 850 (850MHz), GSM 900 (900MHz), DCS 1800 (1.8GHz), and PCS 1900 (1.9GHz). New GSM data standards include HSCSD, GPRS, and EDGE and are referred to as 2.5G. GTP (GPRS Tunneling Protocol) An IP-based protocol used within GSM and UMTS networks. H.323 An ITU standard that defines how audiovisual conferencing data is transmitted across networks. In theory, H.323 should enable users to participate in the same conference even though they are using different videoconferencing equipment. half-duplex Communications in which data can be transmitted between two stations in both directions but in only one direction at a time. Hamming Code A linear error-correcting code that can detect single- and double-bit errors. HAN (home area network) A broadband network in a smart house that connects the vari- ous smart devices. handoff The transfer of duplex signaling as a mobile terminal passes to an adjacent cell in a cellular radio network. handshake The exchange of predetermined signals for control when a connection is estab- lished between two modems or other devices. haptic interface An interface that enables virtual touch. hard wired (1) Referring to a communications link, whether remote phone line or local cable, that permanently connects two nodes, stations, or devices. (2) Descriptive of electronic Glossary hardware circuitry that performs fixed logical operations by virtue of an unalterable circuit layout, rather than under computer or stored-program control. hardware The physical equipment, as opposed to programs or procedures, of a computer system. harmonic distortion A waveform distortion that is usually caused by the nonlinear fre- quency response of a transmission. HARQ (hybrid automatic repeat request) A variation of the ARQ error control method that gives better performance than ordinary ARQ, particularly over wireless channels, albeit at the cost of increased implementation complexity. hash function The process of producing hash values for accessing data or for security. A hash value (or simply hash) is a number gene