A B C D E - H I - L M - P Q - T U - Z   Glossary of Selected Terms (A-D)   Section A 2-wire communication A communication path between end points using only 2 wires. Its difficulties and limitations arise from the shared use by transmit and receive signals of the same path. 2B+D interface An alternative term used to describe the ISDN basic date interface. 4-wire communication A communication path between end points using 4 wires, two each for each direction of transmission, transmit and receive. 4-wire communication paths have better isolation of transmit and receive signals than 2-wire paths and are thus easier to operate. adaptive differential pulse code modulation (ADPCM) A highly efficient, low bit speed method of digital encoding of voice signals. administrator A human responsible for the operation and administration of a network. ADSL (asymmetric digital subscriber line) A transmission technique allowing high speed signals (e.g. 6--8 Mbit/s) to be transmitted to customer premises over existing copper lineplant. In the upstream direction typically a maximum of 384 kbit/s is available. advanced peer to peer networking (APPN) A communications architecture developed by the IBM company. It is used between IBM AS400 computers and between workstations and servers. agent In data communications, an agent is a software program which undertakes a given function on behalf of another program. The function may be delegated for security reasons. Alternatively, the main program may be unable to carry out the function itself. In this case, the agent performs a mediation or conversion function. amplification The boosting of a faint signal---counteracting the effects of attenuation in order to ensure good comprehension by the receiver. amplitude The relative strength or volume of a signal. amplitude modulation (AM) A technique used to enable a low bit speed or comparatively low bandwidth information signal to be carried on a higher frequency carrier signal---by adjustment of the latter's amplitude to match the former's wave shape. analogue transmission A method of signal transmission in which the shape of the electrical current waveform on the telecommunications transmission line is analogous to the air pressure waves of the sound or music signal it represents. application A software program designed to undertake a given function. application layer The uppermost layer (layer 7) of the open systems interconnection (OSI) model. This layer is a software function providing a program in one computer with a communication service to a `cooperating' computer. ARPANET The network of the US Advanced Research Project Agency: the original name for the Internet when first set up in 1968. AS (autonomous system) A network under single ownership and administration---also called an administrative domain. ASCII (American standard code for information interchange) A binary coding scheme using 7-bit or 8-bit binary values to represent textual characters. The original version was a 7-bit code. Various 8-bit extended ASCII codes have subsequently been developed. asynchronous data transfer A form of data transmission in which individual text characters (typically 8-bit binary values) are always preceded by start and stop bits which serve to maintain strict delimitation of consecutive characters. asynchronous transfer mode (ATM) A modern technology for integrating all types of telecommunications traffic (voice, data, video, multimedia) across a single network. ATM forms the technological basis of the broadband integrated services digital network. attenuation The effect of signal dwindling experienced with accumulating line length or distance of radio transmission. attenuation distortion The various frequency components of a complex sound or data signal are often attenuated by the line to different extents. The tonal balance of the original signal is thus distorted---attenuation distortion. audio circuits Copper or aluminium-wired circuits being used to carry a single signal---typically a speech signal in the audio bandwidth. Section B balanced transmission Telecommunications transmission across metallic conductors (i.e. wire pairs) in which both conductors play an equal role. (e.g. twisted pair 120 ohm cabling). bandwidth Measured in Hertz or cycles per second, the range of component frequencies making up a complex speech or data circuit. Thus speech comprises frequencies between 300 Hz and 3400 Hz, a total bandwidth of 3.1 kHz. baseband An analogue signal in its original form (prior to any form of multiplexing). BCP (best current practice) A publication produced by the Internet engineering task force which documents a given best current operational practice when administering or operating Internet protocol (IP)-based networks. bearer service Relating in particular to ISDN, various bit rates or bandwidths (called bearer services) may be made available between the two ends of the connection. Examples include speech bearer service, 64 kbit/s data service. BER (bit error rate) A measure of the quality of a digital transmission line, either quoted as a percentage, or more usually as a ratio, typically 1 error in 100 thousand or 1000 million bits carried. The lower the number of errors, the better quality the line. binary code A 2-state numerical code (values `on'/`off' or 1/0) used to represent text or computer data. B-ISDN (broadband integrated services digital network) A powerful network capable of carrying all types of narrow and broadband telecommunications service types (e.g. voice, data, Internet, video, multimedia etc.) bitrate or bit speed The `bandwidth' of a digital line or signal---governing the rate at which individual alphabetic or numeric characters may be carried by the line. BRI (basic rate interface) The simplest form of network access available on the ISDN (integrated services digital network). The BRI comprises 2B+D channels for carriage of signalling and user information. bridge A device for interconnecting LANs or LAN segments together. broadband A service or system supporting rates greater than 2 Mbit/s. broadcast A service providing unidirectional distribution to multiple receivers. brouter A device comprising both bridge and router functionality. brouter A device comprising both bridge and router functionality. busy hour The period of highest network usage. busy hour traffic A measure of the maximum user demand for network throughput. byte A binary digital signal value of 8 bits (decimal value 0--255). Section C carrier frequency A high frequency signal used in FDM or radio transission. The information is modulated onto the carrier prior to transmission. carrier sense multiple access with collision detection (CSMA/CD) The transmission technique employed in ethernet LANs. Data packets are sent onto a common bus when the bus is seen to be idle. Addresses in the packets allow the correct destination device to take them from the bus. Cat.5 A type of twisted pair cabling commonly used to provide `structured wiring' for data communications sockets in company offices and for patch cabling. (Intended for Baud rates up to 100 MHz.) Cat.6 A shielded foil cabling system similar to Cat.5 but intended for signal Baud rates up to 200 MHz. Cat.7 A shielded foil cabling system similar to Cat.5 but intended for signal Baud rates up to 600 MHz. cell A data block of fixed length (48 byte information field and 5 byte header). Cells are transmitted across a network using ATM. CELP (code excited linear predication) A fast means of low bitrate digital speech compression. Because the algorithm is linear, the processing may be carried out more quickly, thus minimising signal delay. circuit-switching A method of switching provides for a direct connection of fixed bandwidth or bitrate (e.g. 64 kbit/s) between originating and terminating points of a communication for the entire duration of a call. circuit transfer mode A telecommunication transfer technique based on circuit-switching and requiring permanent allocation of bandwidth. client A software program usually designed to run on a human user's workstation or PC to provide a given service or application to the human user. The client software typically provides a graphical user interface (GUI) to the human user and interacts with a server (client/server architecture) to obtain data and functions supporting the application. CMIP (common management information protocol) The standard data protocol defined by ISO and ITU-T as part of the telecommunications management network for requesting management status information from a network element or sending control commands to it. CMIP is used at the Q3-interface. coaxial cable A cable used for high frequency and high bandwidth analogue and digital signal transmission. The cable comprises a single inner conductor plus a conducting outer sheath---in cross-section a dot in a circle. codec (coder/decoder) A device for interfacing a 4-wire digital transmission line to a 4-wire analogue. The coder codes the to-be-sent 2-wire analogue signal in a form suitable for 2-wire digital transmission. The decoder meanwhile decodes the received 2-wire digital signal. configuration management One of the five main network management functions defined by the ISO management model. The others are accounting, security, performance and fault management. connection admission control The procedure by which a network decides whether to accept a new connection. connectionless A type of communication across a telecommunications network in which a datagram (analogous to a postal telegram) is submitted to the network without first checking whether the intended recipient is ready to receive it (and indeed even without checking whether the addressed destination actually exists). connection-oriented A type of communication across a telecommunications network in which a connection is first set-up between caller and destination and the communications path is checked before data is sent. console Another name for a computer terminal. console port A connection port (typically DB-25 or DB-9) on a networking device to which a console can be connected locally (using a serial cable) for purpose of configuration of the device. control plane A conceptual communications connection between data switching devices for communicating control information (e.g. for establishing or clearing connections, etc.). CPE (customer premises equipment) The telephone or data equipment on a customer's premises---that using the public telephone or data network for connection to a remote location. CRC (cyclic redundancy check) A binary checksum enabling the detection and (possibly also) correction of bits within a data frame which have become corrupted during passage over a long distance connection. crosstalk The interference of telephone and other signals resulting from the induction of an unwanted signal from an adjacent telephone line (the crossed-wire). Section D data Data is the term used to describe alphabetic and numeric information stored in and processed by computers. data compression Any of various techniques, of both reversible and irreversible types, designed to reduce the bit speed needed for a particular type of communication (e.g. facsimile). datagram The name given to an Internet protocol (IP) packet---a single block of data or small message carried across a data network. Analogous to postal telegram. datalink layer Layer 2 of the open systems interconnection (OSI) model. This software fuction operates on an individual link within the connection to ensure that individual bits are conveyed without error. The best-known layer 2 protocol is HDLC. DCE (data circuit-terminating equipment) A device which provides for connection of a computing device (data terminal equipment---DTE) to a wide area data network. A DCE (an example of which is a modem) provides an interface between the short range communications capabilities of the DTE and the modulation techniques required for long-distance communication. DNS (domain name system) A service and protocol which provide for resolution of website addess (e.g. www.company.com) and email domain addresses (e.g. martin.clark@company.com) into the IP addresses of the relevant servers. domain name The part of a web address which follows `www.' or the part of an email address which follows the @ sign. In the case of the website address www.company.com or the email address martin.clark@company.com, the domain name is company.com. DTE (data terminal equipment) A computer, computer terminal, PC, server, workstation or similar device connected to a data network for purpose of data communication. duplex (full duplex) A mode of telecommunications transmission in which unrestricted two-way communication is possible at all times. Both parties may `talk-at-once'.