H H.221 ITU-TSS recommendation regarding frame structure for audiovisual teleservices. H.230 ITU-TSS recommendation regarding frame synchronous control and indication signals for audiovisual systems. H.242 ITU-TSS recommendation regarding a system for establishing communication between audiovisual terminals using digital channels up to 2M bps. H.261 ITU-TSS's standard for digitally compressed video. H.320 A family of standards created to aid in the interoperability of videoconferencing systems. They set protocols for decoding, video/audio multiplexing, framing, call set-up and teardown. half-duplex (HDX) transmission Data transmission in one direction only (receive or send). halftone A graphic, usually created from a photograph, in which dots are used to represent continuous tones. hamming code Error correcting code having four information bits and three check bits per character; named after R.W. Hamming of Bell Labs. handheld A scanner design in which the scanning head is passed over the document, paintbrush fashion. handle 1. A small square at the corners and sides of a computer graphic image that represent the spot where the mouse cursor can be placed to manipulate (reduce, move, reshape) the image. 2. A temporary name or number assigned to a file, font, or other object. 3. A nickname used when teleconferencing on a bulletin board. handling zone The part of the optical disc that can be touched by a jukebox's gripping mechanism. handoff The automatic transferring of cellular radio channels when a car phone moves from one cell to another. handover The transfer from one radio site to the next when a user is traveling between different coverage areas in a network. handprint recognition A special discipline within OCR sometimes called ICR, for intelligent character recognition. handset The portion of the telephone that is hand held when the telephone is in use; consists of a receiver and transmitter at the ends of a common handle and is connected by an electrical cord to the handset mounting. handsfree operation When the speakerphone is part of the station set, the user can speak and listen without using the handset. handshaking Exchange of predetermined signals for purposes of control when a connection is established between two data sets. hard disk A direct access storage device (DASD) that uses rotating platters of magnetic recording material, sealed inside an airtight assembly. Data is written and read from a recording head, guided by a mechanical arm. hard RAM Also called Virtual Disk. Setting up a RAM disk enables the use of the computer's conventional, extended, or expanded memory to simulate a disk drive (or drives). hardwire To wire or cable directly between units of equipment. hardwired logic A procedure in which the decision-making elements cannot be altered by external means but only by changing the internal connections. harmonic An alternating signal whose frequency is an integral multiple of fundamental basic frequency. harmonic distortion Problem arising when harmonics of the input frequency appear in the output channel. harmonica A device attached to the end of a connectorized feeder cable that converts the 25 pair into individual four, six, or eight wire modular channels. HD or HDX (half duplex) See channel. HDLC See high-level data link control. HDSL High-rate digital subscriber line. HDTV See high definition television. HDU Hard Disk Unit. head The device which comes in contact with or comes very close to the magnetic storage device (disk, diskette, drum, tape) and reads and/or writes to the medium. In computer devices, it performs the same function as the head on a home cassette tape recorder. head end 1. TV and FM radio receiver which receives broadcast signals, amplifies them and sends them out on a CATV distribution cable. 2. The point in a LAN where the inbound signals are transferred into outbound signals. The head end may be passive or contain an amplifier or frequency translation equipment. header Control information typically afixed to message text, containing information on addressing, priority, or other message-related data. header sheet An instruction sheet for an optical character reader. headset Telephone transmitter and receiver worn on the head to keep the hands free. helical scan A magnetic tape storage method that increases media capacity by laying data out in diagonal strips. Used in video tape recorders. Hertz Measurement term for frequency. Means cycles or vibrations per second. Hertz (Hz) A unit of measurement that indicates frequency in cycles per second (60 Hz is standard electrical frequency). heterogeneous (computer) network Network comprised of different computers, i.e., from different manufacturers. heuristic Using trial and error to solve a problem. hexadecimal A number system with a base of 16. hidden line In computer graphics, a line segment that is obscurred in the two-dimensional representation of a three-dimensional object. hidden line removal A technique that removes hidden lines to decrease the number of bits needed to display or store an image. hierarchical (computer) network Network in which computers, specially suited for particular functions, process and control those functions on several layers. hierarchical data base Data base in which pieces of data are organized above and below one another, in a tree structure (i.e., similar to the way an organizational chart appears). hierarchical network A network in which a designated control device regulates network access. Hierarchical Storage Management (HSM) Managing files spanning several levels of storage devices to balance overall performance with cost. high definition television (HDTV) Transmits TV signals with superior resolution to the NTSC standards.high-frequency (HF) bands","Any frequency above the audible range -- band P: 225 MHz-300 MHz; band J: 350 MHz-530 MHz; band L: 390 MHz-1550 MHz; band S: 1.55 GHz-5.20 GHz; band C: 3.90 GHz-6.20 GHz; band X: 5.20 GHz-10.90 GHz; band K: 10.90 GHz-36.00 GHz; band K: 15.35 GHz-17.25 GHz; band Q: 36 GHz-46 GHz; band V: 46 GHz-56 GHz; band W: 56 GHz-100 GHz." high resolution Basically, any image that is displayed in better quality by increasing the number of dots, or pixels, per inch than normal. Usually refers to better quality computer displays, but can describe printer quality as well. high usage (HU) group Trunk groups that serve as the first choice path between two central offices, and thus manage the highest volume of the communications traffic. high-bit-rate digital subscriber line (HDSL) Allows local exchange carriers to provide T1 and fractional T1 service over existing copper wires. high-frequency (HF) Band of frequencies between 3 to 30 megahertz range. high-frequency (HF) bands Any frequency above the audible range -- band P: 225 MHz-300 MHz; band J: 350 MHz-530 MHz; band L: 390 MHz-1550 MHz; band S: 1.55 GHz-5.20 GHz; band C: 3.90 GHz-6.20 GHz; band X: 5.20 GHz-10.90 GHz; band K: 10.90 GHz-36.00 GHz; band K: 15.35 GHz-17.25 GHz; band Q: 36 GHz-46 GHz; band V: 46 GHz-56 GHz; band W: 56 GHz-100 GHz. High-Level Data-Link Control (HDLC) A bit-oriented data link control protocol endorsed by the International Standards Organization. HDLC is functionally identical to ADCCP. highlight Brightest part of a photograph or halftone. Also, the ability of a display to emphasize a certain area by blinking, reversing video or coloring the section. HIPO Hierarchy Input-Process-Output (design technique). HIPPI See HPPI. HISAM Hierarchical Indexed Sequential Access Method. histogram A chart displaying horizontal or vertical bars. The length of the bars are in proportion to the values of the data items they represent. HLI Host Language Interface. HLL High-Level Language. HLLAPI IBM's High Level Language Application Program Interface; an API that provides a high-level interface to mainframe terminal operations from IBM PCs and compatibles running the OS/2 operating system. HLM Heterogeneous LAN Management. HMI Hub management interface. hold To briefly exit a phone call without disconnecting; there are various methods of placing a call on hold, depending on the type of telephone system in use. hologram The recording of an image on film as the result of spliting a laser beam. holographic optical tracking An optical (laser-based) tracking mechanism that can locate the read/write head in very precise position by matching the alignment of several tracks with a template in its tracking head. By precisely aligning the head with the track, data can be packed closer together. home page The first page of a document or Web site. homogeneous (computer) network Network comprised of many of the same make and/or model computers. hookswitch 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 which opens the telephone circuit, leaving only the bell connected to the line. See switchook. horizontal scan rate The frequency in Hz (hertz) at which the monitor is scanned in a horizontal direction; high horizontal scan rates produce higher resolution and less flicker. host computer A computer that acts as a central processor for a communications network. Hosts systems are typically mainframes or minicomputers. host interface The connection between a network and a hostcomputer. HOSTS An ASCII text file used to associate IP machine names and aliases with IP addresses. hot standby In a redundant hardware configuration, the practice of maintaining backup equipment in operational mode to permit emergency switchover without service interruption. hot swap The ability to remove and replace any drive from an array, for maintenance or upgrade, without removing the array from service. hotel/motel console Specialized attendant station usually located at the hotel front desk. hotline Private line between two telephones. HP-IB Hewlett-Packard Interface Bus. HP-UX Hewlett-Packard UNIX-based operating system. HPGL Hewlett-Packard Graphics Language. HPO High-Performance Option (IBM). HPPI High-Performance Parallel Interface (also HIPPI). HPR High Performance Routing. HSB Hue Saturation Brightness. A color model. Hue (the color); the saturation (the amount of pigment); and brightness (the amount of white included). With the HSB model, all colors can be defined by expressing their levels of hue, saturation, and brightness in percentages. HSSI High speed serial interface. HTML HyperText markup language; the programming code used for creating linkages on the Web. HTTP Hypertext transfer protocol, is the electronic agreement among computers that allows users to move around the Web. hub A wiring concentrator or repeater that combines the connections from multiple network nodes into signals suitable for transmission. hue In computer graphics, a particular shade or tint of a given color. hum Audible interference from a conventional power supply frequency. hundred call seconds See CCS. hunting The act of a PBX switch searching for an available idle circuit of a group. hybrid 1. An electronic circuit that performs the wire conversions necessary for the connection of a local loop with a long-haul facility. 2. A combination of technologies, whose sum is a third "mongrel" technology. hybrid local network 1. A local network in which two or more types of local networks are integrated. 2. A local network engaging two or more components representing different applications or different vendors. hybrid system A communications system that accommodates both digital and analog signals. hyperdensity A 1-inch wide magnetic tape storage medium that holds 3,200 characters per (linear) inch. hypermedia The use of data, text, graphics, video, and voice as elements in a hypertext system. hypertext Documents that cross-reference each other via graphical cues. Hz Abbreviation for Hertz; cycles per second. Often used with metric prefixes, as in kiloHertz (kHz).