Index - A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A-Z
HALF DUPLEX (HDX) - A circuit capable of transmitting or receiving information both ways, but only one direction at a time; a function of both computer protocols and transmission channels. Many computers operate only half duplex on transmission channels capable of full duplex operation.
HAMMING CODE - In data transmission, a code with added redundant bits for error detection purposes.
HANDOFF - In cellular radiotelephony, the protocol by which a call in progress is transferred from one cell to another as the mobile unit moves.
HANDSHAKE - The exchange of control sequences between two locations to set up needed conditions for communications. In the strictest sense, even the steps of dialing a telephone call can be thought of as a "handshake sequence."
HARDWIRE - Using physical wire or cable directly between units of hardware equipment.
HARMONIC - An exact multiple of a base, or " fundamental " frequency. Examples: Second harmonic = double the fundamental frequency; third harmonic = three times, and 12th harmonic = one dozen times and so on. Sometimes an expression of the required bandwidth for transmission, as in the case of binary transmission where it is well established that a bandwidth equal to the tenth odd harmonic is needed for accurate reception of the pulses; this means the fundamental, third, fifth, seventh, ninth, eleventh, thirteenth, fifteenth, seventeenth, nineteenth and twenty-first harmonics must be passed; thus a 50 bps binary pulse requires a bandwidth of 50 x 21 or 1050 Hertz for accurate capture of the pulses ... unless it is somehow encoded for transmission.
HARMONIC DISTORTION - The ratio, expressed in decibels, of the power at the fundamental frequency, to the power of a harmonic of that fundamental. Odd harmonics, particularly third harmonics, are especially harmful to many forms of telecommunications by both wire and radio; as well, they are annoyingly difficult to design out of electronic circuitry. Some forms of telecommunications require measurement of a parameter called " total harmonic distortion ," meaning the sum of all harmonics reaching the receiver along with the base signal.
HDLC (HIGH LEVEL DATA LINK CONTROL) - A bit-oriented international standard data link protocol used in CCITT X.25 packet network links and influencing many others such as SDLC, BDLC and DDCMP; also Honeywell Data Link Control, Honeywell Corporation's bit-oriented protocol for its computer networks.
HEAD END - A broadband LAN term from its progenitor, Cable TV, for a location on the LAN providing remodulation, retiming, message accountability, contention control and diagnostic control.
HEADER - That portion of a message containing information for routing, handling and delivering a message, such as address, size, priority, intermediate routing and synchronization signals.
HEAD-ON COLLISION - Descriptive of a condition in message telephony when two switching exchanges seize a bothways trunk at the same instant and attempt to send outbound call instructions to each other.
HEAD END HOP OFF (HEHO) - A method of traffic engineering whereby calls are completed by using long distance facilities directly off the switch that serves that location.
HERTZ (Hz) - International standard unit of frequency. Replaces, and is identical to, the older unit "Cycles-per- second."
HEXADECIMAL - A number system based on 16, providing convenient notation of the 16 possible combinations of half an 8-bit data processing byte; uses digits 0 through 9 followed by letters A through F to count to 16, thus two " hex " digits can describe one byte in software. Example: ASCII letter capital "A" has the decimal value 65 but is written as 41 in hex software code, while small "z" has the decimal value 122 but is noted as 7A in hex, still requiring only two digits instead of three. Using only two hex digits from 00 to FF, a code of 256 different characters can be described as is done with the adaptation of ASCII used by personal computers; the added characters beyond 128 are often called "Extended ASCII," or "IBM graphics characters."
HOMING - Returning to the starting position, as in a rotary stepping switch when its connection is released.
HOOKSWITCH/SWITCH HOOK (HOOK SWITCH) - The device on which the telephone receiver hangs or on which a telephone handset hangs or rests when not in use. The weight of the receiver or handset operates a switch that disconnects the telephone circuit, and transfers the line connection to the telephone's bell or " ringer " circuit.
HORIZONTAL - Descriptive of the side of a North American wire Distributing Frame on which terminal blocks are mounted horizontally; this is the side equipment is terminated on, as opposed to the "vertical side," on which cables terminate in vertically mounted blocks. In some locations, frames of horizontal blocks only are called Horizontal Intermediate Distributing Frame, or HIDF. Compare to: VIDF.
HOT STANDBY/HOT SPARE - Alternate equipment ready for immediate use if the normal equipment fails.
HOT CUT - Virtually instantaneous replacement of one line with another; when done in multiples to change major facilities, sometimes called a " Flash Cut " or " Flash Cutover ."
HOUSEKEEPING OPERATION - 1.) In data processing, an operation contributing nothing to the throughput or output, but necessary to maintaining operation; 2.) In communications, more commonly called " overhead ."
HYBRID - A very common communications electronics circuit performs the wire conversions necessary for the connection of a 2 wire local loop with a 4 wire long-haul facility; sometimes called a " two wire / four wire converter ." The classic source of echo problems in long- distance telephony, hybrids were once made of expensive, bulky, specially-built transformers; today they are an IC chip full of microelectronics that is even self- adjusting.