rated from a string of text. The hash is substantially smaller than the text itself and is generated by a formula in such a way that it is extremely unlikely that some other text will produce the same hash value. (Hybrid Coordination Function) An IEEE MAC-layer protocol that is an important development in VoWLAN systems. In order to improve service for voice, the standard includes two operating modes: Enhanced Digital Control Access (EDCA) and Polled Access. H-channel A class of high-speed ISDN channels. H-0 is 384Kbps, H-11 is 1.536Mbps, and H-12 is 1.920Mbps. HCSD (high-speed circuit-switched data) A standard for transferring high-speed data over aggregated GSM channels. HCSD provides data rates up to 64Kbps. HDB3 (high-density bipolar 3) A line interface standard for E-1 that is similar to B8ZS. HDB3 eliminates data streams with eight or more consecutive Os, allows for 64Kbps clear chan- nel capacity, and still assures the minimum 1s density required by E-1 lines. HDLC (High-Level Data Link Control) A form of communications line control specified series of bits rather than control characters to control data transmission over a commu- nication line. HDLC is a bit-oriented protocol developed by the ISO. HDSL (High-Bit-Rate DSL) A symmetrical service that can be deployed over a distance of about 2.2 miles km). HDSL is deployed over two twisted-pair cables, and it affords equal bandwidth in both directions. HDSL2 provides symmetrical capacities of up to 1.5Mbps or 2Mbps over a single twisted-pair cable. HDTV (high-definition television) A television format for which several competing stan- dards exist but which normally requires a screen aspect ratio of 16:9 (versus 4:3 with analog TVs) and which is capable of reproducing at least four times more detail than in the analog broadcasting system. headend The control center of a cable TV network. header The initial portion of a message or file, which contains statistical and control information. HELLO The protocol used by a group of cooperative, trusting packet switche