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.

H-channel
A class of high-speed ISDN channels. H-0 is 384Kbps, H-11 is 1.536Mbps, and H-12 is 1.920Mbps.

half-duplex
Communications in which data can be transmitted between two stations in both directions, but only one direction at a time.

HAN (home area network)
A broadband network in a smart house that connects the various 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 established 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 circuitry that performs fixed logical operations by virtue of 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 wave form distortion, usually caused by the nonlinear frequency response of a transmission.

hash function
The process of producing hash values for accessing data or for security. A hash value (or simply hash) is a number generated 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.

HCSD (high-speed circuit-switched data)
A standard for transferring high-speed data over aggregated GSM channels; provides data rates up to 64Kbps.

HDLC (High-Level Data Link Control)
A form of communications line control that uses a specified series of bits rather than control characters to control data transmission over a communication line. 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 (3.6 kilometers). HDSL is deployed over two twisted-pair cables, and it affords equal bandwidth in both directions (i.e., it is symmetrical). 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 standards exist but which normally require a screen aspect ratio of 16:9 (versus 4:3 with current TVs) and which is capable of reproducing at least four times more detail than is the existing broadcasting system.

headend
The control center of a cable TV network.

header
The initial portion of a message or file, which contains statistic and control information.

HELLO
The protocol used by a group of cooperative, trusting packet switches to allow them to discover minimal delay routes.

heuristic
An exploratory method of problem solving in which solutions are arrived at by an interactive, self-learning method.

hexadecimal
A system of numbers in base 16; hexadecimal digits range from 0 (zero) through 9 (nine) and A (10) through F (15). Each hexadecimal digit is represented by 4 binary bits.

HFC (hybrid fiber coax)
A networking arrangement that supports a wide range of services, including traditional telephony, broadcast video, and interactive broadband services. It involves the use of fiber in the backbone and in the access network. The fiber terminates at a neighborhood node, and from that neighborhood node, coax (normally 750MHz or 1,000MHz) is run to the home, in a two-way subsplit system.

hierarchical routing
Routing that is based on a hierarchical addressing scheme. Most TCP/IP routing is based on a two-level hierarchy in which an IP address is divided into a network portion and a host portion. Routers use only the network portion until the datagram reaches a router that can deliver it directly. Subnetting introduces additional levels of hierarchical routing.

high frequency
The portion of electromagnetic spectrum, typically used in shortwave radio applications; frequencies approximately in the 3MHz to 30MHz range.

HIPPI (High Performance Parallel Interface)
A gigabit-per-second OSI Layer 1 and 2 interface standardized by ANSI. HIPP5 supports 800Mbps up to 82 feet (25 meters) using a 32-bit parallel copper connector, and can be extended up to several miles/kilometers by using fiber-optic technology. A higher speed option uses 64 parallel lines to support operation at up to 1.6Gbps.

holding time
The length of time a communication channel is in use for each transmission. Includes both message tone and operating time. Also called connect time.

HomeRF
An open standard for short-range transmission of digital voice and data between mobile devices.

hop
A unit of network distance. In particular, the number of hops between a source and a destination is the number of nodes between them (e.g., number of routers between hosts on the Internet).

hop-by-hop retransmission
A system in which an intermediate device retransmits, so the retransmission travels a shorter path over a fewer number of hops and is therefore less delayed.

horizontal distribution frame
A hub for terminating cables run on a floor.

host
An end user computer system that connects to a network. Hosts range in size from personal computers to supercomputers.

host interface
The link between a communications processor or network and a host computer.

host system
(1) The computing system to which a network is connected and with which other devices can communicate. (2) The primary or controlling computer in a network.

howler tone
The tone that alerts a subscriber when the telephone is off the hook.

HSCSD (High-Speed Circuit-Switched Data)
A high-speed transmission technology that enables users to send and retrieve data over GSM networks at transmission speeds between 28.8Kbps and 43.2Kbps (but the norm is generally around 28.8Kbps) by enabling the concurrent usage of up to four traffic channels of a GSM network.

HSSI (high-speed serial interface)
A physical-layer interface between a DTE, such as a high-speed router or similar device, and a DCE, such as a DS-3 (45Mbps) or SDH/SONET OC-1 DSU.

HTTP (Hypertext Transfer Protocol)
The standard mechanism used on the World Wide Web for the transfer of documents between server and client systems.

hub
A device that extends the maximum physical length of a network by cleaning and retransmitting signals among network segments. A hub provides the central connecting point in a star network topology. Also called a multiport repeater.

Huffman encoding
A particular statistical encoding technique for lossless compression. Statistical encoding is an entropy-encoding method. The Huffman algorithm calculates the frequency of occurrence of each octet for a given portion of data stream. It then determines the minimum number of bits to allocate to each character and assigns an optimal code accordingly. The codes are stored in a codebook. This technique is used in sound, still, and moving image compression.

hybrid circuit
A circuit that has four sets of terminals arranged in two pairs designed so that there is high loss between the two sets of terminals of a pair when the terminals of the other pair are suitably terminated. Commonly used to couple four-wire circuits to two-wire circuits.

hybrid network
A network composed of both public and private facilities.

Hz (Hertz)
A unit of electromagnetic frequency that is equal to one cycle per second.