Communications Systems Glossary

D channel.  A D channel is an integrated services digital network (ISDN) 16 or 64 Kbps packet-switched channel that carries signalling information to control circuit-switched calls (B channels).

Dark fibre.  Dark fibre refers to fibre that has been laid in anticipation of future requirements, but is currently unused.

Data circuit-terminating equipment (DCE).  The data circuit-terminating equipment (DCE) provides the signal conversion and coding between the data terminal equipment (DTE) and the line.

Data Encryption Standard (DES).  The Data Encryption Standard (DEC) is an algorithm that encrypts plain text data into an encoded form, called cipher text.

Data link layer (Layer 2).  The data-link layer is the second layer of the ISO OSI 7-layer reference model. This layer allows point-to-point or node-to-node control. It is often called the frame level because it specifies how to organise data into frames and how to transmit frames over a network.

Data terminal equipment (DTE).  The data terminal equipment (DTE) converts the user information into data signals for transmission, and then converts the received data signals into user information.

Datagram.  In packet switching a datagram is a small packet of data that carries sufficient information to allow it to be routed from the originating data terminal equipment to the destination data terminal equipment without having to establish a connection between the data terminal equipments across the network.

Decibel (dB).  A decibel (dB) is a measure of the relative strength of two signals.

Decryption.  Decryption is the reverse of encryption in which encrypted data (called cipher text) is translated into the original data (called plain text).

Degradation.  Degradation refers to a diminishment of service or quality.

Delay distortion.  Delay distortion is distortion of a signal that arises when the propagation delay for the transmission medium is not constant over the frequency range of the signal. All frequencies do not therefore arrive together, resulting in delay distortion.

Delta modulation.  Delta modulation can be considered to be an incremental pulse-code modulation (PCM) system where, rather than transmitting the absolute amplitude at each sampling, only the changes in signal amplitude from sampling instant to sampling instant are transmitted.

Demand-assigned multiple access (DAMA).  Demand-assigned multiple access (DAMA) is one of the two main techniques for allocating channels to users.

Demodulation.  Demodulation is the opposite function to modulation in which the original baseband signal is recovered from the modulated carrier.

Demultiplexer.  A demultiplexer performs the opposite function to a multiplexer in which a multiplexed channel is broken out into its constituent individual channels.

Deviation.  In an FM signal, deviation is the extent by which the carrier frequency is shifted from its resting frequency.

Differential phase-shift keying (DPSK).  Differential phase-shift keying (DPSK) is a modulation technique that uses phase modulation. DPSK changes phase each time a 1 bit is transmitted and does not change phase when a 0 bit is transmitted.

Digital European Cordless Telecommunications (DECT).  Digital European Cordless Telecommunications is a standard for cordless telephony which provides voice and data services to slow-moving mobile users.

Digital signal.  A digital signal is a discrete or discontinuous signal.

Digital-to-analogue conversion (DAC).  Digital-to-analogue conversion is the opposite of analogue-to-digital conversion (ADC) in which the digital signal is converted to an analogue form.

Digitise.  To digitise a signal is to convert an analogue signal into a digital signal.

Dipole.  A dipole is a simple antenna structure where the two lines of a transmission line are opened up at the end to form an antenna with two arms in a T shape, where each arm of the T is one-quarter of a wavelength long.

Direct current (DC).  When the voltage is supplied by a constant source such as a battery, the current supplied is constant and is called direct current (DC).

Dispersion.  The velocity of propagation of an electromagnetic wave through a medium is frequency-dependent. Lower frequencies travel more slowly than high frequencies meaning that high frequencies arrive first and cause distortion of the received signal, known as dispersion.

Distributed queue dual bus (DQDB).  Distributed queue dual bus (DQDB) is an IEEE 802.6 standard for MAN using a twin optic-fibre loop extending up to 50 km at throughputs of 140 Mbps.

Doppler effect.  The Doppler effect is the change in frequency of an electromagnetic signal that results from the relative speed of the source and the sink.

Double sideband (DSB).  When both sidebands of an AM modulated carrier are transmitted without the carrier, the transmission is called double-sideband suppressed-carrier (DSBSC).

Downlink.  The downlink is the signal from the satellite transponder to a satellite ground station.

Dual-tone multiple frequency (DTMF).  Dual-tone multiple frequency (DTMF) is the touch-tone dialling system where a dual-tone signal is sent to indicate the desired number dialled from a telephone.

Dumb terminal.  A dumb terminal is an input/output terminal that does not perform any local processing.

Duplex transmission.  Duplex transmisison is a data transmission mode in which a transmission path can carry information in both directions simultaneously.

Dynamic range.  Dynamic range is the difference between the maximum signal level in an analogue communications system and the lowest signal level.

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