Telecommunications Dictionary



Two Binary One Quarternary. An ISDN digital subscriber loop encoding technique adapted by ANSI for use in the U.S. This technique encodes a two-bit “ditbit” group using a four state digital line code (two amplitude levels in both polarities) The result is to half the actual transition rate of the digital loop. The 2B1Q plan also calls for substituting digital echo cancellation for time compression multiplexing.


Basic ISDN access. 


Primary ISDN access in North America based on a 1.544Mb/s 24 channel DS-1 standard.


Primary ISDN access in nations using the CEPT digital carrier system.


A set of LAN and MAN standards developed by IEEE.



Analog-to-Digital converter

Intelligent Network (AIN)

A term used to refer to the integration of ISDN and Cellular Radio into a Personal Communications System (PCS) . ISDN provides the ability to assign a phone to a person as opposed to a line termination. When the phone is used it establishes a session with the attached CO switch, and through the services of Signaling System 7, all billing is returned to the proper account. By adding wireless interfaces to ISDN a personal cellular telephone could be attached to the global ISDN from any worldwide location. Problems: the lack of standardization of the radio interface (frequencies and modulation) and the less than uniform deployment of ISDN.


In data communications, a sequence of bits or characters, that identifies a network station, user, or application; used mainly for routing purposes. In telephony, the number entered by the caller that identifies the party called. CCITT X.400 seeks to develop a world wide addressing system for data communications.


Adaptive Differential Pulse Code Modulation. An encoding technique, standardized by the CCITT, that allows a 4kHz voice channel to be carried within a 32Kb/s digital channel. The technique uses 3 or 4 bits to describe the difference between two samples, taken at a sampling rate of 8,000 times a second.


A companding algorithm used as the CEPT standard for DS-0 PCM speech companding.


A feature of network switches, DACS and PBXs, where a call is completed over secondary circuit routes when primary routes are unavailable.



Alternate Mark Inversion. Used in the DS-1 digital signal, where the first mark “1” is a positive voltage and the next mark “1” is a negative voltage. See bipolar. Often used to refer to a DS-1 with robbed bit signaling and without B8ZS coding.


Any electronic component that boosts the strength or amplitude of a transmitted analog signal; functionally equivalent to a repeater in digital transmissions.


The magnitude of variation in an alternating voltage or current from the 0 point. The term is used to refer to the level of an audio or other signal. The adjectives peak, rms, maximum etc. should be used to provide a more accurate description of the signal value.

Mobile Phone Service (AMPS)


The current analog technology used for North American cellular telephone service. Developed in the 1970s and deployed about 1984, AMPS uses a modem driven digital interface for the setup and control of 30kHz analog voice channels.


In communications, transmission employing variable and continuous waveforms to represent information values, where interpretation by the receiver is an estimated approximation (quantization) of the encoded value; compare with digital.


American National Standards Institute. A non-profit organization that performs the administrative functions of the standardization process.


The gain of a radio antenna expressed in dbi (referenced to a theoretical perfect dipole antenna). In microwave radio the gain of a parabolic antenna is calculated by the formula: G (db) =20logF (MHz) +20logD (ft) -52.6.


Layer 7 of the OSI model, defines the final user of the data communications process, normally a data communications program such as VTAM (IBM), or Network Management programs.


American National Standard Code for Information Interchange. A standard seven-bit (eight bits, with parity) character code used for data communications and data processing.

– Arpanet

Advanced Research Projects Agency. Part of the Department of Defense. Developer, through project coordination and funding, of most modern data communications technology, including HDLC, Packet Switching, and TCP/IP. Established the Arpanet, a network of major universities, defense contractors and government agencies, that was used to test and improve TCP/IP.


In data communications, transmission that is not related to a specific timing of the transmission facility. Data transmission characterized by individual characters, or bytes, encapsulated with start and stop bits, from which a receiver derives the necessary timing. Also known as start/stop transmission. In high speed digital transmission, a DS-1 with framing bit is said to be synchronous while the multiplex bit interleaved DS-2 or DS-3 signals are said to be asynchronous.


Asynchronous Transfer Mode. A CCITT term for Broadband ISDN technology using cell relay techniques. Multiplexing and switching is accomplished with 53 byte cells each containing a channel identifier.

Adaptation Layer (AAL)

A physical sub-layer of the ATM standards. The AAL prepares data for placement in cells. AAL has two sub-layers. The Convergence sub-layer forms a large identifiable frame. The segmentation & reassembly sub-layer divides the convergence frame into 44-47 byte segments.


A physical sub-layer of the ATM standards that places segments formed in the ATM Adaptation Layer (AAL) into cells. The ATM layer manages the cell overhead.


Reduction of loss of signal strength, measured in decibels; opposite of gain.


Automatic answering; capability of a terminal, modem, computer, or similar device to respond to an incoming call on a dial-up telephone line, and to establish a data connection with a remote device, without operator intervention; unattended operation for incoming dial-up calls.


Automatic dialing; capability of a terminal, modem, computer, or similar device to place a call over the switched telephone network, and establish a connection, without operator intervention; also, autocall.


Automatic Digital Network; part of U.S. Department of Defense.

Repeat Request (ARQ)

A data communications term referring to the automatic request for retransmission of data found by the error detection system.

Route Selection (ARS)

The capability of a switch, typically a private branch exchange (PBX), to automatically determine an optimal route establishing a circuit; also called least-cost routing (LCR).




Binary 8 Zero Substitution . A technique used with DS-1 signals in order to maintain ones density, where a special code is substituted for eight consecutive zeros and marked by two bipolar violations. The use of B8ZS allows the use of a full 64kb/s DS-0 for data transmission. Not compatible with older “AMI” equipment. (Referred to Bipolar 8 Zero Substitution by early AT&T ref.)


An implied three-wire circuit, where the impedance-to-ground on one wire equals the impedance-to-ground on the other wire; compare with unbalanced-to-ground (two-wire) circuit. A preferable condition for data transmission. Standardized by EIA RS-422.



Balanced/unbalanced, refers to an impedance-matching device used to connect balanced twisted-pair cabling with unbalanced coaxial cable.


The difference, expressed in Hertz (Hz), between the highest and lowest frequencies of a transmission channel. The term is also commonly used to refer to the line rate of a data channel in bits per second. The information carrying capacity of a communications channel is directly proportional to its bandwidth.


(a) In Frequency Division Modulation (FDM) “baseband” describes the band of frequencies occupied by the composite modulated analog carrier frequency. (b) In LAN technology, “baseband” refers to using Time Division Multiplex (TDM) transmission on a single channel versus Frequency Division Multiplex (FDM) “broadband” multi-channel transmission.


In ISDN, basic access consists of two 64kb/s Bearer channels and one 16kb/s “D” channel (2B+D). This is the minimum ISDN service available.


Beginners All Symbolic Instruction Code. A programming high level language.


A measurement of the signaling speed of a data transmission device; equivalent to the maximum number of signaling elements, or symbols, per second that are generated; may be different from bit rate, especially at higher speeds, as several bits may be encoded per symbol, or baud, with advanced encoding techniques such as phase-shift keying.


The original teletype data transmission code. Implemented about 1870 it was composed of marks and spaces so named because of the practice of punching marks into a paper tape. Baudot is using five bits for character representation, one of which is a letters/figures shift. Only upper case letters are used. Transmission was asynchronous and originally accomplished by reversing the polarity of the DC telegraph line. By the 1920s FSK modulation was used. In Telegraphy, Baudot code was replaced by ASCII in the late 1960s.


Block check character ; control character appended to blocks are character-oriented protocols that are used for determining if the block was received in error. They are used in longitudinal and cyclic redundancy checking. The BCC was developed for early teletype and IBM Bisync character transmission.


See Bit Error Rate


Bell Communications Research ; an organization established by the AT&T divestiture, representing and funded by the Regional Bell Carriers RBOCs, for the purposes of establishing telephone-network standards, training materials and quality procedures.


Radius of curvature that a fiber can bend signal loss.


Basic Exchange Radio Telecommunications Service. A system developed in the 1980s to provide wireless access to a standard local loop. BERTS uses TDMA on multiple FDMA channels in the 150-450MHz and 800MHz range. Voice Channels are 16Kb/s ADPCM with DSI. Spectrum efficiency is 80 channels per MHz. Typical applications include rural telephone service, rapidly expanding isolated subdivisions and emergency restoration. Wire line T-1 service from a central office is extended to a transmittersite. A receiver is installed as network equipment and connected as a subscriber loop to the protector block. BERTS is FCC approved and US West has undergone field trials to incorporate this technology in a metropolitan PCN. This test uses a cellular (450MHz) concept connected to a standard digital loop carrier (DLC). This provides loop access with a digital cellular instrument. (Also called BEXR, Basic Exchange Radio)


(a) In digital transmission, an electrical line-signaling method in which the mark value alternates between positive and negative polarities. (b) In semiconductors, a design based on the flow of current across a PN junction.


Broadband ISDN. Refers to ISDN services offered at rates higher than the Primary access rate (23B+D) of 1.544MB/s or 2.048Mb/s. Proposed broadband ISDN service is defined by CCITT as switched services from 34Mb/s to 680Mb/s using cell relay technology. Channels are designated as “H” channels.


Binary synchronous communications (BSC): Character-oriented data communications protocol developed by IBM; oriented toward half-duplex link operation; still widely employed, though replaced in current IBM data communications products by the bit-oriented synchronous data link control (SDLC).


A binary digit, the representation of a signal, wave, or state, as either a binary zero or a one.


The case where the value of an encoded bit is changed in transmission, and interpreted incorrectly by the receiver.

Error Rate (BER)

The percentage of received bits that are in error, relative to a specific amount of bits received; usually expressed as a number referenced to a power of 10; e.g. 1 in 105 .


Describing a communications protocol or transmission procedure where control information is encoded in fields of one or more bits; oriented toward full-duplex link operation; uses less overhead, and is therefore more efficient than character (byte) oriented protocols.


A process of adding bits (marks) to a data stream. Used in bit-oriented data link protocols in order to prevent the “flag” sequence (01111110) from entering the data block. Also used to balance the input/ output flow in asynchronous data communications buffers (DS-2/DS-3).


Bits per second; basic unit of measure for serial data transmission capacity; kb/s or kilobit/s, for thousands of bits per second; Mb/s or megabit/s, for millions of bits per second; Gb/s or gigabit/s for billions of bits per second; Tb/s or terabit/s for trillions of bits per second.


A quantity of transmitted information regarded as a discrete entity and containing its own starting and ending control delimiters, usually self-contained control, routing, and error-checking. Block assembly is normally considered a data link (OSI level 2) function but may be accomplished by level 3 or 4.


The inability of a network, switch, or access node to grant service to a requesting user due to the unavailability of a transmission channel.


In IBMs SNA, a sub area node that can provide certain protocol support for adjacent subarea nodes, including transforming network addresses to local addresses, and visa versa, and performing session-level sequencing and flow control for less-intelligent peripheral nodes.


A device used to connect networks with similar OSI level 1 & 2 protocols. Example: connecting two IEEE 802.5 token ring networks.



Describing transmission equipment and media that can support a wide range of electromagnetic frequencies; typically the technology of CATV transmission, as applied to data communications, that employs coaxial cable as the transmission medium and radio-frequency carrier signals in the 50 to 500MHz range; any communications channel having a bandwidth greater than a voice-grade telecommunications channel, sometimes used synonymously with wideband.




A Local Area Network (LAN) that is distributed via broadband coaxial cable normally utilizing CATV technology and broadband modems. Most commonly used with the Ethernet (CSMA/CD) and Token Bus.


Delivery of transmission to two or more stations at the same time over a common transmission channel. Examples: a bus-type local network, a satellite system, cable or microwave TV.


See bisync.


Process of temporarily storing data in RAM, which allows transmission devices to accommodate differences in data rates and to perform error checking and retransmission of data received in error.


A transmission path or channel; typically an electrical connection, with one or more conductors, wherein all attached devices receive all transmissions at the same time; a local-network topology, such as used in Ethernet and the token bus, where all network nodes “listen” to all transmissions, selecting certain ones based on address identification; involves some sort of contention-control mechanism for accessing the bus transmission medium.


An 8-bit quantity of information, also generally referred to in data communications as an octet or character.




Portion of the electromagnetic spectrum used heavily for satellite and microwave transmission; frequencies of approximately 4 to 6 GHz.


Computer Aided Design.


Computer Aided Instruction.

Detail Recording (CDR)

A feature of private branch exchanges where each phone call is logged, typically by time and charges, and retrievable through a PC by the network operator for departmental billing. Also called Station Message Detail Recording (SMDR).


Computer Aided Manufacturing.


Community Antenna Television (formal), or Cable Television (colloquial); data communications based on radio frequency (RF) transmission, generally using 75-ohm coaxial cable as the transmission medium; communications via coaxial cable where multiple frequency-divided channels allow mixed transmissions to be carried simultaneously; broadband.



A continuous frequency capable of being modulated or impressed with a second data-carrying signal.


Computerized branch exchange; see PBX. The terms “CBX, EPBX and PBX all represent a computer controlled Private Branch Exchange (PBX).


Consulting Committee for International Telegraph and Telephone (from the French, Comite Consultatif International Telegraphique et Telephonique). A committee of the International Telecommunications Union of the UN that advises the United Nations on international telecommunications standards. CCITT standards will be the overall voice/data standards of the 21st century. Sometimes referred to as the ITU-T committee.


Centrum (Hundreds) Call Seconds. A unit of traffic measurement in telephone traffic engineering that represents a circuit, connection, or port where usage of 36ccs, or one Erlang, is in use all the time; typical usage for most voice communications ranges from about 3 to 10 ccs per user station.


(a) Referring to the geographic area served by a single transmitter in a cellular radio network. (b) Referring to a sub-frame, typically 53 bytes, used for ATM statistically multiplexing and Cell Relay switching.


Technology employing low-power radio transmission as an alternative to local loops for accessing the switched telephone network; users may be stationary or mobile. When mobile they are passed under control of a central site from one cell’s transmitter to an adjoining one with minimal switch-over delay. (See AMPS)


A high bit rate asynchronous or synchronous data multiplexing and switching technique based on “Fast Packet” technology. CCITT I.422 specifies a 53 byte cell with a payload of 48 bytes. The 5 byte header contains virtual channel and virtual path identifiers. Cells may be transmitted along permanent virtual circuits or mapped into SONET frames. (See ATM)

Office (CO)

In telephony, the Local Exchange Carrier (LEC) [phone company] switching facility. Also called Local Exchange or wire center. Refers to a Class 5 switching office, at which subscribers’ local loops terminate; handles a specific geographic area, identified by the first three digits of the local telephone number.


A telephone company switching service that uses central-office switching equipment to provide customers PBX features. Typically, the CO is partitioned into 10 number blocks and connected via individual-extension lines to the customer. The Centrex CO is programmed to provide advanced telephone features. Nearly all telephone features can be supplied including direct inward calling dialing (DID), attendant consoles, group pickup and least cost routing. This service has become the best selling LEC service in the 1990s. It is very attractive economically to small and medium size business.


Conference of European Postal and Telecommunications administrations. This standards group represents most international telephone companies outside of North America & Japan.


In communications, a physical or logical path allowing the transmission of information; the path connecting a data source and a data sink, or receiver.


DS-0 to DS-1 multiplex equipment primarily used for analog voice to Pulse Code Modulation (PCM) conversion and multiplexing. Typically used in a telephone central office as a subscriber loop carrier device. The channel bank also detects and transmits signaling information for each channel, and inserts a framing bit so time slots allocated to each channel can be identified by the receiver. Current technology is the D4 channel bank with ESF and B8ZS enhancements.


Describes the attachment of devices directly to the input/output channels of a computer mainframe (IBM). Refers to devices attached to a controlling unit by cables rather than by telecommunications circuits.


Standard bit representation of a symbol, letter, number, or punctuation mark; generally contained within a byte.


One of several standard sets of binary representations for the alphabet, numerals, and common symbols, such as ASCII, EBCDIC, and BCD.


Describing a data communications protocol that carries control information encoded as character strings. Example: “ETX” {end-of-transmission}; “SOH” {start-of-header}, etc.


The term associated with the result of a CRC calculation used for frame error checking. See Frame Check Sequence (FCS) and Cyclic Redundancy Check (CRC).


Customer Information Control System; and IBM program product and mainframe operating environment, designed to enable transactions entered at remote terminals to be processed concurrently by user-written applications programs; includes facilities for building and maintaining databases.


Generally referring to a transmission medium connecting two or more electronic devices.


Theprocess of establishing and maintaining a circuit between two or more users on demand, so the users have exclusive use of the circuit until the connection is released. In digital switching “Space Switching” refers to the circuit switching function for a dedicated time slot.


Cellular Telecommunications Industry Association (CTIA) , also called TIA. A group of manufacturers and service providers organized to improve cellular technology. Primarily functions in the U.S..


In optical fiber, that low refractive index material that surrounds the core and provides optical insulation and protection to the core.

X office

Designation of a phone company switching facility in the telephone hierarchy, where Class 5 is an end office, Class 4 is a toll center, Class 3 is a primary center, Class 2 is a sectional center, and Class 1 is a regional center. Since 1983, only Class 5 and Class 3 switches are employed.

Of Service (COS)

Designation for one of several variable network connection services available to the user of a network, usually distinguished by security offered (such as encryption), transmission priority, and bandwidth; the network user designates class of service at connection establishment, typically using a symbolic name mapped into a list of potential routes, any of which may provide the requested service.


In digital communications, the ability to provide a 64kb/s DS-0 for the
customer. Implies that telephone signaling information is out-of-band
and that a density of ones (1s) is maintained by B8ZS. See B8ZS.


An oscillator-generated signal that provides a timing reference for a
transmission link; used to control the timing functions such as sampling
interval, signaling rate, and duration of signal elements. A digital
network typically has a “master” clock.


A device that handles the remote communications processing for multiple
(usually dumb) terminals or workstations; generally considered as
an IBM 3270-family controller, such as the IBM 3274, or compatible


Complementary Metal Oxide Semiconductor. A semiconductor that utilizes a thin layer
of metal oxide to conduct current between two identical PN junctions.


Communications Satellite Corporation; private U.S. satellite carrier, established
by Congress in 1962, for the coordination and construction of satellite
communications and facilities for international voice and data communications.


A popular transmission medium consisting of a central wire conductor
surrounded by dielectric insulator and encased in a wire mesh. Coaxial
cable provides excellent high frequency transmission (50 – 500MHz)
and data rates to 45Mb/s. Commonly uses as CATV transmission cable,
56kb/ s-45Mb/s data cables and Ethernet LAN connections. Also called


Coder/decoder; an integrated circuit (IC), or series of ICs, that performs analog-to-digital
conversion of a 0-4kHz voice signal to a 64Kb/s PCM digital bit stream.
The term also refers to an analog-to-digital television signal converter,
Video CODEC.

Division Multiple Access (CDMA)

A spread spectrum access technique that addresses individual bit groupings
to specific radio receivers, either satellite or cellular telephones.
The signal is transmitted just above noise level across the available
bandwidth. CDMA is in use as a satellite technology but will require
much additional development for cellular telephones.


In the United States, any supplier of transmission facilities or services
to the general public that is authorized to provide such facilities
or services by the appropriate regulatory authority. A supplier of
the Public Switched Telephone Network (PSTN). Either a Local Exchange
Carrier (LEC) or an Inter eXchange Carrier (IXC).

Channel Interoffice Signaling (CCIS)

A telephone signaling technique that places multiple channel signaling
information in a separate channel along with network management information.
CCIS was introduced by AT&T in the 1970s and used modems to carry
signaling information. CCITT Signaling System 7 (SS7) is the current
standard CCIS and is used by nearly all IXC and many LEC carriers.
SS7 uses a separate fast packet switched network of DS-0 channels
for network control.


Compressing/expanding; the process of reducing the bandwidth required for representation
of an analog waveform for transmission, and then reconstructing (most
of) the original waveform at the receiving end. In digital voice (PCM),
a nonlinear allocation of sampling intervals used to lower quantization
error over a wider dynamic range of input signals.


Any communications device that allows a shared transmission medium to
accommodate more data sources than there are channels currently available
within the transmission medium.


Extra-cost options that users may apply to leased, or dedicated, voice-grade
telephone company circuits used for data transmission, that will generally
allow for higher-quality and/or higher-speed data transmission. Frequency
response (attenuation distortion) and envelope delay distortion are
improved by C-conditioning, available in grades C1, C2, or C4. Signal-to-noise
ratio is improved by D1 conditioning.


In data communications, extra transmitted characters used to control
or facilitate data transmission between terminal equipment (DTE) and
data circuit-terminating equipment (DCE), or peripheral equipment.
See byte-oriented protocol.


The central light transmission part of the optical fiber with a refractive
index higher than that of the cladding.


Customer premises equipment; in telephony, equipment that interfaces to the
telephone network and physically resides at the user’s location; includes
most, but not all, gear referred to as network channel terminating
equipment (NTE).


Central Processing Unit.


Cyclic Redundancy Check. A register mathematical calculation using a polynomial
algorithm applied to each bit in a transmitted frame, that is matched
with an identical algorithm within the receiver. A match produces
a 99.99+% error free result (typically based on a 16-bit field appended
to the transmitted frame). Used within all modern data communications


An electromechanical telephone switch that uses moving electronic bars
to connect vertical and horizontal leads. Introduced by AT&T in
the 1930s, this technology is still employed in some telephone central


Unwanted transference of electrical energy from one transmission channel to
another, usually adjacent, channel.


Carrier-Sense Multiple Access with Collision Detection. A popular Media Access Control
(MAC) technology, where all devices attached to a local network listen
for transmissions in progress before attempting to transmit and, if
two or more begin transmitting at the same time, each backs off (defers)
for a random period of time (determined by a set algorithm) before
again attempting to transmit again. Used in packet radio and the Ethernet.


Channel Service Unit. A customer premises equipment (CPE) device used to terminate a DS-1 or DS-0 [56/64 kb/s] digital circuit. The CSU must comply with FCC rules and store error information provided by the Extended Super Frame (ESF). The CSU also performs line-conditioning, protection, loop-back and timing functions.


Cathode-ray tube


Clear To Send; modem control code.


Continuous Variable-Slope Delta modulation. A speech digitizing and encoding
technique that uses a one-bit sample to encode to difference between
two successive signal levels; sampling usually done at 32,000 times
a second, although some implementations employ lower sampling rates.

Redundancy Check (CRC)

An error checking procedure commonly used in data communications in which each transmitted
bit in a frame is applied to a register containing series of XOR
gates thereby solving a polynomial equation. The result of this
operation is appended to the completed frame and compared to an
identical register in the receiver. A match equals an error-free
frame. There are several standard CRC checks; CRC-6 used in the
DS-1 ESF, CRC-16 used as the ISO data communications standard &
CRC-32 used in the IEEE 802 LAN standards. See FCS; Checksum.




Digital Access and Cross Connect System. A TSI switch that switches DS-0 channels
among DS-1 data streams. Computer controlled and designed for keyboard
entry as opposed to automatic switching. There are many uses, including
semi-automatic test boards, digital circuit concentration, trunk switching,
and alternate routing. The term “DACS” was extended for DS-0/ DS-1
switching but has been applied to DS-1/ DS-3 switching. Many large
“DACS” units include combined switching systems that allow DS-0 switching
among DS-3s, always with demultiplexing of the DS-3 data streams.
IXC and LEC carriers offer partitioned DACS service under customer
control, which allows the customer to control a Virtual Private Network
(VPN) within the carrier’s network.


Application of several techniques that reduce the number of bits required to represent
information in data transmission or storage, therefore conserving
bandwidth and/or memory. CCITT V.42 is the modem transmission standard.

Encryption Standard (DES)

Cryptographic algorithm designed by the National Bureau of Standards to encipher
and decipher data using a 64bit key; specified in Federal Information
Processing Standard Publication 46, dated January 15, 1977. There
have been several modifications to this standard.


In packet switching, a self contained packet message. Each datagram carries
routing information. Datagrams are required for connectionless data
service. No longer offered in the X.25 standard. The DDN (ARPANET)
is organized around a series of datagram messages.


A PBX switch that allows digital transmission (without modems) as well
as voice. An option offered today by nearly all manufacturers.


Refers to level 2 of the OSI model. A serial data communications connection
between two adjacent node.


A modem; infrequently used today except within the telephone carrier


A service and trademark of AT&T; generically refers to the transmission
of data over the phone network (Dataphone Digital Service, or DDS)
using 56Kb/s or 64Kb/s channels.


Decibel; a unit of measurement used to express the ratio of two values, usually
the power of electrical signals; equal to 10 times the logarithm of
the power output/input of the channel. When voltage or current are
compared, the formula would be 20log output/input.


Decibel referred to one milliwatt. A milliwatt is a power measurement, therefore,
the formula would be dBm=10log measured power in milliwatts/1. Used
in telephony to refer to relative strength of a signal.


Decibel referred to a 0 value set by the telephone company.


Decibel referenced to the existing noise level of a telephone circuit. Used
in telephony as a signal-to-noise ratio measurement.


Data Circuit-terminating Equipment; sometimes, Data Communications Equipment.
Equipment that is either part of the network, an access point to the
network, a network node, or equipment at which a network circuit terminates.
In the case of an RS-232-D connection, the modem is usually regarded
as DCE, while the user device is DTE, or data terminal equipment;
in a CCITT X.25 connection, the network access and packet-switching
node is viewed as the DCE.


Digital Data Communications Message Protocol (Digital Equipment Corporation).
Similar to HDLC.


Dataphone Digital Service. The first private-line digital service offered by
AT&T, with data rates typically at 2.4, 4.8, 9.6 and 56Kb/s; now
a part of the services listed by AT&T under the Accunet family
of offerings. All LEC and IXC carriers offer similar services.


See Data Encryption Standard (National Bureau of Standards).


A dedicated circuit, a non-switched channel, or a private line; See
leased line.


The process of retrieving an original signal from a modulated carrier
wave. See modulation.


Describing the process of establishing a temporary connection via the switched
telephone network.


Referring to communications procedures, techniques, and equipment where information
is encoded as either a binary “1” or “0”.


Referring to cellular telephony using compressed digital speech and digital
modulation, as opposed to analog voice channels. Digital techniques
will improve the use of available spectrum by factors of 3 to 7 while
reducing noise and allowing the efficient transmission of digital
information (Fax & Data). Current plans call for a gradual migration
to digital cellular technology.

Local Loop

In ISDN, describing the existing two wire telephone connection between
the CO or remote and the customer (NT-1) over which Basic Access is


The process of establishing and maintaining a connection, under stored
program control, where binary-encoded information is routed between
an input and an output port through a virtual circuit established
by a time shared matrix (Time Multiplex Switching {TMS}) or by buffering
data streams and interchanging time slots (Time Slot Interchange {TSI}).
Both techniques are more efficient than dedicated matrix circuits.

Inward Dialing (DID)

A PBX option wherein the CO transfers the last four digits of an incoming
call to the PBX and the extension is then reached directly without
going through an operator.

Outward Dialing (DOD)

A PBX feature allowing an internal caller at an extension to dial an
external number without going through an operator. Typically accessed
by dialing 9 followed by the external number.


The cause of bandwidth limitations in a fiber. Dispersion causes a broadening
of input pulses along the length of the fiber. Two major types are:
a) modal dispersion caused by differential optical path lengths in
a multimode fiber, and b) material dispersion caused by a differential
delay of various wavelengths of light in a waveguide material.


The corruption of a signal caused by time delay, harmonics, frequency
response noise or other disturbances.

Data Processing (DDP)

Describing a network of geographically dispersed, though logically interconnected,
data processing nodes; generally configured so that nodes can share
common resources, such as a file server, a print server, host applications,
or a database; communications between DDP nodes may be sporadic or
intensive, interactive or batch; also, distributed processing.


A structure (typically wall-mounted) for terminating telephone wiring.
Usually consisting of 50 pair blocks designed for “punching” permanent
wires to interconnect telephone and/or data (LAN) circuits.


The breakup of AT&T, mandated by the federal courts based on an antitrust
accord reached between AT&T and the U.S. Department of Justice;
effective January 1, 1984; most notable effects include the separation
of 22 AT&T-owned local Bell operating companies (BOC) into seven
independent regional Bell holding companies, the requirement for AT&T
to manufacture and market customer premises equipment through a separate
subsidiary, and use of the Bell name an logo only by the divested


In IBM’s Systems Network Architecture (SNA), a host-based Systems Services
Control Point (SSCP) and its Sub-Area nodes. Domains are interconnecting
through a boundary function gateway.


Usually a software module that, under control of the processor, manages an
I/O port to an external device, such as a serial RS-232-C port to
a modem.


Distributed Queue Dual Bus. A metropolitan area network (MAN) technique standardized
by IEEE 802.6 in late 1991. The DQDB name describes the LLC/MAC technique
of the MAN. The DQDB MAN is designed to provide LECs with a high speed
open access data bus to serve customers with packet type requirements
at speeds of 45Mb/s and above. The future of this technology will
depend on user requirements and the cost advantage over direct DS-3
or SONET connection.


Digital Signal level 0. The bandwidth of a digitized PCM voice signal, 64kb/s.
May carry voice or data.


Digital Signal level 1. A 1.544Mb/s digital signal consisting of 24 DS-0s
and a framing bit (193 bits) transmitted 8000 times per second. May
be carried on a T1 facility or other transmission medium.


Digital Signal level 1C. Term describing a 3.152Mb/s digital signal comprised
of two DS-1s. Used in many older LEC installations but rarely installed


Digital Signal level 2. A multiplexed 6.312Mb/s digital signal comprised of
four DS-1s and 96 DS-0s. Often carried on a T2 facility which is the
basis of the Subscriber Line Carrier (SLC-96) used as a common LEC
remote concentrator.


Digital Signal level 3. A multiplexed 44.736Mb/s digital signal comprised
of 7 DS-2 or 28 DS-1 signals. Contains 672 DS-0 channels. Multiple
DS-3s are multiplexed onto fiber optical carriers.


Data Service Unit. Customer premises equipment (CPE) used to interface
to a digital circuit. Commonly associated with DDS termination. Provides
DTE termination of data circuit and is now generally combined with
a Channel Service Unit (CSU). Performs conversion of customer’s data
stream to bipolar format for transmission.


Digital signal cross-connect. A patch panel for the testing and cross connection
of DS (as in DS-1, DS-2) circuits.


Data terminal equipment; generally end-user devices, such as terminals
and computers, that connect to DCE, which either generate or receive
the data carried by the network. The RS-232, V.35 and X.21 connections
are common DCE to DTE interfaces.


Dual Tone Multi-Frequency. The signal, consisting of two tones, produced by a standard telephone
push-button (Touch-Tone) 12 key pad. The signal produced replaced the older pulse dialing of rotary phones.




The reflection back to the sender of transmitted signal energy. The amount
of delay in an echo depends on the distance from the transmitter to
the point of reflection. Eliminated by signal processors called Echo


A new technique that samples the transmission and echo signal and inserts
an out-of-phase cancellation signal into the return channel. Requires
advanced signal processing techniques.


Electronic Data Processing.


Electronic Industries Association. A standards organization that developed the
RS-232 and other interfaces.


Electromagnetic interference. Radiation leakage outside of a transmission medium that
results (mainly) from the use of high-frequency wave energy. May be
reduced by shielding.


The addition of control information to a data packet for use by a higher
level protocol.


In the telephone network, the spacing and operation of amplifiers so
the gain provided by the amplifier coincides with the signal loss
at the same frequency. Usually accomplished by a combination of adjustable
coils, capacitors, and/or resistors. See Conditioning.


Standard unit of measurement of telecommunications traffic capacity and usage
demand. Used in traffic engineering for throughput and capacity planning.
An Erlang equals 36 ccs, which represents a conventional telecommunications
traffic plan that is in use full time.


Extended Super Frame. A technique of combining 24 DS-1 (T-1) frames so that
the repeating 24 framing bits (F-bits) can be used to carry encoded
network management data as well as a CRC-6 frame check sequence.


Electronic Switching System; one of a family of AT&T-manufactured, stored-program-control,
central office switches; most prevalent are the Nos. 1, 1A, 4, and
5 switches.


A Local Area Network (LAN) trademarked product of Xerox Corp. characterized
by 10Mb/s base band transmission over a shielded coaxial cable and
employing CSMA/CD media access-control mechanism. Standardized by
IEEE 802.3 which also allows other cable designs.


Referring to the local telephone Central Office (CO), or to any subscriber telephone
switch, as in Private Branch Exchange (PBX).


Extended Super Frame. A group of 24 D4 frames used together. In addition to a synchronization
pattern, the framing bits also carry CRC-6 error information from
the preceding ESF and a data channel, the Facilities Data Link,
that reports detected errors. Replaced the super frame standard
in 1983.




The communications process in which graphics or text documents are scanned,
bit encoded and transmitted via a modem dial-up phone line. The CCITT
standards for information representation and transmission (Group 1
analog, with page transmission in four or six minutes; Group 2, with
page transmission in two or three minutes; and Group 3 digital with
page transmission in less than one minute). Also called fax.


A phenomenon, in microwave radio transmission, where atmospheric refraction
or rain attenuation causes a drop in receive signal level.

Packet Switching

A message switching approach that attempts to minimize packet processing
time at each node to utilize high speed data channels effectively.
Frame Relay, Cell Relay, and SS7 are examples.


See facsimile.


Federal Communications Commission; board of commissioners appointed by the
President under the Communications Act of 1934, with the authority
to regulate all interstate telecommunications originating in the United


See Frame Check Sequence


Fiber Distributed Data Interface. A LAN or MAN, network standard (IEEE 802.7)
based on two counter rotating fiber rings operating at 100Mb/s. The
LLC/MAC is a modification of the token ring. Designed as a LAN backbone,
or campus ring, for distributing data to other Local Area Networks.
The bandwidth is considered too low to meet the requirements for a
MAN although metropolitan area distances are possible. Gaining wide
acceptance in campus environments.


Frequency Division Multiplexing. A technique that translates several low frequency
channels onto one broadband high frequency channel. Used in telephony,
LAN, CATV applications. See Frequency Division Multiplexing.

Division Multiple Access (FDMA)

An access technique that assigns several users of a radio channel to
a specific frequency slot. Frequency Division Multiplexing (FDM) is
identical except that FDMA has a control channel that dynamically
reassigns frequency channels among the users. Used as a multiple access
technique for satellite and cellular telephone receivers.


Full duplex. The use of two separate communications channels for one circuit;
where the endpoints are “A” and “B”, one channel is transmitted from
“A” and received at “B” while the other channel is transmitted from
“B” and received at “A”.


Front-End Processor. A dedicated computer often called a communications processor,
channel attached to one or more host computers or multi-user minicomputers
and performs data communications functions. In IBM SNA networks, an
IBM 3704, 3705, or 3725 communications processor when channel attached
to a host node.


Transmission technology where modulated lightwave signals, generated by a laser
or LED, are propagated along a glass or plastic lightguide and then
demodulated back into electrical signals by a light-sensitive receiver.


In Local Area Networks, a station dedicated to providing file and mass
data storage services to the other stations on the LAN.


Electronic circuitry that removes energy in unwanted frequencies, such as noise,
from a transmission channel; may be analog or digital in operation.


Federal Information Processing Standard. A U.S. Federal Government standard
procedure that must be met by any government supplier.


In data communications, a bit pattern of 01111110, used by many bit-oriented
protocols to mark the beginning and end of a frame.

Error Correction (FEC)

Technique for correcting errors incurred in data transmission by the receiving
DCE. Typically involves a convolution of the transmitted bits, and
the appending of extra bits, using an algorithm common to both the
receiver and transmitter. Accomplished within OSI layer 1 and independent
of higher layers.


Frequency modulation.


In data communications, a group of bits sent serially over a communications
channel and containing some delimitation, address and control information.
Normally associated with OSI layer 2, i.e. frame generally refers
to layer 2 while packet generally refers layer 3. In video, refers
to a single scanned image.

Check Sequence (FCS)

Used in bit-oriented protocols, a 16-bit field usually appended at the
end of a frame that contains transmission error-checking information.
A technique known as Cyclic Redundancy Check (CRC) is used to compute
the FCS and the term “CRC” is often substituted for FCS.


A fast packet data communications standard that segments and packetizes
(frames) data into discreet transmission units that are routed along
the frame relay network via permanent virtual connections. Newer X.25
packet switches may be reprogrammed for Frame Relay. Frame relay is
independent of originating protocols. Standardized by CCITT and ANSI.
Major IXC companies offer Frame Relay switching services.


A bit added to byte interleaved time multiplexed data streams to mark
the beginning of each sequence of repetitive bytes. Multiple framing
bit patterns provide data stream synchronization.

Space Loss

The signal loss between the receiver and the transmitter of a radio system.
Calculated by the formula: Loss = 36.6 +20logD (miles) +20logf (MHz)


The number of repetitions per second of a complete waveform normally expressed
in Hertz (Hz).

Modulation (FM)

A Method of placing an intelligent signal (analog or digital) on a carrier
wave by varying the frequency of the transmitted signal.


The variation in relative strength (measured in decibels) between frequencies
in a given frequency band.

Division Multiplexing (FDM)

Technique for sharing a transmission channel wherein carrier signals of different
frequencies are transmitted simultaneously. FDM was widely used in
telephony, especially on microwave radio channels until the mid-1980s.
Broadcast radio, TV and broadband LANs are the major applications
currently in use.


Frequency Shift Keying. Sometimes called Voice Frequency Teletype (VFT). An
early modulation technique used for teletype transmission over voice
channels. A mark was represented by a frequency shift above and a
space by a shift below the carrier frequency. The term VFT implies
multiplexing several telegraph channels onto a signal voice channel
by using a number of different carrier frequencies.


File Transfer Program. An application level data communications program
that transfers files. Files are identified, located, segmented, tagged
with a header and passed to the next protocol level for transmission.
The name comes from TCP but the function is a typical application
level program.

duplex (FDX)

The use of two separate communications channels for one circuit; where
the endpoints are “A” and “B”, one channel is transmitted from “A”
and received at “B” while the other channel is transmitted from “B”
and received at “A”.


Television transmission where images are sent and displayed in real-time and
motion is continuous; compare with freeze frame. Used in all broadcast
TV, whereas slow scan TV is often used in conference video.


Foreign Exchange. A telephony term for an exchange switch outside the LATA. Often
refers to a dedicated leased line from a PBX to the FX, so a call
placed to the FX number is answered by the PBX.




Prefix meaning one billion (e.g. Gb/s).


Increased signal power, usually the result of amplification; measured in decibels
for the ratio of an output signal level; opposite of loss, or attenuation.


An actual protocol translation computer or logical boundary area within
a network computer that serves to interconnect data communications
networks with different OSI layer 4 and above protocols. See Bridge
and Router. *In TCP/IP protocols Gateway refers to a network protocol
node, often called more correctly a Router .


The position where communications satellites will remain in orbit over
the same location above the earth’s equator. An orbit of about 23,300
miles above the earth’s surface, where a satellite’s orbital velocity
matches the rotation of the earth, causing it to remain stationary
relative to a point on the earth.


A type of optical fiber where the refractive index of the core varies
smoothly with the radius. Graded-index cores are used in multi-mode
fibers utilized in LAN applications.


An electrical connection or common conductor that, at some point, connects
to the earth.


A telephony term describing a signaling method where one station detects
that a circuit is grounded at the other end. This is opposed to Loop
Start which signals using both tip & ring. Ground start offers
supervision, the ability to transfer calls back to the CO.


An assemblage of communications equipment, including signal generator,
transmitter, receiver, antenna, and communications control computers
that transmit and receivers signals to/from a communications satellite.
Also earth station.


Global System for Mobile Communications Service . The European digital cellular
standard. GMA uses TDMA to provide 8 channels of 13kb/s voice on
an 200kHz carrier channel. Setup and control is also TDMA. The standard
also calls for half rate service using 6.5kb/s voice to provide
16 channels in 200kHz. GSM therefore offers a spectrum use improvement
of from 1.2 to 2.4 over analog AMPS. To this GSM adds a frequency
hopping technique to improve access.



duplex (HDX)

The operational of a communications channel where transmission and reception
occurs in both directions alternately based on an established channel
control protocol.


In communications, a predefined exchange of signals or control characters
between two devices or nodes that sets up the conditions for data
transfer or transmission; also, handshaking.


Referring to a communications link that is not switched and is therefore dedicated
to a single set of users.


Signal distortion (normally analog) caused by harmonic frequencies generated
from the interference among the primary signal frequencies on the
channel. The power of the harmonic frequencies is measured in decibels
as compared to the power of the input signal.


See High-level Data Link Control.


Half duplex.


A passive component in a broadband transmission network that translates
one range of frequencies (transmit) to a different frequency band
(receive) and allows devices on a single-cable network to send and
receive, without the signals interfering with each other. Generally
refers to the beginning end of a broadband communications system.


The unit for the measurement of frequency, where one Hertz equals one
cycle per second.


(HF) Portion of the electromagnetic spectrum, typically used in short-wave
radio applications; frequencies approximately in the 3-to-30-MHz range.

Data Link Control (HDLC)

CCITT standard, bit-oriented, data-link-control protocol that is the foundation
on which most other bit-oriented protocols are based. HDLC is the
underlying protocol of ADCCP, DDCPM, SDLC, LAP-B, and LAP-D.


High Speed Serial Interface . A DTE to DCE interface developed to support
serial speeds up to 52Mb/s. (V.35 is standards limited to 6Mb/s and
RS-449/422 to 10Mb/s.) HSSI is the defacto standard for DS-3/STS-1
interface. Uses a 50pin connector identical to a SCSI external connection,
but only 12 pins are identified in the standard.


In telephony, an induction coil and related circuitry at a central office
that interfaces a two-wire local loop to a four-wire circuit, allowing
for the physical separation of the transmit and receive signals.


See hertz.






International Communications Association. A major trade organization for large communications
users based in Dallas, Texas.


Institute of Electrical and Electronics Engineers.


The opposition to an electrical wave based on frequency, resistance, inductance,
and capacitance. Measured in ohms. Impedance is said to be matched
when all components of a communications channel present the same (normally
standardized) average impedance to the communications signal.


Portion of the electromagnetic spectrum used for optical-fiber transmission
and also for short-haul open-air data transmission; transmission wavelengths
longer than about 0.7 microns. The band is divided into three sub-bands;
Near Infrared 1.75 micrometers to 3 micrometers, Middle Infrared 3
micrometers to 30 micrometers, Far Infrared 30 micrometers to 1000

Laser Diode (ILD)

Sometimes called the semiconductor diode. A laser in which the lasing occurs
at the junction of the n-type and p-type semiconductor materials.
A coherent light source. Special dopants in the materials emit photons
when excited by an electronic current. A feedback loop provides the
lasing effect.

Digital Network (IDN)

The integration of transmission, switching and protocols into a circuit
switched data network. Refers to networks other than ISDN.


A direct circuit between telephone central offices.


Integrated Services Digital Network. A set of standards developed by CCITT to
define a global public access voice/data network for the 21st century.
The future telephone network. The 1988 CCITT standards release makes
possible to manufacture of hardware for network implementation. Broadband
and International standards will be implemented in the 1990s.


International Standards Organization. Performs ANSI functions internationally.


A method of synchronous transmission where the timing relationship between
the transmitter and the receiver is maintained by the clock rate of
marks on the transmission path. Timing is passed from one transmitter
to the next. Used in DS-1 related signals.


Inter eXchange Carrier. Any carrier registered with the FCC that is authorized
to carry customer transmissions between LATAs interstate or, if
approved by a state public utility commission, intrastate. Examples:
AT&T Communications, MCI, U. S. Sprint, WillTel, and so on.



Digital Cellular (JDC)


The Japanese digital cellular telephone standard. JDC uses TDMA to provide
3 channels of 8kb/s voice over a 25kHz carrier channel providing a
spectrum use improvement of 3.6 over AMPS.


The slight movement of a transmission signal in time or phase that can
introduce errors and loss of synchronization for high-speed synchronous


A wire or cable used to establish a circuit, often temporarily, for
testing or diagnostics.



Notation for one thousand (e.g., kilobit/s).


Expression representing a Kilo or 1000. In Hertz, kHz = 1000Hz. In data storage
a k = 1,024, or 2 to the 10th power. A Kbyte of memory = 1,0204 bytes
of computer memory.


Portion of the electromagnetic spectrum; frequencies approximately in the
12-to-30-GHz range.


Kilobits per second. A standard measure of data rate and transmission capacity.

Telephone System (KTS)

In telephony; a Key station or key telephone equipment. Describing multiline
telephone equipment. A telephone system that offers multiple access
to 2 to 12 trunk lines, and 4 to 40 extensions. Originally this equipment
was electromechanical, leased from AT&T and required 25 pair cable
to connect it. It has been functionally replaced by low cost, 3-pair
electronic equipment.


Portion of the electromagnetic spectrum, being used increasingly for satellite
communications; frequencies approximately in the 10-to-12-GHz range.




Portion of the electromagnetic spectrum commonly used in satellite and microwave
applications, with frequencies approximately in the 1 GHz region.


Local Area Network


Link Access Procedure. A term used to describe a data-link-level used for
packet switched networks. The CCITT X.25 packet switch standard uses
LAP-B (LAP-Balanced) and LAP-D is standardized for use on the ISDN
“D” channel.


Local Access and Transport Area. Defined as part of the AT&T Bell system
breakup (1983) one of 161 local telephone serving areas in the United
States. A Local Exchange Company (LEC) has the exclusive rights to
operate within a LATA, subject to the state PUC regulations. Any inter-LATA
communications must be transferred to an Inter eXchange Company (IXC).


An acronym for Light Amplification by Stimulated Emission of Radiation,
a device which transmits an extremely narrow and coherent beam of
electromagnetic energy in the visible light spectrum. Primary light-signal
source for optical-fiber transmission.


A dedicated circuit, typically supplied by the local exchange company.
The term applies to any voice, data, modem or FAX circuit either Off-Premises
Extension (OPX) , Foreign Exchange, Tie line, T-1 or trunk circuit.

Emitting Diode (LED)

A semiconductor device that emits incoherent light formed by the P-N
function. Light intensity is roughly proportional to electrical current
flow. A principal light source for optical-fiber transmission used
mainly with multimode fiber.


Referring to electromagnetic wavelengths in the region of visible light; wavelengths
of approximately 0.8 to 1.6 microns; referring to the technology of
fiber-optic transmission.


Refers to a channel or telephone line.


In SONET; part of the transmission overhead, between major equipment
such as multiplexer to DXC, mux to Mux, etc. Provides timing alignment
and protection switching. (See Section & Path)


The logical entity in the OSI model concerned with transmission of data
between adjacent network nodes. Dee data-link.


An induction device employed in telephone local loops, generally those
exceeding 18,000 feet in length, that compensates for the wire capacitance
by adding inductance.

Area Network (LAN)

Local Area Network. A data network with transmission media optimized for
high speed, short distance, multiple access applications. Typical
applications are PC-to-PC, PC-to-mini or PC-to-mainframe. LAN standards
are provided by IEEE 802.X documentation.

Exchange Company (LEC)

Local Exchange Company. Name for the telephone company in which local service
of your telephone is connected. Provides the first level of connection
to the Public Switched Telephone Network (PSTN). Prior to 1996 was
protected by law from competition in a specific franchise area.


In telephony, the wire pair that connects a subscriber to a phone company
central office or remote. Typically operating at 48 volts with a 20Hz
70 volt ring signal and limited to a 5mi length.

Link Control (LLC)

A protocol standardized by the IEEE 802.2 common to all LAN applications
and used for data link-level transmission control. LLC works with
the Media Access Control (MAC) protocols.

Unit (LU)

In IBM’s SNA, a logical port through which a user gains access to the
services of a network.


In telephony, a local loop that signals an off-hook condition by allowing
DC current flow between the tip and ring conductors. Loop start is
common for single line telephones.


A testing procedure commonly in telecommunications, normally on full
duplex (FDX) channels, in which a test message is sent to the far-end
of the channel and then resent back to the originator and compared
with the original test transmission. Noise, distortion, signal loss,
BER etc. can be determined by loopback testing. Loopback testing requires
manual or computer control patch panels in order to connect test generation
and measurement equipment to the channel and to establish the far-end


Reduction in signal strength, expressed in decibels; also attenuation.


Longitudinal Redundancy Check. Part of the original Block Check Character (BCC)
based on longitudinal and vertical parity.




The designation for one million (e.g. Mb/s).


An X.25 feature, or facility, that notifies the receiver all data from
the sender (or transport-layer entity) has been transmitted.


Media Access Control. The lower sublayer of the IEEE 802.X defined link
layer, which complements the Logical Link Control (LLC). This is a
media-specific access control protocol within IEEE 802 specifications
and includes variations for the token ring, token bus, and CSMA/CD.
MAC deals with the electrical signals and access protocols of 802.X


Metropolitan Area Network. A carrier operated, high speed, easily accessed data
network operating within a LATA. See DQDB.

Distributing Frame (MDF)

In telephony, a board containing many punch blocks where telephone/PBX
subscriber lines are terminated and interconnected by patch wires.


Digital line encoding technique in which a transition occurs in the middle
of each bit period; a negative-to-positive (voltage) transition of
the bit period designates a binary “1”, while a positive-to-negative
transition represents a “0”. Primarily used on coaxial wire systems.
This encoding technique also allows for network clocking from the
line frequency. Differential Manchester coding, used on Token Ring
networks over twisted pair, is nearly identical except a relative
(as opposed to absolute) change in phase indicates a binary “1”.


Manufacturing Automation Protocol. A network under development by major U.S. manufacturers
with the objective of combining factory management and robotics into
an integrated computer manufacturing process. MAP utilizes the Token
Bus LAN technology.


In switch technology. That portion of the switch architecture where input
and output are physically interconnected.


The basic unit of measurement of mass data storage, equal to 1,024 Kbytes.


A statistical multiplex and switching technique of storing messages
(today data packets) on hard disk and, after selecting the proper
outgoing channel, placing the message in queue for retransmitting
it toward its destination.


Micrometer. One-millionth of a meter.


An electronic integrated circuit, typically a single-chip package, capable
of receiving and executing coded instructions (e.g., INTEL 80286,
80386, Motorola 68000 are popular microprocessors).


One-millionth of a second.


Portion of the electromagnetic spectrum above about 890 megahertz. Also describing
a line-of-sight open-air radio transmission of a radio beam of energy
within the microwave frequency band used as a carrier for multiple
communications channels.


One-thousandth of a second.


Million Instructions Per Second. A general comparison gauge of CPU raw processing


Modulator/demodulator. A Digital to Analog (D/A) or (A/D) converter that alters a carrier
signal to represent a binary “1” or “0”. A simple example is On/Off
keying, where the presence of the carrier signal is equal to a “1”.
However, any signal property (s), frequency, phase, amplitude, may
be used. Modems are used over telephone wire lines and other transmission

Huffman Code

A common data compression technique.


The process of placing intelligence, represented by either an analog wave
or a digital code, on a transmission path. Normally accomplished by
varying a characteristic of a carrier wave in accordance with another
signal wave or binary digit. In analog transmission, Amplitude Modulation
(AM) varies the carrier amplitude; while Frequency Modulation (FM)
varies the frequency in accordance with the amplitude of the signal
wave. Carrier Wave (CW) [also called on/off (OOF) keying] and phase
modulation are common with digital information.


In data communications, refers to a quantity, such as of messages or
frames, that can be counted before the counter resets to zero. Often
referring to the number of messages (N-1) that can be outstanding
on the network before an acknowledgment is required from the receiver.
(e.g. in HDLC related protocols, Modulo 8 = 3 bit counter and Modulo
128 = 7 bit counter).


The original character code used for the telegraph (1841). Morse code
is composed of long (dash) and short (dot) electromagnetic pulses
used together to form characters. At first these were actually written
on moving paper tape. Morse code became very widely used for radio
telegraphy because of its ease of modulation, but was replaced in
telegraphy by the marks and spaces of Baudot Code (1870).


Metal-oxide semiconductor. A semiconductor that uses a thin layer of metal oxide
to conduct electrons across P-N junctions. This technique produces
lower heat and current requirements compared to bipolar construction,
thereby allowing large scale integrated circuits.


Mean time between failure. The average time between reported equipment
failures. May be calculated for systems by the formula 1/MTBF=1/A+1/B+…1/X.


In IBM’s SNA, a network consisting of two or more host-based system services
control points; typically, a network with more than one host mainframe.


A communications arrangement where multiple devices share a common transmission
channel, though only one may transmit at a time. See multipoint.


In optical fiber communications, describing a lightguide (optical fiber),
with a 50 or 62 micron core capable of propagating light signals along
various paths (modes). Such paths (modes) will differ in time delay
thereby causing pulse spreading and limiting the speed at which the
lightguide may be modulated. Compare with single-mode fiber.


In microwave radio, signal fading caused by the reflection of part of
the radio beam off of the ground in such a manner as to arrive at
the receive antenna 180 degrees out-of-phase thereby canceling the
primary signal.


A device that combines several communications channels on to a single
channel of higher bandwidth ( or line speed). The term multiplexer
commonly assumes both multiplexing and demultiplexing within the same
device. See TDM and FDM.


The combining of multiple data channels onto a single high bandwidth transmission
channel. Data streams are interleaved on bit or byte basis [Time Division
Multiplexing (TDM)] while analog signals are frequency translated
and placed on a high frequency carrier [Frequency Division Multiplexing


A single communications channel (typically a leased telephone circuit)
to which more than one station is attached and addressed separately
under a polling protocol controlled by the master station or computer.






A speech compending
algorithm used for North American DS-0 PCM.




(nm) One billionth of a meter.


One-billionth of a second.


Designed by Motorola as an interim technique before digital cellular, NAMPS
used a 10kHz analog voice channel instead of the 30kHz uses in AMPS.
It provides the same spectrum use improvement as AMPS but requires
added processing to maintain channel quality. NAMPS degrades data


North American Telecommunications Association. A trade association of telephone
equipment suppliers & distributors. Trade show called UNICOM.


Network Addressable Unit. In SNA, the combination of a logical unit (LU) and
a physical unit (PU) that allows the use of the SNA network services.


National Bureau of Standards. U.S. government agency that maintains standards,
operates WWV time station and distributes the Data Encryption Standard.


name given IBM’s network management software package. NETVIEW runs
on IBM mainframes using SNA but will interface network equipment supplied
by many manufacturers.


Trade name given to Novell’s LAN software packages. 


A group of nodes (voice or data terminals) interconnected by a series
communications channels; through an assortment of modems, multiplexers,
and transmission equipment.


A binary address, usually translated from human language by a communications
computer, that is placed within the network call packet or datagram
and allows that packet or datagram to find its intended node. The
number of bits required will depend on the number of nodes and sub-networks
interconnected. CCITT X.400 seeks to find a global data network addressing
system. Your telephone number with country and area code is a global
voice network address.


OSI layer 3 responsible for routing data through the network.

Management System

A comprehensive system of equipment used in monitoring, controlling,
and managing a data communications network. Usually consists of testing
devices, CRT displays and printers, patch panels, and circuitry for
diagnostics and reconfiguration of channels, generally housed together
in an operator console unit. The location of the network management
system is often called the Network Control Facility (NCF).


Describing the physical and logical relationship of nodes in a network; the schematic
arrangement of the links and nodes of a network (IBM); networks are
typically of either a star, ring, tree, or bus topology, or some hybrid
combination thereof.


A point where one or more functional units interconnect transmission
lines (ISO). A physical device that allows for the transmission of
data within a network; an end point of a link or a function common
to two or more links in a network, typically includes host processors,
communications controllers, cluster controllers, and terminals.


A method of assigning a specific value, in numerical readings, to the
transmission impairment due to the noise encountered by an average
user operating a particular class of telephone subset. Noise Weighting
has been established by the agencies concerned with public telephone
service. They represent successive stages of technological development.


In switching, describing matrix through which a traffic path always exists
for each attached station. Generically, a switch or switching environment
designed never to experience a busy condition due to call volume.


Network Termination type 1. In ISDN the principal network-to-customer interface.
It is a rough equivalent of a CSU. Terminates the digital subscriber
loop and interfaces to the CPE.


Network Termination type 2. In ISDN a specified interface to an NT-1 from
multiple user CPE. Normally, the NT-2 will be contained within the
PBX or other customer node. The NT-2 provides local address resolution
and contention functions.


In communications theory, states that two samples per cycle is sufficient to characterize
a band-limited analog signal; in other words, the sampling rate
must be twice the highest frequency component of the signal (e.g.,
sampling at 8 kHz for a 4 kHz analog signal).




Executable machine code, programs that have been compiled or assembled; compare
with source code.


Optical Character Recognition; the process of reading characters by scanning
text and converting it into machine readable digital codes.


Original Equipment Manufacturer. The maker of equipment marketed by another
vendor, usually under the name of the reseller. The OEM may manufacture
certain components, or complete systems.


In telephony, condition indicating the active state of a subscriber’s
telephone circuit; a line state that signals a central office that
a user requires service; opposite of on-hook.


Condition in which user, terminal, or other device is not connected to a computer,
or is not actively transmitting via a network; operation of a functional
unit without the continual control of a computer; compare with on-line.


Functional ranking of a telephone network switching center depending on transmission
requirements and hierarchical relationship to other switching centers;
see Class X central office.


Deactivated condition of a subscriber’s telephone circuit, where the telephone
or circuit is not in use; opposite of off-hook.


Condition in which a user, terminal, or other device is actively connected with
the facilities of a communications network or computer; pertaining
to the operation of a functional unit that is under the continual
control of a computer; opposite of off-line.

System (OS)

The software of a computer that controls the execution of programs, typically
handling the functions of input/output control, resource scheduling,
and data management (e.g., CP/M, MS-DOS, VM/370, UNIX).


Any filament or fiber, made of dielectric materials used to transmit laser
or LED-generated light signals. Optical fiber usually consists of
a core, which carries the signal, and cladding, a substance with a
slightly higher refractive index than the core, which surrounds the
core and serves to reflect the light signal back into it; also, lightguide.


Off Premises Extension. An extension not located in the same area as a
PBX; the Local Exchange Company (LEC) provides a dedicated leased
pair from the PBX location to the distant location, normally within
the same LATA.


An electronic device used to produce repeating signals of a given amplitude
or frequency.


See operating system.


Open Systems Interconnection. Referring to an International Standards Organization
(ISO) conceptional model for data communications protocols. The OSI
reference model provides a logical structure for data communications
network standards. The OSI model is based on a seven-layer architecture;
(1) Physical, (2) Data-Link, (3) Network, (4) Transport (5) Session,
(6) Presentation & (7) Application. Comment: While an OSI standard
data communications architecture may never materialize, all future
data communications standards will be based on the logical approach
of the OSI model.


In communications, all information, such as control, routing, and error checking characters,
in addition to user-transmitted data; includes information that
carries network status or operational instructions, network routing
information, and retransmissions of user-data messages that are
received in error.




Private Automatic Branch Exchange. See PBX. Note: Electronic Private Branch
Exchange (EPBX) and Computerized Branch Exchange (CBX) are all identical


A group of bits arranged with a sync character (s) address, control,
message data, frame check sum and ending flag. The term “packet” is
usually associated with a network frame, where the address is the
network address of the sender and receiver and control information
is related to the network as opposed to the data-link. The network
packet may contain combined or segmented data-link frames. The packet
can be sent along a network, stored and forwarded. The term was developed
with X.25 standard and applies to other “packet” networks.


A system once used to form a permanent record of transmitted and received
data in telecommunications centers by punching holes representing
data codes, normally Baudot, on paper tape. This technology became
obsolete with advent of reliable magnetic storage media in the 1960s.


Concurrent or simultaneous execution of two or more processes or programs, within
the same processor; compare with serial or sequential processing.

data transmission

A type of data transfer in which all bits of a byte or computer word
are sent simultaneously during one clock cycle, either over separate
conductors or multiplexed over a single channel.


An additional non-information bit appended to a group of bits, typically
to a 7- or 8-bit byte, which indicates whether the number of ones
in the group is an odd or even number. A basic and elementary mechanism
for error checking.


Process of error checking using the parity bit; varied methods include longitudinal
parity check and vertical parity check.


The connection between two communicating devices.


The calculation of the availability of a microwave radio path expressed
in the percentage of time per year the path is available to the user.
This considers radio propagation only. Thus, a path with an availability
of 99.9999% has a propagation outage of .0001 X 525,960 min/yr. =
5.26 minutes/yr.

Control layer

In IBM’s SNA, the network processing layer that handles primarily the
routing of data units as they travel through the network and manages
share link resources.


In SONET; the overhead user device (first optical Mux) to user device.
Placed in SPE, it identifies the payload by trace number and contents.
(See Line Overhead & Section Overhead.)


Private Branch Exchange. A telephone switch located on a customer’s premises
that provides voice-grade circuits with advanced features for the
local premises, and is connected to LEC and IXC service providers
by trunk access lines, and may also be connected with other PBX switches
over tie-lines. Numerous other enhanced features, such as least-cost
routing and call-detail recording are available. The PBX is the major
equipment requirement for a private voice network. Also called Electronic
PBX (EPBX) and Computer PBX (CBPX).


Pulse Code Modulation. The primary standard for voice digitization. Based
on the Nyquist theorem, the 0-4000Hz telephone voice signal amplitude
is sampled at twice its highest frequency (8000 samples/second) and
each sample value is assigned an 8-bit binary value. At the receiver,
the samples are read and converted back to analog voltages which are
passed through a low-pass filter to recreate the original voice analog
signal. PCM signal is comprised of 8000 8-bit bytes per second or

Communications Network (PCN)

See Personal Communications Service.

Communications Service (PCS)

ability to provide PBX features to a widely moving Cellular Telephone.
This would include telephone, voice mail, FAX and Page services to
one instrument with plug in accessories. If this could be integrated
with office and residence wire telephone services it would form the
ultimate PCN.


In telephony, the measurement, in degrees out of phase, that an analog
signal deviates form the referenced phase of the main data-carrying
signal; often caused by alternating current components in a telecommunications


A data transmission encoding method wherein the phase angle of the carrier
wave is varied to represent different bit values to the receiver.
Typically eight phase angles are used to represent 3 bits per phase

Shift Keying (PSK)

Phase-modulation encoding technique employed by many modems. See phase modulation.


Within the OSI model, the lowest level of network processing. This level
is concerned with the electrical, mechanical, and handshaking procedures
over the interface that connects a device to a transmission medium.
Some physical layer standards are RS-232 or V.21.

Unit (PU)

In IBM’s SNA, the component that manages and monitors the resources of
a node, such as attached links and adjacent link stations; PU types
follow the same classification as node types; see node types.


One-trillionth of a second; one millionth of a microsecond.

Element (pel)

The smallest discrete scanning line sample of a facsimile system containing
only black/white information.


A device used to convert optical signals to electrical signals in a
light wave receiver. For relatively fast speeds and moderate sensitivity
in the 0.75 micron to 1.1 micron area wavelength, the silicon photodiode
is most commonly used. Avalanche photodiodes (APD) combine the detection
of optical signals with internal amplification of the current produce.
The internal gain is realized through avalanche multiplications of
carriers in the junction region. The advantage in using an APD is
its higher signal-to-noise ratio, especially at high bit rates.

of Presence (POP)

In telephony, the physical access location within a LATA of a IXC; the
point to which the local telephone company terminates subscribers’
circuits for the IXC. Commonly, a distribution frame connected to
a lightwave transmission system.


Any condition where there are two opposing charges, such as positive and


A data communications control procedure where a master station systematically
invites tributary stations on a multipoint circuit to transmit data.


A point of access into a computer, a network, or other electronic device.
The physical or logical interface through which one gains access.

Telegraph and Telephone (PTT)

A government organization (normally European) that operates a nationalized
public telecommunications network. See CEPT.


In the OSI model, the layer of processing that provides services to the
application layer, allowing it to interpret the data exchange and
to structure data messages to be transmitted in a specific display
and control format.


A leased telephone line, attached to one single user.


A network established and operated by a private organization or corporation
for users within that organization or corporation. A private network
may contain circuits leased from public carriers.


The time it takes a signal, composed of electromagnetic energy, to travel
from one point to another over a transmission channel; normally the
speed-of-light delay through open air or a vacuum, or a slightly decreased
speed depending on the propagation characteristics of the transmission


Formal set of rules governing the format, timing, sequencing, and error control
of data exchange across a data network. Note that many protocols may
be required and used on a single network. The OSI model outlines a
protocol for each layer.

Data Unit (PDU)

Information delivered as a unit between peer entities (layers) of an OSI network
containing protocol control information.


A data encoding technique used in basic access ISDN. This is bipolar
alternate space (negative) inversion. Replaced in the U.S. by 2B1Q

Data Network (PDN)

A packet switched data network publicly available to subscribers. The
term is more commonly used outside the United States. In the U.S.
examples would be Telenet or Compuserve etc. packet network services.


Generically, a network established and operated by communications common carriers
or telecommunications administrators for the provision of circuit-switched,
packet-switched, and leased-line circuits to the public; compare with
private network.

Switched Telephone Network (PSTN)

The public telephone network. The term is more commonly applied outside the United States.
In the U.S. both LEC and IXC carriers combine to form the PSTN.



Amplitude Modulation (QAM)

A modulation technique, using variations in both signal amplitude and
phase. This technique allows up to 128 data-encoded symbols to be
represented per hertz and is used in high density digital microwave
radio and modem applications. Commonly implemented as 64 QAM.


In telecommunications, a feature that allows calls or packets to be “held” or delayed at
the origination switch while waiting for a channel to become available.




Scattering caused by submicroscopic particle fluctuation in material density
causing minute changes in refractive index.


The level of circuit noise that will produce a measured reading equal
to that produced by 1 Pico watt (-90 dBm) of electric power at 1,000


The ratio of light velocity in a vacuum to its velocity in the transmitting

Operations Service Element (ROSE)

A sub-layer program within layer 7 of the OSI protocol.


In digital transmission, equipment that receives a pulse train, regenerates
and retimes it, and then reconstructs the signal for retransmission.


For interactive sessions, the elapsed time between the end of an inquiry
and the beginning of the response; the interval between a user data
entry and the reply from a CPU or destination device.

to Zero (RZ)

A convention in digital line coding that returns the signal voltage
to a zero level after slightly more than one-half bit time. See Non-Return
Zero (NRZ).


In telephony, a 20Hz, 40-105volt (70v. typical) signal used for on-hook
loop signaling. Distinctive ringing is possible by varying the frequency
within a 15-68Hz range.


A device used to interconnect data communications networks with different
OSI level 3 protocols. The term is generally applied to a device that
integrates router and bridge functions with leased service interfaces.
These Routers often use Internet Protocol (IP) to route datagrams
among networks. Also called Internodal Processors (INP) , the device
can be interfaced to SNA networks and have revolutionized corporate
network design. The leading manufacturers are Cisco Systems and Wellfleet
Inc. See Bridge and Gateway.


An EIA-specified physical interface, with associated electrical signaling,
between data circuit-terminating equipment (DCE) and data terminal
equipment (DTE); the most commonly employed interface between computers
and modems.


Electrical characteristics of balanced-voltage digital interface circuits. Utilizes
the RS-449 interface standard.


Electrical characteristics of unbalanced-voltage digital interface circuits. Utilizes the RS-449
interface standard.




In ISDN, the interface between the NT-2 network adapter and the user
often combined with the “T” interface to connect the user directly
to the NT-1. The “S” connector is an RJ-45 connector and is considered
the normal user interface.


The use of geostationary orbiting satellites to relay transmissions from
one sending earth station to another, perhaps multiple other, earth


Cause of lightwave signal loss in optical fiber transmission; diffusion
of a light beam caused by microscopic variations in the material density
of the transmission medium.


Synchronous Data Link Control. The bit-oriented IBM version of the HDLC protocol
used in SNA communications.


See Software Defined Network. Also VPN.


In SONET: part of the transmission overhead, from MUX/DXC to Repeater
and from Repeater to Repeater. Provides error monitoring on each optical
segment. (See Line Overhead & Path Overhead)


The process of dividing a user data message into smaller frames, blocks,
or packets for transmission, where each has an integral sequence number
for reassembly of the complete message at the destination end. Often
called fragmentation.


The sequential transmission of the bits constituting an entity of data
over a data circuit (ISO).


Region surrounding a broadcasting station where signal strength is at or
above a stated medium; the geographic area handled by a telephone
central-office facility; generally equivalent to a LATA.

Access Point (SAP)

The interface point between layers of the OSI model where a protocol entity
can access the services of that layer. Lower layers provide SAPs for
the use of higher layers.


A connection between two stations that allows them to communicate (ISO);
the time period that a user engages in a dialog with an interactive
computer; in IBM’s SNA, the logical connection between two network
addressable units (NAUs).


In the OSI model, the layer responsible for binding and unbinding logical
links between end users and maintaining an orderly dialog between


Protective enclosure surrounding a transmission medium, such as coaxial cable,
designated to minimize electromagnetic leakage.


One-way data transmission, with no capabilities for changing direction.


Relationship of the magnitude of a transmission signal to the noise of a channel;
measurement of signal strength compared to error-inducing circuit
noise; given in decibels.


A lightguide (optical fiber) with an 8 micron core that allows only
one light path. Used in nearly all LEC and IXC applications. Normally
requires a LASER source. Current technology allows approximately 2Gb/s
with 35 miles between repeaters.


Type of radio transmission where one sideband of the carrier signal is
transmitted while the other is suppressed; the main carrier wave itself
may either be transmitted or suppressed.


A method of flow control used between two DCE devices that regulates
the number of unacknowledged data packets or frames on the network.
Various network protocols have specific techniques regulating the
window size.


Switched Multimegabit Data Service. Access to switched data service above 45Mb/s.
Initial service will be accessed by dedicated DS-3s, later through
802.6 MANs. Switching will be via Cell Relay.


Simple Mail Transfer Program. Part of the IP protocol suite, an applications
level program that provides electronic mail services. One of the options
that enhances the popularity or TCP/IP


See Systems Network Architecture (IBM).


Simple Network Management Protocol (Version 2). An object-oriented network
management application protocol for use with TCP/IP. SNMP uses the
connectionless UDP protocol of the TCP/IP suite with messages formatted
by ICMP. Objects are also available to support OSI networks, making
SNMP very popular among network users.

Defined Network (SDN)

Also called “Virtual Private Network (VPN)”. An offering of the public
network (IXC & LEC), that gives users the appearance of a private
network. The IXC or LEC allows the user access to partitions within
their DACS equipment so the user can reconfigure leased circuits within
the public network. Network management information is also provided
to the user for their circuits. The result is a network that appears
private in terms of flexibility without requiring the purchase of
nodal multiplex equipment.


Synchronous Optical NETwork. A set of standards under development by CCITT &
ANSI aimed at synchronizing the information carried on a optical fiber
network. Before SONET, optical fiber signals were composed of asynchronous
DS-3s multiplexed onto the optical path by bit interleaving. DS-3s
are also asynchronous but are composed of synchronous DS-1s. SONET
will add path overhead bytes identifying each optical system input,
DS-1, DS-3 or Cell. Then a 125 microsecond wide and 9 rows deep frame
is constructed, where the first three bytes of each row contain, among
other overhead, pointers to the start of each path overhead byte.
This procedure allows the direct extraction of any input tributary
within the frame. SONET allows direct tributary switching without
frame disassembly or electronic demultiplexing which is a significant
improvement over DACS switching. Significant network control advantages
are also included in the overhead bytes. Therefore, IXC and LEC carriers
are rapidly installing SONET equipment by using the technology for
all new and replacement optical equipment.


A switching term for a circuit-switch; a switch with an actual physical
path through the matrix. This may be time-shared (see Time-Multiplexed
Switching) but not time-slot interchanged.


Payload Envelope. A SONET term for the data carrying part of SONET
frames. The SPE contains the user data and Path Overhead.

of light

2.998 x 108 meters or 186,000 miles per second.


See System Services Control Point.


A lightguide (optical fiber), the core of which exhibits a uniform refractive
index. Used for single-mode fibers.


In asynchronous transmission, the last transmission element in each character,
which permits the receiver to come to an idle condition before accepting
another character.

and Forward

Describing operation of a data network where packets, messages, or frames are
temporarily stored within a network node before they are transmitted
toward the destination.


In IBM’s SNA, a type 4 node, defined by a communications processor (37XX)
and its attached peripheral nodes, all of which share a common sub
area address.


A group of 12 D4 frames used together so that the framing bits provide
a synchronization pattern. Replaced by the Extended Super frame in


Data communications where characters or bits are sent at a fixed rate,
with the transmitting and receiving devices synchronized. Significantly
increases data throughput rates. Accomplished by sync characters,
flags, or digital line coding techniques.


A collection of components that perform a defined function, as in a
transmission system, or a computer system.


The gain in db of a microwave receiver\transmitter calculated algebraically
by adding the transmitter power output to the receiver threshold.

Services Control Point (SSCP)

In IBM’s SNA, a host-based data communications subprogram that manages
the network configuration, coordinates network operator and problem-determination
requests, maintains network address and mapping tables, and provides
directory support and session services.

Network Architecture (SNA)

In IBM networks, the layered logical structure, formats, protocols, and procedures
that govern data communications and information exchange.





Another name for a T-1.


A telephone circuit or cable through which a T-carrier runs.


AT&T term for a digital carrier facility used to transmit a DS-1 formatted
digital signal at 1.544 Mb/s. Assumes the use of two-pair of copper
wires as the transmission medium.


AT&T term for a digital carrier facility used to transmit a DS-1C formatted
digital signal at 3.152 Mb/s. Assumes the use of two-pair of copper
wires as the transmission medium.


AT&T term for a digital carrier facility used to transmit a DS-2 formatted
digital signal at 6.312 Mb/s. Assumes the use of two-pair of copper
wires as the transmission medium.


Terminal Adapter. An ISDN term for a standard device used to adapt non-ISDN
data equipment to an ISDN NT.


A telephone switching center designed to switch 4-wire trunks among
CO switches or other tandem switches. A trunk office/switch. Primarily
used for long distance service by IXC companies but also used within
a large LATAs to route calls among CO switches.



A document filed by a regulated telephone company with the state public
utilities commission or the FCC in order to establish rates charged
for services offered. A regulated utility must provide services to
all who request them at a fixed rate. The tariff defines the service
and the rate. Tariffs are approved by a PUC based on a formula that
allows the regulated utility a certain percentage of profit. Tariffs
may be challenged before the PUC or in the courts. If the court finds
a tariff unjustified or if the services specified were not provided,
the utility might have to refund moneys collected.


Transport Control Protocol/Internet Protocol. A protocol suite developed by
several universities under government grants that became the U. S.
government data network protocol until 1990. TCP/IP is a connectionless
protocol system designed to work with a very wide assortment of computer
equipment. While it is not formally standardized, it is widely used
and highly developed, and therefore, very popular with private organizations.
Despite the U.S. government’s formal announcement of its intent to
switch to OSI protocols, TCP/IP is the fastest growing network protocol
in terms of new network users. OSI has adopted many of the features
of TCP\IP, but the protocol remains attractive because of its ease
of implementation.


See Time Division Multiplexing.

Division Multiple Access (TDMA)

A multiple access technique where multiple users of a radio channel
share the channel by time usage. Messages are always digital and transmitted
in frames. Transmission time slots are controlled by a master station
either from a reference frame or from a echo of transmitted frames
reclocked by the master station. Used as a multiple access technique
in satellite and cellular telephone systems.


Telephone central office, in most usage; but also, generic abbreviation for
the telephone company or LEC.

Access Method (TCAM)

A widely employed communications management software package from IBM
that runs on IBM 370 and compatible mainframes; depending on version
and options, can support either SNA or pre-SNA networking; supplanted
by VTAM in recent years, especially where networks are primarily SNA.


Generic term describing voice telecommunications.


Remote-access data processing (ISO); the use of data-link communications to accomplish
a computer-based task; distinguished from distributed data processing
(DDP), where remote communications are not a prerequisite to all processing.


Generic term for a teleprinter terminal; Teletype is a trademark of Teletype


Teleprinter exchange; a worldwide switched message-exchange service, characterized
by Baudot-coded data (though numerous conversion facilities are now
available), provided worldwide by Western Union in the United States.


A one-way information retrieval service normally provided by a cable
TV channel with a special decoder that allows page selection from
a computer.


A telephone circuit with an impedance of 600 ohms and terminated with
a load having the characteristics of a standard telephone set.


A leased or private dedicated telephone circuit connecting two PBX switches.
Also known as an interswitch trunk.

Compression Multiplexing (TCM)

A technique of time multiplexing both the transmit and receive channel
on one path using a “ping-pong” system of sending frames. Used for
the ISDN Basic Access (2B+D). Replaced within the U.S. by the 2B1Q

Multiplexing (TDM)

Interleaving digital data from many users onto one two-serial communications link
by dividing channel capacity into time slices. (Example DS-1)

Switching (TMS)

In space-division switching (circuit-switching) where, under computer
control, the path through the matrix is time configured for each TDM
code word. Used in nearly all digital telephone switches.


Expiration of predefined time period, at which time some specified action occurs;
in communications, timeouts are employed to avoid unnecessary delays
and improve traffic flow; used, for example, to specify maximum response
times to polling and addressing, before a procedure is automatically

Interchange (TSI)

A switching technique where the TDM PCM channels are moved in time between
the input and output TDM data streams. This technique requires that
each input data stream, normally a DS-1, be buffered, then DO-0s are
interchanged among various DS-1s. Currently used as initial and final
stages on large switches combined with a TMS matrix. Also, the technology
used for DACS equipment.


See Time-Multiplexed Switching.


A local network access media and topology in which all stations actively
attached to the bus listen for the broadcast token or supervisory
frame; stations wishing to transmit must receive the token before
doing so; however, the next physical station to transmit is not necessarily
the next physical station on the bus; bus access is controlled by
preassigned priority algorithms. Defined by IEEE 802.4.


A local network media access control and topology in which a supervisory
frame or token is passed from station to station in sequential order;
stations wishing to gain access to the network must wait for the token
to arrive before transmitting data; in a token ring, the next logical
station receiving the token is also the next physical station on the
ring; compare with token bus. Defined by IEEE 802.5.


Registered AT&T trademark for push-button dialing; see DTMF.


In data communications, a message destined for an application program;
a computer-processed task that accomplishes a particular action or
result; in interactive communications, an exchange between a terminal
and another device, usually a computer; in batch or remote job entry,
a job or job step.


Generic term describing any device, usually a terminal, that can both transmit
and receive.


The dispatching of a signal, message, or other form of intelligence by
wire, radio, telegraphy, telephony, facsimile, or other means (ISO);
a series of characters, messages or blocks, including control information
and user data; the signaling of data over communications channels.


In satellite communications, the circuitry that receives an up-link signal,
translates it to another, usually higher, frequency, amplifies it,
and then retransmits it as the down-link signal.


In the OSI model, the network processing entity responsible, in conjunction
with the underlying network, data link, a physical layer, for end-to-end
control of transmitted data and the optimized use of network resources.


A dedicated aggregate telephone circuit connecting two switching centers,
central offices, or data concentration devices.


A telephone exchange dedicated primarily to interconnecting trunks.


Multiple trunk circuits between the same two switching centers that can be
accessed by dialing a single trunk number and use the same multiplexing
equipment at both ends.


See Time-Slot Interchange.


Transistor-to-transistor logic.


Teletypewriter communications; generally, basic asynchronous ASCII-coded data communications.




User Datagram Protocol. A connectionless datagram protocol that is
an option to TCP protocol for use with IP. Used with SMTP.


Ultra High Frequency. The portion of the electromagnetic spectrum ranging
from about 300 MHz to about 3 GHz; the frequency band that includes
television channels 14 through 83, and cellular radio frequencies.


User Network Interface . The term applied to a user interface to ATM services.
Bellcore currently is looking to a specification within SMDS for the
UNI for ATM services. (See ATM and SMDS.)


Universal synchronous/asynchronous receiver transmitter . An integrated circuitry
common to many data communications devices; converts data in parallel
form, the CPU into serial form for transmission.


Universal synchronous receiver/transmitter . An integrated circuit that performs
conversion of parallel data to serial form for transmission over a
synchronous data channel.


Portion of the electromagnetic spectrum between the short wavelength extreme
of the visible spectrum (about 0.4 microns) and 0.04 microns.


With a two-wire circuit, where the impedance-to-ground on one wire is measurably
different from that on the other; compare with balanced-to-ground.


Bell Laboratories’ trademark for a computer operating system designated for telecommunications
and multi-user environments, developed by Bell Laboratories; a popular
multi-user operating system for 32-bit computers.




Value-added network. A data network that sells services to the public. GTE Telenet
and Timenet are primary examples.


Very High Frequency; portion of the electromagnetic spectrum with frequencies
between approximately 30 to 300 MHz; operating band for television
channels 2 to 13 and most FM radio.

redundancy check

(VRC) A parity check performed on each character of an ASCII block as the
block is received.


An interactive data communications applications designated to allow unsophisticated
users to converse with a remote database, enter data for transactions,
and retrieve textual and graphics information for display on subscriber
television sets of (typically) lower cost terminals.


In packet switching, network facilities that give the appearance to the
user of an actual end-to-end circuit; in contrast to a physical circuit,
a dynamically visitable network connection where sequential user data
packets may be routed differently during the course of a “virtual
connection”; enables transmission facilities to be shared by many
virtual circuits simultaneously.


concept of storage space that may be viewed as addressable main storage
to a computer user, but is actually auxiliary storage (usually peripheral
mass storage) mapped into real addresses; amount of virtual storage
is limited by the addressing scheme of the computer.


Virtual Private Network. An IXC service now offered by LECs, that allows the
customer to program portions of the carrier’s DACS equipment to define
the connections and bandwidth required by the customer. To some extent
VPNs allow the customer bandwidth on demand as well as alternate routing
and network management information. A VPN brings with it far more
flexibility and reliability than a truly private network, because
the customer has the use of the carrier’s resources. Also called SDN.

Frequency (VF)

Describing an analog signal within the range of transmitted speech, typically
from 300 to 3,400 Hz; contained within a 0-4000z channel.


A telecommunications circuit used primarily for speech transmission typically supporting
frequency range of 300 to 3,400 Hz; also, voice band.




Wide Area Telephone Service. An IXC service allowing reduced rates for
bulk long distance services; may be In-WATS, or 800-number service,
where calls can be placed to a location from anywhere at no cost to
the calling party, or Out-WATS, where calls are placed out from a
central location; cost is based on hourly usage per WATS circuit and
on distance based on zones.


A rectangular metallic pipe with the dimensions of one-half by one-quarter
wave length used for directing microwave electromagnetic radiation.
It has the advantage of low signal loss per unit of distance.


Distance between successive peaks of a sine wave. Wavelength = 1/ frequency
x the speed-of-light.

Division Multiplexing (WDM)

A technique for increasing the fiber optic capacity by signaling a more
than one frequency. One fiber can be used to send data at multiple
frequencies simultaneously. This technique is similar to frequency
division multiplexing but is used in optical transmissions.


Generally, a communications channel offering a transmission bandwidth wider than
a voice-grade channel.


A flow-control mechanism in data communications, the size of which is
equal to the number of frames, packets, or messages that can be sent
from a transmitter to a receiver before any reverse acknowledgment
is required.


Termination point for customer premises wiring, offering access to service personnel;
generally serves a specific area, with multiple wiring closets that
are cross-connected.




Transmitter off/transmitter on; a commonly used peripheral-device flow-control
protocol, used extensively for modem control by an attached terminal
or processor.


Cross connect.



transmission level point (0 TLP)

In telephony, a reference point for measuring the signal power gain and losses
of a circuit, at which a zero dBr0 signal level is established.


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