[H]

H.221 – CCITT recommendation regarding frame structure for audiovisual teleservices.

H.230 – CCITT recommendation regarding frame-synchronous control and indication signals for audiovisual systems.

H.242 – CCITT recommendation regarding a system for establishing communication between audiovisual terminals using digital channels up to 2 Mbps.

H.261 – CCITT recommendation regarding a video codec for audiovisual services.

Half duplex – Alternating the direction of transmission of signals between two terminals or computers but not transmitting in both directions simultaneously. Contrast with duplex.

Handoff – Process by which a cellular radio telephone call is transferred from one cell site to another.

Handset – That portion of the telephone containing the transmitter and receiver which is designed to be hand-held when the telephone is in use.

Handshake – A preliminary exchange of predetermined signals performed by modems and/or terminals and computers to verify that a communications link has been established and data transmission can proceed.

Hardwire – To directly attach or dedicate a cable or line between units of equipment.

Hertz (Hz) - International standard unit of frequency. [Replaces, and is identical to, the older unit cycles-per-second.]

Heuristic – Pertaining to exploratory methods of problem solving in which solutions are arrived at by an interactive, self-learning method.

Hierarchical network – (1) A system, in which processing and control functions are performed at designated levels by computers specifically designed for those functions. (2) A switching architecture arranged in classes of offices with defined levels of responsibility. [Such as the American Class 1 to Class 5 network or the CCITT international network.]

Hierarchy – The relationship of various accounts or entities within a multi-level structure.

Hit-on-the-line – Description of the errors caused by external interferences such as impulse noise caused by lightning.

Holding time – The total period a communications channel or facility is in use for each transmission, including both message and operating times.

Homing – (1) Returning to the starting position, as in a rotary stepping switch. (2) Fixing a signal on the originating equipment to determine service quality or problems.

Hookswitch – The device on which the telephone receiver hangs or rests when not in use.

Housekeeping operation – The execution of a computer program that doesn’t directly contribute to a product or service but is essential to the running of the system. For example, the initialization of storage areas or the execution of a calling sequence. Synonym: overhead operation.

Hubbing – Converting analog lines to a digital T1 facility.

Hunting – (1) Automatic routing of calls to an idle circuit in a prearranged group when the circuit called is busy. (2) The movement of a call as it progresses through a group of lines. [Typically, the call will try to be connected on the first line of the group; if that line is busy it will try the second line and then the third, etc.]