head-of-bus functionThe function that generates management information and empty bus slots at the point on each bus where data flow begins.
head-on collisionA collision that occurs on a communications channel when two or more users begin to transmit on the channel at approximately the same instant.
headendA central control device required by some networks (e.g., LANs or MANs) to provide such centralized functions as remodulation, retiming, message…
HeaderA masonry wall unit of brick which is laid so that its short end is exposed.
header check sequence (HCS)A 16-bit cyclic redundancy check (CRC) check sequence (CS) that is derived from bits from the first 8 octets (excluding flags) of a UIH format packet. [T1.
height gainFor a given propagation mode of an electromagnetic wave, the ratio of the field strength at a specified height to the field strength at the surface of the…
helical antennaAn antenna that has the form of a helix. Note: When the helix circumference is much smaller than one wavelength, the antenna radiates at right angles to…
helical scanA method of recording video information diagonally on a tape, used in home and professional video cassette recorders (VCRs).
heterochronousA relationship between two signals such that their corresponding significant instants do not necessarily occur at the same time.
heterodyneTo generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer.
heterodyne repeaterIn radio reception and retransmission, a repeater that converts the original band of frequencies of the received signal to a different frequency band for…
heterogeneous multiplexingMultiplexing in which not all the information-bearer channels operate at the same data signaling rate.
heuristic routingRouting in which data, such as time delay, extracted from incoming messages, during specified periods and over different routes, are used to determine the…
hexadecimalA number system with a base of 16. The digits are 0-9 and A-F, where F equals the decimal value of 15.
HFDFAbbreviation for high-frequency distribution frame.
hierarchical computer networkA computer network in which processing and control functions are performed at several levels by computers specially suited for the functions performed…
hierarchical routingRouting that is based on hierarchical addressing. Note: Most Transmission Control Protocol/Internet Protocol (TCP/IP) routing is based on a two-level…
hierarchically synchronized networkA mutually synchronized network in which some clocks exert more control than others, the network operating frequency being a weighted mean of the natural…
High Definition Television (HDTV)Television that has approximately twice the horizontal and twice the vertical emitted resolution specified by the NTSC standard.
High Frequency (HF)The portion of the radio frequency spectrum between between 3 and 30 MHz
high probability of completion (HPC)With reference to American National Standard ANSI T1.631-1993, an aspect of National Security and Emergency Preparedness (NS/EP) telephone calls, as…
High Sierra formatA standard format for placing fields and directories on CD ROM, revised and adopted by the International Organization for Standardization as ISO 9660.
high-frequency distribution frame (HFDF)A distribution frame that provides terminating and interconnecting facilities for those combined supergroup modulator output circuits and combined…
high-level controlIn the hierarchical structure of a primary or secondary data transmission station, the conceptual level of control or processing logic that (a) is above…
high-level language (HLL)A computer programming language that is primarily designed for, and syntactically oriented to, particular classes of problems and that is essentially…
high-pass filterA filter that passes frequencies above a given frequency and attenuates all others.
high-performance equipmentEquipment that (a) has the performance characteristics required for use in trunks or links, (b) is designed primarily for use in global and tactical…
high-priority emergency service callA call identified as receiving emergency service treatment, which has been marked as requiring special treatment by the emergency service administration.
high-usage trunk groupA group of trunks for which an alternate route has been provided to absorb the relatively high rate of overflow traffic.
highwayA digital serial-coded bit stream with time slots allotted to each call on a sequential basis. 2.
hissNoise in the audio frequency range, having subjective characteristics analogous to prolonged sibilant sounds.
history listA record of the documents visited during a Web session, which list allows users to access previously visited pages more quickly or to generate a record of…
holding timeThe total length of time that a call makes use of a trunk or channel. Note: Holding time is usually measured in seconds. 2.
home networkThe carrier's own Signaling System No. 7 (SS7) network. [T1.226-1992]
home pageMain page of a Web site. Usually has hyperlinks to other pages, both inside and outside the site.
homingA process in which a mobile station is directed, or directs itself, toward an electromagnetic, thermal, sonic, or other source of energy, whether primary…
Homing Tandem Operating CompanyThe Homing Tandem Operating Company is the company identified in the Telcordia LERG Routing Guide as being the owner of the tandem to which the NXX code…
homochronousThe relationship between two signals such that their corresponding significant instants are displaced by a constant interval of time.
homogeneous claddingIn an optical fiber, a cladding in which the nominal refractive index is constant throughout.
homogeneous multiplexingMultiplexing in which all of the information-bearer channels operate at the same data signaling rate.
hopAn intermediate stop on the path along which mail is routed when the sender's server and recipient's server are not directly connected.
hop countIn a data communications network, the number of legs traversed by a packet between its source and destination.
horizon angleOf a directional antenna, the angle, in a vertical plane, subtended by the lines extending (a) from the antenna to the radio horizon and (b) from the…
horizontal resolution1. In facsimile, the number of pixels per unit distance in the direction of scanning or recording. 2.
hornIn radio transmission, an open-ended waveguide, of increasing cross-sectional area, which radiates directly in a desired direction or feeds a reflector…
horn gap switchA switch provided with arcing horns, ordinarily used for disconnecting or breaking the charging current of overhead transmission and distribution lines.
hostIn a network, the central computer which controls the remote computers and holds the central databases.
host addressA fully qualified domain name (usually alphabetic) identifying the address of one specific host computer on the Internet.
host central officeAn electronic analog or digital base switching unit containing the central call processing functions which service the host office and its remote…
host computerIn a computer network, a computer that provides end users with services such as computation and database access and that usually performs network control…
host nameA fully qualified domain name identifying one specific host computer within the Internet.
hot siteAn alternate facility that has the equipment and resources to immediately recover the business functions affected by the occurrence of a disaster See also…
hot swapThe ability to remove a failed member of a redundant disk array and replace it with a good drive without bringing down the server or interrupting…
hotlineA point-to-point communications link in which a call is automatically originated to the preselected destination without any additional action by the user…
hotlistA list of frequently used web locations and URLs (Uniform Resource Locators). Note: Hotlists may consist of a) lists of bookmarks accessed by a browser or…
house cableDeprecated term. Communication cable within a building or a complex of buildings.
HubA network device that connects multiple computers/peripherals together and allows them to share data.
Huffman codingA coding technique used to compact data by representing the more common events with short codes and the less common events with longer codes.
Human-computer interfaceThe boundary across which physical interaction between a human being and the application platform takes place [POSIX.0].
human-machine interface (HMI)Human-machine interface between user and terminal/system that consists of a physical section (electro-acoustic, electro-optic transducer, keys, etc.
HuntingIncludes big- and small-game hunting, waterfowl hunting, and trapping.
HybridA class that is a mixture or combination of 2 or more classes. For example, a Druid can be viewed as a Warrior/Rogue/Priest hybrid.
hybrid balanceAn expression of the degree of electrical symmetry between two impedances connected to two conjugate sides of a hybrid set or resistance hybrid.
hybrid cableAn optical communications cable having two or more different types of optical fibers, e.g., single-mode and multimode fibers.
hybrid coilA single transformer that effectively has three windings, and which is designed to be configured as a circuit having four branches, i.e.
hybrid communications networkA communications network that uses a combination of line facilities, i.e., trunks, loops, or links, some of which use only analog or quasi-analog signals…
hybrid computerA computer that processes both analog and digital data.
hybrid connectorA connector that contains contacts for more than one type of service. Note: Examples of hybrid connectors are those that have contacts for both optical…
hybrid couplerIn an antenna system, a hybrid junction used as a directional coupler. Note: The loss through a hybrid coupler is usually 3 dB.