Fiber Optics Illustrated Dictionary - Part 47 pptx

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 a battery sour

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 a battery 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 a number

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 of a service or operation

hoot'n'hoIler, holler down, shout down, squawk

box A dedicated, 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 of information 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 Inradio, the extent of a transmission from Earth

to ionosphere and back 3.In frame relay, the extent

ofan individual trunk line transmission path between

two switches 4.Inan IBM Token-Ring network, the

extent ofan individual transmission path between two

bridges 5 In cellular communications, where the user

may be traveling through several transmission zones

during the course of a 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

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 storeinthe 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, a radio

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 ofhops that make

up a route between its source and destination, or be-tween a specified 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 ofa call Hop counts are one means

to gauge the efficiency of a system and to configure

or tune it for better performance

hop off To exit one type of system and complete the

route on another For example, you may initiate a fac-simile transmission on the Internet, that thenhops 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, thehop offis from

the phone system to the Internet, or, conversely, you can consider it ahop on to the Internet, ifyou 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

of fiber optics, lenses, and the improvement of these

systems In the 1960s, he took a position as a

profes-sor of optics at Reading University See endoscope;

Kapany, Narinder

Grace Hopper - Programming Pioneer

Grace Murray Hopper was an American

mathema-tician, physicist, lecturer, and one of the first

com-puter programmers in the days when programming

involved rearranging the wires within a vacuum-tube

computer system.

Hopper, Grace Murray (nee Grace Brewster

Murray, 1906-1992)AnAmerican mathematician,

physicist, and educator, Hopper was an originator of

software compilers and developer ofthe COBOL

pro-gramming language She earned a Ph.D from Yale

in 1934 and spent many years as a lecturer, 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 ofAmerican 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-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 ofcomputers to a time when electronics would be used to channel signals auto-matically) In spite ofdetractors, 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

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sociation Computer Science Man-of-the-YearAward

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 ofTechnology by the U.S President See A-O, B-O, bug, Harvard Mark1

hopsSee hop count

horizontal blanking interval, horizontal blanking timeThe period during which a display is suppressed

on a cathode-ray tube (CRT) to allow the electron gun

to return from the right side ofthe 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 connectThe 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 frameThe 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 linkInATM, a link between two logical nodes belonging to the same peer group

horizontal resolutionAquantification ofthe amount

of information that is contained on a single horizon-tal line of a rasterized output device such as a moni-tor or printer On raster monimoni-tors, horizontal resolu-tion is expressed in terms ofpixels, usually about800

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 A resolution

of about 72 pixels per inch or higher on a grayscale

Fiber Optics Illustrated Dictionary

or color display is preferred for video displays

Col-ors and shades of gray can be used to antialias a

dis-play to give it a higher perceptual resolution

horizontal scan rateA measure of the scan speed of

electron beam display devices, usually described in

hertz(Hz),as in cathode-ray tubes (CRTs) thatsweep

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 ofscan rates and

reso-lutions, most ranging from about 40 to 75Hz See

cathode-ray tube

horizontal segmentIn wiring distribution systems,

the wiring route from individual NAM or10

loca-tions to the riser closets through ceilings or floors,

usually up to a maximum of about 250 feet

horn alertAnelectronic 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

horsepowerhp 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 of power equal to 746 watts

See watt

host1 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 givenhostandclient

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 adapterHBA 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 Iffiber optic connection

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

a vendor-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 A software configuration utility may be used to establish a relationship between the HBA physical device and the logical HBA number as-signed

host carrierIn 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 computerA 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 site1 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 A computer bulletin board sys-tem, which typically hosts email, chats, games, and file uploads and downloads

hot1 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 cathodeA hot cathode is one that produces a stream ofelectrons (a cathode ray)

by means of thermionic 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

use with electronic devices, such as the waveguide

tubes used in micro- and millimeter-wave

commu-nications Barium-dispenser thermionic cathodes

have been ofparticular 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 ofinterest 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

For 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-accelemttest-ing scanntest-ing electron

microscopes (SEMs) See cathode-ray tube

hot cut, flash cutThe 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 switchwir-ing 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.).(RAIDsystems 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 dockingInserting 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 offall 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 combination1 A combination of keys that

when pressed simultaneously will perform a specific

function or engage a memory resident progmm, such

cesses that are frequently needed but would be dis-tracting ifrunning 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

hot lineAprivate, 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

of a 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 servicePhone service that expedites an au-tomatic connection through a dedicated private phone See hot line

hot linksIncomputer 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 listIncomputing, a list of frequently used appli-cations progmms, directories, or Internet newsgroups, Web sites, or archives Ahot 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, hotspot1 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 Abottleneck or area ofcon-gestion in a network, component, or software routine

4.Anarea of a circuit in which some component is generating more heat than would normally be ex-pected and that may signify a potential problem 5 A region of a document or image that includes an

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 standby Abackup 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 Aprotocol 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 of a 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-ingi~.See FireWire, hot cut

HotJavaAnadjunct 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 a browser's

capa-bilities See Applets, Java

Hotline Vu1ual Private Line Service Acommercial

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.AnAmerican inventor who

devel-oped one ofthe 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 ofkeys 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 halfofthe 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

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 a particular 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 if a phone is left off-hook, "If you'd like to make a call, please hang up andtryagain ," 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 ofthe 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.Itcan't really be considered a personal computer, as it listed at just under $6,000 (more than the price of a 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, a color model that allows settings to be adjusted along these three properties Hue is the color, saturation is the amount or richness ofthe color, and value is the light-ness or darklight-ness 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 Aconnecting point on a network to centralize wiring and connection management Ahub may be passive or active and is often used in systems with star topologies 2.Aconnection 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

router, switcher.

hub siteThe 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 ofthe system The hub

site allows easy cabling and administrative access to

a variety of connections Hubs are often located at

main wiring or logical junctions and may connect to

external systems

hueA color of the visible spectrum Hue does not

include white, black, or shades ofgray, 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 calledtoneortint.See

inten-sity, saturation

Huffman encodingHuffman 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 ofa message 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.In1858, 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- ) AWest 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 SystemsA 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 of associated 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

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 a grandfather 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 a piano-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 Comcom-pany 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 ofmore 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)AnAmerican 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 theAlEE Journal,V.40 The magne-tron became important in the development of radar and satellite communications systems See magnetron Human Computer Interface standardsHCI A se-ries of protocol platform standards from the IETF, including but not 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 hearhear-ing 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, huntingA process through which a call is routed by seeking the best path or fIrst available path

or device

Fiber Optics Illustrated Dictionary

hunt groupIntelephone systems, a series of lines

set up so calls can be assigned to the next available

line in a group ifthe first accessed line is busy There

are different ways to organize hunt groups, from a

straight sequential hunt to random hunts

Hush-a-Phone decisionA 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 principleWavefronts

can be mathematically decomposed (integrated) into

a series ofpoint sources, with each seen as the origin

ofan 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 ofwhat

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 principleFresnel 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 ofall the

wave-lets as to their amplitudes and relative phases

This led to a more complete though still not fully

de-veloped description ofwavefront 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-16A 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

bands and can still be used over those frequencies

Hybrid Fiber CoaxHFC 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 insdigi-tallation 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 Fiber Coax architectureAhybrid 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 ofa 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)AnAmerican 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

hydrawSee octopus

hydroelectricElectrical power derived from the energy

ter under pressure Large dam projects are usually

linked to a desire for hydroelectric power for

residen-tial, urban, and industrial purposes

hydrolysisAprocess ofchemical decomposition that

occursittthe presence ofmoisture such that new

com-pounds result from the reaction Hydrolysis is

ofcon-cern in maintaining insulating materials in

underwa-ter cable installations Hydroxyl ions (OH-) may

re-sult from the ionization ofwater See hydroxyl group

hydrometerAn instrument that measures, by

dis-placement, the specific gravities ofliquids Used, for

example, to measure the electrolytes in batteries

hydroxyl groupA group of atoms that occurs in

or-ganic molecules, a type ofJunctionalgroup 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 a molecule's

chemi-cal properties Hydrocarbons containing a hydroxyl

functional group are known as alcohols A metal

joined with a hydroxyl groups is called ametal

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

hygroscopeAninstrument 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 ofthe air in order to adjust printing

materi-als and processes for quality control See

densitom-eter

hygroscopic1 Amaterial with a tendency to absorb

and retain moisture 2 A material that is able to

ab-sorb and retain moisture

hyperlink, hypertext link Alogical 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 ofvirtual cards in a computer

card catalog or as browser-accessible links on the

also applies to interconnected visual image links, where the user clicks on an icon or a part ofa picture 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

hypertextAmeans ofaccessing 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.Inthe 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 ofWeb 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 LanguageHTML 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

Fiber Optics Illustrated Dictionary

Since then, the number ofusers 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 ofupdates 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)

Hypertext Transfer Protocol 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.Italso 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 ofeffects

upon a body from a force, when the force acting upon

the body changes For example, in abody 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 ofthe body

lag (due to friction or viscosity, etc.) behind those of

the magnetizing force 2 The difference in response

of a 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

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 oftheir original magnetism after removal ofthe magnetic in-fluence This property can be shown to vary among substances by means of a hysteresis curve diagram Thus, materials with a narrow curve are suitable for the cores ofelectromagnets 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.Anexample 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 a reserve system only after a sustained 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 Ifthe user

is in a locality where there are several signals ofsimi-lar strength, the unit couldtryto constantly switch from one to another, causing interruptions and inef-ficient use ofthe resources Ahysteresis circuit helps

to ensure that excessive adjustments are not made

Hz See hertz

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 ofthe display

I interface, Inter-Service Provider interfaceAn

interface between two Cellular Digital Packet Data

(CDPD) networks deployed over AMPS See A

in-terface, E interface

I &Rabbrev 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 ofthe 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

oftelecom-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 thein-phase or I signal; the other is

thequadrature-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 ofthe transmission in a computer

simulation before building the hardware Thus IIQ

cording to their known mathematical properties See quadrature amplitude modulation

I-frame intra-coded frame.InMPEG 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-wayslang 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 Aprocessor subjected

to a processing load in excess ofwhat 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 of110 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 ofperipheral 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 engagein 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