rdware module, such as a multiplexer or of options from international base standards, SO as to perform PABX. certain functions. There is greater chance of interoperability Fixed assignment. A method for allocation of capacity on the between implementors of the same functional standard. These medium. This is fixed and cannot be varied by user demand. standards are also known as profiles. (See also demand assignment.) Flag. In a communication system, it refers to a predefined bit pattern used in a transmitted protocol to define some event, such as the beginning of a transmitted frame. Gain. The increase in signal power, usually caused by an amplifi- Flooding. Routing technique in which the node receiving the data cation device. It is measured in decibels, as the ratio of the sends it out again on every outgoing link. output to the input signal level. Flow control. A technique used to modulate the information from Gain hit. The surge of signal which causes errors in transmitted a device so as to suit line or equipment speeds. Usually done by data. the use of control signals to enable or disable the information Gateway. Usually refers to a device which is used to interface flow from a transmitting device. between two other devices or networks, providing protocol Focal length. The distance between an antenna feed point and the conversion between the two sytems. centre of the antenna reflector. Geostationary orbit. Refers to the orbit at which a satellite re- Footprint. In communication it refers to the area covered on the mains over the same Earth location. (Approximately 23300 earth's surface by a satellite beam. Also commonly used to refer miles above the earth's surface.) It is the orbit where the to the physical space occupied on the ground by an equipment. velocity of the satellite matches the rotational velocity of the Foreign exchange circuit. A circuit from a switching exchange to Earth, SO the satellite appears to be stationary. a subscriber who would normally be served by another switch- Glare. Situation whe