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Telecommunications Terms

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2B1Q :

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 "dibit" 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. 2B+D :

Basic ISDN access.

23B+D :

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

30B+2D :

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

802.X :

A set of LAN and MAN standards developed by IEEE.

A

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AD

Analog-to-Digital converter

Advanced 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.

Address

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.

ADPCM

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-Law

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

Alternate routing

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

AMI

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.

Amplifier

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

Amplitude

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.

Advanced 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.

Analog

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.

ANSI

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

Antenna Gain

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.

Application Layer

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.

ASCII

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

ARPA - 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.

Asynchronous

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 s ignals are said to be asynchronous.

ATM

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.

ATM 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.

ATM Layer

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.

Attenuation Reduction of loss of signal strength, measured in decibels; opposite of gain. Auto-answer

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.

Autodial

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.

Autodin

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

Automatic Repeat Request (ARQ)

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

Automatic 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).

B

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B8ZS

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.)

Balanced-to-ground

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.

Balun

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

Bandwidth

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.

Baseband

(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.

Basic Access

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.

BASIC Language

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

Baud

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.

Baudot Code

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.

BCC

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.

BER

See Bit Error Rate

Bellcore

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.

Bend radius

Radius of curvature that a fiber can bend signal loss.

BERTS

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 transmitter site. 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)

Bipolar

(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.

BISDN

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.

Bisync

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).

Bit

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

Bit error

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

Bit 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 .

Bit Oriented

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

Bit stuffing

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).

Bit/s (b/s)

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.

Block

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.

Blocking

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

Boundary node

In IBMs SNA, a subarea 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.

Bridge

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

Broadband

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.

Broadband ISDN

See BISDN.

Broadband LAN

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.

Broadcast

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.

BSC

See bisync.

Buffering

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.

Bus

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.

Byte

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

C

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C band Portion of the electromagnetic spectrum used heavily for satellite and microwave transmission; frequencies of approximately 4 to 6 GHz. CAD Computer Aided Design. CAI Computer Aided Instruction. Call 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). CAM Computer Aided Manufacturing. CATV 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. Carrier Signal A continuous frequency capable of being modulated or impressed with a second data-carrying signal. CBX Computerized branch exchange; see PBX. The terms "CBX, EPBX and PBX all represent a computer controlled Private Branch Exchange (PBX). CCITT 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. CCS 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. Cell (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. Cellular Radio 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) Cell Relay 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) Central 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. Centrex 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. CEPT Conference of European Postal and Telecommunications administrations. This standards group represents most international telephone companies outside of North America & Japan. Channel In communications, a physical or logical path allowing the transmission of information; the path connecting a data source and a data sink, or receiver. Channel bank 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. Channel Attached 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. Character Standard bit representation of a symbol, letter, number, or punctuation mark; generally contained within a byte. Character code One of several standard sets of binary representations for the alphabet, numerals, and common symbols, such as ASCII, EBCDIC, and BCD. Character oriented Describing a data communications protocol that carries control information encoded as character strings. Example: "ETX" {end-of-transmission}; "SOH" {start-of-header}, etc. Checksum 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). CICS 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. Circuit Generally referring to a transmission medium connecting two or more electronic devices. Circuit switching The process 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. CTIA Cellular Telecommunications Industry Association (CITA) , also called TIA. A group of manufacturers and service providers organized to improve cellular technology. Primarily functions in the U.S.. Cladding In optical fiber, that low refractive index material that surrounds the core and provides optical insulation and protection to the core. Class 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. Class 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. Clear channel 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. Clock 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. Cluster controller 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 device. CMOS Complementary Metal Oxide Semiconductor. A semiconductor that utilizes a thin layer of metal oxide to conduct current between two identical PN junctions. COMSAT 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. Coaxial cable 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 coax. CODEC 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. Code 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. Common carrier 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). Common 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. Companding 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. Concentrator Any communications device that allows a shared transmission medium to accommodate more data sources than there are channels currently available within the transmission medium. Conditioning 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. Control characters 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. Core The central light transmission part of the optical fiber with a refractive index higher than that of the cladding. CPE 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). CPU Central Processing Unit. CRC 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 protocols. Crossbar switch 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 offices. Crosstalk Unwanted transference of electrical energy from one transmission channel to another, usually adjacent, channel. CSMA/CD 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. CSU 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. CRT Cathode-ray tube CTS Clear To Send; modem control code. CVSD 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. Cyclic 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. -------------------------------------------------------------------------------- D Top -------------------------------------------------------------------------------- DACS 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. Data Compression 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. Data 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. Datagram 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. Data PBX A PBX switch that allows digital transmission (without modems) as well as voice. An option offered today by nearly all manufacturers. Data link Refers to level 2 of the OSI model. A serial data communications connection between two adjacent node. Data set A modem; infrequently used today except within the telephone carrier industry. Dataphone 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. dB 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. dBm 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. dBr0 Decibel referred to a 0 value set by the telephone company. dBrn0 Decibel referenced to the existing noise level of a telephone circuit. Used in telephony as a signal-to-noise ratio measurement. DCE 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. DDCMP Digital Data Communications Message Protocol (Digital Equipment Corporation). Similar to HDLC. DDS 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. DES See Data Encryption Standard (National Bureau of Standards). Dedicated line A dedicated circuit, a non-switched channel, or a private line; See leased line. Demodulation The process of retrieving an original signal from a modulated carrier wave. See modulation. Dial-up Describing the process of establishing a temporary connection via the switched telephone network. Digital Referring to communications procedures, techniques, and equipment where information is encoded as either a binary "1" or "0". Digital Cellular 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. Digital 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 provided. Digital Switching 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. Direct 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. Direct 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. Dispersion 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. Distortion The corruption of a signal caused by time delay, harmonics, frequency response noise or other disturbances. Distributed 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. Distribution frame 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. Divestiture 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 BOCs. Domain 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. Driver 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. DQDB 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. DS-0 Digital Signal level 0. The bandwidth of a digitized PCM voice signal, 64kb/s. May carry voice or data. DS-1 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. DS-1C 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 today. DS-2 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. DS-3 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. DSU 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. DSX Digital signal cross-connect. A patch panel for the testing and cross connection of DS (as in DS-1, DS-2) circuits. DTE 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. DTMF 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. -------------------------------------------------------------------------------- E Top -------------------------------------------------------------------------------- Echo 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 Cancellers. Echo cancellation 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. EDP Electronic Data Processing. EIA Electronic Industries Association. A standards organization that developed the RS-232 and other interfaces. EMI 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. Encapsulation The addition of control information to a data packet for use by a higher level protocol. Equalization 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. Erlang 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. ESF 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. ESS 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. Ethernet A Local Area Network (LAN) trademarked product of Xerox Corp. characterized by 10Mb/s baseband 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. Exchange Referring to the local telephone Central Office (CO), or to any subscriber telephone switch, as in Private Branch Exchange (PBX). ESF 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 superframe standard in 1983. -------------------------------------------------------------------------------- F Top -------------------------------------------------------------------------------- Facsimile 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. Fading A phenomenon, in microwave radio transmission, where atmospheric refraction or rain attenuation causes a drop in receive signal level. Fast 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. Fax See facsimile. FCC 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 States. FCS See Frame Check Sequence FDDI 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. FDM 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. Frequency 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. FDX 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". FEP 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. Fiber Optics 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. File server In Local Area Networks, a station dedicated to providing file and mass data storage services to the other stations on the LAN. Filter Electronic circuitry that removes energy in unwanted frequencies, such as noise, from a transmission channel; may be analog or digital in operation. FIPS Federal Information Processing Standard. A U.S. Federal Government standard procedure that must be met by any government supplier. Flag In data communications, a bit pattern of 01111110, used by many bit-oriented protocols to mark the beginning and end of a frame. Forward 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. FM Frequency modulation. Frame 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. Frame 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. Frame Relay 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. Framing Bit 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. Free 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) . Frequency The number of repetitions per second of a complete waveform normally expressed in Hertz (Hz). Frequency Modulation (FM) A Method of placing an intelligent signal (analog or digital) on a carrier wave by varying the frequency of the transmitted signal. Frequency response The variation in relative strength (measured in decibels) between frequencies in a given frequency band. Frequency 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. FSK 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. FTP 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. Full 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". Full-motion video 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. FX 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. -------------------------------------------------------------------------------- G Top -------------------------------------------------------------------------------- G (Giga) Prefix meaning one billion (e.g. Gb/s). Gain Increased signal power, usually the result of amplification; measured in decibels for the ratio of an output signal level; opposite of loss, or attenuation. Gateway 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 . Geosynchronous orbit 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. Graded-index 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. Ground An electrical connection or common conductor that, at some point, connects to the earth. Ground start 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. Ground station 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. GSM 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. -------------------------------------------------------------------------------- H Top -------------------------------------------------------------------------------- Half duplex (HDX) The operational of a communications channel where transmission and reception occurs in both directions alternately based on an established channel control protocol. Handshake 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. Hard wired Referring to a communications link that is not switched and is therefore dedicated to a single set of users. Harmonic distortion 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. HDLC See High-level Data Link Control. HDX Half duplex. Head end 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. Hertz (Hz) The unit for the measurement of frequency, where one Hertz equals one cycle per second. High Frequency (HF) Portion of the electromagnetic spectrum, typically used in short-wave radio applications; frequencies approximately in the 3-to-30-MHz range. High-Level 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. HSSI 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. Hybrid 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. Hz See hertz. I/O Input/Output. -------------------------------------------------------------------------------- I Top -------------------------------------------------------------------------------- ICA International Communications Association. A major trade organization for large communications users based in Dallas, Texas. IEEE Institute of Electrical and Electronics Engineers. Impedance 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. Infrared 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 micrometers. Injection 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. Integrated Digital Network (IDN) The integration of transmission, switching and protocols into a circuit switched data network. Refers to networks other than ISDN. Interoffice trunk A direct circuit between telephone central offices. ISDN 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. ISO International Standards Organization. Performs ANSI functions internationally. Isochronous Transmission 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. IXC 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. -------------------------------------------------------------------------------- J Top -------------------------------------------------------------------------------- Japanese 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. Jitter The slight movement of a transmission signal in time or phase that can introduce errors and loss of synchronization for high-speed synchronous communications. Jumper A wire or cable used to establish a circuit, often temporarily, for testing or diagnostics. -------------------------------------------------------------------------------- K Top -------------------------------------------------------------------------------- K (kilo) Notation for one thousand (e.g., kilobit/s). k 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. Ka band Portion of the electromagnetic spectrum; frequencies approximately in the 12-to-30-GHz range. Kb/s Kilobits per second. A standard measure of data rate and transmission capacity. Key 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. Ku band Portion of the electromagnetic spectrum, being used increasingly for satellite communications; frequencies approximately in the 10-to-12-GHz range. -------------------------------------------------------------------------------- L Top -------------------------------------------------------------------------------- L band Portion of the electromagnetic spectrum commonly used in satellite and microwave applications, with frequencies approximately in the 1 GHz region. LAN Local Area Network LAP 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. LATA 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). LASER 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. Leased line 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. Light 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. Lightwave 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. Line Refers to a channel or telephone line. Line Overhead 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) Link layer The logical entity in the OSI model concerned with transmission of data between adjacent network nodes. Dee data-link. Loading coil 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. Local 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. Local 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. Local Loop 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. Logical 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. Logical Unit (LU) In IBM's SNA, a logical port through which a user gains access to the services of a network. Loop start 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. Loopback 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 loopback. Loss Reduction in signal strength, expressed in decibels; also attenuation. LRC Longitudinal Redundancy Check. Part of the original Block Check Character (BCC) based on longitudinal and vertical parity. -------------------------------------------------------------------------------- M Top -------------------------------------------------------------------------------- M (Mega) The designation for one million (e.g. Mb/s). M bit An X.25 feature, or facility, that notifies the receiver all data from the sender (or transport-layer entity) has been transmitted. MAC 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 networks. MAN Metropolitan Area Network. A carrier operated, high speed, easily accessed data network operating within a LATA. See DQDB. Main Distributing Frame (MDF) In telephony, a board containing many punch blocks where telephone/PBX subscriber lines are terminated and interconnected by patch wires. Manchester encoding 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". MAP 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. Matrix In switch technology. That portion of the switch architecture where input and output are physically interconnected. Megabyte (Mbyte) The basic unit of measurement of mass data storage, equal to 1,024 Kbytes. Message switching 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. Micron Micrometer. One-millionth of a meter. Microprocessor 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). Microsecond One-millionth of a second. Microwave 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. Millisecond One-thousandth of a second. MIPS Million Instructions Per Second. A general comparison gauge of CPU raw processing power. Modem 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 media. Modified Huffman Code A common data compression technique. Modulation 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. Modulo-N 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). Morse Code The original character code used for the telegraph (1841). Morse code is composed of long (dash) and short (dot) electromagnetic pluses 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). MOS 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. MTBF 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. Multidomain network 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. Multidrop A communications arrangement where multiple devices share a common transmission channel, though only one may transmit at a time. See multipoint. Multimode 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. Multipath Fading 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. Multiplexer 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. Multiplexing 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 (FDM)]. Multipoint line 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. Mux Multiplexer MULDEM Multiplexer/Demultiplexer Mu-Law A speech companding algorithm used for North American DS-0 PCM. -------------------------------------------------------------------------------- N Top -------------------------------------------------------------------------------- Nanometer (nm) One billionth of a meter. Nanosecond One-billionth of a second. Narrowband-AMPS (NAMPS) 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 capacity. NATA North American Telecommunications Association. A trade association of telephone equipment suppliers & distributors. Trade show called UNICOM. NAU 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. NBS National Bureau of Standards. U.S. government agency that maintains standards, operates WWV time station and distributes the Data Encryption Standard. NETVIEW Trade name given IBM's network management software package. NETVIEW runs on IBM mainframes using SNA but will interface network equipment supplied by many manufacturers. NetWare Trade name given to Novell's LAN software packages. Novell is the industry leader in LAN software. Network A group of nodes (voice or data terminals) interconnected by a series communications channels; through an assortment of modems, multiplexers, and transmission equipment. Network Address 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. Network Layer OSI layer 3 responsible for routing data through the network. 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). Network Topology 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. Node 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. Noise weighting 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. Non-blocking 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. NT-1 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. NT-2 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. Nyquist theorem 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). -------------------------------------------------------------------------------- O Top -------------------------------------------------------------------------------- Object code Executable machine code, programs that have been compiled or assembled; compare with source code. OCR Optical Character Recognition; the process of reading characters by scanning text and converting it into machine readable digital codes, OEM 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. Off-hook 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. Off-line 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. Office class Functional ranking of a telephone network switching center depending on transmission requirements and hierarchical relationship to other switching centers; see Class X central office. On-hook Deactivated condition of a subscriber's telephone circuit, where the telephone or circuit is not in use; opposite of off-hook. On-line 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. Operating 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). Optical fiber 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. OPX 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. Oscillator An electronic device used to produce repeating signals of a given amplitude or frequency. OS See operating system. OSI 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. Overhead 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. -------------------------------------------------------------------------------- P Top -------------------------------------------------------------------------------- PABX Private Automatic Branch Exchange. See PBX. Note: Electronic Private Branch Exchange (EPBX) and Computerized Branch Exchange (CBX) are all identical terms. Packet 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. Paper tape 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. Parallel processing Concurrent or simultaneous execution of two or more processes or programs, within the same processor; compare with serial or sequential processing. Parallel 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. Parity bit 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. Parity check Process of error checking using the parity bit; varied methods include longitudinal parity check and vertical parity check. Path The connection between two communicating devices. Path Availability 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. Path 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. Path Overhead 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.) PBX 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). PCM 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 64kb/s. Personal Communications Network(PCN) See Personal Communications Service. Personal Communications Service (PCS) The 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. Phase jitter 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 network. Phase modulation 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. Phase Shift Keying (PSK) Phase-modulation encoding technique employed by many modems. See phase modulation. Physical layer 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. Physical 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. Picosecond One-trillionth of a second; one millionth of a microsecond. Picture Element (pel) The smallest discrete scanning line sample of a facsimile system containing only black/white information. PIN-diode A device used to convert optical signals to electrical signals in a lightwave 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. Point 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. Polarity Any condition where there are two opposing charges, such as positive and negative. Polling A data communications control procedure where a master station systematically invites tributary stations on a multipoint circuit to transmit data. Port A point of access into a computer, a network, or other electronic device. The physical or logical interface through which one gains access. Postal, Telegraph and Telephone (PTT) A government organization (normally European) that operates a nationalized public telecommunications network. See CEPT. Presentation Layer 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. Private line A leased telephone line, attached to one single user. Private network 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. Propagation delay 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 medium. Protocol 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. Protocol Data Unit (PDU) Information delivered as a unit between peer entities (layers) of an OSI network containing protocol control information. Pseudoternary Coding A data encoding technique used in basic access ISDN. This is bipolar alternate space (negative) inversion. Replaced in the U.S. by 2B1Q coding. Public 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. Public network 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. Public 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. -------------------------------------------------------------------------------- Q Top -------------------------------------------------------------------------------- Quadrature 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. Queuing 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. -------------------------------------------------------------------------------- R Top -------------------------------------------------------------------------------- Rayleigh scattering Scattering caused by submicroscopic particle fluctuation in material density causing minute changes in refractive index. Reference noise The level of circuit noise that will produce a measured reading equal to that produced by 1 picowatt (-90 dBm) of electric power at 1,000 Hz. Refractive index The ratio of light velocity in a vacuum to its velocity in the transmitting medium. Remote Operations Service Element (ROSE) A sub-layer program within layer 7 of the OSI protocol. Repeater In digital transmission, equipment that receives a pulse train, regenerates and retimes it, and then reconstructs the signal for retransmission. Response time 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. Return 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). Ring Signal 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. Router A device used to interconnect data communications networks with different OSI level 3 protocols. The term is generally applied to a device that intragrates 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. RS-232-D 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. RS-422-A Electrical characteristics of balanced-voltage digital interface circuits. Utilizes the RS-449 interface standard. RS-423-A Electrical characteristics of unbalanced-voltage digital interface circuits. Utilizes the RS-449 interface standard. -------------------------------------------------------------------------------- S Top -------------------------------------------------------------------------------- S Interface 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. Satellite communications The use of geostationary orbiting satellites to relay transmissions from one sending earth station to another, perhaps multiple other, earth stations. Scattering 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. SDLC Synchronous Data Link Control. The bit-oriented IBM version of the HDLC protocol used in SNA communications. SDN See Software Defined Network. Also VPN. Section Overhead 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) Segmentation 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. Serial transmission The sequential transmission of the bits constituting an entity of data over a data circuit (ISO). Serving area 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. Service 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. Session 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). Session Layer In the OSI model, the layer responsible for binding and unbinding logical links between end users and maintaining an orderly dialog between them. Shielding Protective enclosure surrounding a transmission medium, such as coaxial cable, designated to minimize electromagnetic leakage. Simplex One-way data transmission, with no capabilities for changing direction. Signal-to-Noise Ratio 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. Single mode 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. Single-sideband transmission 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. Sliding Window 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. SMDS 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. SMTP 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 SNA See Systems Network Architecture (IBM). SNMP (SNMPv2) 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. Software 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. SONET 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. Space-Division Switching 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. Speed of light 2.998 x 108 meters or 186,000 miles per second. SPE Synchronous Payload Envelope. A SONET term for the data carrying part of SONET frames. The SPE contains the user data and Path Overhead. SSCP See System Services Control Point. Step Index A lightguide (optical fiber), the core of which exhibits a uniform refractive index. Used for single-mode fibers. Stop bit In asynchronous transmission, the last transmission element in each character, which permits the receiver to come to an idle condition before accepting another character. Store 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. Subarea 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 subarea address. Superframe A group of 12 D4 frames used together so that the framing bits provide a synchronization pattern. Replaced by the Extended Superframe in 1983. Synchronous Transmission 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. System A collection of components that perform a defined function, as in a transmission system, or a computer system. System Gain The gain in db of a microwave receiver\transmitter calculated algebraically by adding the transmitter power output to the receiver threshold. System 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. Systems Network Architecture (SNA) In IBM networks, the layered logical structure, formats, protocols, and procedures that govern data communications and information exchange. -------------------------------------------------------------------------------- T Top -------------------------------------------------------------------------------- T carrier Another name for a T-1. T-span A telephone circuit or cable through which a T-carrier runs. T1 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. T1C 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. T2 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. TA Terminal Adapter. An ISDN term for a standard device used to adapt non-ISDN data equipment to an ISDN NT. Tandem office/switch 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. Tariff 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 ithe services specified were not provided, the utility might have to refund moneys collected. TCP/IP 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. TDM See Time Division Multiplexing. Time 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. Telco Telephone central office, in most usage; but also, generic abbreviation for the telephone company or LEC. Telecommunications 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. Telephony Generic term describing voice telecommunications. Teleprocessing 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. Teletypewriter Generic term for a teleprinter terminal; Teletype is a trademark of Teletype Corporation. Telex 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. Teletext A one-way information retrieval service normally provided by a cable TV channel with a special decoder that allows page selection from a computer. Terminated line A telephone circuit with an impedance of 600 ohms and terminated with a load having the characteristics of a standard telephone set. Tie line A leased or private dedicated telephone circuit connecting two PBX switches. Also known as an interswitch trunk. Time 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 procedure. Time-Division Multiplexing (TDM) Interleaving digital data from many users onto one two-serial communications link by dividing channel capacity into time slices. (Example DS-1) Time-Multiplexed 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. Timeout 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 restarted. Time-Slot 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. TMS See Time-Multiplexed Switching. Token Bus 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. Token Ring 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. Touch-Tone Registered AT&T trademark for push-button dialing; see DTMF. Transaction 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. Transceiver Generic term describing any device, usually a terminal, that can both transmit and receive. Transmission 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. Transponder 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. Transport Layer 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. Trunk A dedicated aggregate telephone circuit connecting two switching centers, central offices, or data concentration devices. Trunk exchange A telephone exchange dedicated primarily to interconnecting trunks. Trunk group 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. TSI See Time-Slot Interchange. TTL Transistor-to-transistor logic. TTY transmission Teletypewriter communications; generally, basic asynchronous ASCII-coded data communications. -------------------------------------------------------------------------------- U Top -------------------------------------------------------------------------------- UDP User Datagram Protocol. A connectionless datagram protocol that is an option to TCP protocol for use with IP. Used with SMTP. UHF 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. UNI User Network Interface . The term applied to a user interface to ATM services. Belcore currently is looking to a specification within SMDS for the UNI for ATM services. (See ATM and SMDS.) USART 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. USRT Universal synchronous receiver/transmitter . An integrated circuit that performs conversion of parallel data to serial form for transmission over a synchronous data channel. Ultraviolet Portion of the electromagnetic spectrum between the short wavelength extreme of the visible spectrum (about 0.4 microns) and 0.04 microns. Unbalanced-to-ground 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. Unix 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. -------------------------------------------------------------------------------- V Top -------------------------------------------------------------------------------- VAN Value-added network. A data network that sells services to the public. GTE Telenet and Timenet are primary examples. VHF 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. Vertical redundancy check (VCR) A parity check performed on each character of an ASCII block as the block is received. Videotex 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. Virtual Circuit 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. Virtual storage The 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. VPN 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. Voice 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. Voice-grade channel A telecommunications circuit used primarily for speech transmission typically supporting frequency range of 300 to 3,400 Hz; also, voice band. -------------------------------------------------------------------------------- W Top -------------------------------------------------------------------------------- WATS 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. Waveguide 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. Wavelength Distance between successive peaks of a sine wave. Wavelength = 1/ frequency x the speed-of-light. Wavelength 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. Wideband Generally, a communications channel offering a transmission bandwidth wider than a voice-grade channel. Window 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. Wiring closet Termination point for customer premises wiring, offering access to service personnel; generally serves a specific area, with multiple wiring closets that are cross-connected. -------------------------------------------------------------------------------- X Top -------------------------------------------------------------------------------- X-off/X-on Transmitter off/transmitter on; a commonly used peripheral-device flow-control protocol, used extensively for modem control by an attached terminal or processor. XC Cross connect. -------------------------------------------------------------------------------- Z Top -------------------------------------------------------------------------------- Zero 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.