Telecom Dictionary - Definitions of terms

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

H

H.323 - The standard call protocol for voice and videoconferencing over LANs, WANs, and the Internet, allowing these activities on a real-time basis as opposed to a packet-switched network. Initially designed to allow multimedia to function over unreliable networks, it's the oldest and most established of the VoIP protocols. See also SIP and MGCP.

Hack - 1. To break into or use a computer network or use a system without authorization, as a hacker might do. 2. Referring to a track used to bypass a flaw or a bug in an application program or application.

Hacker - 1. A person who breaks into, or attempts to break into, or use, a computer network or system without authorization, often at random, for personal amusement or gratification, and not necessarily with malicious intent. 2. [An] unauthorized user who attempts to or gains access to an information system. 3. A technically sophisticated computer expert who intentionally gains unauthorized access to targeted protected resources. 4. Loosely, a computer enthusiast. 5. A person who uses a computer resource in a manner for which it is not intended or which is in conflict with the terms of an acceptable-use policy, but (unlike the work of a cracker) is not necessarily malicious in intent. hack: 1. To break into or use a computer network or use a system without authorization, as a hacker might do. 2. Referring to a track used to bypass a flaw or a bug in an application program or application.

Hagelbarger Code - A convolutional code that enables error bursts to be corrected provided that there are relatively long error-free intervals between the error bursts. Note: In the Hagelbarger code, inserted parity check bits are spread out in time so that an error burst is not likely to affect more than one of the groups in which parity is checked.

Half-Duplex (HDX) Operation - Operation in which communication between two terminals occurs in either direction, but in only one direction at a time. Note: Half-duplex operation may occur on a half-duplex circuit or on a duplex circuit, but it may not occur on a simplex circuit. Synonyms one-way reversible operation, two-way alternate operation.

Halftone - Any photomechanical printing surface or the impression therefrom in which detail and tone values are represented by a series of evenly spaced dots in varying size and shape, varying in direct proportion to the intensity of tones they represent

Halftone Characteristic - 1. In facsimile systems, the relationship between the density of the recorded copy and the density of the object, i.e., the original. 2. In facsimile systems, the relationship between the amplitude of the facsimile signal to either the density of the object or the density of the recorded copy when only a portion of the system is under consideration. Note: In an FM facsimile system, an appropriate parameter other than the amplitude is used.

Hamming code: An error-detecting and error-correcting binary code, used in data transmission, that can (a) detect all single- and double-bit errors and (b) correct all single-bit errors. Note: A Hamming code satisfies the relation 2 ^m n +1, where n is the total number of bits in the block, k is the number of information bits in the block, and m is the number of check bits in the block, where m = n-k.

Hamming Distance - The number of digit positions in which the corresponding digits of two binary words of the same length are different. Note 1: The Hamming distance between 1011101 and 1001001 is two. Note 2: The concept can be extended to other notation systems. For example, the Hamming distance between 2143896 and 2233796 is three, and between "toned" and "roses" it is also three. Synonym signal distance.

Hamming Weight - The number of non-zero symbols in a symbol sequence. Note: For binary signaling, Hamming weight is the number of "1" bits in the binary sequence.

Handoff - 1. In cellular mobile systems, the process of transferring a phone call in progress from one cell transmitter and receiver and frequency pair to another cell transmitter and receiver using a different frequency pair without interruption of the call. Synonym handover. 2. In satellite communications, the process of transferring ground-station control responsibility from one ground station to another without loss or interruption of service.

Handover - 1. In telephony, synonym handoff. 2. The transfer (permanent or temporary) of a component or series of components to another application process. 3. The automatic rerouting of the radio portion of a call for signal quality, traffic management, or other reasons.

Handshaking - 1. In data communications, a sequence of events governed by hardware or software, requiring mutual agreement of the state of the operational modes prior to information exchange. 2. The process used to establish communications parameters between two stations. Note: Handshaking follows the establishment of a circuit between the stations and precedes information transfer. It is used to agree upon such parameters as information transfer rate, alphabet, parity, interrupt procedure, and other protocol features.

Handshaking Procedures - [The] dialogue between two information systems for synchronizing, identifying, and authenticating themselves to one another.

Hangover - Synonym tailing. In facsimile systems, the excessive prolongation of the decay of the signal.

Hang-Up - Calling- or called-user placement of a telephone set or other unit of telecommunications equipment in the quiescent state.

Hang-Up Signal - An on-hook signal sent from an end office toward the disconnect-control office indicating either calling or called user hang-up and requesting the connection be disconnected. The interface remains dedicated to the call until the disconnect-control office responds to the hang-up signal.

HA1-Receiver Weighting - A noise weighting used in a noise measuring set to measure noise across the HA1-receiver of a 302-type or similar instrument. Note 1: The meter scale readings of an HA1 test set are in dBa (HA1). Note 2: HA1 noise weighting is obsolete for new DOD applications.

Hard Copy - In computer graphics and in telecommunications, a permanent reproduction, on any media suitable for direct use by a person, of displayed or transmitted data. Note 1: Examples of hard copy include teletypewriter pages, continuous printed tapes, facsimile pages, computer printouts, and radiophoto prints. Note 2: Magnetic tapes, diskettes, and nonprinted punched paper tapes are not hard copy.

Hard-Copy Key - [A] physical keying material, such as printed key lists, punched or printed key tapes, or programmable, read-only memories (pROM).

Hard Disk - A flat, circular, rigid plate with a magnetizable surface on one or both sides of which data can be stored. Note: A hard disk is distinguished from a diskette by virtue of the fact that it is rigid. Early in the development of computer technology, hard disks, often multiple disks mounted on a common spindle, were interchangeable and removable from their drives, which were separate from the processor chassis. This technology is still in use, especially in conjunction with large mainframe computers, but physically smaller computers use hard disks that are in sealed units, along with their control electronics and read /write heads. The sealed units are usually installed permanently in the same chassis that contains the processor.

Hard-Drawn Copper Wire - Copper wire that has not been annealed after being drawn.

Hardened - Pertaining to the condition of a facility with protective features that enable it to withstand destructive forces, such as explosions, natural disasters, or ionizing radiation.

Hard Limiting - See limiting. Any process by which a specified characteristic (usually amplitude) of the output of a device is prevented from exceeding a predetermined value. Note 1: Hard limiting (" clipping ") is a limiting action in which there is (a) over the permitted dynamic range, negligible variation in the expected characteristic of the output signal, and (b) a steady-state signal, at the maximum permitted level, for the duration of each period when the output would otherwise be required to exceed the permitted dynamic range in order to correspond to the transfer function of the device. Note 2: Soft limiting is limiting in which the transfer function of a device is a function of its instantaneous or integrated output level. The output waveform is therefore distorted, but not clipped.

Hard Sectoring - In magnetic or optical disk storage, sectoring that uses a physical mark on the disk, from which mark sector locations are referenced. Note: Hard sectoring may be done, for example, by punching an index hole in a floppy diskette. When the presence of the index hole is recognized by an optical reader, a reference signal is generated. All sector locations can be referenced from this signal.

Hardware - 1. Physical equipment as opposed to programs, procedures, rules, and associated documentation. 2. The generic term dealing with physical items as distinguished from its capability or function such as equipment, tools, implements, instruments, devices, sets, fittings, trimmings, assemblies, subassemblies, components, and parts. The term is often used in regard to the stage of development, as in the passage of a device or component from the design stage into the hardware stage as the finished object. 3. In data automation, the physical equipment or devices forming a computer and peripheral components.

Hardware Platform - Synonym platform. 1. The type of computer on which a given operating system or application runs. Synonym hardware platform. 2. The operating system in use on a given computer. Synonym operating system platform. 3. The application program in use on a given computer and operating system. Synonym application platform. Note 1: The term platform, usually with some kind of accompanying qualifying verbiage, may also be applied to any combination of the foregoing. Note 2: The term cross-platform may be used to characterize an application program or operating system that may be run on more than one platform.

Hardwire - 1. To connect equipment or components permanently in contrast to using switches, plugs, or connectors. 2. To wire in fixed logic or read-only storage that cannot be altered by program changes.

Hardwired Key - [A] permanently installed key. Information (usually a sequence of random or pseudorandom binary digits) used initially to set up and permanently installed for the operations performed in crypto-equipment for the purpose of encrypting or decrypting electronic signals, for determining electronic counter-countermeasures patterns (e.g., frequency hopping or spread spectrum), or for producing other key. Note: "Key" has replaced the terms "variable," "key(ing) variable," and "cryptovariable."

Harmful Interference - 1. Any emission, radiation, or induction interference that endangers the functioning or seriously degrades, obstructs, or repeatedly interrupts a communications system, such as a radio navigation service, telecommunications service, radio communications service, search and rescue service, or weather service, operating in accordance with approved standards, regulations, and procedures. Note: To be considered harmful interference, the interference must cause serious detrimental effects, such as circuit outages and message losses, as opposed to interference that is merely a nuisance or annoyance that can be overcome by appropriate measures. 2. Interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with these [Radio] Regulations.

Harmonic - 1. Of a sinusoidal wave, an integral multiple of the frequency of the wave. Note: The frequency of the sine wave is called the fundamental frequency or the first harmonic, the second harmonic is twice the fundamental frequency, the third harmonic is thrice the fundamental frequency, etc. 2. Of a periodic signal or other periodic phenomenon, such as an electromagnetic wave or a sound wave, a component frequency of the signal that is an integral multiple of the fundamental frequency. Note: The fundamental frequency is the reciprocal of the period of the periodic phenomenon.

Harmonic Distortion - In the output signal of a device, distortion caused by the presence of frequencies that are not present in the input signal. Note: Harmonic distortion is caused by nonlinearities within the device.

Hash Function - A mathematical function that maps values from a large (or very large) domain into a smaller range, and that reduces a potentially long message into a "message digest" or "hash value" or that is sufficiently compact to be input into a digital signature algorithm. Note: A "good" hash function is one that results from applying the function to a (large) set of values that are evenly (and randomly) distributed over the range.

Hashing - Computation of a hash total, which is: 1. The result obtained by subjecting a set of data to an algorithm for purposes of checking the data at the time the algorithm is applied or for use at a later time such as after transmission or retrieval from storage. 2. [A] value computed on data to detect error or manipulation.

Hash Total - 1. The result obtained by subjecting a set of data to an algorithm for purposes of checking the data at the time the algorithm is applied or for use at a later time such as after transmission or retrieval from storage. 2. [A] value computed on data to detect error or manipulation. See checksum.

Hash Value - In cryptography, the result of applying a cryptologic hash function to a message.

Hashword - [The] memory address containing [a] hash total.

Hazards of Electromagnetic Radiation to Fuel (HERF) - The potential for electromagnetic radiation to cause ignition or detonation of volatile combustibles, such as aircraft fuels.

Hazards of Electromagnetic Radiation to Ordnance (HERO) - The potential for electromagnetic radiation to affect adversely munitions or electroexplosive devices.

Hazards of Electromagnetic Radiation to Personnel (HERP) - The potential for electromagnetic radiation to produce harmful biological effects in humans.

H-bend - A smooth change in the direction of the axis of a waveguide, throughout which the axis remains in a plane parallel to the direction of magnetic H-field (transverse) polarization. Synonym H-plane bend.

H-channel - In Integrated Services Digital Networks (ISDN), a 384-kb/s, 1472-kb/s, or 1536-kb/s channel, designated as "H₀", "H₁₀", and "H₁₁", respectively, accompanied by timing signals used to carry a wide variety of user information. Note: Examples of types of user information representation forms include fast facsimile, video, high-speed data, high-quality audio, packet-switched data, bit streams at rates less than the respective H-channel bit rate that have been rate-adapted or multiplexed together, and packet-switched information.

HDCP - Abbreviation for High Definition Content Protection. Technology used to prevent piracy of high-quality uncompressed video, primarily over DVI connections.

HDLC - Abbreviation for high-level data link control. A Link-Level protocol used to facilitate reliable point-to-point transmission of a data packet. Note: A subset of HDLC, known as " LAP-B," is the Layer-two protocol for ITU-T Recommendation X.25.

HDMI - Abbreviation for High Definition Multimedia Interface. A high-quality digital connector. Similar to DVI and sometimes with HDCP, HDMI can digitally transmit uncompressed high-definition video and audio on the same cable, preserving picture and sound quality.

HDTV - Abbreviation for high-definition television. Television that has approximately twice the horizontal and twice the vertical emitted resolution specified by the NTSC standard. The highest quality digital television, generally widescreen 16:9. It consists of either 1080 interlaced or 720 progressively scanned lines of resolution and includes the ability to transmit digital surround sound. Note that HDTV and DTV are not the same thing — HDTV is one format of DTV. Note 1: In HDTV, the total number of pixels is therefore approximately four times that of the NTSC standard. Note 2: HDTV may include any or all improved-definition television (IDTV) and extended-television (EDTV) improvements. Note 3: HDTV employs a wide aspect ratio.

HDTV Monitor (also HDTV-Ready) - A TV set with the inputs and capability to become an HDTV with the addition of an HDTV tuner, HD cable set-top box, or HD satellite receiver.

HDTV Tuner (also known as Decoder or Receiver) - A device capable of receiving and decoding HDTV signals. HDTV tuners can either be built into a TV set (called an Integrated Digital TV Set) or be a stand-alone device, such as a set-top box.

HDX - Abbreviation for half-duplex (HDX) operation. Operation in which communication between two terminals occurs in either direction, but in only one direction at a time. Note: Half-duplex operation may occur on a half-duplex circuit or on a duplex circuit, but it may not occur on a simplex circuit. Synonyms one-way reversible operation, two-way alternate operation.

Head - A device that reads, writes, and/or erases data on a storage medium.

Head End - See cable headend, headend. 1. A central control device required by some networks (e.g., LANs or MANs) to provide such centralized functions as remodulation, retiming, message accountability, contention control, diagnostic control, and access to a gateway. 2. A control center of a CATV system, where incoming signals are amplified, converted, processed, and combined into a common cable for transmission to customers. The headend usually includes antennas, preamplifiers, frequency converters, demodulators, modulators, processors, and other related equipment. Synonym [in this sense] cable headend.

Headend - 1. A central control device required by some networks (e.g., LANs or MANs) to provide such centralized functions as remodulation, retiming, message accountability, contention control, diagnostic control, and access to a gateway. 2. A control center of a CATV system, where incoming signals are amplified, converted, processed, and combined into a common cable for transmission to customers. The headend usually includes antennas, preamplifiers, frequency converters, demodulators, modulators, processors, and other related equipment.

Header - The portion of a message that contains information used to guide the message to the correct destination. Note: Examples of items that may be in a header are the addresses of the sender and receiver, precedence level, routing instructions, and synchronizing bits.

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.

Head-of-Bus Function - The function that generates management information and empty bus slots at the point on each bus where data flow begins.

Head-on Collision - A collision that occurs on a communications channel when two or more users begin to transmit on the channel at approximately the same instant.

Heaviside Layer -That portion of the ionosphere existing between approximately 95 and 130 km above the surface of the Earth. Note: The Heaviside Layer ( E Region) lies between the D and F regions. Synonym E region, Kennelly-Heaviside layer.

Height Gain - For 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 Earth.

Helical Antenna - An 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 the axis of the helix. When the helix circumference is one wavelength, maximum radiation is along the helix axis.

Helical Scan - A method of recording video information diagonally on a tape, used in home and professional video cassette recorders (VCRs). Note: High-speed rotating video heads scan these diagonal video tracks, giving an effective tape speed much higher than the actual tape speed, allowing more information to be recorded on a given length of magnetic tape.

HEMP - Abbreviation for high-altitude electromagnetic pulse. An electromagnetic pulse produced at an altitude effectively above the sensible atmosphere, i.e., above about 120 km.

HE₁₁ Mode - Designation for the fundamental hybrid mode of an optical fiber.

HERF - Abbreviation for hazards of electromagnetic radiation to fuel. The potential for electromagnetic radiation to cause ignition or detonation of volatile combustibles, such as aircraft fuels.

HERO - Abbreviation for hazards of electromagnetic radiation to ordnance. The potential for electromagnetic radiation to affect adversely munitions or electroexplosive devices.

HERP - Abbreviation for hazards of electromagnetic radiation to personnel. - The potential for electromagnetic radiation to produce harmful biological effects in humans.

Hertz (Hz) - 1. The SI unit of frequency, equal to one cycle per second. Note: A periodic phenomenon that has a period of one second has a frequency of one hertz. 2. A unit of frequency which is equivalent to one cycle per second.

Hertzian Wave - Synonym radio wave. An electromagnetic wave of a frequency arbitrarily lower than 3000 GHz.

Heterochronous - A relationship between two signals such that their corresponding significant instants do not necessarily occur at the same time. Note: Two signals having different nominal signaling rates and not stemming from the same clock or from homochronous clocks are usually heterochronous.

Heterodyne - 1. To generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer. Note: A superheterodyne receiver converts any selected incoming frequency by heterodyne action to a common intermediate frequency where amplification and selectivity (filtering) are provided. 2. A frequency produced by mixing two or more signals in a nonlinear device.

Heterodyne Repeater - In radio reception and retransmission, a repeater that converts the original band of frequencies of the received signal to a different frequency band for retransmission after amplification. Note: Heterodyne repeaters are used, for example, in microwave systems, to avoid undesired feedback between the receiving and transmitting antennas. Synonym IF repeater.

Heterogeneous Multiplexing - Multiplexing in which not all the information-bearer channels operate at the same data signaling rate.

Heuristic Routing - Routing in which data, such as time delay, extracted from incoming messages, during specified periods and over different routes, are used to determine the optimum routing for transmitting data back to the sources. Note: Heuristic routing allows a measure of route optimization based on recent empirical knowledge of the state of the network.

Hexadecimal - 1. Characterized by a selection, choice or condition that has sixteen possible different values or states. Synonym sexadecimal. 2. Pertaining to a fixed-radix numeration system in which the radix is sixteen.

HF - Abbreviation for high frequency. Frequencies from 3 MHz to 30 MHz.

HFDF - Abbreviation for high-frequency distribution frame (HFDF): A distribution frame that provides terminating and interconnecting facilities for those combined supergroup modulator output circuits and combined supergroup demodulator input circuits that contain signals occupying the baseband spectrum.

Hierarchical Computer Network - A computer network in which processing and control functions are performed at several levels by computers specially suited for the functions performed, such as industrial process control, inventory control, database control, or hospital automation.

Hierarchically Synchronized Network - A mutually synchronized network in which some clocks exert more control than others, the network operating frequency being a weighted mean of the natural frequencies of the population of clocks.

Hierarchical Routing - Routing that is based on hierarchical addressing. Note: Most Transmission Control Protocol / Internet Protocol (TCP/IP) routing is based on a two-level hierarchical routing in which an IP address is divided into a network portion and a host portion. Gateways use only the network portion until an IP datagram reaches a gateway that can deliver it directly. Additional levels of hierarchical routing are introduced by the addition of subnetworks.

High-Altitude Electromagnetic Pulse (HEMP) - An electromagnetic pulse produced at an altitude effectively above the sensible atmosphere, i.e., above about 120 km.

High-Definition Television (HDTV) - Television that has approximately twice the horizontal and twice the vertical emitted resolution specified by the NTSC standard. Note 1: In HDTV, the total number of pixels is therefore approximately four times that of the NTSC standard. Note 2: HDTV may include any or all improved-definition television (IDTV) and extended-television (EDTV) improvements. Note 3: HDTV employs a wide aspect ratio.

Higher Frequency Ground - Deprecated name for facility grounding system. The electrically interconnected system of conductors and conductive elements that (a) provides multiple current paths to the earth electrode subsystem, and (b) consists of the earth electrode subsystem, the lightning protection subsystem, and the fault protection subsystem.

High Frequency (HF): Frequencies from 3 MHz to 30 MHz. See electromagnetic spectrum.

High-Frequency Distribution Frame (HFDF) - A distribution frame that provides terminating and interconnecting facilities for those combined supergroup modulator output circuits and combined supergroup demodulator input circuits that contain signals occupying the baseband spectrum.

High-Level Control - In the hierarchical structure of a primary or secondary data transmission station, the conceptual level of control or processing logic that (a) is above the Link Level and (b) controls Link Level functions, such as device control, buffer allocation, and station management.

High-Level Data Link Control (HDLC) - A Link-Level protocol used to facilitate reliable point-to-point transmission of a data packet. Note: A subset of HDLC, known as " LAP-B," is the Layer-two protocol for ITU-T Recommendation X.25.

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 independent of the structure of a specific computer or class of computers; for example, Ada ®, COBOL, Fortran, Pascal. Synonym high-order language.

High-Order Language - Synonym high-level language.

High-Pass Filter - A filter that passes frequencies above a given frequency and attenuates all others.

High-Performance Equipment - Equipment that (a) has the performance characteristics required for use in trunks or links, (b) is designed primarily for use in global and tactical systems, and (c) sufficiently withstands electromagnetic interference when operating in a variety of network or point-to-point circuits. Note: Requirements for global and tactical high-performance equipment may differ.

High-Priority Emergency Service Call - A call identified as receiving emergency service treatment, which has been marked as requiring special treatment by the emergency service administration. The call has an associated CESID and transfer numbers.

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 implemented in the public switched network (PSN) for the Government Emergency Telecommunications Service (GETS).

High Sierra Format - A standard format for placing fields and directories on CD ROM, revised and adopted by the International Organization for Standardization as ISO 9660.

High-Usage Trunk Group - A group of trunks for which an alternate route has been provided to absorb the relatively high rate of overflow traffic.

Highway - 1. A digital serial-coded bit stream with time slots allotted to each call on a sequential basis. 2. A common path or a set of parallel paths over which signals from more than one channel pass with separation achieved by time division.

Hiss - Noise in the audio frequency range, having subjective characteristics analogous to prolonged sibilant sounds. Note: Noise in which there are no pronounced low-frequency components may be considered as hiss.

History List - A 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 a user's progress. Synonym bookmark list, go list, hotlist.

Hit - 1. A transient disturbance to, or momentary interruption of, a communication channel, power supply (especially that provided via a commercial electric power distribution network or grid, but not limited to same), etc. 2. A match of a data item to a prescribed criterion or criteria. Note: For example, each of the matches from a search engine is called a "hit." 3. The access of any item, such as a page or a graphic image, from a Web server. Note 1: Hits are recorded for the purpose of assessing traffic /interest in a web site; many web sites have a commercial sponsor who pays a small amount per hit for displaying the commercial advertisement on that site. Note 2: The number of hits on a web site is not synonymous with the number of distinct visitors. 4. The number of document requests being answered by a server.

Hit Counter - An indicator on a Web page that graphically displays the number of previous users that have accessed the page.

HLL: Abbreviation for high-level language. A computer programming language that is primarily designed for, and syntactically oriented to, particular classes of problems and that is essentially independent of the structure of a specific computer or class of computers; for example, Ada ®, COBOL, Fortran, Pascal.

Hockey Puck - A polishing fixture used to facilitate the manual finishing of the endfaces of certain types of optical fiber connectors. Note 1: The hockey puck consists of the appropriate mating sleeve for the connector in question, mounted at right angles to, and in the center of, a disk of stainless steel or other hard material. When the unfinished connector, secured to the fiber-optic cable, is mounted in the hockey puck, excess material (e.g., fiber end, bead of adhesive material, and excess connector length, if present) protrudes from the opposite side of the disk. The excess is then ground away as the fixture is manually swept to and fro, usually in a figure-8 pattern, in contact with a piece of microfinishing film which is in turn supported by a rigid flat substrate. Two to four grades of microfinishing film, with abrasive particles ranging in size from 15 m to 0.3 m, are commonly used. Note 2: Various manufacturers use proprietary names to identify this device; however "hockey puck" has become ubiquitous.

Hold-in Frequency Range - The range of frequencies over which a phase-locked loop can vary and still maintain frequency lock.

Holding Time - 1. The total length of time that a call makes use of a trunk or channel. Note: Holding time is usually measured in seconds. 2. The time in which an item of telephone plant is in actual use either by a customer or an operator. For example, on a completed telephone call, holding time includes conversation time as well as other time in use. At local dial offices any measured minutes which result from other than customer attempts to place calls (as evidenced by the dialing of at least one digit) are not treated as holding time.

Hold-in Range - See hold-in frequency range. The range of frequencies over which a phase-locked loop can vary and still maintain frequency lock.

Home Network - The carrier's own Signaling System No. 7 (SS7) network.

Home Page - The Web document that your browser displays when you access a site - typically the main page of a Web site. 1. The document that is configured to be displayed first when a Web browser is opened. 2. The document designed to be the user's point of entry into a Web site, or the page that the user first sees when he or she first visits a company's Web site.

Homing - 1. A process in which a mobile station is directed, or directs itself, toward an electromagnetic, thermal, sonic, or other source of energy, whether primary or reflected, or follows a vector force field or a gradient of a scalar force field. 2. In radio direction-finding, the locating of a moving signal source by a moving direction-finding station that has a mobile advantage. 3. The act of approaching a source of electromagnetic radiation in which the approaching vehicle is guided by a receiver with a directional antenna. 4. Seeking, finding, intercepting, and engaging an object, i.e., a target (fixed or mobile) that may contain a signal source.

Homochronous - The relationship between two signals such that their corresponding significant instants are displaced by a constant interval of time.

Homogeneous Cladding - In an optical fiber, a cladding in which the nominal refractive index is constant throughout. Note: An optical fiber may have several homogeneous claddings, each having a different refractive index.

Homogeneous Multiplexing - Multiplexing in which all of the information-bearer channels operate at the same data signaling rate.

Hop - 1. The excursion of a radio wave from the Earth to the ionosphere and back to the Earth. Note: The number of hops is synonymous with the number of reflections from the ionosphere. 2. A waveform transmitted for the duration of each relocation of the carrier frequency of a frequency-hopped system. 3. To modify a modulated waveform with constant center frequency so that it frequency hops. 4. An intermediate network connection consisting of a leg from one router to another router and over which a packet travels to reach its destination. Note: Hops can be traced using ping or other trace utilities.

Hop Count - 1. In a data communications network, the number of legs traversed by a packet between its source and destination. Note: Hop count may be used to determine the time-to-live for some packets. 2. The number of signal regenerating devices (such as repeaters, bridges, routers, and gateways) through which data must pass to reach their destination.

Horizon Angle - Of 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 antenna in its direction of maximum radiation.

Horizontal Redundancy Check - Synonym longitudinal redundancy check. - A system of error control based on the formation of a block check following preset rules. Note 1: The block check formation rules are applied in the same manner to each character. Note 2: A combination of longitudinal and vertical redundancy check allows the detection and correction of single bit errors.

Horizontal Resolution - 1. In facsimile, the number of pixels per unit distance in the direction of scanning or recording. 2. In digital telegraphy, the number of pixels in the horizontal direction. 3. In raster-scanned television, the number of picture elements in a scan line. 4. In a computer monitor, the number of pixels per unit distance in the horizontal direction. Note: This value is inversely proportional to the dot pitch of the monitor.

Horn - 1. In radio transmission, an open-ended waveguide, of increasing cross-sectional area, which radiates directly in a desired direction or feeds a reflector that forms a desired beam. Note 1: Horns may have one or more expansion curves, i.e., longitudinal cross sections, such as elliptical, conical, hyperbolic, or parabolic curves, and not necessarily the same expansion curve in each (E-plane and H-plane) cross section. Note 2: A very wide range of beam patterns may be formed by controlling horn dimensions and shapes, placement of the reflector, and reflector shape and dimensions. 2. A portion of a waveguide in which the cross section is smoothly increased along the axial direction. 3. In audio systems, a tube, usually having a rectangular transverse cross section and a linearly or exponentially increasing cross-sectional area, used for radiating or receiving acoustic waves.

Horn Gap Switch - A switch provided with arcing horns, ordinarily used for disconnecting or breaking the charging current of overhead transmission and distribution lines.

Host - A computer directly connected to the Internet. Also refers to a computer on a network that provides services to other computers on the network. 1. In packet- and message-switching communications networks, the collection of hardware and software that makes use of packet or message switching to support user-to-user, i.e., end-to-end, communications, interprocess communications, and distributed data processing. 2. Synonym host computer.

Host Address - A fully qualified domain name (usually alphabetic) identifying the address of one specific host computer on the Internet. Note: The host address is a subset of the IP address.

Host Central Office - An electronic analog or digital base switching unit containing the central call processing functions which service the host office and its remote locations.

Host Computer - 1. In a computer network, a computer that provides end users with services such as computation and database access and that usually performs network control functions. Synonym host. 2. A computer on which is developed software intended to be used on another computer.

Host-Host Protocol - End-to-end (transport) protocol.

Hosting - Synonym (in Internet usage) content hosting. Storage and management of databases by a content provider.

Host Name - A fully qualified domain name identifying one specific host computer within the Internet.

Hot Boot - Synonym warm restart. 1. A sequence of operations that is performed to reset a previously running system, after an unintentional shutdown. Synonym warm start. 2. In computer operations, the restarting of equipment, after a sudden shutdown, that allows reuse of previously retained initialized input data, retained programs, and retained output queues. Note 1: A warm restart may be needed after a program failure. Note 2: A warm start or restart cannot occur if initial data, programs, and files are not retained after closedown.

Hotbot - Synonym droid. 1. In the Internet, an intelligent search tool that automatically seeks out relevant online information based on the user's specifications. Synonyms agent, bot, crawler, hotbot, infobot, information agent, intelligent agent, Internet search engine, knowbot, knowledge robot, personal agent, robotic librarian, search robot, spider, Web crawler, Web spider, wizard. 2. In artificial intelligence, an entity with the ability to sense its environment and to act in such a way or to affect that environment; typically knowledge-based entities that can communicate with each other through some message-passing scheme.

Hotline - A point-to-point communications link in which a call is automatically originated to the preselected destination without any additional action by the user when the end instrument goes off-hook. Note 1: Hotlines cannot be used to originate calls other than to preselected destinations. Note 2: Various priority services that require dialing are not properly termed "hotlines." Synonyms automatic signaling service, off-hook service.

Hotlink - See hyperlink. 1. A software function that (a) is manifest to the user as displayed, selectable words or icons, and (b) allows viewers of an HTML document to navigate thereby to another HTML document or file. 2. The link created, as in 1.

Hotlist - A 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 b) lists of URLs at a web site for linking to other relevant web sites. Synonyms bookmark list, go list, history list.

Hot Site - In computer security, synonym for hot standby. See standby. 1. In computer and communications systems operations, pertaining to a power-saving condition or status of operation of equipment that is ready for use but not in use. Note: An example of a standby condition is a radio station operating condition in which the operator can receive but is not transmitting. 2. Pertaining to a dormant operating condition or state of a system or equipment that permits complete resumption of operation in a stable state within a short time. 3. Pertaining to spare equipment that is placed in operation only when other, in-use equipment becomes inoperative. Note: Standby equipment is usually classified as (a) hot standby equipment, which is warmed up, i.e., powered and ready for immediate service, and which may be switched into service automatically upon detection of a failure in the regular equipment, or (b) cold standby equipment, which is turned off or not connected to a primary power source, and which must be placed into service manually.

Hot Standby - 1. In computer and communications systems operations, pertaining to a power-saving condition or status of operation of equipment that is ready for use but not in use. Note: An example of a standby condition is a radio station operating condition in which the operator can receive but is not transmitting. 2. Pertaining to a dormant operating condition or state of a system or equipment that permits complete resumption of operation in a stable state within a short time. 3. Pertaining to spare equipment that is placed in operation only when other, in-use equipment becomes inoperative. Note: Standby equipment is usually classified as (a) hot standby equipment, which is warmed up, i.e., powered and ready for immediate service, and which may be switched into service automatically upon detection of a failure in the regular equipment, or (b) cold standby equipment, which is turned off or not connected to a primary power source, and which must be placed into service manually. See standby.

Hot Swap - In an electronic device, of a subassembly or component therein (e.g. circuit card), the act or process of removing and replacing the subassembly or component without first powering down the device. Note: Hot swapping of components in devices or environments not designed to support such practice may result in damage to the component or device, or may pose a spark hazard even if the primary power has been removed. And, in some computing devices, hot swapping may not be appropriate because new components may be recognized only during boot-up.

House Cable - Deprecated term. Communication cable within a building or a complex of buildings. Note: House cable owned before divestiture by the Bell System and after divestiture by the Regional Bell Operating Companies will eventually be fully depreciated and will then belong to the customer. See on-premises wiring.

Housekeeping Signals - Synonym service signals. Signals that enable data systems equipment to function correctly, and possibly to provide ancillary facilities.

HPC - Abbreviation for high probability of completion.

H-Plane Bend - Synonym H-bend. A smooth change in the direction of the axis of a waveguide, throughout which the axis remains in a plane parallel to the direction of magnetic H-field (transverse) polarization. Synonym H-plane bend.

HTML - See HyperText Markup Language Abbreviation for Hypertext Markup Language. An application of SGML (Standard Generalized Markup Language) implemented in conjunction with the World Wide Web to facilitate the electronic exchange and display of simple documents using the Internet.

HTTP - Abbreviation for Hypertext Transfer Protocol. In the World Wide Web, a protocol that facilitates the transfer of hypertext-based files between local and remote systems.

Hub - 1. A distribution point in a network. 2. A device that accepts a signal from one point and redistributes it to one or more points.

Huffman Coding - A coding technique used to compact data by representing the more common events with short codes and the less common events with longer codes. Note: Huffman coding is used in Group 3 facsimile.

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.) and a logical section dealing with functional operation states.

Hundred Call-Seconds (CCS): See call-second. The CCS, equivalent to 100 call-seconds, is often used. Note 4: 3600 call-seconds = 36 CCS = 1 call-hour. Note 5: 3600 call-seconds per hour = 36 CCS per hour = 1 call-hour per hour = 1 erlang = 1 traffic unit.

Hunt Group -A service provided by most phone companies that allows several lines to be tied together. A single number is generally dialed by all callers. If that line is busy, the phone company will "roll-over" the call to another line in that group. Also called "rollover" and more recently "busy-line transfer".

Hunting - 1. In telephony, pertaining to the operation of a selector or other similar device to find and establish a connection with an idle circuit of a chosen group. 2. Pertaining to the failure of a device to achieve a state of equilibrium, usually by alternately overshooting and undershooting the point of equilibrium.

Hybrid - 1. A functional unit in which two or more different technologies are combined to satisfy a given requirement. Note: Examples of hybrids include (a) an electronic circuit having both vacuum tubes and transistors, (b) a mixture of thin-film and discrete integrated circuits, and (c) a computer that has both analog and digital capability. 2. A resistance hybrid. 3. A hybrid coil.

Hybrid Balance - An expression of the degree of electrical symmetry between two impedances connected to two conjugate sides of a hybrid set or resistance hybrid. Note 1: Hybrid balance is usually expressed in dB. Note 2: If the respective impedances of the branches of the hybrid that are connected to the conjugate sides of the hybrid are known, hybrid balance may be computed by the formula for return loss.

Hybrid Cable - An optical communications cable having two or more different types of optical fibers, e.g., single-mode and multimode fibers.

Hybrid Coil - A single transformer that effectively has three windings, and which is designed to be configured as a circuit having four branches, i.e., ports, that are conjugate in pairs. Note: The primary use of a hybrid coil is to convert between 2-wire and 4-wire operation in concatenated sections of a communications circuit. Such conversion is necessary when repeaters are introduced in a 2-wire circuit. Synonym bridge transformer.

Hybrid Communications Network - A 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 and some of which use only digital signals. Synonyms hybrid network, hybrid system.

Hybrid Computer - A computer that processes both analog and digital data.

Hybrid Connector - A connector that contains contacts for more than one type of service. Note: Examples of hybrid connectors are those that have contacts for both optical fibers and twisted pairs, electric power and twisted pairs, or shielded and unshielded twisted pairs.

Hybrid Coupler - In an antenna system, a hybrid junction used as a directional coupler. Note: The loss through a hybrid coupler is usually 3 dB.

Hybrid Fiber Coaxial Cable (HFC) - A telecommunications cable in which optical fiber cable and coaxial cable constitute different portions of a network carrying broadband content (such as video, data, and voice). Typically, a local CATV company might use fiber optic cable from the cable headend (distribution center) to the serving node located near business and residential users, and then use coaxial cable from these nodes to individual businesses and homes. An advantage of HFC is that some of the characteristics of fiber optic cable (high bandwidth and low noise /low interference susceptibility) can be brought close to the user without having to replace the entire existing coaxial cable that is installed.

Hybrid Interface Structure - In integrated services digital networks (ISDN), an interface structure that uses both labeled and positioned channels.

Hybrid Junction - A waveguide or transmission line arranged such that (a) there are four ports, (b) each port is terminated in its characteristic impedance, and (c) energy entering any one port is transferred, usually equally, to two of the three remaining ports. Note: Hybrid junctions are used as mixing or dividing devices.

Hybrid Mode - A mode consisting of components of both electrical and magnetic field vectors in the direction of propagation. Note: In fiber optics, such modes correspond to skew (nonmeridional) rays.

Hybrid Network - See hybrid communications network. A 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 and some of which use only digital signals. Synonyms hybrid network, hybrid system.

Hybrid Routing - Routing in which numbering plans and routing tables are used to permit the collocation, in the same area code, of switches using a deterministic routing scheme with switches using a nondeterministic routing scheme, such as flood search routing. Note: Routing tables are constructed with no duplicate numbers, so that direct dial service can be provided to all network subscribers. This may require the use of 10-digit numbers.

Hybrid Set - Two or more transformers interconnected to form a network having four ports that are conjugate in pairs. Note: The primary use of a hybrid set is to convert between 2-wire and 4-wire operation in concatenated sections of a communications circuit. Such conversion is necessary when repeaters are introduced in a 2-wire circuit.

Hybrid Spread Spectrum - A combination of frequency hopping spread spectrum and direct-sequence spread spectrum.

Hybrid System - Synonym hybrid communications network. A 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 and some of which use only digital signals. Synonyms hybrid network, hybrid system.

Hybrid Topology - See network topology. A combination of any two or more network topologies. Note 1: Instances can occur where two basic network topologies, when connected together, can still retain the basic network character, and therefore not be a hybrid network. For example, a tree network connected to a tree network is still a tree network. Therefore, a hybrid network accrues only when two basic networks are connected and the resulting network topology fails to meet one of the basic topology definitions. For example, two star networks connected together exhibit hybrid network topologies. Note 2: A hybrid topology always accrues when two different basic network topologies are connected.

Hydroxyl ion Absorption - In optical fibers, the absorption of electromagnetic waves, including the near-infrared, due to the presence of trapped hydroxyl ions remaining from water as a contaminant. Note: The hydroxyl (OH^-) ion can penetrate glass during or after product fabrication, resulting in significant attenuation of discrete optical wavelengths, e.g., approximately 1.3 m, used for communications via optical fibers.

Hyperlink - 1. A software function that (a) is manifest to the user as displayed, selectable words or icons, and (b) allows viewers of an HTML document to navigate thereby to another HTML document or file. 2. The link created, as in 1.

Hypermedia - Computer-addressable files that contain pointers for linking to multimedia information, such as text, graphics, video, or audio in the same or other documents. Note: The use of hypertext links is known as navigating.

Hypertext - The system of coding that is used to create or navigate hypermedia in a nonsequential manner

HyperText Link - A connection between two Web documents; usually underlined text or a graphic that you click on to display another Web document.

HyperText Markup Language (HTML) - The basic coding language used to create WWW documents.

Hypertext Transfer Protocol - See HTTP.

HyperText Transport Protocol (HTTP) - The protocol for moving HyperText files across the Internet

HTTP - See HyperText Transport Protocol

Hz - : Abbreviation for hertz. 1. The SI unit of frequency, equal to one cycle per second. Note: A periodic phenomenon that has a period of one second has a frequency of one hertz. 2. A unit of frequency which is equivalent to one cycle per second.

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