sa- from its optimum path, usually caused by atmospheric effects. tion and timing. Fall time. The time needed for a signal pulse to fall from 90% to Frame alignment. Situation where the frame of the receiving 10% of its peak value. See also rise time. equipment is correctly phased with respect to that of the re- False start. Refers to an attempt at dialling which is aborted before ceived signal. it is completed. Frame alignment recovery time. Elapsed time between the receipt Far end crosstalk. Crosstalk which travels in the same direction as of a correct frame alignment signal and the establishment of the signal being affected. (See also near end crosstalk.) frame alignment in the equipment. Fibre loss. Signal loss in a communication fibre cable, due to Frequency. Usually specified as the number of cycles of an alter- attenuation of the light in the fibre medium. nating signal per unit of time, often one second. It is expressed Figure of merit. Measure of a quality of a system, such as a satel- in Hertz. lite earth station. Frequency diversity. The use of back up equipment and trans- File server. Refers to a central station, serving several terminals mission paths within a radio system in order to protect against connected to it via a local area network, and containing mass atmospheric fading effects. data storage and other filing facilities. Frequency re-use. Capability of using the same frequency in sev- Filter. Refers to a device for removing unwanted signals at speci- eral locations without interfering with each each, for example fied freqencies, e.g. removing noise from a transmitted signal. due to physical distance or different polarisation. Firmware. A computer programme resident on a microprocessor Fresnel zone. A method for expressing the amount of clearance of which is part of the equipment. It implies a state in-between a microwave signal over an obstacle. hardware and software. Also called imbedded software i.e. it is Functional standards. Standards formed by selecting a limited set imbedded in the ha