splitting up" a wider bandwidth into several nar- rower bandwidths. Frequency modulation (FM). The process of modifying the fre- quency of a carrier wave in step with the amplitude variations of the signal to be transmitted. Frequency shift keying (FSK). A commonly used method of fre- quency modulation in which a one and a zero (the two possible states) are each transmitted on separate frequencies. Front end. Device which interfaces the communications network to a computer's input/output channel. Front-end processor. A communications computer associated with a host computer. It may perform line control, message handling, code conversion, error control, and applications functions such as control and operation of special purpose terminals. Full-duplex. Refers to a communications system or equipment ca- pable of transmission simultaneously in two directions. 124 GLOSSARY Garble. An unintelligible received message. Half-duplex circuit. A circuit which provides transmission alter- nately in either direction. Host computer. A computer interconnected with a network that performs the computational jobs. Host interface. An interface which connects a host computer with a communication processor. Intelligent terminal. A terminal that contains a processing element. It can perform local data processing and storage activities. Jitter. Short term line or circuit instability. Keyboard send/receive (KSR). A teleprinter that enables an oper- ator to send or receive data. Leased line. A communication channel provided for the exclusive use of a customer, by a common carrier, at a fixed monthly rate. Line printer. A printing device which prints an entire line of data at a time and then advances to the next line. Local loop. The part of a communication circuit from the sub- scriber's equipment of the line-terminating equipment. Mean time between failures (MTBF). A measure of reliability usu- ally expressed as the average number of hours between one random failure and the next. Message switch. A network node used to route data transact