Fiber Optics Illustrated Dictionary - Part 30 pps

domain name server A computer with server capa-bilities that communicates with the Domain Name Service DNS on the Internet after having registered its own unique ID.. Fiber Optics Illust

Fiber Optics Illustrated Dictionary

Anemail server at the local domain handles the

processing of mail once it is received from the ISP,

anditcan route it according to the needs of the

lo-cal system

The domain name is expressed alphanumerically with

dots between each of the levels or portions of the

domain, with the assigned domain as the last two parts

of the name Since 1995, the number of registered

domain names has risen from 100,000 to over 10

million

Within the U.S., domain names are subdivided into

various categories Outside the U.S., the extension is

usually a designator for the country The North

American Domain Name Extensions chart shows a

sample offamiliar domain name extensions The

ap-pendix includes a more complete list ofover 200

In-ternet domain name extensions from around the

world See Internet Corporation for Assigned Names

and Numbers

domain name registration A necessary

administra-tive step for individuals and organizations who wish

to have a unique domain name on the Internet

Un-like state registrations of company names, or

com-panies in different industries that share a common

name, there can be only one of a particular name on

the global Internet This fact has made domain names

hot commodites, with companies buying and selling

domain names the way logos are sometimes bought

and sold To register a domain name, you must have

a site ready to come on line, and you will usually go

through your ISP to establish the name for the site

In 1995 registration changed to a fee system; in 1998,

it was about $100 for initial registration and $50 per

year to retain the name Now it's much less See

Do-main Name System, InterNIC, lANA

domain name server A computer with server

capa-bilities that communicates with the Domain Name

Service (DNS) on the Internet after having registered

its own unique ID The serving computer provides

an Internet Protocol(IP)address to the DNS for a

do-main name that is not fully qualified (does not end in

a dot) For individual users, this is usually handled

by a serving computer at a local Internet Services

Pro-vider (ISP) For campus/commercial users, it may be

handled by a local campus backbone server The

lo-cal Start ofAuthority is delegated to provide Domain

Name Server services for the assigned domain This

information is essential to providing connections

be-tween individual systems or local area networks

(LANs) and the Internet at large See Domain Name

System

Domain Name System, Domain Name Service

DNS Adomain name distributed database established

in the early 1980s at the University of Wisconsin

DNS provides mapping between host names and

In-ternet Protocol (IP) addresses DNS evolved out of a

need for a distributed system to handle a very large

number ofdomain names In older systems, host files

were regularly distributed, until they became too large

to be managed on most systems

To become a node on the Internet, it is necessary to

fonnally register aunique domain name The extensions

familiar to Intemetusers as.com, edu, net, biz, and infoare part of the domain naming scheme called

zones.Each domain name is stored in a central re-pository on the Net, and addresses are resolved through this database

Demand for domain names on the Internet has risen from a trickle to a flood Businesses are realizing that the unique name requirement is different from tradi-tional business naming schemes For example, it is possible for two businesses in the same state to have the same business name if they are in different lines

of business On the Internet, however, there can be only one of each name in the world, assigned on a first-come, first-serve basis This exclusivity has cre-ated an unprecedented demand for names, leaving second comers with little choice but to come up with

a less desirable or memorable name or to change their signs, stationery, and other marketing materials, an expensive proposition

To relieve some of the demand for domain names, new domain name extensions were introduced in

2001, theoretically increasing the availability of names to other comers However, the demand prob-lem wasn't fully resolved for two reasons First, anew extension doesn't necessarily aid certain types of business (e.g., a book store isn't helped much by the

fact that there are new aero extensions available for

aeronautics companies) Second, big corporations with large marketing budgets typically register mul-tiple versions of their names, thus leaving smaller companies out of the running even when additional extensions are added See domain, domain name server, InterNIC, RFC 830

Domain Name System security extensions DNSSE Specifications developed to improve the weak secu-rity aspects of the Domain Name System As the In-ternet Domain Name System became increasingly important for the transmission of secure documents, such as sensitive government communications and financial information and services, it was felt that extensions for the use of cryptographic digital sig-natures should be developed See RFC 2065 domain organization In its general sense, the orga-nizational structure of a digital network domain in-cludes the operations, devices, and other elements under the general control of a processor, system, or network The overall controlling and administrating

entity may be called a host and may function as a

server or have jurisdiction over a number of servers The host maintains some type ofaccess and manage-ment mechanism to a database or other record ofother computers and devices on the system This entity may

be a single computer or software program, or a logi-cal amalgamation of several computers or software programs Similarly, security mechanisms are gen-erally orchestrated by the host or other controlling member of the domain The organization of the ele-ments associated with the host and contained within the domain can vary substantially with the type of network and various devices that are included domestic arc Aportion ofan orbiting satellite's path

or range that provides transmissions between the

ber of domestic satellites in use that specifically are

launched to cover a particular country or territory, as

in Alaska and India Others serve domestic needs

during a particular portion oftheir orbit See satellite

dominant carrierA designation for a long-distance

telecommunications provider that dominates a

par-ticular region or market.Inmost cases, a dominant

carrier is more stringently regulated in order to

bal-ance a monopolistic advantage with opportunities for

competition

dominant mode 1.The most effective or most

preva-lent mode of transmission or conduction within a

material For example, conduction (rather than

con-vection or radiation) may be the dominant mode for

heat transmission in a particular material In fiber

pulling systems in a furnace with a gaseous

environ-ment, thermal radiation has been found to be the

dominant mode for heat transfer 2 In a single-mode

waveguide or one that can carry more than one

wave-length at a time (e.g., multimode fiber) the mode that

has the best propagation characteristics for that

waveguide, which will depend upon the frequencies

used and physical characteristics of the guide Thus,

the mode with the least loss or distortion of signal

over the link is the dominant mode or the wavelength

that is not cut offbelow the V parameter See

numeri-cal aperture,Vnumber

DOMSATDomestic Communications Satellite

A geosynchronous broadcast relay satellite NOAA

uses it, for example, in conjunction with the

Geosta-tionary Operational Environmental Satellite (GOES)

system, for relaying meteorological data

dongleA small hardware device orsecurity keyused

for software or system security Dongles were widely

introduced on microcomputers to protect commercial

products in the 1980s and were raucously opposed

by hackers, technical users, and enough general

us-ers that their use has been almost abandoned in North

America

donorAnn-type dopant, such as phosphorus, which

is used in solar photovoltaic devices The dopant puts

an additional electron into an energy level near the

conduction band to increase electrical conductivity

Doping is a common way of manipulating the

prop-erties of electromagnetic materials See doping

doorA term often used on computer bulletin board

systems (BBSs) to indicate a category ofuser access

activity that is external to the BBS software itself

Thus, a separate software program, such as a game

or quiz, external to the BBS software is launched

when the user selects a specific door

dopantAn industrial chemical added in minute

amounts to pure semiconductor materials, usually to

improve the conducting properties of the materials

See donor, doping

doped fiber amplifierDFA A fiber optic cable that

has been impregnated or doped with substances,

usu-ally rare earths, which alter its transmission

proper-ties See doping

dopingAmeans of adding small amounts

ofmateri-als with particular properties to another in order to

For example, in semiconductor manufacture, mate-rials are doped to enhance or inhibit particular ten-dencies to give up electrons or form "holes." Optical fibers are often doped with rare earth elements to al-ter their transmission characal-teristics See germanium

Doppler shiftA perceived or measured shift in fre-quency when the source ofradiant energy moves rela-tive to the position of the observer or receiver

DoSSee Denial of Service

DOSSee disk operating system

double densityDD Aterm for the physical configu-ration of magnetic particles on a floppy diskette, and hence the maximum amount of digital data it can store Three and one-half inch, double density dis-kettes hold approximately 720-880 kilobytes ofdata, depending on the platform The diskettes themselves are interchangeable between systems, provided they are formatted for the operating system on which they are being used Double density diskettes have given way to high density diskettes (1.4 Mbytes), SuperDisks (100+ Mbytes), higher capacity cartridge formats, and USB and FireWire devices

double-pass autocollimation testAtype ofnull test

in the sense that it tests a mirrored optical assembly, such as a parabolic mirror, against itself This is a very accurate test with the light path reflecting offthe mir-rored surface twice and is useful for assessing tele-scoping components A Ronchi grating, with evenly spaced lines and facets, can be used as the reference image Any spherical aberrations in a flat mirror under test will usually manifest as concave or con-vex out-of-focus images if the mirror is parabolic, the bands will be curved and should be compared against an ideal reference See Ronchi test

double-superheterodyneDSH A technology used

in audio tuners as a conversion method for maximiz-ing the selectivity of tuners and reducmaximiz-ing distortion

as much as possible in order to take full advantage

of programming from broadcast sources that pack a lot ofprogramming into limited frequencies and from shortwave sources See superheterodyne

DOVdata over voice Technology that allows data transmissions to be carried over traditional phone connections, usually copper twisted pair See ISDN

down converterAtechnique and device used in com-munications in which the incoming frequencies are shifted There are two common reasons for doing this

By shifting incoming frequencies so they are differ-ent from the outgoing frequencies, it is possible to reduce interference between the two sets of signals Some frequencies are shifted up when they are trans-mitted, in order to put them into a particular broad-cast band or slot A down converter is then needed at the receiving end to downshift the frequencies back

to levels that can be used by the playing or viewing equipment that provides the information to the user

downlinkThe common name for thesatellite to

Earth to satellite portion The downlink is often frequency-shifted from the uplink in order to reduce interference between the two sets of signals Uplink

Fiber Optics Illustrated Dictionary

and downlink services may be carried by different

providers and may be subject to different usage

re-strictions or billing arrangements

downloadTo receive computer data from a source

on another system, usually through a network or

modem connection Downloading is typically done

with a communications program file transfer

proto-col (ZModem is commonly used), FTP client, or a

Web browser Common types of downloads include

files from BBSs or Internet file archives and World

Wide Web images and text

When using browsers on the Web, the information

that you view is typically downloaded orcached on

your machine (because loading the source code ofthe

Web page on your local computer allows the browser

to redisplay the previously viewed pages more

quickly.) This is usually convenient, but it has

disad-vantages as well and constitutes a security hole on

your system It may also fill up your hard drive Make

sure your cache is flushed (erased) when you are

fin-ished with the Web files You can always save ones

of interestinappropriate directories for later

view-ing while offline

While downloading files from FTP sites or Bulletin

Board Systems, make sure you don't accidentally

overwrite an existing file of the same name Not all

download software will inform you ofa duplicate

file-name It is a good policy, at any time, to download

into a separate directory or even a separate partition

until you have run a virus checker on the downloaded

files All foreign files should be suspected ofpossible

viruses until you have determined that they are

prob-lem- and virus-free See FTP, upload

downstreamA designation for any of the systems,

nodes, legs, or hops in a transmissions pathway that

are subsequent to the current one Thus, a printer is

generally downstream from a computer, a radio

lis-tener is downstream from the radio broadcaster, and

various workstations may be downstream from a

server See download Contrast with upstream

downtimeThe block of time during which a system

is nonfunctional Downtime on a computer may be

caused by software or hard drive crashes, broken

net-work connections, etc Downtime on phone systems

may be caused by power outages, overloaded lines,

or breaks in the lines

downwardly compatibleSoftware or hardware

de-signed to work in some way with older software or

hardware, sometimes calledlegacy applications or

equipment Often the downward compatibility is only

partial For example, a software program may be able

to export a file in the older format, but it may not

in-clude all the characteristics of the file when loaded

into the earlier application Similarly, a new computer

may work with an older monitor, but that doesn't

mean the monitor can support all the graphics modes

that might be built into the graphics controller on the

new computer Downward compatibility is a way of

safeguarding a financial investment and

ofmaintain-ing a minimum level ofcontinued data access and use

of existing software Contrast with upwardly

com-patible

DP 1 See data processing 2 See Deflate Protocol

3 See demarcation point 4 Dial Pulse A standard Hayes modem command with the letter "p" used to designate a pulse dial setting for subsequent dialouts, thus ATP (ATtention, dial Pulse) See ATD DPA 1 Dearborn Protocol Adapter A commercial Microsoft Windows PC board for interfacing host computers with automotive communications net-works, from the Dearborn Group, Inc 2 Defence/ Defense Procurement Agency 3 Demand Protocol Architecture.Anarchitecture for dynamically load-ing protocol stacks as they are required 4 digital port adapter 5 See Differential Power Analysis 6 See Digital Performance Archive 7 Disabled Persons Assembly Telecommunications technologies are an important tool for enabling disabled persons to lead fuller, more active lives as well as being a means for people with similar problems to intercommunicate and support one another through difficult challenges http://www.dpa.org.nZ/8 distributed processing ar-chitecture 9 See Document Printing Application DPBX digital private branch exchange (PBX) Most private branches in North America are becoming digi-tal, so the D is now commonly assumed when using PBX See private branch exchange

DPCMSee differential pulse code modulation DPEdistributed processing environment See distrib-uted computing

DPLBDigital Private Line Billing

DPNSS See Digital Private Network Signaling System

DPODirect Public Offering See Initial Public Of-fering for description of this specific state-regulated subcategory for securities offerings

DPP1 See Demand Priority Protocol 2 See Direct Print Protocol 3 Director of Public Prosecutions (Australia) 4 Distributed Pipe Protocol A client-server protocol that was implemented over TCP us-ing remote method invocation (RMI) by Wang and Ouyang in 2000 5 Distributed Processing Periph-eral 6 See distributed programming platfonn DPX See DataPath Loop Extension

DQDBSee Distributed Queue Dual Bus

Dr Dobb's Journal ofSoftware Tools This journal

has been a perennial favorite with programmers, pro-viding technical information on a wide variety ofplat-forms and programming languages since 1976 It originated as a newsletter in 1975 documenting Tiny BASIC The name is somewhat a collapse of the originators' first names, Dennis and Bob

Draft RFC Aformal stage in the Request for Com-ments standards and information distribution process

in which the proposal is submitted for evaluation and comment On the Internet, this process is widely used

to encourage open standards and professional and public participation See Request for Comments drag lineAwire, rope, or other line for threading wire and cable through narrow channels (pipes, conduits, walls, etc.) The drag line may be preinserted during building construction and left for later use See birdie dragonAprogram running low-level secondary sys-tems tasks, especially on Unix syssys-tems, which are

tistical programs are often run in the background as

dragons, and the results of their activities may be

viewable by the system administrator or those with

sufficient security clearance See daemon

DRAM 1 digital recorder, announce mode 2 See

dynamicRAM

Draper, Henry(1837-1882) An American physician

and inventor and the son ofJ William Draper, Henry

Draper was a pioneer in spectral analysis, carrying

on research in astronomy and photography that was

begun by his father Henry made major contributions

to spectroscopy and astronomical photography and

created the first photographs of stellar spectral lines,

in 1872 See Draper Catalogue, Henry

Draper Catalogue, HenryThe Henry Draper

Cata-logue is a spectral classification of stars established

in 1890 by Edward Pickering, Anna Palmer Draper,

Willamina Fleming, Annie Jump Cannon et a1 in

honor of Henry Draper By 1915, more than a

quar-ter million stars had been cataloged, and, in 1918, the

first volume of the Catalogue was published The

Catalogue is still an important astronomical reference

and has now been published online

Draper, John(ca 1943- ) See Cap'n Crunch

Draper, John William(1811-1882) An

English-born American scientist, educator, and historian who

developed some ofthe early photographic processes

Draper researched incandescent substances and his

son, Henry Draper, carried on this line ofresearch to

become a pioneer in spectrum analysis

Some of Draper's photographic research in the late

1830s apparently predated Daguerre's He

investi-gated photography of the very small and of the very

distant (e.g., the Moon) His bookHuman

Physiol-ogy(1856) contained the first-published

micro-pho-tographs He is credited with creating the first

Ameri-can portrait, in 1840 Draper's ten-minute exposure

was a big improvement over previous techniques that

required many hours of exposure

Draper was an associate of Samuel Morse and they

cooperated on some projects.In1862, he published

The History of the Intellectual Development

ofEu-rope,in 1874,The History of the Conflict Between

Religion and Scienceand,in 1878,Scientific Memoirs.

drift1 Variation from a desired signal or current over

time from factors other than line, load, environment,

or warmup period See calibration 2.Inradio

tech-nology, signal drift is not uncommon For example,

if you set an analog radio to a favorite station and

gradually lose the setting as the tuning changes, this

is drift

DRiP See Duplicate Ring Protocol

drop pointA protrusion on an insulator or other

ob-ject designed to channel moisture away from

sensi-tive components or electrical currents Drop points

(sometimes called drip points) are typically on the

lowest protruding edge where water is channeled and

may have a variety of shapes and spacings See

in-sulator

drive typeThe make, model, and configuration of a

hard drive Adrive can conform to one ofthe common

once it is formatted, it is important for the system to have a record of the drive type in order to keep track

of sectors, blocks, partitions, and files, the various data configurations that can be set up dynamically on the drive during use The drive type is really a com-bination of the cabling, data bus characteristics, physical properties of the drive, and magnetic data configuration which is superimposed on the magnetic recording surface

Some types ofdrives can be used in combination with one another, and others cannot For example, SCSI and IDE drives are not mixed on one data bus A dif-ferent controlling mechanism is used for each type

SCSI devices can chain up to six devices (the con-troller counts as the seventh device), while IDE drives can chain up to two, with one designated as amaster

and the other as aslave.

When a formatting software program isrunwith a new drive, or one that is being formatted for another computer system, it may query the drive for infor-mation about its characteristics and display that in-formation on the screen For example, it may show the brand, model, and size of the drive, and whether there are any existing partitions Many drives now come preformatted, but it may still be necessary to set up partitions, if desired

driver1 In software, a program that includes code that can translate commands into instructions recog-nizable by a specific device, such as a facsimile mo-dem, printer, scanner, hard drive, etc Desktop publishing programs typically include a directory full

of drivers for various printers which translates the print instructions from the software into the closest approximation possible by the printer through the print driver software 2 In software event-process-ing, code that receives commands and distributes them appropriately for execution

droidcolloq abbrev. android 1 An anthropomor-phic robot, generally more human than machine which may be a combination of biological and me-chanicaVelectronic parts.2 Acompany drone, some-one who unquestioningly follows instructions and mechanically goes about the business ofwork (or liv-ing) without much enthusiasm or introspection, pos-sibly due to apathy or unquestioned acceptance of authority 3 A person hired as a human robot to do a mindless, repetitious, production-line job that offers few opportunities for variety or creative interaction

These are the kinds of positions that should be handled by machines, in order for people to have more leisure and creative time

dropA short cable connection, often between a util-ity pole and a building, or between one panel and another, or a panel and other distribution entity, com-monly used for supplying telephone, cable TV, or computer network services

drop frameIn television video broadcast recording and playback, North American television was de-signed to play at 30 frames per second Then when color signals were introduced, the differences be-tween black and white and color technology resulted

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Fiber Optics Illustrated Dictionary

ina compensatory adjustment ofthe frame rate down

to 29.97 frames per second, a situation that altered

the time code and reduced its usefulness for station

timing

Drop frame mode, also known ascompensated mode,

is a technique in which the system skips ahead a very

small amount at specified intervals, skipping over the

fIrst two bits in each minute However, this is

mi-nutely too much of an adjustment, so each ten

min-utes, only a single bit is skipped It's similar to the

way in which we adjust our calendar to celestial

events by introducing leap years (except that a day is

added rather than skipped in a leap year) where

needed, to better synchronize them

Drop frame modes are important to broadcasters

be-cause programming is interspersed with commercial

announcements, shorts, special programming, and

other timing-related items, that need to be sequenced

in a smooth, seamless way See SMPTE time code

drop loopIntelephone wiring between the

switch-ing office and local subscribers, the circuit is called

aloealloopand the specific section ofthe circuit from

the utility pole or other nearby junction point to the

subscriber's home or office is the drop loop

dropout, drop-outAnundesirable low-level,

irregu-lar loss of information when transferring from one

system, medium, or format to another Dropout

happens, for example, in video editing, when copy-ing or editcopy-ing tapes, especially with less robust for-mats and inexpensive equipment Dropout can some-times be seen as white dots appearing somewhat ran-domly on the screen They are especially noticeable

if the screen has large areas of dark or solid colors dropout, transmission A short interruption in a trans-mission, usually caused by a problem in the transmit-ting or receiving equipment Different industries have different objective measures for the length of inter-ruption that constitutes a dropout

dropped call A call terminated without the express desire ofthe parties engaged in the call In radio phone communications, dropped calls are not uncommon,

as the signal can easily be interrupted by terrain, weather, or a stronger signal from another source or distance

dry cable, raw cable, dark cable Conductive cable

or wire with no added electronics and no signal pass-ing through The cable you find on spools in the hard-ware store isdrycable Raw fiber optic cable is called dark cable

dry cell A common, compact type of battery de-scended from the wet cell, but differing in that it em-ploys nonliquid electrolytes in the form of paste or gel Dry cells were invented by Gassner in 1888 and manufactured in the early 1900s Since they do not

Digital Transmission Speed Categories Signal Level/Description

Os-o A 64,000 bps standard for transmitting digital data through pulse code modulation (PCM)

A sampled signal is quantized and transmitted with bits that represent quantization levels being transmitted separately A standard used in telephone systems

DS-I A frame format standard for transmitting data at 1.544 Mbps, developed in 1962 Used on

Tl systems It incorporates time division multiplexing (TOM) to combine 24 OS-O signals, and adds a single framing bit Signals are transmitted with bipolar (B8ZS) pulses or

alternate mark inversion (AMI) In 1969, the standard was extended to SuperFrame to increase the signal-to-noise ratio, and later it was further modified to create Extended

I SuperFrame which is more robust Europe uses a 32-channeI2.048 Mbps systemwhi~his somewhat similar but incorporates different synchronization and signaling formats See Extended SuperFrame, SuperFrame

OS-IC This signal system was designated 1C because it fits somewhere between OS-1 and OS-2 in

terms of its 3.152 Mbps signaling rates Used on TIC systems It was introduced by AT&T

in 1975 OS-1 signaling bits are bit-interleaved into the information bits

OS-2 A frame format developed for longer transmission lines and to accommodate AT&T's

Picturephone technology (which was developed many years before the technology to use and support it became sufficiently widespread) Used on T2 systems It combines four

OS-1signals or 96 OS-O signals, employs two framing stages, and transmits at 6.312 Mbps Europe uses a different lTV-defined system that operates at 8.448 Mbps (2.048 Mbps

primary rate)

OS-3 A frame format developed for signaling over broad bandwidth signaling systems Used on

T3 systems OS-3 uses Bipolar with Six Zero Substitution (B3ZS) The OS-3 signal

combines 7 OS-2 or 672 OS-O signals, is framed in two stages, and transmits at 44.736 Mbps Through multiplexing, the asynchronous signals are transmitted over synchronous links Europe uses a different lTV-defined system that operates at 34.368 Mbps (2.048 Mbps primary rate)

portable than wet cells, and can be used in any

orien-tation They are commonly used in flashlights, small

appliances, and many handheld devices Manydry

cells include toxic chemicals and heavy metals and

should be recycled through local centers, not thrown

in the trash See Gassner, wet cell

DS 1 Dansk Standardiseringsrad The Danish

Stan-dards Institute, located in Hellerup 2 digital system

3 See Distributed Single Layer Test Method

DS- A series of signal speeds for transmitting digital

data through a variety of modulation and

multiplex-ing schemes, designated DS-l through DS-4, with

higher numbers representing faster possible

transmis-sion speeds This system is primarily used in North

America and Japan A similar system, which differs

in data rates, encoding, and numbers ofchannels, the

E-system, is used in Europe The DS-system first was

initially used by phone carriers for connecting main

switching centers Gradually, as the technology

be-came less expensive, it began to be used in the

back-bones oflarger private branch exchanges, and now it

is used in telephone feeder plants, and local area

net-work backbones as well See Digital Transmission

Speed Categories chart See E-carrier

DSFacility A categorization system for describing

digital transmission capacity See DS-, Digital

Trans-mission Speed Categories chart

DSA 1 data service adapter 2 Digital Signature

Al-gorithm See Digital Signature Standard, Electronic

Certification 3 See Direct Selling Association 4 See

Directory System Agent

DSAT 1 digital satellite 2 digital satellite TV 3 See

Digital Supervisory Audio Tone

DSC 1 See digital selective calling 2 See Digital

Subtitle Encoder 3 Disability Statistics Center

4 document supply center 5 distributed statistical

computing

DSCS See Defense Satellite Communications System

DSE 1 See Deep Sky Exploration 2 See

Distrib-uted Single-Layer Embedded 3 distribDistrib-uted software/

systems engineering See distributed programming

platfonn 4 See Dynamic Systems Estimation library

DSH See double-superheterodyne

DSL See Digital Subscriber Line

DSLAM DSL access multiplexer See add/drop

mul-tiplexer, Digital Subscriber Line

DSM 1 See Design Structure Matrix/Dependency

Structure Matrix 2 See digital switching matrix

DSP See Defense Support Program

DSR See data set ready

DSRC See Dedicated Short-Range Communications

DSRR See Digital Short-Range Radio

DSS 1 digital satellite system 2 See Digital

Signa-ture Standard 3 direct satellite system See direct

broadcast satellite 4 direct selling support 5 direct

station selector A consumer broadcast system

com-ponent that enhances channel selection for desired

programs; the term is especially applied to satellite

programming services selection

DstlAnorganization formed in July 2001 when the

U.K Defence Evaluation and Research Agency

agency, which supports a number of sites supporting research in science and technology Dstl traces some ofits lineage back to organizations originating in the 15th century

DSTO See Defence Science and Technology Organisation

DSU See Digital Service Unit

transmission line and a router Some units combine

a digital modem, router, and terminal server, a com-bination that is popular with Internet Services Pro-viders See digital service unit, channel service unit

DSXpanel See digital signal cross-connect panel

DT See Deutsche Telekom

DTE See Data Terminal Equipment, End Device

DTL 1 database template library 2 See Designated Transit List 3 See diode transistor logic 4 distance teaching and learning

DTMF See dual tone multifrequency

DTMX digital trunk manual (telephony) exchange DTP See desktop publishing

DTR 1 See Data Terminal Ready 2 detailed trans-action report 3 discrete tone relations

DTRS 1 digital tape recording system 2 digital te-lemetry recording system 3 Digital Trunked Radio System

DTS Digital Television Standard See ATSC Digital Television Standard

DTSR 1 Dial Tone Speed Recording 2 digital tape system recording 3 digital temporary storage record-ing

DTT 1 digital tape transfer 2 digital telecommuni-cations/telephone trunk 3 digital tie trunk A digital telephonytrunkline providing a direction connection between private branch exchanges (PBXs), in other words, tying them together 4 digital trunk testing

DTTB Digital Terrestrial Broadcasting See Digital Video Broadcasting

DTU 1 digital test unit 2 digitaltrunkunit

Du Fay, CharlesFran~oisde Cisternay (1698-1739)

A French soldier and scientist who discovered that electricity had two basic attracting and repelling prop-erties, which could be demonstrated, for example, by rubbing amber with wool and rubbing glass with silk

He called theseresinous electricity and vitreous elec-tricity, making the distinction in a context that had

eluded previous researchers

DuFay developed some ofthe ideas first investigated and described by S Gray in England and made some important observations about the composition of the materials of the conducting medium Perhaps most important ofOll Fay's observations is that

" an electrified body attracts all those that are not themselves electrified, and repels them as soon as they become electrified by the electrified body."

DUA See Directory User Agent

dual attachment concentrator DAC Aconnecting device used in double ring Fiber Distributed Data Interface networks which employ a token-passing scheme over a redundant ring network Dual ports and

Fiber Optics Illustrated Dictionary

attachment points are used in connection with the

concentrator to reroute data if a problem arises See

Fiber Distributed Data Interface for more detailed

information

Dual Attachment StationDAS A configuration of

a Fiber Distributed Data Interface (FDDI)

token-pass-ing, dual attachment network The dual attachments

provide fault tolerance They consist ofa primary ring

and a secondary ring, the first ofwhich is usually used

for data transmissions, and the second as a backup in

case ofproblems A Class A, dual attachment station

(DAS) connects to both rings and a concentrator

which, inturn,ensures the ring transmission is not

interrupted Failure in a ring causes a series

ofadap-tations such that the ring wraps back on itselfand

tem-porarily eliminates the failed station See A port,

B port

dual cableA two-cable configuration, usually in a

local area network (LAN), often implemented to

pro-vide redundancy and fault tolerance, as in ring-based

Fiber Distributed Data Interface (FDDI) systems See

Fiber Distributed Data Interface

dual homing1 A means of providing backup and

fault tolerance on a network system, particularly

char-acteristic ofFiber Distributed Data Interface (FDDI)

networks FDDI networks utilize stations, which can

be eliminated through rerouting ifa problem is found,

without interrupting transmissions Dual homing

uti-lizes two concentrators, a primary and a secondary,

with the secondary used as backup that is

automati-cally activated ifa problem occurs See Dual

Attach-ment Station, optical bypass 2 In aFrame Relay

net-work, a means of providing fault tolerance by using

dual port connections in different locations

dual modeThere are many dual mode devices in

tele-communications Many modems are dual mode in

order to support both vendor proprietary protocols

and industry standard protocols Dual mode monitors

will sometimes support both NTSC and RGB signals

Some phones have dual pulse and tone dialing

capa-bilities Many cellular phones are now designed to

support traditional analog signals and emerging

tech-nologies that use digital signals Dual mode devices

tend to come about when there are competing

stan-dards, or when technology is transitioning from one

stage to the next Some are autosensing, switching

to the correct setting unaided, and some have to be

explicitly set with a switch or software

dual output fiber optic sensorA device with two

sensors in one assembly Thus, dual independent

read-ings may be taken for redunancy or comparison The

sensors may be analog, digital, or hybrid

dual tone multifrequencyDTMF Touchtone

signal-ing on a phone system The tones are actually a

bination of two frequencies that are variously

com-bined to provide unique codes for each key on the

telephone, when pressed This signal is sent through

the line to indicate the desired number to be dialed

or the desired selection from an automated phone

menu system The tones were chosen for frequencies

that carry well on voice-grade lines

Phone phreakers used to exploit these tones for dialing

unauthorized long-distance numbers with small de-vices called blue boxes With the increased use ofout-of-band signaling systems, such as Signaling System

7, which send the signals separately from the conver-sational information, this practice is becoming less prevalent and will eventually be impossible See touchtone phone

Dublin CoreAstandard metadata scheme for describ-ing document-like informational objects in order to facilitate data discovery on computer networks Thus,

core objects such as Date, Creator, and Description

can be quickly and easily located and utilized for in-dexing and archiving purposes The Dublin core is intended to supplement rather than supplant existing Web metadata searching and indexing methods and can be applied to physical as well as electronic ob-jects The Dublin Core Metadata Element Set (DCMES) became the first IETF metadata standard arising from this scheme See Government Informa-tion Locator Service, InternaInforma-tional Development Markup Language

Dublin Core Metadata InitiativeDCMI.Anopen forum first held in Dublin, Ohio, in 1995 as a result

of discussions in October 1994 at the 2nd Interna-tional World Wide Web Conference DCMI promotes the understanding and development of interoperable online metadata standards and specialized vocabular-ies intrinsic and related to the Dublin Core Consen-sus is evaluated and administered by the Dublin Core Directorate http://www.dublincore.org/

ductProtective pipe or tube through which lines or fluids are run See conduit

ductileAproperty ofbeing malleable, a material that can be shaped, drawn out, flattened, or othelWise bent

or manipulated without significant stress or breakage Duddell, William du Bois (1869-1942)AnEnglish experimenter and engineer who discovered that elec-tric arcs created in a circuit with coils and condensers could generate very high-frequency audible tones in the low radio wave frequencies Duddell fashioned a keyboard connection to control the oscillations, thus varying the pitch and creating the "Singing Arc," in

1899,arguably inventing the fIrst electronic instru-ment and creating possibilities for wireless commu-nications The Duddell Medal and Prize was estab-lished in 1923 by the Council of the Physical Soci-ety See Poulsen arc

Dulbecco's observationThe observation that credit for a scientific discovery seems to go to the one who gets the most publicity as opposed to the one who cre-ated the original invention or made the original ob-servation As stated by Nobel laureate Renato Dulbecco, "Credit generally goes to the most famous discoverer, not to the first." A closely related senti-ment was expressed earlier by William Osler as "In science the credit goes to the man who convinces the world, not to the man to whom the idea first occurs."

To emphasize the importance ofcredit to a scientific career, Broad and Wade have stated,"Itis difficult for a nonscientist to appreciate the overriding impor-tance to the researcher ofpriority ofdiscovery The desire to win credit, to gain the respect ofone's peers,

Priority for scientific discovery carries not only

rec-ognition and personal satisfaction, but has a highly

significant bearing on future scientific opportunities

and funding for future research

dumb switch A switching device that channels

sig-nals through a desired pathway as needed without

automation through digital intelligence Dumb

switches are commonly manually set passive devices

A dumb AlB switch can be used for two computers

to share one printer or modem Dumb switches are

very common, particularly as AlB orAlBICswitches,

because they are inexpensive and easy to set up and

use They make no logical or electrical evaluations

or decisions about the incoming or outgoing data;

they simply route it mechanically

Inits simplest form, a dumb switch does not alter or

boost the electrical connection, though some may be

equipped to amplify or condition a signal, without

changing its infonnational content Thus, switches

may be electrically passive or active In video, where

a great deal of switching occurs, banks of both

pas-sive and active switches channel the desired video or

audio feeds into the broadcast orrecordin~channel

In contrast, on automated networks, "smart' switches

may evaluate the incoming data and perform some

rudimentary routing Some switches are so smart, in

fact, that the distinction between switches and

rout-ers at the high end is blurred On layered networks,

switches typically operate at the second layer

Other than the inconvenience of threading cables

be-hind desks and through walls, installing a dumb

de-vice switch is pretty straightforward Ensure that the

interconnected devices are compatible, and use

gen-der bengen-ders and converters that are the right sizes and

numbers of pins to hook everything together Most

dumb switches for computer applications have

25-pin female D connectors Make sure all the systems

are powered offbefore making any connections, and

test new connections individually rather than all at

once, so that a problem can be isolated and corrected

right away See AlB switchbox

dumb terminal A minimally configured computer

tenninal, with no direct processing capabilities, that

isn't very useful unless it is networked to a central

processing system Universities and libraries often

use dumb terminals to provide cost-effective user

ac-cess to the main system The advantage is that the

dumb terminals are inexpensive, easy to maintain,

and not highly subject to abuse or vandalism In their

simplest form, they consist only of a touchscreen

monitor or teletype interface; one step up is a

key-board or mouse and a monitor Some

not-quite-as-dumb terminals will include both keyboard and

mouse, monitor, and sometimes a floppy drive or

CD-ROM drive, but they still rely on a remote system for

actual processing ofdata and commands Hybrid

ter-minals may include a processor for simpler tasks

but still rely on the remote system for most of their

computing power

dumpster diving Searching for mechanical parts,

discarded electronics, trade secrets, software and

passwords in large outside trash cans called dumpsters Dumpster diving is practiced by corpo-rate espionage agents and computer hackers The practice has been around for a long time, but the term gained media prominence in the mid-1980s when a group ofteenagers in San Diego was apprehended for computer piracy and revealed some of the ways by which it had come into possession of source code, computer passwords, phone numbers of BBSs and timeshare systems, and other confidential materials Shredders are employed by many companies to pro-tect sensitive documents from prying eyes and some will incinerate the documents rather than disposing

of them in a dumpster In the military, even greater precautions against theft are taken, with codebooks and other sensitive documents sometimes being printed in inks that will disappear if photocopied or exposed to moisture

DUN See dialup networking

Dunwoody, Henry Harrison Chase Dunwoody pat-ented the use of carborundum in early radio wave crystal detectors in 1906 (U.S patent #837,616) Car-borundum (silicon carbide) was robust in the sense that it could be clamped down, and thus it was used

in portable wireless telegraphy (radio) units Tube radios, which could be readily amplified, eventually superseded crystal detectors

duopoly Amarket situation in which two major sell-ers greatly influence the market, though they may not necessarily control it In industries requiring licenses,

a situation where exclusive operating licenses are is-sued to two businesses rather than one

duplex Double, bidirectional

duplex connection, duplex transmission Data trans-missions in which a message can be sent in both di-rections along the same transmissions line or path

In many duplex systems, the messages can be alter- nately sent in one direction or the other, but not in both directions simultaneously, whereas in full du-plex, the messages can be sent in both directions at the same time Serial communications software of-ten has half- and full-duplex settings

duplex telegraphy A historic innovation in tele-graphic communications in which two messages were sent in opposite directions, at the same time, over the same line by varying the strength ofthe current This innovation was first put into practical use in the 1850s and was thereafter ofgreat interest, since it could sig-nificantly improve the efficiency oftelegraphic com-munications, in effect doubling the capacity of the line in the 1800s Since two-way or duplex telegra-phy had many commercial advantages, there was much interest in this idea and in putting it into com-mercial use

J.W Gintl, an Austrian telegraphic director and physi-cist, was one ofthe first to propose a practical means

of duplex communications using two batteries Two years later, Siemens& Halske were to patent a du-plex telegraph, using only a single battery In North America, Joseph B Stearns refined the concept and Thomas Edison extended it, patenting a quadraplex

Fiber Optics Illustrated Dictionary

telegraph (U.S #480,567) that could transmit two

signals in each direction by varying not only the

strength of the current, but the direction as well

(fol-lowed by patents for a number of refinements and

variations)

Duplex or multiplex communications are now

gen-erally the norm for many types of communications,

although wireless communications over the same

fre-quency are still sometimes carried out in one-way or

simplex mode, as are some simple homebrew

com-munications systems See Farmer, Moses; Frischen,

Carl; Gintl, Julius; Stearns, Joseph; telegraph history

duplexer A switching device that provides

alternat-ing transmittalternat-ing and receivalternat-ing through the same

trans-missions system (data line or antenna)

Duplicate Ring Protocol DRiP ACisco protocol for

Virtual LAN (VLAN) switches and routers that

iden-tifies active Token-Ring VLANs DRiP data is

uti-lized for detecting duplicate configurations and for

all-routes explorer filtering DRiP has a Cisco HOLC

protocoltypevalue ofOxO102 See Token-Ring network

DVB See Digital Video Broadcasting

DVBG See Digital Video Broadcasting Group

DVCPRO See D-7

DVD See Digital Versatile Disc

DVD Forum An international association ofsoftware

and hardware developers and manufacturers

ofDigi-tal Versatile Discs (DVD) technology.Itwas

origi-nally founded as the DVD Consortium in 1995 The

original founding companies included Hitachi,

Matsushita Electric Industrial, Mitsubishi Electric,

Pioneer Electronic, Royal Philips Electronics, Sony,

Thomson Multimedia, Time Warner, Toshiba, and

Victor There are now more than 200 member

com-panies, with headquarters in Tokyo, Japan

The Forum promotes acceptance of DVD-related

products in the entertainment, IT, and consumer

electronics industries.Itdefines DVD format

speci-fications, publishes educational materials, creates

DVD format books, and administers the DVD

Veri-fication Laboratories Membership is open to

orga-nizations engaged in DVD research and development

or manufacturing See Digital Versatile Disc, Digital

Versatile Disc player http://www.dvdforum.org/

DVD+RW Alliance A voluntary alliance of

indus-try-leading personal computer and optical storage

developers and manufacturers, including

Hewlett-Packard, MCCNerbatim, Philips Electronics, Ricoh

Company, Sony, Thomson multimedia, and Yamaha

The Alliance disseminates information relating to

DVD+RWtechnologies through various events and

publications It has been working independently of

the DVD Forum

In spite ofits quick acceptance and widespread

popu-larity, the DVD format will likely be superseded by

other optical formats, most probably blue laser

tech-nologies Blue laser diodes have the potential to write

optical media with more than five times as much data

as current DVD formats Thus, about 40 hours of

broadcast recording or 2 hours of high quality

cin-ematic entertainment could be recorded on a single

side See DVD Forum http://www.dvdrw.com/

Dvorak keyboard A type of keyboard layout de-signed by August Dvorak and William Dealey after they studied the natural movement offingers and of the hand over typewriter keys and researched ways

in which to conform the key positions to the com-fortable hand use, rather than conforming the hand

to unnatural keyboard lettering layouts

DVST See direct view storage tube

DWDM See dense wavelength division multiplexing DWS See Dialable Wideband Service

DXC See digital cross-connect

DXI See data exchange interface

dynamic bandwidth allocation The process of as-signing bandwidth on demand or according to algo-rithms to maximize the efficiency of the system, rather than transmitting on particular frequencies or

at particular times

dynamic beam focusing In cathode-ray tubes (CRTs), the sweep of the beam from an electron gun across the inside surface of the screen that displays the image This beam forms a series of motions which, ifkept equal, are curved To keep the distance equal across the sweep of the beams, earlier televi-sion screens and computer monitors were curved to match the length ofthe beams Early flat screen moni-tors were rare and expensive With more sophisticated hardware and software algorithms, manufacturers have devised ways of compensating the travel dis-tance of the beam to adjust to the characteristics ofa flat surface One ofthese techniques is dynamic beam focusing, adjusting the beam focus as needed, de-pending on which part of the screen is illuminated, and its distance from the gun

dynamic data exchange DDE Any process in which data is transferred between systems or between applications without intermediary steps, such as sav-ing the information and transmittsav-ing with a different application See drag and drop, Object Linking and Embedding

Dynamic Host Configuration Protocol DHCP.An expanded client/server configuration protocol de-scended from, and downwardly compatible with, Bootstrap Protocol (BOOTP) DHCP provides manual, automatic, and dynamic allocation ofIP ad-dresses and a complete set of TCP/IP configuration values It utilizes ports 67 and 68 and retains BOOTP'sbootrequest and bootreply packet formats.

See RFC 1533, RFC 1541

dynamic IP addressing When logging onto the In-ternet, or any system using the Internet Protocol (IP),

itis necessary for a unique number to be assigned to the session to handle the flow of data to and from the user Dynamic IP addressing is a scheme for automat-ing the process of assignautomat-ing an address when a user connects to an Internet Access Provider (lAP) or other network access point As part of the user login, a unique number is assigned for that session The num-ber is typically freed when the user logs off, so the lAP can reassign it to the next user, ifneeded Free-ing the address is an important part of the process on large distributed networks like the Internet, where there may be millions of users online, with some of

taneous users.

dynamic gain equalization processorDGEP An

electronic processor for providing dynamic

gain-flat-tening and tilt control in optical amplifiers In

com-mercial systems, DGEP provides gain equalization

through patented all-fiber acousto-optical tunable

fil-ter (AOTF) technology In single-mode optical fibers,

acoustic waves can be applied directly to produce

what Novera Optics calls notch filters for optical

sig-nal transmission Notches may be shaped by

control-ling the frequency and amplitude of the acoustic

wave See acoustic wave

Dynamic Link LibraryDLL In software

program-ming, a Microsoft product format for consolidating

a number offrequently used routines, or routines that

may not be available by defaultinan application, such

as Visual BASIC The DLL is an organizational

pro-gramming tool that allows a 'library' of routines to

be written once, bundled together, and thereafter

linked into a program and called by the application

program as needed

dynamic RAMDRAM Random access memory

that requires a supply of current through the chip at

all times in order to retain and refresh the stored

in-formation When youtuma computer off, the data

currently in RAM is lost RAM is one of the most

prevalent types ofdynamic fast storage used in

com-puters Most systems these days require about 16 or

32Mbytes of RAM for basic functioning This

amount is in stark contrast to desktop computers in

the 1970s, which couldruntelecommunications

pro-grams, word processors, and spreadsheets in less that

8 kilobytes (not megabytes) of RAM, and systems

in the mid-1980s, which could run music and

graph-ics simultaneouslyina fully multitasking

environ-ment in only 4 megabytes ofRAM See static RAM

Dynamic Random Access MemorySee dynamic

RAM

dynamic range Arange of intensities, between the

minimum and maximum extremes It's a phrase that

is often applied to concepts of light or sound In

im-agery, the dynamic range of a scanner, for example,

is the range of light levels, from the brightest

high-light to the darkest shadow, that can be picked up and

transmitted In music, the dynamic range of a

re-corded symphony performance is the range from the

softest note to the loudest, expressed in terms

ofdeci-bels Dynamic range is sometimes described more

objectively in terms of the maximum and minimum

levels of a parameter as measured by an instrument

designed for that use See gamut

dynamic resource allocationIn various types of

communications, the administration, allocation, and

dynamic reallocation of resources, such as

frequen-cies, channels, processes, programs, and access to

shared peripheral devices Dynamic resource

alloca-tion usually entails intelligent algorithms for

deter-mining authorizations, priorities, and needs, and

of-ten includes sophisticated queuing, routing and

multi-tasking capabilities

dynamic routingIn general, the creation and

adjust-optimized basis, so paths will change to fit the needs ofa situation as specified In data networks, dynamic routing allows the system as a whole to stay online even if individual systems or routes change or are unavailable This is accomplished through routers, which can communicate with other routers, usually those topologically nearby, and which may increase,

or modify routing tables as needed

Dynamic routing works well on large, changeable, packet-switched systems like the Internet Routers can relay data around distressed or suddenly unavail-able systems or trunks On small systems, the over-head of dynamic routing may not be worth the loss

of speed that the processing takes Static routing may

be used quite effectively on small systems with known, stable characteristics See router, Routing Information Protocol

dynamic sector repair Afault correction and pre-vention system built into hard drive systems, particu-larly multiple disk arrays such as RAID, that seeks faulty sectors on a disk, repairs the data if possible, and records bad sectors to prevent the system from trying to write to those sections in the future See re-dundant array of inexpensive disks, SMART

dynamic storageIn computing, the allocation of

tem-porary or permanent storage space in an intelligent manner, so unused space can be optimally used, and unneeded data is removed to allow the reuse of stor-age for other applications It may also involve occa-sional reorganization ofinformation ifextra process-ing cycles are available See garbage collection

Dynamic Systems Estimation libraryDSE An ob-ject-oriented noncommercial software library for studying multivariate time series analysis techniques and forecasting models DSE runsinSplus andR The library is suitable for applications such as studying the statistical implications ofequivalence among dif-ferent model representations, studying the forecast-ing properties of models, or studyforecast-ing small sample properties of estimators

dynamically controllable magneticDCM Mag-netic materials that can change permeability in real-time when stimulated by a magnetic field DCM ma-terials that have this property have been found in the VHF to microwave frequency ranges Some commu-nications antennas need to be transparent at some fre-quencies and reflective at others, and DCM materi-als are being tested for their effectiveness for this use

dynamoThe historical name for what is now termed

a generator Adynamo is a machine that converts me-chanical energy into electrical energy (direct current)

A friction-based bicycle light is an example of a simple dynamo When the cyclist pedals, the wheel spins and rubs against the contact point for the gen-erator The generator takes this mechanical energy and converts it into light, so the cyclist can see and

be seen at night See alternator; generator; Siemens, Werner 2 An energetic, dynamic individual

dynamometer, electrodynamometerA sensitive current, voltage, and power detecting instrument similar to aD'Arsonval meter except that it uses a