mmunications line into multiple smaller units of bandwidth, each of which supports an inde- pendent information stream. FDMA (Frequency Division Multiple Access) A multiple-access technique used in ana- log cellular systems, in which each user is assigned to a different frequency. FEC (forward error correction) An error control system for data transmission that differs from standard error detection and correction in that the technique is specifically designed to allow the receiver to correct some errors without having to request a retransmission of data. FEC (Forwarding Equivalence Class) In MPLS, a term used to describe a set of packets with similar or identical characteristics that may be forwarded the same way. FECN (forward explicit congestion notification) A bit in the Frame Relay header which the network can inform the receiver of network congestion. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow control action, as appropriate. femtosecond 0.000000000000001 (i.e., 10-15) second. FEP (front-end processor) FEP (front-end processor) A dedicated communications system that intercepts and han- dles activity for the host. It may perform line control, message handling, code conversion, and error control, as well as such application functions as control and operation of special-purpose terminals. FHSS (Frequency Hopping Spread Spectrum) A spread spectrum technique which the frequency hopping varies in a known pattern, and separate error correction must be included. fiber-optic waveguides Thin filaments of glass through which a light beam can be trans- mitted for long distances by means of multiple internal reflections. Occasionally, other transpar- ent materials, such as plastic, are used. fiber optics A technology that uses light as a digital information carrier. Fiber-optic cables (light guides) are a direct replacement for coaxial cables and twisted-wire pairs. The glass-based transmission facilities occupy far less physical volume yet provide a tremendous amount of t