nect lower speed clusters of devices or lower nalling and analogue speech. speed networks. Answerback. Facility in some equipment to respond to a remote Backoff. It is the delay, in a multiple access system, which a control signal. Usually takes the form of handshake sequences transmitting station adopts if a collision has occurred in its between devices, when they set up, before they start to inter- earlier transmission. change information. Balanced to ground. A two wire transmission circuit in which the Antenna. A device used to send or collect radio signals from a impedance to ground on one wire equals that on the other wire. source or receiver. Bandwidth. It is the difference between the highest and lowest Antipodal signalling. Technique for encoding binary signals, frequencies of a transmission channel, measured in Hertz. It is which provides good error performance in relation to signal to therefore often used to define the information carrying capacity noise ratio. of a channel. Aperture. That area of the antenna which is able to interact with Baseband. Usually refers to the frequency band containing the the transmitted radio signal. basic information being transmitted, before it is modulated and other characters have been added. Apogee. The peak altitude taken by a satellite in orbit. Base station. Usually refers to the radio station in a mobile com- Application layer. A logical entity forming the top or seventh layer of the OSI seven layer model. It is largely defined by the munication system, containing the transmitters and receivers for communication with a mobile. user to perform his applications. Batch processing. Data processing system in which transactions Application process. This usually refers to a collection of soft- are sent for processing in groups or batches. ware modules which carry out all the processing needed to fulfil a certain function. Battery feed. The battery supply from the switching centre to the subscriber's telephone. Assembler. A programme which converts an assembly level com- B