a condition in various parts of a communications system when devices or lines are dedicated to the momentary use of one user, generally in response to a request for service.
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.
SERIAL TRANSMISSION - transmitting data characters or bytes one bit at a time, in sequence. Contrast with: Parallel Transmission.
SERVER - 1.) In telecommunications traffic engineering, the generic term for a register or outgoing line that would provide the connection a subscriber demands; 2.) In LANs, a processor providing specific services to network users, as a file server to manage orders for file access, a gateway server to manage exit and entry of information to LAN users and similar LAN operations.
SERVICE AND EQUIPMENT RECORD - A list of equipment billed to customer by type, quantity, monthly charge, location and billing dates.
SESSION - 1.) Engaging two terminal nodes of a network in a logical connection for information transfer, closely akin to a circuit connection in switched telephony; 2.) The fifth layer of ISO's Reference Model for Open Systems Interconnection.
SF SIGNALING (SINGLE-FREQUENCY) - A North American tone signaling system for analog telephone circuits using 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. Cataloged by the CCITT as Signaling System R-1.
SHANNON'S LAW - a statement defining the theoretical maximum at which error-free digits can be transmitted over a bandwidth-limited channel in the presence of noise; the rough equation works out to about 10 bits per hertz of bandwidth in practical analog circuits, making the Shannon limit about 30,000 bps for voice-grade lines.
SHORT HAUL - Generally descriptive of carrier systems designed for use over distances of 50-250 miles length on paired-wire cables; a rapidly-fading distinction as fiber optics becomes common. More appropriate these days as intra-LATA distances.
SIDEBAND - The resultant added signals above and below a carrier frequency resulting from the modulation process placing information on the carrier. Since the process produces two identical sidebands, telephony has in most systems for years filtered one out to save transmission space in AM analog systems, resulting in so-called "single sideband" transmission; more recently similar treatment has been applied to FM microwave radio as well, doubling its transmission capacity.
SIGNAL - Energy intentionally introduced into a transmission path for the purpose of transmitting information. Contrast with noise.
SIGNALING - In telephony, the process of transferring information between two parts of a network to control the establishment of communications between long distance carrier terminal points, and customer equipment required for voice grade dedicated circuits.
SIGNALING CONVERTER - In telephony. a device with input and output signals containing the same information but employing different electrical signals 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 in North America) 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 tone is audible, the signaling equipment is arranged for "tone on when idle" operation; when the channel is seized for use, the tone cuts off.
SIGNAL TO NOISE RATIO - Ratio of the signal power to the noise power in a specified bandwidth, usually expressed in decibels; the smaller the ratio, the poorer the channel. Generally speaking, a ratio of 20 db or more is a channel subjectively "excellent" for telephony, while broadcast television video requires 30 b or more, but 1200 bps modems can function with only 12 db, requiring more as the data speed increases.
SIMPLEX (SX) 1.) A channel capable of transmitting in one direction only; 2.) A derived signaling path over a dry talking wire pair using the two wires of the circuit in parallel, obtained by connecting the midpoints of repeating coils at each end of the channel to signaling units that use the earth for a return path.
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, a reason why loss is often deliberately put into transmission channels.
SMART TERMINAL - a data terminal capable of operating in either a conversational or a block mode; containing a full set of local editing capabilities without reliance on a controlling external computer.
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 (VPN).
SPACE - The communications signal state corresponding to binary zero. Variously represented as no current, no hole in paper tape, (usually) positive voltage. A very "long space" is used in telegraphy as a circuit failure alarm and a disconnect signal for Telex lines. Contrast with Mark.
SPECIAL GRADE NETWORK TRUNK - 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); rapidly being made archaic by improvements in transmission facilities.
SPECIALIZED COMMON CARRIER (SCC) - Common carriers specializing in one type of technology or service. Examples: Mobile radio common carriers, microwave radio common carriers, radiopaging common carriers. Even specialists in marine radio common carriage have existed for decades.
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.
SPREAD SPECTRUM - Nascent to civilian telecommunications, a method of transmitting (particularly radio) signals as a very wideband but low-powered signal that appears almost like noise; at its receiver all the components of the desired signal are filtered from the noise and sum up to a respectable signal. Offers significant potential for local radio and satellite use as a means to increase capacity on crowded radio bands.
START BIT - The leading bit of every asynchronous character, needed to trigger the receiver that a new character is starting; must intrinsically be a space (0) bit.
START-STOP TRANSMISSION - Telegraphic and IBM synonym for asynchronous transmission.
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.
STATISTICAL MULTIPLEXING - A method of adding dynamic buffering and time slot assignment to time division multiplexing, such that time slots are allocated only to active ports; effective capacity increases as high as 800% are achieved in applications such as manual remote inquiry data transmission.
STD - Subscriber Trunk Dialing; the international equivalent of Direct Distance Dialing (DDD), which see elsewhere.
STOP BIT - A mandatory mark bit (1) that must follow the information bits of an asynchronous character to inform the receiver the character has ended; it can be a single bit when sent to electronic receivers, but mechanical printers have classically required a stop bit in excess of 1 bit time. The CCITT long ago standardized it at 1.42 bit times for teleprinters.
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 control 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.
STRAP - Alluding to long-standing optioning of telephone hardware with soldered wire straps, data modems call their optioning, even if done with DIP switches, "straps," while now even software option commands to processing devices have become called "software straps."
SUPERGROUP (SG) - 5 groups of 12 circuits totaling 60 circuits carried as a unit through an analog FDM carrier system; first used in Bell's type K carrier on toll cable with low-capacitance pairs between cities.
SUPERMASTERGROUP (SMG) - 6 mastergroups each comprised of 10 supergroups each comprised of 5 groups of 12 circuits totaling 3600 circuits carried as a unit in an analog FDM carrier system; first used in Bell's L4 coaxial cable carrier systems.
SUPERVISION/SUPERVISORY SIGNALS - Signals of the type of "on-hook" or "off-hook," that indicate the operational status of a circuit; the Bell equivalent of "line signals" in CCITT parlance.
SWITCH - Mechanical, electromechanical or electronic devices for making, breaking or changing connections among circuits.
SWITCHED ACCESS - Connection between a caller's phone and the switching machine of their chosen IEC when a regular long distance call is made. Also the connection between the switching machine of the caller's IEC and the phone in the distant city.
SWITCHED MESSAGE NETWORK - a network service, such as Telex or TWX, providing interconnection of message devices such as teletypewriters.
SWITCHED SERVICE NETWORK (SSN) - a network consisting of terminals, transmission links and at least one exchange on which any user can communicate with any other user at any time.
SWITCHING - The operations involved in interconnecting circuits in order to establish communications.
SWITCHING CENTER - A location at which telephone traffic, either local or toll, is switched or connected from one circuit or line to another.
SWITCHING OFFICE - A telephone company office containing a switch.
SVD (SIMULTANEOUS VOICE/DATA) - An IRC term for Frequency Division Multiplexing of an international voice grade circuit to provide a small number of low-speed data circuits (typically telegraphy) along with a telephone tie line between PBXs in two nations. Used by AT&T to describe its implementation of Data Over Voice on local wire in its Datakit product.
SYNCHRONIZATION - The function of getting a receiver into step with a transmitter, preparatory to starting transmission.
SYNCHRONOUS - Having a regular time relationship between successive bits, characters, blocks, frames, messages or other elements. Even so-called "asynchronous" data has synchronism during the reception of the information bits of each character. On a broader scale, synchronous "networks," of the type of mainframe data networks, establish and maintain synchronism for long periods of time; in this way they avoid the need and overhead of regaining synchronism each character time. Where throughput time is saved, there are tradeoffs of complexity and hardware economy. Compelling arguments can be raise for each approach.
T CARRIER - AT&T's sequential letter designation for its PCM telephone carrier systems, suffixed by a number indicative of the capacity of the system.
T-1 - 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.
T1C - 48 voice channels digitized at 64,000 bps, combined into a single 3.152 mbps digital stream (2 x 8,000 bps signaling plus 64,000 bps framing control), for carriage over intercity "toll grade" twisted pair cable.
T-2 - 96 voice channels digitized at 64,000 bps, combined into a single 6.312 mbps digital stream (4 x 8,000 bps signaling plus 2 x 64,000 bps sub-framing plus 8,000 bps framing) is often placed on bypass microwave; this arrangement is not the original plan of Bell Labs, which expected to use a T-2 system on coaxial cable that never was built, thus "T-2" is somewhat of a misnomer.
T3 - 672 voice channels digitized at 64,000 bps, combined into a single 44.736 mbps digital stream in the form of 28 T-1's plus added framing for transmission on local fiber optic lightguide or microwave radio.
T4 - 4032 voice channels digitized at 64,000 bps, combined into a single 274.176 mbps digital stream in the form of 168 T-1's plus added plus added framing for transmission on intercity fiber optic lightguide or microwave radio.
TABLE-DRIVEN - Describes a logical computer process widespread in the operation of communications devices and networks, where a user-entered variable is matched against an array of predefined values; this is a frequently used logical process in network routing, access security and modem operation.
TAIL END HOP OFF (TEHO) - In a private network, a call carried over flat rate facilities (Intermachine Trunks or IMT) to the closest switching node to the destination of the call, and then connected into the public network as a local call.
TANDEM - 1.) General telecommunications: Connecting the output terminals of one network, circuit or link directly to the input terminals of another, as in tandem linkage of microwave relays; 2.)Specific to telephone switching: A switching arrangement in which the trunk from the calling office is connected to a trunk to the called office through an intermediate point (called a tandem exchange or tandem switch).
TANDEM DATA CIRCUIT - A data circuit relayed by connecting two DCEs back-to-back on their DTE interfaces. At the relay point, transmitted and received data, clock and other control signals must be transposed; if not accomplished in a switching system, a cable between the two must do so, often called a "rollover cable," or "null modem."
TANDEM SWITCHING SYSTEM - - 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.
TCAM (TELECOMMUNICATIONS ACCESS METHOD) - An IBM macro language for creating communications applications programs and message control.
TELCO - Local telephone company; now properly called an LEC.
TELECOMMUNICATIONS - The transmission of voice and/or data through a medium by means of electrical impulses and includes all aspects of transmitting information.
TELEGRAPH - A system employing the interruption of, or change in, the polarity of DC current signaling to convey coded information. Many techniques of data communications follow precepts first established in telegraphy.
TELEMETRY - Technology of measuring a quantity or quantities, transmitting the results to a distant point and there interpreting, indicating or recording the quantities measured; with so many elements common to classic public communications, it can be seen to fit under the umbrella of telecommunications.
TELEPHONE - A device converting acoustical (sound) energy into electrical energy for transmission to a distant point.
TELEPROCESSING - 1.) Remote access data processing; 2.) Use of data link communications to accomplish a computer-based task; distinguished from Distributed Data Processing in that an application processor is not required at each and every node as in DDP.
TELETEX - A communications service enabling document correspondence on an automatic memory-to-memory basis over a switched public communications network; the CCITT's successor to Telex, with a transmission speed of 1200 bps using CCITT International Telegraph Alphabet #5 ... for all practical purposes, ASCII.
TELETEXT - Generic term for essentially one-way broadcast of graphics and text for display on user television sets or low cost CRT's. Many forms simply transmit frame after frame of video repeatedly and the user selects the desire frame(s) at the receiver. Rather long-established and highly evolved in Europe, all North American efforts have been rather ineffective to date, with a number of commercial failures. Typical transmission medium is as a data signal hidden in the blank space of broadcast television vertical synchronizing pulses, although other means are possible on TV broadcast signals as well. Videotex is the interactive form of Teletext (which also see).
TELETYPEWRITER - A machine used to transmit and/or receive communications on printed page and/or tape.
TELETYPEWRITER EXCHANGE SERVICE (TWX) - Originally an AT&T communications service product, now the property of Western Union Corporation under the product name Telex II, transmitting ASCII code at 110 bps; interfaced to a number of other services of different codes and speeds through a mainframe-based protocol converter Western Union calls InfoMaster.
TELEX - The CCITT's global teleprinter exchange switched service based on 50 bps transmission of Baudot code. While archaic in view of recent advances, Telex remains the most conveniently widespread telecommunications service to any corner of the world, reaching more places than the telephone.
TERMINAL - 1.) A point at which information can enter or leave a communications network; 2.) Any device capable of sending or receiving information over a communications channel.
TERMINAL EQUIPMENT - 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. 4.) The end time or end point of a contract for communications services.
THROUGHPUT - A widely varying measurement of the number of bits, blocks, characters, messages or interactive transactions passing though a data communications system. Measurement methods are highly variable and subject to many influences that must be identified in order to obtain a meaningful assessment.
TIE-LINE /TIE LINE/TIELINE - A private leased line linking two phones or phone systems directly. Can ring distant phone automatically when telephone is lifted from its mounting (ARD), when a short code is dialed (OPX), or may relay random calls between two switching machines (DRTL).
TIME DIVISION MULTIPLE ACCESS (TDMA) - a satellite communications technique for sharing use of a satellite transponder by dynamically allocating time slots among its users, under control of a common signaling channel; has many technical commonalities with a land-based "common channel signaling system" for telephony.
TIME DIVISION MULTIPLEXING (TDM) - Equipment enabling transmission of a number of independent signals over a common path by transmitting them sequentially at different instants of time.
TIME-DIVISION SIGNALING - In TDMs, use of a common signaling channel time slot for the telephone supervisory signaling of all telephone channels borne on the TDM link.
TIME-DIVISION SWITCHING - Switching method used by assigning the data entering one port at one end of a data stream to a different port at the other end; used in both electronic telephone exchanges an so-called "switching TDMs."
TIME-SHARING - The sharing of use of a processor among multiple simultaneous users; often using telecommunications to connect users at a distance.
TIME-OUT/TIMEOUT/TIME OUT - A technique used throughout telecommunications to cause an action to default to a preset course, alarm a failure, or control traffic loads. Essentially amounts to placing a timer in the path to run its course, initiating an action if it reaches the end of its time; otherwise resetting the timer and holding it off.
TOKEN PASSING - A LAN access method in which a "token" message constantly circulates around the network, inviting users to attach their messages for transmission; first developed by Eric Soderblom of Sweden. A station may not transmit unless it has the token, meaning the medium is clear for transmission. While most commonly associated with IBM's ring topology LANs, can be applied to any topology.
TOLL CALL - Any call to a point outside the user's immediate local calling area.
TOLL CENTER - 1.) A central office where operators (human or mechanical) are present to assist in completing incoming toll calls. 2.) Name of a Class 4 switching center in the original Bell DDD hierarchy of long distance switching centers, providing links to the local exchanges of a city or metropolitan area; the point of interconnection between local networks and intercity networks.
TOLL OFFICE - A center for the switching of toll calls and the termination of intercity transmission facilities.
TOLL PLANT - The facilities that connect toll offices throughout the country.
TOLL RESTRICTION - 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.
TOP (TECHNICAL OFFICE PROTOCOL) - General Motors' data protocol optimized for use on LANs serving engineering offices, with their typically long graphics file transfers. Contrast: MAP.
TOPOLOGY - The logical or physical arrangement of nodes on a network in relation to each other; common terms are ring, star, bus and tree.
TOUCH-TONE ADAPTOR - A device that can be connected to a rotary dial telephone to allow for DTMF signaling.
TRACE PACKET - A special test packet in packet networks that causes a report to be sent back to the network control center from each node it transits in its path through the network.
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.
TRANSCODER - A digital device converting the coding of one system to another; most common is the T-1/CEPT conversion of 1.544 megabit T-1 streams to 2.048 CCITT digital carrier streams.
TRANSMISSION - The electrical transfer of a signal, message or other form of information from one location to another; most desirably in unaffected form at the receiver.
TRANSMISSION LEVEL - In telephony, the level of power of a signal, (at 1,000 Hertz in North American practice; 800 Hertz globally), which should be measured at a particular reference point.
TRANSMISSION SPEED - Number of pulses or bits transmitted in a given period of time, expressed variably in Bits Per Second (BPS), Words Per Minute (WPM), Characters per Second (CPM), an occasionally as Lines per Minute (LPM) in printer transmission. Skilled technologists can translate one to the other.
TRANSPARENCY ("TRANSPARENT COMMUNICATIONS") - 1.) A basic objective of telecommunications systems, to make the transportation of information invisible to the user; 2.) In data communications, a suspension of control character recognition in certain systems while information transfer is in progress.
TRANSPONDER - 1.) Generally, a telecommunications device that receives a signal in one form and relays it out in another form; 2.) In satellite communications, the prime function aboard a satellite that receives signals on one microwave channel an retransmits them on another; many satellites have 24 to 36 transponders on board.
TRANSPORT LAYER - The fourth layer of the ISO Open Systems Interconnection Reference Model; performs the function of end-to-end control of transmitted information and is responsible for optimized use of network resources, implying a parallel to switched telephony routing control resides here.
TREE - a bus network topology in which branches feed off the bus at various points; closely akin to multipoint lines in telephony; - Synonym: Branching Bus (archaic).
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 - An group of telecommunications switched channels between two locations; the physical path of all may be variable, so long as the logical path is the same.
TRUNK TYPE (TT) - Trunks that use the same type of circuit operation between the same terminating locations.
TRUNK UTILIZATION REPORT (TUR) - A computer printout detailing the traffic use of a trunk.
TSO (TIME SHARING OPTION) - An archaic IBM environment for implementing time-shared use of a mainframe computer; operates under IBM's obsolescent OS-based operating system.
TTL (Transistor-to-Transistor Logic) - An internal transfer standard for electronics devices in which a 1 state is +5 Volts and a zero state is ) volts; communications systems are sometimes expected to interface to this and provide transmission converters to telecommunications standards.
TTY (TELETYPE) - 1.) The registered trade name for teleprinters and data terminals of the Teletype Corporation; 2.) Used generically in the telecommunications industry for teleprinters or data terminals that emulate teleprinter operations.
TURNAROUND TIME - 1.) Time required to reverse the direction on a half-duplex communications channel; 2.) Somewhat misapplied if used in data communications to describe the total response time, including processor time, for interactive data transactions.
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 a twisted pair of wires, connecting a telephone to a switch. Implies half-duplex communications operations.
TELETYPEWRITER EXCHANGE SERVICE (TWX) - A service whereby a customer's leased teletypewriter is connected to a "TWX" switching exchange; thence to a teletypewriter of any customer who subscribes to a similar service. Originally AT&T's equivalent to Telex, TWX is now owned by Western Union and sold under their product name Telex II. Interconnection from and to
TWX an Telex with a wide variety of telecommunications services is available on-line via carrier-provided protocol conversion. TWX operation as a PSN is available in North America only; Telex is the global equivalent.
UDLC (UNIVAC DATA LINK CONTROL) - Sperry-Univac (now UNISYS) version of a bit-oriented computer protocol based on the CCITT's HDLC.
UNATTENDED OPERATION - Transmission automatically controlled; not required a human operator to function.
UNIFORM CALL DISTRIBUTOR (UCD) - A device located at the telephone office or in a PABX which distributes incoming calls evenly among individuals; called a "call sequencer" in some non-Bell LECs.
UNIFORM-SPECTRUM RANDOM NOISE - The laboratory name for "white noise," a test signal made of noise that is constant in its power for every unit of bandwidth; used to test the crosstalk characteristics of multichannel analog transmission systems.
UNIVERSAL SERVICE ORDER CODE (USOC) - The information in coded form for billing purposes use by the local telephone company pertaining to information on service orders and service equipment records. Utterly anachronistic in its name as every operating company has wide differences in terminology.
UPLINK - That portion of a communications link reaching upward from the Earth to a satellite or airborne platform.
USASCII - The North American version of ASCII code; differences from CCITT International Telegraph Alphabet #5 are so minor as to be insignificant to most applications.
VALUE-ADDED NETWORK SERVICE (VANS) - A data transmission network routing transmissions 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. Closely akin to courier services or shipping forwarders in physical commerce.
VERTICAL - Descriptive of the "vertical side" of a North American wire distributing frame, on which terminal blocks for cables are mounted vertically, as opposed to the horizontally-mounted blocks on the equipment side of the frame. Frames of vertical-only blocks are not uncommon at intermediate cable connection points between main frames, hence the term Vertical Intermediate Distributing Frame, or VIDF. Compare: HIDF. English colloquial: VERT; VERTS.
VIA NET LOSS (VNL) - The earlier Bell plan for allocating a fixed amount of loss per mile on long-haul telephone trunks and certain fixed losses to shorter circuits in order to realize a nationwide transmission plant with the lowest possible loss while minimizing echoes and "singing" circuits.
VIDEOTEX - Interactive version of teletext, with a return channel using variable forms of low-speed data. English "Prestel" typifies one realization of videotex.
VIRTUAL CIRCUIT - in packet switching, network facilities with the appearance of a dedicated private line, even though individual packets may constantly be taking variable routes.
VOCODER - an early form of digital speech coder, consisting of a speech analyzer and synthesizer. While unsuccessful in its experiments for general public communications, it became the basis for numerous speech encryption devices because its output could be run through cryptographic equipment to create secure voice communications.
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. Also called Protective Connecting Arrangement (PCA).
VOICE FREQUENCY (VF) - Any of the frequencies in the band 300-3,400 Hz which are transmitted in telephony systems to reproduce human speech voice with reasonable fidelity. Some Oriental languages have less than satisfactory results with this narrow a band, and emerging ISDN implementations are increasing it for them.
VOICE GRADE CHANNEL / VOICE GRADE FACILITY (VGF) - A line suitable for voice, low-speed data, facsimile, or telegraph service. Generally, it has a frequency range of about 300-3000 Hz.
VRC / VERTICAL REDUNDANCY CHECKING - Synonomous with Parity Checking (which also see).
VTAM / VIRTUAL TELECOMMUNICATIONS ACCESS METHOD - IBM's communications I/O control programming software that unburdens the central processor from most routine communications functions; requires a subsidiary processor in its telecommunications adapters running a program called Network Control Program (NCP).
WATS AGGREGATOR - A brokering billing agent having an arrangement to obtain volume discounts from common carriers for the totality of WATS services used by multiple firms; need not be the operator of a switching system.
WAVEGUIDE - Physical transmission medium using boundaries of conductors to guide electromagnetic signals; widely used in microwave radio since WWII; closely akin to fiber optic light guide operations.
WAVELENGTH DIVISION MULTIPLEXING (WDM) - Essentially FDM for fiber optic lines (wavelength is simply the inverse measure of frequency), using optical prisms to combine and separate different optical streams at each end of an intercity line; fiberoptic capacities are readily overbuilt to double original size by this means.
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, originally by means of dedicated WATS Access Lines (WAL) directly connected to the public telephone network at WATS-billing equipped central offices. The dedicated access line operation permits inward or outward service, but not both. Recent evolution permits WATS connection via regular user local PSTN dial lines.
WIDEBAND - A term applied to facilities or circuits where bandwidths are greater than that required for one voice channel; sometimes inaccurately called "broadband" in the common carrier sense of the term.
WINDOWING - 1.) A split screen in some data CRT displays, permitting display of two events simultaneously; 2.) a technique in (mostly PC) data communications protocols that permits the sender to run ahead in transmission, backing up to resend if the receiver signals an error in a recently-sent block; closely akin to "Go Back N" in IBM's SDLC.
WIRE CENTER - The physical structure that houses one or more central office switching systems; the place in a locality at which local distribution cabling focuses.
"0" or "0-" - Zero minus dialing. Allows a caller to dial zero and nothing else to get the Operator.
"0+" - Zero plus dialing; the user also inputs the desired area code and number. An operator assisted long distance call that incurs extra charge for the operator's labor.
"00+" or "00-" - Double zero dialing. Allows a caller to get an IEC Assistance Operator in areas where dialing only one zero would connect the caller with the local Operator; occurs as a result of the division of services into Intra- and InterLATA.
"010+" DIALING - The access code for operator-assisted intercon tinental calls; after the user inputs the required codes and number, an operator is signaled to come on the line, as in domestic "0+" dialing.
"011+" DIALING - The access code for subscriber-dialed intercontinental calls; following the 011, the user inputs the CCITT Country Code followed by any in-country City Code, followed by the destination telephone number.
"1+" DIALING - The capability to dial "1" plus the long distance number for calls within the North American Numbering Plan area. With 1+ dialing, Intra-LATA calls are carried by the local telephone company, while Inter-LATA calls are carried by the caller's primary carrier. Also sometimes called "default" dialing.
"10-XXX" DIALING - The ability to send calls over a carrier other than a caller's primary carrier by dialing "10-XXX" then "1+" the long distance number, where "XXX" is the 3-digit Carrier Code of the alternative long distance company (also called a secondary carrier). Available to all Equal Access customers.
800 SERVICE - 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.
900 SERVICE - 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 $.50 for the first minute of any call and $.35 for each additional minute. Recent evolution has extended to the taking of automatic orders for consumer products with a price of $45 and more, billed by the telephone company; abusive business practices have clouded the image of "900" services as a result, but thoughtful users can make good of it.
976 NUMBERS - 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.
