he transmission line or trunk, for example due to a power cable system. falling on to the telephone lines. Exchange area. The area served by one or more telephone ex- Forward channel. Refers to the communication channel carrying changes, grouped together by trunks and tandem offices to form data from the calling party to the called party. (Reverse channel a unit. flows in the opposite direction.) Expansion. Usually refers to the arrangement where there are a Four wire circuit. A transmission system with separate channels greater number of links available at the outlet of the equipment, for each direction of transmission. Usually this is provided by than there are entering the equipment. two different pairs of wires in each direction. Extended addressing. Facility within a protocol which allows it to Fractional speech loss. Speech that is clipped in a TASI system address a larger range than would normally be the case. due to the fact that all the channels are full when talk begins. Extinction threshold. The minimum voltage level below which a Fractional T1. Full T1 (1.544Mbit/s) delivered to the user but only conducting gas discharge lamp will go off. a proportion (discrete number of channels) used and paid for. Frame alignment. The situation where the frames of the receiving equipment are correctly phased with respect to the received signal. Facsimile. The process of scanning a document, containing text or Full access network. A network in which any input can access any graphics, and transmitting the information over the telephone output. lines, for printing at the other end. Frame. A sequence of bits, arranged in a logical pattern, and used Fade margin. It is the amount the signal can fade by, measured in for transmitting information over a communication channel. decibels, before it goes below the threshold of the receiver. Different protocols have different frame structures, which in- Fading. Loss of signal strength, due to diversion of a radio signal clude such ancillary information as error checking, sychroni