Fiber Optics Illustrated Dictionary and status lights on the front panel. The Kitchen Com- puter was technically known as the H-316 Pedestal Model. Series-16 rack- and table-top models were available as well. Despite its spectacular lack of success in the kitchen market, the Series-16 technology was not completely lost, as it eventually made its way into components used in the ARPANET (as interface message proces- sors for connecting hosts), a more suitable market for cutting-edge, pioneer computing technologies. See Altair, PCP series computers. hook switch, switch hook The hook switch was originally designed not just to terminate a connection so the next call could come through, but served also to disconnect from abattery source so it wouldn't be quickly used up, and later an electrical source. Modern telephones draw current from the line and don't require a separate battery to operate the basic calling and receiving functions, but the hook switch, the hook on the side of an old traditional box phone or the buttons (plungers) on top of a traditional ro- tary desk phone, are still used for disconnecting a call, and sometimes for generating a tone (if they are held down briefly, which doesn't cause immediate discon- nection). See hooking signal. hook switch dialing On older wall box phones and rotary pulse phones, it was possible to dial anumber by depressing the hook carefully for each number you wanted to dial. (Depressing the hook switch for too long would disconnect the line.) This is even possible on some of the older pay phones. See hook switch. hookflash Asignal-sending mechanism whereby the hook on an old-style phone or button plunger on a newer phone is quickly depressed to signal the ini- tiation ofa service or operation. hoot'n'hoIler, holler down, shout down, squawk box Adedicated, four-wire, open phone circuit con- necting speakers or speakerphones at each end of the connection round the clock. It's like a 24-hour pub- lic address system using phone lines with full duplex, two-way communication. Other phones on the sys- tem can be picked in order to listen to the conversa- tions ongoing on the speaker system. Hoot'n'holler systems are useful in industrial yards, institutions, and fast-paced financial floors where numbers of free-moving individuals look to central- ized sources ofinformation or engage in communal dialog at different locations. hop n. I. The extent of an individual transmission path between two nodes (with no intermediate nodes). 2. In radio, the extent of a transmission from Earth to ionosphere and back. 3.In frame relay, the extent of an individual trunk line transmission path between two switches. 4. In an IBM Token-Ring network, the extent of an individual transmission path between two bridges. 5. In cellular communications, where the user may be traveling through several transmission zones during the course ofa call, a hop is a change in the radio frequency channel. hop by hop/hop-by-hop routing In contrast to a sys- tem that predetermines a route before sending a trans- mission, hop-by-hop routing creates a route along the 452 transmission path, a step at a time, by using routing information at switchers along the way. There are ad- vantages to both. A predetermined route may be an efficient one, designed to speed the transmission through faster links or perhaps by choosing the short- est path. This is common on small or local systems. On the other hand, on a large system like the Inter- net, there may be millions of possible routes, too many to store in the routing tables at the source of the transmission. In this case, hop-by-hop routing is a scalable technique that makes use of the best infor- mation at each station to progressively build a path for the data. It has been suggested that ATM imple- mentations of hop-by-hop datagram forwarding on the Internet are no longer adequate to handle traffic volume and improvements have been suggested. See cell switch router, RFC 2098. hop channel In cellular communications, aradio fre- quency (RF) channel that is available to continue transmissions for a user with a call in progress who is moving through zones. Available channels are needed to continue uninterrupted transmission while the user is on the move. See cellular, hop, mobile communications. hop count The sum of the number of hops that make up a route between its source and destination, or be- tween aspecified segment of the route. In radio com- munications, the number of times the wave bounces from the Earth to the ionosphere and back. In networking, the hop count is the number of seg- ments between individual nodes or routers, a num- ber that is recorded in Internet Protocol (lP) packets on packet-switched data networks. In cellular, the number of times a radio frequency change occurred during the course of a call. Hop counts are one means to gauge the efficiency ofa system and to configure or tune it for better performance. hop off To exit one type ofsystem and complete the route on another. For example, you may initiate a fac- simile transmission on the Internet, that then hops off to a phone line and a dedicated facsimile machine. Or, you may make a voice call from a telephone that is routed through a voice translation program and in- terfaces with the Internet and becomes an email mes- sage at the destination. In this case, the hop offis from the phone system to the Internet, or, conversely, you can consider it a hop on to the Internet, if you are con- sidering the Internet as the main portion of the trans- mission route. Hopkins, Harold Horace (1918-1994) A British mathematician and physicist with an early interest in optics. After World War II, Hopkins took a position with the Imperial College in London. Hopkins applied for and received a grant from the Royal Society to develop glass fiber bundles for use in an endoscope. He hired N. Kapany, who became a significant pio- neer in the field, to work on the project in 1952 and communicated his ideas to colleagues, including F. Zemicke. The idea spread and the race to publish re- sulted in A. van Heel describing the topic in June 1953. Hopkins observed that a fiber could act as a filter as © 2003 by CRC Press LLC well as a lightguide and continued researching aspects of fiber optics, lenses, and the improvement of these systems. In the 1960s, he took aposition as aprofes- sor of optics at Reading University. See endoscope; Kapany, N arinder. Grace Hopper - Programming Pioneer Grace Murray Hopperwas an American mathema- tician, physicist, lecturer, and one of the first com- puter programmers in the days when programming involved rearrangingthe wires within a vacuum-tube computer system. Hopper, Grace Murray (nee Grace Brewster Murray, 1906-1992) An American mathematician, physicist, and educator, Hopper was an originator of software compilers and developer of the COBOL pro- gramming language. She earned a Ph.D. from Yale in 1934 and spent many years as alecturer, research scientist, and programmer for various organiza- tions, including the U.S. Naval Reserve. Hopper became involved in many of the important computer development projects at the end of World War II. In 1944, she joined Howard Aiken's Harvard Mark I project as its third programmer and later worked on the Harvard Mark II. Hopper is perhaps best known for relating a story in which a technician found a bug inside a Harvard Mark II and solved a problem by removing it. She appar- ently glued the bug into the computer logbook and, in the 1970s, announced that she would be contrib- uting it to the National Museum of American History. Thus, the term computer bug was popularized. In the late 1940s, Hopper left the academic world to join the Eckert-Mauchley Computer Corporation where she had the opportunity to put her software theories to practical use. In the early 1950s, when new ideas about programming and reusing existing code began to evolve, she made what was probably her big- gest contribution to the field. She became a cham- pion of ideas that led to high-level languages, com- piled software, and more efficient coding methods, even though many professionals claimed at the time that such things were impossible (they couldn't see past the physical wiring of computers to a time when electronics would be used to channel signals auto- matically). In spite of detractors, Hopper contributed significantly in the transition from paper tape and punch cards to coding languages such as C-IO. She further proposed that computers could be pro- grammed in English, an idea that was ridiculed, and developed a compiler for business use with an En- glish-like syntax for nontechnical programmers that evolved into COBOL. Hopper's contributions were too significant to pass ;~~~::i~t~~~r~:~:~i5~~:~i~;z~;. sociation Computer Science Man-of-the- Year Award. In 1973, she became the first woman recognized as a Distinguished Fellow of the British Computer Soci- ety and, in September 1991, was awarded the Na- tional Medal of Technology by the U.S. President. See A-O, B-O, bug, Harvard Mark 1. hops See hop count. horizontal blankinginterval,horizontalblanking time The period during which a display is suppressed on a cathode-ray tube (CRT) to allow the electron gun to return from the right side of the screen to the next display position down and on the left side of the screen (assuming left to right and top to bottom scan- ning as is present on most standardized frame-based video display systems). See blanking, cathode-ray tube, frame, sweep. horizontal cross connect The interconnection be- tween a horizontal distribution system and a telecom- munications central wiring location such as an equip- ment or patch panel closet or bay. horizontal distribution frame The equipment and structural elements that facilitate the interconnection of interfacility cabling configurations, as between subscribers and substations and central offices. The frame technically does not include the wiring but di- rects and contains it. Horizontal distribution frames are usually built into flooring or crawl spaces, hence the name. See distribution frame. horizontal link, inside linkIn ATM, a link between two logical nodes belonging to the same peer group. horizontal resolutionA quantification of the amount ofinformation that is contained on a single horizon- tal line ofa rasterized output device such as a moni- tor or printer. On raster monitors, horizontal resolu- tion is expressed in terms of pixels, usually about 800 to 1024. On black and white laser printers, horizon- tal resolution on consumer machines ranges from 300 to 1000 dots per inch (dpi), and on prosumer and in- dustrial printers from 1000 to 2700 dpi. Thus, the to- tal would be the number of inches times the dpi. A resolution of about 600 dpi or greater is needed to show clean lines and curves, without staircased arti- facts, for common printed documents. Aresolution of about 72 pixels per inch or higher on a grayscale 453 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary or color display is preferred for video displays. Col- ors and shades of gray can be used to anti alias a dis- play to give it a higher perceptual resolution. horizontalscan rateA measure of the scan speed of electron beam display devices, usually described in hertz (Hz), as in cathode-ray tubes (CRTs) that sweep repetitively from left to right and top to bottom. The horizontal scan rate describes how many hori- zontal scan lines per unit of time (usually seconds) can be displayed. At a particular scan rate, the num- ber of lines that can be displayed decreases propor- tionally as the refresh rate increases. Multiscan com- puter monitors permit a variety of scan rates and reso- lutions, most ranging from about 40 to 75Hz. See cathode-ray tube. horizontal segment In wiring distribution systems, the wiring route from individual NAM or 10 loca- tions to the riser closets through ceilings or floors, usually up to a maximum of about 250 feet. horn alertAn electronic connection for sounding a hom or loud buzzer to signal an incoming transmis- sion during times when the user might be some dis- tance from the communications device. Hom alerts are used for after-hours phone calls or doorbells, for cellular phones in cars, and for a variety of security systems. horsepower hp. Aunit ofpower designated as equal to raising 33,000 pounds one foot in one minute, which can also be expressed as an English gravita- tional unit ofraising 550 pounds one foot in one sec- ond. In the U.S., a unit ofpower equal to 746 watts. See watt. host 1. One upon whom others depend for shelter or sustenance. 2. The main organizer and holder of an event. 3. It's a little difficult to define host as it re- lates to computer systems because different groups of computer personnel have given host and client opposite meanings in the past. For consistency with the English meaning of the word and popular usage, this dictionary defines host as a main server or con- trolling system, and the client as a subservient sys- tem in terms ofpriority or capabilities. See client. host bus adapter HBA. A computer storage or net- work transmissions connectivity device. HBAs can be built into the motherboard or they may be avail- able as optional peripheral cards. PCI Local Bus-com- patible HBAs are prevalent, but there are also HBAs for other formats, including FC-AL (Fibre Channel) and CompactFlash. The HBAhandles low-level hard- ware controller interaction, including data I/O to the controller registers and data transmissions. HBAs are of interest for storage area networks (SANs), data warehouses, RAID systems, signal pro- cessing, video editing systems, and other high-capac- ity/high-throughput applications. HBAs for other for- mats such as CompactFlash can be used to insert very small CompactFlash or hard drive devices (e.g., matchbox-sized 320-MByte hard drive) directly onto a peripheral card. Depending upon the type ofHBA, there is typically a connection point on the card for attaching a fiber optic or copper duplex cable. If fiber optic connection 454 is provided, there may be a gigabit interface converter (GBIC) as well. The HBAmay have LED status lights to indicate power and port activity, similar to the sta- tus lights on a modem or network hub. As of Sum- mer 2001, speeds up to 2 gigabits/second were pos- sible, and most PCI -based HBA devices supported 32- or 64-bit addressing. There has been a trend to- ward adding larger buffers to increase performance. Although PCI is a widespread standard for periph- eral cards, not all HBA PCI cards support all operat- ing systems; there are different flavors ofHBAs. Mul- tiple HBA cards may be inserted into a system, up to avendor-specified maximum, depending upon power and configuration. HBAs are shipped with individual IEEE standard unique address identifiers. For Fibre Channel connec- tivity, a World Wide Name (WWN) is derived from the given IEEE address identifier to handle arbitrated loop activity. Asoftware configuration utility may be used to establish a relationship between the HBA physical device and the logical HBA number as- signed. host carrier In telecommunications, the main carrier through which billing is channeled. In systems where a call goes through various networks or providers, carriers may have arrangements with the host carrier to bill through them to save papelWork and other ad- ministrative costs. host computer A computer in a network providing primary operations and applications that are run through clients or remote terminals at other locations. A network may have more than one host, and some hosts may be specialized for modem access, email distribution, printing, and other tasks. The term host is related more to function than raw hardware capa- bilities but, due to resource sharing economics, the host frequently has greater capabilities (more memory, storage, peripherals, etc.) than the clients accessing it. host site 1. Arepository or other archive site accessed by remote users through client programs such as Telnet, FTP, Web browsers, and others. The host site is the one on which the administrative tasks and stor- age are carried out. 2. Acomputer bulletin board sys- tem, which typically hosts email, chats, games, and file uploads and downloads. hot 1. Connected; live; ungrounded current-carrying conductor. A term frequently applied to electrical wires. 2. A hot chip is one that either runs at a high temperature and requires cooling, or one that has a fault that causes it to emit more heat than is normal and is likely to fail soon. See heat sink. 3. Stolen. 4. Topical, popular, desired by a large following. 5. Tit- illating, arousing. See hot chat. hot cathode, thermionic cathode A hot cathode is one that produces a stream of electrons (a cathode ray) by means ofthermionic emission. Thus, heat (ther- mal energy) that is associated with the cathode pro- vides the energy boost needed to liberate the beam of electrons that comprises the cathode ray. Electri- cal current is used to provide the heat and replace the electrons that stream away from the cathode. © 2003 by CRC Press LLC New types of hot cathodes have been developed for use with electronic devices, such as the waveguide tubes used in micro- and millimeter-wave commu- nications. Barium-dispenser thermionic cathodes have been of particular interest to researchers. These new cathodes are suitable for environments where small size and low power requirements are advan- tages, as in Earth-orbiting satellites and deep space probes. NASA has a facility for cathode research and development and the evaluation of new materials, called the Thermionic and Non-Thermionic Cathode Research and Development Test Facility. Thermionic cathodes are used in electronically pulsed injectors for high energy physics research on super- conductors. They have also been considered for use as electron-beam scrubbing devices, that is, the beam could be used to clean hard-to-clean surfaces for spe- cialized industrial applications. Thermionic cathodes are also of interest in consumer applications. For example, Philips Display Compo- nents, the largest manufacturer ofCRTs for computer monitors and television sets, has teams of materials scientists working on new generation versions and applications of cathodes. F or electron beams that are significantly brighter than conventional thermionic cathodes, thermal field emis- sion (TFE) cathodes are being developed. These high- resolution emitters can be used in electron beam test- ing equipment and low-accelemting scanning electron microscopes (SEMs). See cathode-ray tube. hot cut, flash cut The transition from one circuit to another while the system is in operation, hopefully without disruption to components or current users. Hot cuts are used when switching from an old wir- ing system to a new one, or when switching around physical routing paths. On individual computer sys- tems, components are sometimes hot swapped, al- though it is never recommended. Never hot cut a com- ponent that is being accessed. It is especially inad- visable to hot cut most types of drives (floppy, CD- ROM, hard drive, etc.). (RAID systems are an excep- tion.) Keyboards and mice are not usually damaged by hot cuts, but make it a habit to power offa system before making hardware configumtion changes. See half tap, hot swap, redundant array of inexpensive disks. hot docking Inserting a component into a docking bay (as in laptop docks or video bays) while the sys- tem is powered on. This is generally inadvisable. Whenever possible, power off all components before connecting electrical circuits. Some newer compo- nents are being developed for use in hot docking en- vironments, such as high-capacity disk storage sys- tems and consumer storage devices such as USB pe- ripheral devices that include hard drives, card read- ers, and gmphics tablets. Hot docking is very conve- nient for the user as it is not necessary to close appli- cations and power down a system to attach or swap a peripheral device. hot key combination 1. A combination of keys that when pressed simultaneously will perform a specific function or engage a memory resident progmm, such as a printer utility. It is handy for background pro- cesses that are frequently needed but would be dis- tracting if running in the foreground along with other current process. 2. A combination of keys pressed simultaneously to perform a specific operating sys- tem function. For example, on anAmiga,Amiga-Am- iga-Ctrl reboots the machine. On an mM-compat- ible running MS-DOS, Ctrl-Alt-Del perfonns a simi- lar function. 3. A combination of keys to access text style attributes and search and replace functions in older word processing programs developed before graphical user interfaces became common. hotline A private, dedicated phone connection, some- times indicated by the color of the phone. On a land line, when you pick up the line, it either connects automatically or does so quickly through the touch ofa button or speed dialing. On a wireless service, the system may be configured so the phone can con- nect only with a specific number. Hot lines are used as emergency phones in buildings, on roadways, in brokerage firms, and by important personnel in gov- ernment or military positions. Fiber optic connections are particularly suited to se- cured hotlines used for sensitive emergency or gov- ernment communications due to their speed, wide bandwidth, and relative immunity to electrical taps. hot line service Phone service that expedites an au- tomatic connection through a dedicated private phone. See hot line. hot links In computer software applications, virtual links that form a connection between information in one document (such as text or images) and another, even if their native formats differ. For example, in a desktop-published document, there may be a hot link to text in a word processor and another to an image in a graphics program. Depending upon the system and the software, changing text in the word proces- sor or in the graphics program may immediately ef- fect a change in the corresponding desktop published document, or may effect a change when the page is refreshed or when update links is selected from a menu. As systems become more capable (multitask- ing, faster CPUs, more memory), hot links are more prevalent and updates happen more automatically. See drag and drop. hot listIn computing, a list of frequently used appli- cations progmms, directories, or Internet newsgroups, Web sites, or archives. A hot list is usually displayed as a text list or pull-down menu from which the user can quickly select the desired destination. See book- mark. hot spot, hotspot 1. A location on a touch sensitive device that alerts the software to respond in some fashion to user input. 2. A screen location that re- sponds when a cursor is moved into the region, or if the cursor is positioned and a mouse or key clicked to activate the hot spot. 3. A bottleneck or area of con- gestion in a network, component, or software routine. 4. An area ofa circuit in which some component is generating more heat than would normally be ex- pected and that may signify apotential problem. 5. A region of a document or image that includes an 455 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary embedded link so some further action happens if the region is selected or activated. Hyperpage applica- tions use hot spots for various links; graphics pro- grams sometimes use hot spots to activate palettes or specialized drawing menus. hot standbyA backup or background system or pro- gram that is operating, but idle and available to take over if failure of the regular system occurs. Hot Standby Router Protocol HSRP. A protocol that provides resiliency, fault-tolerance, and transparent network topology support for network routers. Standby routers inherit the lead position if the lead router in a group fails. hot swap The process of connecting or disconnect- ing an electric circuit, component, or peripheral while the system is powered up. Hot swapping is done to minimize disruption to users ofa system. It is highly inadvisable in most circumstances. Some systems are designed to handle hot swaps (some types of video components or redundant hard drive systems), but be sure you know what you are doing before attempt- ing i~. See FireWire, hot cut. HotJavaAn adjunct to Java, the widespread, object- oriented, cross-platform programming language from Sun Microsystems that continues to grow in popu- larity for use on the Web. HotJava is a Java-enabled Web browser with support for JDK and SSL that is installed on the local computer system and enables Web sites with Java applications to run from a desk- top system. Java support enhances abrowser's capa- bilities. See Applets, Java. Hotline Vu1ual Private Line Service A commercial Nynex subscriber service that uses public lines spe- cially programmed and configured to operate as though they were private dedicated lines, with the connection activated when picking up the handset. See hot line. House, Royal E. An American inventor who devel- oped one of the first practical direct paper tape print- ing telegraphic receivers, patented in 1846. House continued to improve upon the original design and patented the improved version in 1852. See House telegraph. House telegraph The House telegraph (U.S. #4,464) was a relatively complex printing telegraph. Two people were required to operate it, as one had to tum a crank to run the mechanism while the other oper- ated the telegraph. It had a wooden base with the cir- cuitry mounted on the top and a piano-keyboard-like series of keys underneath a hinged flap. The message was printed on a strip of paper similar to the stock ticker machines that evolved out of printing tele- graphs. The House telegraph is said to have been capable of transmitting up to 40 words per minute and was in common use in the U.S. in the latter halfof the 1800s. The House telegraph formed part of the inspiration for the subsequent Phelps Combination Printer tele- graph, designed by George Phelps to improve upon the House and Hughes telegraph systems. See House, Royal E.; Hughes telegraph; Phelps Combination Printer; telegraph, printing. 456 housing A protective enclosure commonly used to insulate, protect, or manage wires or electrical con- nections. Many housings are shaped like boxes, with one side open to provide access. Splice enclosures are aparticular type of housing used to connect fiber optic cables between the head end and the node. howl An irritating, unwanted wailing or screeching sound from acoustic or electric feedback that may occur, for example, when a speaker and microphone from the same transmission are placed too close to- gether. Noise and echo canceling equipment can pre- vent or reduce howling. howler, howler tone In telephone communications, a unit that creates a loud sound to signal that a phone has been left off-hook. For example, on some public exchanges, a recording will play first ifa phone is left off-hook, "Ifyou'd like to make a call, please hang up and try again ," followed by a series ofrau- cous beeps that can be heard up to about 15 feet from the phone. HP See Hewlett-Packard. HP9830 Historically, one of the earliest desktop com- puters, coming out a few months after the Kenbak-l and at almost the same time as the Intel SIM4, in 1972. The HP 9830 was the first desktop computer to really look like modem desktops, with a typewriter- style keyboard, numeric keypad, function keys, and status lights. It can't really be considered a personal computer, as it listed at just under $6,000 (more than the price ofa car, in those days) and was primarily marketed to institutions and the scientific community. See Altair, Kenbak-l, Sim4. HPA See high power amplifier. HRPT high resolution picture transmission. A spe- cialized image communication for very high resolu- tion images such as those transmitted by satellites. HSCI High-Speed Communications Interface. A single-port interface from Cisco Systems that provides full duplex synchronous serial communications. HSCS high speed circuit switched. HSDA high speed data access. HSDU See High Speed Data Unit. HSRP See Hot Standby Router Protocol. HST See High Speed Technology. HSV hue, saturation, value. In color imaging, acolor model that allows settings to be adjusted along these three properties. Hue is the color, saturation is the amount or richness of the color, and value is the light- ness or darkness. HSV systems for adjusting palettes on computer desktops, applications, and graphics pro- grams are common. HTL See high-threshold logic. HTTP See Hypertext Transfer Protocol. HTTPS See Secure Hypertext Transfer Protocol. hub Focal point, center of attachment or activity. hub, network 1. A connecting point on a network to centralize wiring and connection management. A hub may be passive or active and is often used in systems with star topologies. 2. A connection box on video or audio systems that permits centralization of cables and easy reconfiguration of devices. Often used in connection with switchers and, in many cases, the © 2003 by CRC Press LLC switcher itself may double as a hub. See bridge, router, switcher. hub site The location ofa hub, which may vary from a small box on a desk or rack to an entire closet or room, depending upon the size of the system. The hub site allows easy cabling and administrative access to a variety ofconnections. Hubs are often located at main wiring or logical junctions and may connect to external systems. hue A color of the visible spectrum. Hue does not include white, black, or shades of gray, which are the presence of all colors (white) or absence of color (black) in various intensities (grays). Most people are familiar with hues as the colors of the rainbow: red, orange, yellow, green, blue, indigo, violet. In soft- ware, hue may also be called tone or tint. See inten- sity, saturation. Huffman encoding Huffman is a fast, variable- length, tree-oriented encoding scheme developed in the early 1950s by David Huffman. It optimizes on the basis of more frequently occurring characters in order to achieve compression in fewer bits. Since it produces a coding table that can be reused for addi- tional image encodings, it is efficient for certain types of multi-image applications, though not especially efficient for very short messages (due to header over- head). Data corruption on a small scale can signifi- cantly affect the content of amessage when decoded, so Huffman is best used with robust transmissions protocols with built-in error-checking. JPEG image compression is based, in part, on Huffman encoding. Modified Huffman (MH) is widely used in facsimile transmissions. Hughes, David E. (1831-1900) An English-born American music teacher who developed one of the first printing telegraphs, in essence the telegram and later teletype machines. The telegraph, with improve- ments by George Phelps, formed the basis of the American Telegraph Company, in competition with Western Union. In 1858, Hughes returned to Europe to demonstrate and promote the system and remained there as a resident. Hughes also invented the carbon microphone, in 1877, an important contribution to telephony. See House, Royal; Hughes telegraph; Morse, Samuel F.B.; Phelps, George; telegram. Hughes, David R. (ca. 1929- ) A West Point gradu- ate and retired U.S. Army Colonel, Hughes is an ac- knowledged pioneer in internetworking and educa- tional applications in distance learning. He is cred- ited with teaching the first online college credit courses (1983). Hughes designed and supported the Big Sky Telegraph network and the Montana state METNET. Hughes Network Systems A company (actually a group of companies under the Hughes umbrella) which has been involved in satellite communications since the early launches and has developed a num- ber ofassociated innovative technologies. One such product is DirecPC, which allows a satellite feed to connect with a personal computer for data commu- nications. See Applications Technology Network Program, DirecPC. Hughes telegraph The Hughes telegraph began as an idea for transcribing music and ended up as a print- ing telegraph that used a tone to synchronize the mechanism between the transmitting and receiving printers. It was developed by David E. Hughes at about the same time the House printing telegraph was being marketed commercially and was patented in 1856 (U.S. #14,917). Power to the Hughes telegraph was provided by a weight-driven clock system, similar to agrandfather clock, thus making it possible for a single operator to use the system, as opposed to two operators for the competing House telegraph. The essential designs of the House and Hughes systems was the same, a wood cabinet equipped with apiano-style alphabetic keyboard and the various mechanisms mounted on top of the cabinet. They differed mainly in detail and in the way they were powered. The mechanism was not perfect and needed some refinement to be commercially successful, but there was a demand for telegraph machines at the time, and the rights were purchased by a newly forming com- pany called the American Telegraph Company. ATC turned to George Phelps to improve the system, a move that made the Hughes telegraph moderately successful in North America and highly successful in Europe after Hughes traveled there to demonstrate the system, beginning in 1858. The Hughes printing telegraph enjoyed a long working life of more than a century in some European locations. In North America, the House and Hughes telegraphs led to improved designs that were highly successful early teletype machines. See House telegraph; Hughes, David E.; Phelps Combination Printer. Hull, Albert Wallace (1880-1966) An American physicist who made important contributions to X -ray crystallography and who developed a number of types of electron tubes, including the magnetron, a tube capable of generating microwave frequencies. Hull published a description of magnetron technol- ogy in 1921 in the AlEE Journal, V.40. The magne- tron became important in the development ofradar and satellite communications systems. See magnetron. Human ComputerInterface standards HCI. A se- ries of protocol platform standards from the IETF, including butnot limited to Common Desktop Envi- ronment (CDE), Complex Text Layout (CTL), Mo- tif, etc. Hunnings, Henry (1843-1886)ABritish clergyman inventor who developed a carbon granule-based hear- ing aid in 1878, which replaced the electric hearing aid developed by A. Graham Bell in 1876. Hunnings also applied the carbon granule technology to im- prove upon microphones invented by Thomas Edi- son. Hunnings' improvements led to sturdy devices with better sensitivity. Similar carbon granule tech- nology was also developed by Francis Blake, Jr., to improve telephone transmissions, and both Blake and Hunnings telephones were prevalent at the time. hunt, hunting A process through which a call is routed by seeking the best path or fIrst available path or device. 457 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary hunt group In telephone systems, a series of lines set up so calls can be assigned to the next available line in a group if the first accessed line is busy. There are different ways to organize hunt groups, from a straight sequential hunt to random hunts. Hush-a-Phone decision A landmark communica- tions case in 1956 which challenged the AT&T mo- nopoly of phone line access. The company market- ing Hush-a-Phone Company and Harry C. Tuttle wanted to attach a mechanical device to a phone set in order to screen out background noise. AT &T ar- gued against it, supported by the Federal Communi- cations Commission (FCC), but the decision was later overturned in the court of appeals, the main argu- ments including the mechanical rather than electri- cal nature of the device and the fact that it in no way harmed the phone equipment. A Hush-a-Phone is on exhibit at the Museum of Independent Telephony in Abilene, Kansas. See Carterfone decision. Huygen's integral, Huygen's principle Wavefronts can be mathematically decomposed (integrated) into a series of point sources, with each seen as the origin of an expanding, spherical wavelet that can be repre- sented as a free space Green's function. At any par- ticular moment, the wavefront's shape comprises the envelope that includes the secondary wavelets. This is a useful though not complete representation of what actually occurs, as it does not fully account for dif- fraction interactions or differences in wavelength (e.g., between light and radio waves). See Huygen- Fresnel principle, Snell's law. Huygen-Fresnel principle Fresnel and other scien- tists studied Huygen's principle, filled in the math- ematics, and made some adaptations to the concept to account more other aspects, such as interference. The Huygen-Fresnel principle states that at a given instant, every unobstructed point of a wavefront serves as a source of expanding spherical secondary wavelets with the same frequency as the primary wave and that the amplitude of the optical field at points beyond the envelope (that was specified by Huygen) comprises the superposition of all the wave- lets as to their amplitudes and relative phases. This led to a more complete though still not fully de- veloped description of wavefront behavior. Kirchhoff enlarged on the concept further by taking into con- sideration obliquity and his approach is still widely used. From the Huygen concept, the Fraunhofer approxi- mation, which is important in the study of optics and ultrasound, can be derived. See diffraction, Fourier transfonn, Huygen's integral, interference, wavelet. HW-16 A three-band radio transmitter and receiver housed together in one case, by Heathkit. The trans- mitter was crystal-controlled and the device was elec- tron-tube based, with the exception of one transistor component. It was popular with novice amateur ra- dio operators in the late 1960s and early 1970s. The unit could be upgraded as the radio operator learned more and received an operator's license. Sockets on the front panel enabled crystals to be inserted. The HW-16 was most effective over the 40- and 80-meter 458 bands and can still be used over those frequencies. Hybrid Fiber Coax HFC. A transmission system combining fiber optic cable with coaxial cable that can handle simultaneous analog and digital signals. It is less expensive than a full fiber or switched digi- tal video installation but still provides greater band- width than traditional technologies built entirely on copper wire or coaxial. Network technologies such as ATM, SONET, and frame relay can be transmit- ted over HFC. Discrete wavelet multitone (DWMT) is being proposed as a suitable modulation scheme for existing HFC installations. See discrete wavelet multitone, Hybrid Fiber Coax architecture, SONET. Hybrid FiberCoax architectureA hybrid fiber coax technology for carrying video or telephone services, or a combination of both. For video, the bandwidth is typically divided into channels which can further be subdivided into phone lines. It is primarily a down- stream technology, which serves broadcast TV very well, but it may not be as flexible for interactive TV and phone services. The downstream nature is not inherent in the cable, but rather in the transmission and amplification technology. Typically, optical fiber runs from the central office to a node servicing an area neighborhood. From that point, the signal can be con- verted to be carried via coaxial cables to individual subscribers. At the subscriber point, a device splits the video and telephone signals so they can be di- rected to the appropriate lines or devices within the premises. This hybrid system balances some of the speed and bandwidth of a full fiber-based system, with some of the economic advantages of coaxial servicing indi- vidual neighborhoods. One disadvantage is that there is not an unlimited amount of bandwidth available for phone lines, and phone service must be planned and adjusted as needed. HFC technologies can put cable companies in a position to compete with tele- phone providers, which may create a shift in future market share. See Hybrid Fiber Coax. Hyde, J. Franklin (1903-1999) An American chem- ist and inventor educated at Syracuse University, Franklin worked as a post-doctoral organic.chemist at Harvard, then went to work at Coming Glass Works, in 1930. He remained with Coming Glass Works and then Dow Coming Corporation (which was founded as a result of his work) until 1975. Hyde developed a means for converting silicon com- pounds to silicone and conducted extensive research on silicone rubbers. He is a significant pioneer in glass manufacture as the inventor of fused silica glass (patent #2,272,342). This invention is important to many areas of optics, including semiconductor tech- nologies and optical lenses. It also fonned the his- toric basis for practical fiber optic waveguides made from low-loss pure glass. Hyde was inducted into the National Inventor's Hall of Fame in 2000 for his de- velopment of silica glass. In 1992, the J. Franklin Hyde Scholarship in Science Education award was established in his honor. See Kao, Charles K. hydraw See octopus. hydroelectric Electrical power derived from the energy © 2003 by CRC Press LLC provided by rapidly moving water or channeled wa- ter under pressure. Large dam projects are usually linked to a desire for hydroelectric power for residen- tial, urban, and industrial purposes. hydrolysisA process of chemical decomposition that occurs itt the presence of moisture such that new com- pounds result from the reaction. Hydrolysis is of con- cern in maintaining insulating materials in underwa- ter cable installations. Hydroxyl ions (OH-) may re- sult from the ionization of water. See hydroxyl group. hydrometer An instrument that measures, by dis- placement, the specific gravities of liquids. Used, for example, to measure the electrolytes in batteries. hydroxyl group A group of atoms that occurs in or- ganic molecules, a type ofJunctional group. Hydroxyl comprises a polar group with a single oxygen atom (0) bound to a single hydrogen atom (H). The pres- ence ofa functional group alters amolecule's chemi- cal properties. Hydrocarbons containing a hydroxyl functional group are known as alcohols. A metal joined with a hydroxyl groups is called a metal hy- droxide. Acidic concentrations have a low hydroxyl ion (OH-) to hydrogen ion (H) ratio. Base concentrations have a high hydroxyl ion to hydrogen ratio. If they are in balance, the concentration is neutral (pH 7). Hydroxyl occurs naturally through hydrolysis and ozone photolysis reactions. Hydroxyl radicals are important in hydrocarbon reactions in the troposphere during the day. Hydroxyl groups also occur naturally in "spectral tuning" proteins in the eyes of creatures that sense color. In the optoelectronics industries, the hydroxyl ions present in water molecules can interfere with optical transmission by absorbing some wavelengths. The term High-OH stands for "high hydroxyl content" and refers to transmission systems that are most efficient in the ultraviolet and visible spectra. Similarly, low- OH is most efficient in the near-infrared and visible spectra. Low-OH, clear, fused quartz is used in UV lamps, thennocouples, lightguides, fiber optic probes, and a variety of types of semiconductors. Low-OH synthetic fused quartz is used in components such as semiconductors and UV-illuminator sleeve tubings. See hydrolysis. hygroscope An instrument for measuring the amount of moisture in a material. Handheld paper hygro- scopes are commonly used in the printing industry to monitor paper moisture balance and the relative humidity of the air in order to adjust printing materi- als and processes for quality control. See densitom- eter. hygroscopic 1. A material with a tendency to absorb and retain moisture. 2. A material that is able to ab- sorb and retain moisture. hyperlink, hypertext link A logical link between meaningful data organized within a random access database or markup language. Hyperlinks can be hi- erarchical or flat. They can be one-directional or bi- directional. Although hypertext links are most famil- iar to users in the form of virtual cards in a computer card catalog or as browser-accessible links on the World Wide Web, a hyperlink in its broadest sense also applies to interconnected visual image links, where the user clicks on an icon or a part ofapicture rather than on a word or block of text. Hyperlinks on the Web have opened up global Inter- net interactions and cross references to immense, shared information storehouses. There are a number of popular games that are navigated through text or visual links. See browser, hypertext transfer proto- col, World Wide Web. hypertextA means of accessing information through referential links. This idea has been around for a long time and has had various implementations, with Bush developing a microfilm system and suggesting asso- ciative indexing in the 1940s. In the 1960s, D. En- ri:~i?l~i:'~;~~~~:~;~£:~1~~fE~;~: :. terns have been developed by various researchers, but the implementation of the concept on computer net- works did not become commonly understoqd and rec- ognized until the distribution of HyperText on the Macintosh computer in the late 1980s. The most significant implementation of hypertext, which serves as a simple front-end to the Internet in the form of Web pages, is the Hypertext Markup Lan- guage. Hypertext tags can be imbedded in Web pages to allow them to connect to any other public page on the Internet. See Hypertext Markup Language. Hypertext Markup Language HTML. A simple markup language for creating platform-independent hypertext documents for display and distribution over a computer network. HTML is a generic semantics implementation of Standard Generalized Markup Language (SGML - ISO 8879: 1986). HTML has a simple tag-based syntax that can be readily learned, for basic page display, by individu- als without much prior programming skills. It can be readily configured to link to graphics and documents in other locations on the Web. Thus, HTML has be- come widely used on the Web to represent hyper- media, documentation with inline graphics, database query results, news, stock reports, course outlines, storefronts, and discussion lists. The formal definition ofHTML syntax is described in the HTML Document Type Definition (DTD). HTML was designed by Tim Berners-Lee at CERN and has been in use since 1990 by the World Wide Web global information initiative. The Document Type Definition (DTD) was written by Dan Connolly in 1992. In 1993 a number of contributors provided enhancements, and the incorporation ofNCSA Mo- saic software allowed the inclusion of inline graph- ics. Dave Raggett derived forms material from the HTML+ specification. In 1994, the HTML Specification was rewritten by Dan Connolly and Karen Olson and edited by the HTML Working Group, with updates by Eric Schieler, Mike Knezovich, and Eric Sink from Spy- glass, Inc. Finally, the entire draft was restructured by Roy Fielding. The development and use of Web browsers began to spread. 459 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Since then, the number of users of the Web, interact- ing through HTML, has climbed to more than 40 million, and many millions have authored personal, institutional, and commercial Web pages using HTML. HTML has undergone a number of updates and revi- sions since its initial introduction. HTML pages are often enhanced with applets created with Sun's Java programming language and may also be enhanced with ActiveX objects. See browser, hypertext, RFC 2070 (Internationalization). HypertextTransferProtocol HTTP. An application- level, generic, stateless, object-oriented network pro- tocol intended for quick-access, distributed, collabo- rative, hypennedia systems. HTTP uses typed data representation, allowing system-independent data transfer. HTTP use is widespread, as it has been an intrinsic part of the World Wide Web initiative since 1990 and has been widely incorporated into Web servers and clients. It also provides a generic means of communication between user agents and proxies/ gateways, and Internet protocols for email, search en- gines, and file servers. HTTP communications on the Internet are typically over TCP/IP connections, with a default port ofTCP 80. A message is the basic unit ofHTTP communi- cation, which uses a request/response paradigm for serving information. Once a connection is established between the client and the server, the client sends a request, and the server responds with control and er- ror infonnation and, if the request is successful, the requested content. The syntax of the HTTPURL is as follows: http://<host>:~rt>/~th>?<searchpart> There are other transfer protocols as well, including File Transfer Protocol (FTP), for the transmission of files over the Web. When used with a Web browser, it has a similar syntax to an HTTP URL, except that the prefix ftp: is used rather than http:. See File Trans- fer Protocol, MIME, Secure HTTP. hysteresis 1. The diminution or retardation of effects upon a body from a force, when the force acting upon the body changes. For example, in a body that is mag- netized by a changing magnetizing force (e.g., an electromagnet with a varying current), hysteresis is the amount by which the magnetic values of the body lag (due to friction or viscosity, etc.) behind those of the magnetizing force. 2. The difference in response ofa system to a varying force or signal. 3. The dif- ference in the ability ofa system or device to respond and change according to a sudden force upon it. To give a simplified example, stomping on a car accel- erator or brake does not result in an instantaneous 460 change to a new speed. Hysteresis is the delay effect between the stomping action and the response of the vehicle to the action. Sports car drivers experience less hysteresis than motorhome drivers. hysteresis curve A diagrammatic representation of a magnetizing force and its related magnetic flux. In a hysteresis curve for magnetic materials that are subj ected to a magnetic influence, then separated from the influence, then magnetized and separated again, it can be seen that materials retain some of their original magnetism after removal of the magnetic in- fluence. This property can be shown to vary among substances by means ofa hysteresis curve diagram. Thus, materials with a narrow curve are suitable for the cores of electromagnets in industrial applications; those with wide curves can retain their magnetic prop- erties and are used accordingly. hysteresis device A device or circuit intended to me- diate a situation in which power levels, or other im- portant operating aspects, are fluctuating outside of nonnal parameters. An example of such a device would be an emergency system that switches to re- serve generators or battery power when voltages drop or which draw off extra power if voltages spike. In this case, a delay mechanism (a hysteresis device) may be deliberately introduced in order to prevent constant fluctuation or fluttering, so that the system switches to areserve system only after asustained or significant change in power levels occurs. Without the hysteresis device, the system might othelWise be constantly switching back and forth between main and reserve systems, a situation that would be imprac- tical and perhaps even dangerous. Hysteresis devices/circuits are also important in mo- bile communications based upon cell transmissions. In these systems, a person using a mobile phone, for example, may be traveling along a path that passes several transceiving stations. As the user moves from one transceiving region to another, the signal will change. Buildings and bridges will also affect the strength of the signal reaching the mobile unit. Mo- bile systems are designed to assess the incoming sig- nals and select the best one, a situation that may change from moment to moment, especially in a moving vehicle. Whenever there is a switch from one transceiver to another, there is a slight interruption during which the system adjusts its settings. If the user is in a locality where there are several signals of simi- lar strength, the unit could try to constantly switch from one to another, causing interruptions and inef- ficient use of the resources. A hysteresis circuit helps to ensure that excessive adjustments are not made. Hz See hertz. © 2003 by CRC Press LLC I 1. symb. current. 2. symb. incident (as in incident light). 3. A symbol commonly used to designate the "on" position on a rocker switch, with 0 commonly used for "off." 3. abbrev. intensity. The I is usually indicated on or near an analog dial on a computer monitor or TV screen, to allow the user to increase or decrease the amount of illumination of the display. I interface, Inter-Service Provider interface An interface between two Cellular Digital Packet Data (CDPD) networks deployed over AMPS. See A in- terface, E interface. I & R abbrev. installation and repair. I Series Recommendations A set oflTU-T recom- mendations that provides guidelines for ISDN. These are available as publications from the lTU- T for pur- chase and a few may be downloadable without charge from the Net. Some of the related general categories and specific I category recommendations that give a sense of the breadth and scope of the topics are listed here. Since lTU-T specifications and recommenda- tions are widely followed by vendors in the telecom- munications industry, those wanting to maximize interoperability with other systems need to be aware of the infonnation disseminated by the lTU-T. See also similar listings under Q, V, and X Series Rec- ommendations that describe other aspects of telecom- munications. See lTU-T I Series Recommendations chart following. I signal One stream in a split signal in certain modu- lation systems. The transmission may be split into two streams: one is the in-phase or I signal; the other is the quadrature-phase or Q signal. In various data transmission schemes, it is common to split a signal and to alter the characteristics of one or both of the data streams so that they can be transmitted together without interfering with one another or creating ex- cessive crosstalk. The signals may then be recom- bined or synchronized at the receiving end. Streams may also be split according to their differ- ent transmissions needs, as in speech, which can be sent on voice grade lines, and graphics, which require better and wider transmissions media. When prototyping hardware transmission devices (e.g., GPS receivers), it is sometimes expedient to model the behavior of the transmission in a computer simulation before building the hardware. Thus IIQ signals and other signal schemes are simulated ac- cording to their known mathematical properties. See quadrature amplitude modulation. I-frame intra-coded frame. In MPEG animations, a picture that has been encoded into a video frame with- out reference to past or later frames, using predicted motion compensation algorithms. See B- frame, I -pic- ture. I-picture intra-coded picture. In MPEG animations, a picture that is to be encoded into a video frame with- out reference to past or later frames to prevent refer- ence image errors. Once it is encoded, it is consid- ered to be an I-frame. See MPEG encoder. 1- TV See interactive television. I-way slang. An expression for the growing global telecommunications network, derived from a short- ening of the phrase "Information Super Highway." 110 input! output. Generally used in the context of computers as meaning input from users, applications, or processes and output to devices, applications, or processes. See input, input device, output, output de- vice. 110 bound input/output bound A processor subjected to a processing load in excess of what it was designed to handle, or which causes processes and response time to be uncomfortably slow for the user, is said to be liD bound. There are a number of ways to reduce the incidence of 110 congestion: more efficient algo- rithms; co-processing chips for computing intensive operations such as graphics, sound, or device man- agement to ease the load on the central processing unit (CPU); faster CPUs; reconfiguration or reorga- nization of peripheral devices; distributed processing over a network, etc. 110 device input/output device. Apiece of computer hardware physically interfaced with a system, and electrically and logically configured to engage in two- way communication with the operating system and relevant applications. Many computing devices are primarily input or output devices. Joysticks and mice are primarily input devices; speakers and printers are primarily output devices. (Few devices are strictly one or the other, since signal processing, device sta- tus, and handshaking signals often are returned by the device to the system to improve the efficiency oftheir use.) Most monitors are output devices, but touchscreen 461 © 2003 by CRC Press LLC . of keys pressed simultaneously to perform a specific operating sys- tem function. For example, on anAmiga,Amiga-Am- iga-Ctrl reboots the machine. On an mM-compat- ible running MS-DOS, Ctrl-Alt-Del perfonns a simi- lar function. 3. A combination of keys to access text style attributes and search and replace functions in older word processing programs developed before graphical user interfaces became common. hotline. Fiber Optics Illustrated Dictionary and status lights on the front panel. The Kitchen Com- puter was technically known as the H-316 Pedestal Model. Series-16 rack- and table-top models were available as well. Despite its spectacular lack of success in the kitchen market, the Series-16 technology was not completely lost, as it eventually made its way into components used in the ARPANET (as interface message proces- sors for connecting hosts), a more suitable. LLC Fiber Optics Illustrated Dictionary hunt group In telephone systems, a series of lines set up so calls can be assigned to the next available line in a group if the first accessed line is busy. There are different ways to organize hunt groups, from a straight sequential hunt to random hunts. Hush-a-Phone decision A landmark communica- tions case in 1956 which challenged the AT&T mo- nopoly of phone line access. The company market- ing Hush-a-Phone Company and Harry C. Tuttle wanted to attach a mechanical device to a phone set in order to screen out background noise. AT &T ar- gued against it, supported by the Federal Communi- cations Commission (FCC), but the decision was later overturned in the court of appeals, the main argu- ments including the mechanical rather than electri- cal nature of the device and the fact that it in no way harmed the phone equipment. A Hush-a-Phone is on exhibit at the Museum of Independent Telephony in Abilene, Kansas. See Carterfone decision. Huygen's