Fiber Optics Illustrated Dictionary - Part 29 ppt

Discrete line illuminators are useful in many technologies, including communications devices and telescopic and industrial quality assurance calibration systems.. disk, diskette A round,

Fiber Optics Illustrated Dictionary

with power conditioners and good quality surge

sup-pressors

DISA 1 Data Interchange Standards Association

2 Defense Infonnation Systems Agency 3 Direct

In-ward System Access A subscriber option for external

access to a private branch exchange (PBX), usually

through a security code

disable 1.v.To prevent from functioning There are

many ways to disable a process or mechanism, but

the two most common are interrupting the power or

the transmission 2 n A signal, tone, or command

sent through a circuit to disable a device at the end

of the transmission Telephone companies have the

capability of electronically enabling and disabling a

number ofphone services

Disabilities Rights Office DRO.Anoffice of the

Federal Communications Commission (FCC) that

helps to ensure that people with

communications-re-lated disabilities are given the same opportunities as

others to access and use communications services

http://www.fcc.gov/cib/drol

disaster recovery The procedures and resources for

bringing a system back online after a disaster

Disas-ter recovery applies to everything from repairing or

replacing facilities and equipment damaged by

floods, hurricanes, or bombing to recovering data

from a hard drive damaged by a lightning strike or a

toddler poking paper clips into the circuit board Data

recovery is a fact of life Whether or not you archive

your data, your drivewillfail at some point, or you

willinadvertently overwrite an important file

Pre-vention is usually better but, if recovery of data

be-comes necessary, there are many shareware and

com-mercial tools to assist with the task

disc Around, flat data storage and retrieval medium

Usually these arewrite once/read many(WORM)

devices, since optical media are not as easy to write,

and rewrite,asfloppy disks Although in English disc

and disk are used somewhat interchangeably, in

com-puter parlance, disc tends to be used more often to

designate optical storage media such as audio CDs,

laserdiscs, etc., whereas disk is used more often as a

contraction ofdisketteto designate an encased floppy

diskette (flexible magnetic medium) See disk

discard To throwaway, be rid of, toss aside This is

usually said ofsomething that is ofno interest or

fur-ther use (or anticipated future use) or that cannot be

dealt with or used at a particular instance in time The

concept of discard is important in networks since

there are many instances when too much data or bad

datamay create bottlenecks or other types ofproblems

discard-eligible OE In data-segmented networking

systems, there are often packets orframesthat are

dis-carded for various reasons, including redundancy,

congestion, timing out, low priority status, etc

Some-times the decision to discard the packet is made on

the part of the network's lower level functions, and

sometimes the discard status is explicitly represented

by a signal bit.InFrame Relay, there is a bit in the

HOLC frame address field that can be set to mark the

frame as expendable in case ofcongestion See leaky

bucket

disclaimer A refutation ofresponsibility or liability Software license agreements often have disclaimers

of liability for any damage to computers on which the software isrun.It's pretty unusual for software

to damage hardware (yes, it is possible), but it doesn't happen under normal operating conditions Neverthe-less, attorneys prefer to have all bases covered disco techslangA rather good pun fordisconnect technician.

disconnect 1 To separate two discreteunitsor de-vices from one another, usually with the implication that the separation broke some type of electrical, in-ductive, or communications link between the two 2

To terminate a communication, as in a phone call 3

To break a circuit

Discoverer XIV Ahistoric satellite launched in Aug

1960, it was the first satellite to be ejected from an orbiting space vehicle and recovered in midair The satellite was launched into a north-south polar orbit

in an orbit of 116 miles altitude at the perigee and

502 miles at the apogee On the 17th pass, the Agena ejected Discoverer XIV from its nose and the re-en-try vehicle fired retrorockets to slow the return trip Once in Earth's atmosphere, it parachuted toward Earth and was recovered by a C-119 on the third pass over the parachute

discrete Individual, separate, distinct

discrete cosine transform DCT A mathematical means of manipulating information by 'overlaying' cosines in order to analyze or use it from another point ofview OCT techniques are used in a number ofdigi-tal compression schemes, including a lossy compres-sion technique that provides a practical balance be-tween a high degree of compression and relatively good perceptual clarity in the decompressed image (although very high compression ratios may create a blocky effect).Anadaptive version ofthis technique

is used in JPEG image compression JPEG is one of the primary image representation formats used to play graphics on the World Wide Web One ofthe dis-advantages ofDCT is a visual artifact called Gibb's effect that manifests as ghostly ripples running along distinct edges See discrete wavelet multitone, Fou-rier transform, fractal transfonn, JPEG, MPEG, lossy compression, wavelet

discrete line illumination, line illumination DLI

A light source with a narrow spectral width Discrete line illuminators are useful in many technologies, including communications devices and telescopic and industrial quality assurance calibration systems Some have connections for fiber optic cables for wavelength calibration of fiber-equipped spec-trometers

DLI sources include single-mode laser lights and cooled, very low pressure spectral calibration lamps Spectral lamps are available commercially to provide

a variety ofdefined wavelengths in materials such as neon, argon, xenon, and mercury

discrete multitone OMT Amulticarrier technology used for transmitting multiple media over existing copper wires It lends itself to a variety ofdata deliv-eryservices, including Digital Subscriber Line (OSL)

by the American National Standards Institute (ANSI)

as a standard line code for T1.413

DMT uses discrete Fourier transforms for creating

harmonics along the main lobes (and demodulates at

the receiving end) Thus, DMT uses harmonics, or

tones, as a means to divide the bandwidth into

sub-channels The number of bits per tone is dependent

upon the frequency of the tone since signal-to-noise

ratios vary at different pitches The utilization ofthese

tones is somewhat like FM broadcasting; some tones

are used for data and some are used as guard bands

A signaling tone is used in each data stream for

tim-ing By dividing the available bandwidth into smaller

units, portions of the available bandwidth can be

in-dividually tested to evaluate speed, availability, and

suitability for transmission This allows the

optimi-zation oftransmissions over existing twisted-pair

in-stallations, which can vary widely in their

character-istics from region to region This scheme is currently

used in ADSL installations and is being evaluated as

a potential standard for VDSL See carrierless

am-plitude and phase modulation, Digital Subscriber

Ser-vices, discrete wavelet multitone

discrete wavelet multitone DWMT A multicarrier

technology developed by Aware, Inc for transmitting

multiple media over existing copper wires, based on

the same general principles as discrete multitone

DWMT is being promoted for use with Digital

Sub-scriber Line (DSL) and Hybrid Fiber-Coax (HFC)

DWMT divides the available bandwidth into smaller

units to utilize suitable portions.Itdiffers from DMT,

which uses Fourier transforms, by using wavelet

transforms for encoding subchannel bits DWMT

produces lower energy harmonics than DMT,

mak-ing it easier to demodulate the encoded signal at the

receiving end, where a fOlWard fast wavelet transform

(FWT) is used It is less susceptible than other

schemes to channel distortion and requires less

over-head See discrete multitone, wavelet

discussion list Amechanism for people to

intercom-municate and debate topics ofinterest over computer

networks In the early days, when most

communica-academic institutions and individual subscribers to a

list, they were called mailing lists However, now it

is best to call them discussion lists to distinguish them

from mailing lists that are used for bulk commercial

or junk email messages containing product informa-tion or solicitainforma-tions (similar to bulk mail delivered

by the postal service) Discussion lists are ubiquitous and account for a significant proportion of commu-nications over local networks Because participation

in discussion lists is so high, they are typically hier-archical and many are now moderated, to minimize abuse, commercial messages, and personal slurs The content of most discussion lists is text messages sent through email In recent years, however, Web-based lists are growing in prevalence, and some lists now incorporate HTML tags and graphics As band-width and transmission speeds increase, it is likely that video images and sound will become established

in discussion lists See mail distributor, USENET

dish colloq Common terminology foraparabolic satellite or terrestrial receiving or transmitting dish dish aperture The diameter of a parabolic commu-nications receiving dish In general, the larger the aperture, the broader the scope of signal apprehen-sion (there are exceptions depending upon the nature ofthe signal and the shape and materials ofthe dish) The amount ofaperture is also partly dependent upon the size and position of the feed apparatus, as it will block some of the signals

dish focal point The distance, in a parabolic antenna, from the reflective surface to the focusing point of the signal It is important to know this pointinorder

to position the feed mechanisms efficiently The fo-cal point is dependent on the breadth and CUlVature

of the dish, with flatter dishes generally having a fo-cal point that is farther away from the reflective sur-face than more concave dishes ofthe same diameter disk, diskette A round, flat, flexible, encased me-dium, typically used for data storage and retrieval The flexible medium inside the case is coated with magnetic particles that can be rearranged, thus pro-viding read/write capabilities

Common Modulation and Signal Subdivision Schemes Modulation or Subdivision Scheme Abbreviation/Notes

carrierless amplitude/phase modulation CAP Carrier signal is suppressed and reassembled at the

receiving end Single channel makes it more susceptible to interference

quadrature amplitude modulation QAM Variations in signal amplitude are used to represent

data.

discrete multitone DMT Frequencies are divided into discrete subchannels in

order to optimize the throughput of each channel; faster, less susceptible to interference

discrete wavelet multitone DWMT Provides some interesting means of implementing

better performance and low-loss compression with less susceptibility to interference

Fiber Optics Illustrated Dictionary

Inthe late 1970s and early 1980s, desktop

comput-ers were equipped with 8" disk drives, and the

ac-companying diskettes were flexible and highly

sub-ject to damage By the mid-I980s, 3.5" disk drives

were becoming popular, even though the diskettes

were expensive at $6.00 each, because they had a hard

shelled protective covering Prices dropped until

dis-kettes ofdouble the capacity were only $.30 each By

the mid-1990s, superdisks were starting to be

devel-oped, but it was not until 1998 that they were widely

announced The superdisks are downwardly

compat-ible with regular floppy disks but can hold over 100

Mbytes each Diskettes are being superseded by USB

and FireWire storge devices

Disk also is short fordisk drive,a high capacity hard

storage medium See disc, diskette, hard drive

disk controllerAhardware peripheral circuit

provid-ing an interface between a computer's main circuitry

and a floppy or hard disk drive The most common

formats for hard disk controllers are SCSI, IDE, USB,

and FireWire SCSI is predominantly used on Sun,

Amiga, Macintosh, DEC, HP, NeXT, SGI, and

server-level ffiM-licensed desktop computers IDE is

pre-dominantly used on ffiM-licensed consumer-model

desktops and some recent Macintosh computers

SCSI allows up to six devices to be chained to each

controller port (the controller is considered the

sev-enth device) IDE allows up to two devices,

desig-nated master and slave, to be chained to each

con-troller port SCSI disk concon-trollers can be used to

in-terface with hard drives, scanners, cartridge drives,

CD-ROM drives, and redundant array ofinexpensive

disk(RAID)systems IDE can interface with hard

drives, cartridge drives, some CD-ROM drives, and

some scanners To enhance the limited capabilities

ofIDE, an Enhanced IDE specification has been

de-veloped See FireWire, Universal Serial Bus

disk mirroringSee mirror, RAID

disk operating systemDOS The low level

operat-ing functions of a computer that are used to read or

write a floppy disk or hard disk and are engaged when

the system boots up The phrase has since taken on a

more general meaning to encompassall the low level

operating functions of a computer.Inthe late 1970s,

there were several disk operating systems made

avail-able for the various microcomputers For example,

the Radio Shack TRS-80 Model I could be run with

CP/M, TRS-DOS, LDOS and others, and you could

select the one you wanted to run by reading it from

tape (technically, a tape operating system), or from a

floppy disk (and later from a hard drive) Gradually,

however, as microcomputers came into the

main-stream, people began to associate the operating

soft-ware with the computer on which it came, and many

do not realize that each computer hardware platform

is capable ofrunning a variety ofoperating systems

For modem computers, there are many choices,

in-cluding Rhapsody, BeOS, Windows, MacOS,

Open-Step, Linux with The X Windows System, etc The

user doesn't have to be tied to the operating system

that comes bundled with the computer In the future,

ifmore developers adapt the various emerging

inter-national standards for open systems, particularly ob-ject-oriented technologies, applications may finally become operating system independent, and then us-ers will be able to choose different computus-ers the way they choose different cars, with the common object format being the "gas" that is standardized to power them all

disk serverA system dedicated to file storage, re-trieval, and handling on a network with shared disk resources It may also implement user access pass-words, file locking, license restrictions, and other administrative tasks associated with dynamic file sharing on a multiuser network On large computer systems, the disk server may manage many dozens

or hundreds of disks and may require a room of its own with special fast cabling, such as Fibre Channel cabling See RAID

diskless computerA computer with RAM access only and no disk drives or which has no user-acces-sible hard disks Typically, diskless PCs are simpli-fied systems for inexperienced users or secured sys-tems for preventing vandalism, theft, or the introduc-tion of viruses Diskless PCs are generally attached

by a network cable to a secure console housing a stor-age medium such as a hard drive Some, however, may allow the user to insert removable flash storage Diskless systems are often found in kiosks, copying centers that offer access to computers by the hour, public shopping centers, libraries, amusement parks, and science and technology museum exhibits DISN See Defense Information Systems Network dispatch1.~To send out a verbal or written com-munication or package 2.n Amissive, telegram, or

other written or electronic communication, usually intended for someone out of normal hearing range,

or intended to provide a record of the communica-tion 3.n.A communication intended for a group of recipients, such as field workers, taxi drivers, law enforcement officials, etc Often sent verbally as a radio communication

dispersionThe spreading and gradual loss of signal strength that occur in electromagnetic or optical data transmissions over distance Different media have different dispersion characteristics Dispersion is usu-ally, though not always, undesirable, and steps are taken to minimize it in most transmission technologies There are a number of types of dispersion related to the wavelength composition, characteristics, and angles of reflectance of propagating light beams.In fiber optics, multimode cables are more subject to dispersion than single-mode cables, and hence have

a shorter effective distance (this can be mitigated somewhat with graded index fiber)

In airwave broadcasts, undesirable dispersion, called scattering, can occur as a result ofmoisture, particles, and terrain See diffraction, graded index, reflection, stepped index

dispersion, chromaticInfiber waveguides, the dis-crepancy in the rate ofpropagation oflight beams of varying wavelength Since light is made up of a wide variety ofwavelengths which have different properties

in terms of speed, visibility, and ability to travel

length regions will interact in specific ways with the

conducting core and the reflecting cladding resulting

in varying times of arrival and varying amounts of

power (Le., some wavelengths may be absorbed more

than others), especially over longer cable runs

Graded index fiber that has been doped to customize

its light-carrying property may be used to mitigate

some of the effects of chromatic dispersion See

dis-persion, modal

dispersion, modalIn fiber waveguides, the pulse

spreading that occurs as the different beams oflight,

reflecting from the cladding back to the core, vary in

how far they travel in a given period of time In

mul-timode fibers and those with larger core diameters,

there is a greater prevalence of dispersion ofthe time

ofarrival ofalight pulse due to the larger area through

which the beams may reflect and disperse from one

another Graded index fiber, which has been doped

to influence its light-carrying properties, may be used

to mitigate the effects ofmodel dispersion by

match-ing the indexes of portions of the fiber to the beams

that will be traveling through See dispersion,

chro-matic

dispersion, waveThe separation of an

electromag-netic wave into its component frequencies For

ex-ample, light rays passing through a prism will be

dis-persed into individual color frequencies See

diffrac-tion, dispersion

displayn.Apresentation ofvisual information,

usu-ally on a display medium such as a screen, wall, or

computer display device such as a cathode-ray tube

(CRT), plasma display, liquid crystal diode (LCD),

or light-emitting diode (LED) surface See

cathode-ray tube, light-emitting diode, liquid crystal diode,

multisync

display driverThe software that translates

instruc-tions from a computer processor to the correct

oper-ating instructions for an attached display device A

display driver may be written to control one specific

ety of devices Most computers have a set ofgeneric display drivers compatible with the operating system

However, as there are many different types of prod-ucts, with new ones all the time, many vendors will provide display driver software with their product

Sometimes the display driver will be incorporated into the peripheral card that interfaces with the

dis-~~!s~~=~lO~:~~~:~~~; ~:~~~~~~=~~}~~=;II mation to the driver about their capabilities and

con-figuration parameters, so the driver can configure it-selfto optimally take advantage ofthe installed device

display interface cardA peripheral card that is in-terfaced between the computer processor and the physical display device in order for electrical signals and control data from the device driver to be trans-mitted between the computer and the display device

Sometimes the peripheral card itselfcomes installed with display driver software

Some display interface cards have hardware settings

to indicate the parameters ofthe installed display de-vice Thus, there may be a dial, jumpers, or dip switches that have to be set to configure the screen resolution, number of colors, refresh rate, and other parameters

Display Support Protocolnsp.An assigned well-knownport 33 on TCPIUDP systems

Display Systems Protocolnsp.An assigned weI/-knownport 246 on TCPIUDP systems

distance learning, distant learningReceiving edu-cation through telecommuniedu-cations media, including mail correspondence, email, Web sites, audio and video tapes, videoconferencing, etc Anumber ofedu-cational institutions are providing course content, ref-erences, etc., on the Internet, especially on the Web, and with digital signatures they may also eventually provide testing, assignments, and critiques through electronic means See audiographics, videoconfer-encing, whiteboarding

Modal Dispersion Concepts

_ !iiiliidi r i M • ;&1

•

Light beams traveling along afiber optic lightguide do not all reflect at the same angles at the same time Imagine

two pulses sent through afiber core at slightly different angles (1a and 1b) with 1a parallel to the reflective cladding

and1b at an angle such that it reflects back andforth in relation to the cladding and the conducting core but in the

same general line oftravel as the straight beam.

If you were to stretch out (calculate) the paths oftwo beams starting at the same time (2), b would be longer, and

the discrepancy between the time of arrival ofa and b would increase as the diameter ofthe core and/or the length

ofthe transmission link increased Light travels very fast, so the effects are subtle, but may become significant over

long haul multimode networks.

Fiber Optics Illustrated Dictionary

distinctive dial tonesThis is a method by which dial

tones are distinguished from one another by

proper-ties such as pitch, so that the caller can tell what type

of call is being connected This is particularly useful

on private branch systems (PPBX) where internal and

external calls require different sequences ofnumbers

For example, it is common to dial "9" to get an

out-side line on private systems, and the change in tone

helps the caller know the call has been given outside

access and will now accept an outside number

distinctive ringing Asubscriber option or feature of

some phones that uses a different tone or ringing

se-quence to identify incoming calls as coming from

inside or outside a branch system, or to signal another

extension Different phone carriers offer this option

under a variety of names, such as Feature Ring and

Ident-a-Call

While distinctive ringing is common in businesses

with private exchanges, there are now phones and

peripherals for residential and small business users

to distinguish calls on separate lines, or to identify

another callee on a single line, with codes that make

a different ringing sound or sequence It's handy if

you get many calls for teenagers

distortTo defonn, contort, warp or pervert out of a

normal sound, shape, or condition Distortion is

sometimes intentional, as in distorting the sound from

an electric guitar or synthesizer to create some

spe-cial effect In most cases, however, distortion of a

transmission is an undesirable fault condition

distortion Anundesirable change in the basic

char-acteristics of a wave or data transmission sufficient

to interfere with the information or its perception

Extraneous noise is not technically considered to be

distortion In visual images, distortion usually

in-volves undesirable aberrations in the basic

character-istics ofthe image, such as color, shapes, or lines that

misform or obscure the original features, or of the

entire image, in which case the outer contours may

become squeezed or twisted.Insound distortion, the

pitch, speed, or timbre of the sound may be altered,

making it difficult to discern the content or source of

the sound

distributeTo apportion, spread out, scatter, dole, deal

out, or dispense; to give out, broadcast, or deliver

to members in a group

distributed backbone Abackbone is a central artery

ortrunkin a network A distributed backbone is one

in which network segments are interconnected

through hubs joined with backbone cables Thus,

there may be multiple segments or rings joined to one

another through a backbone segment See backbone,

collapsed backbone

distributed computing Asystem in which the

com-puting processes are divided, parcelled out, or

other-wise handled simultaneously, or in which computing

services are apportioned, broadcast, or delivered to

users.Alocal area network (LAN) or wide area

net-work(WAN)are examples ofdistributed computing

environments Users of individual workstations can

work independently of one another on tasks that are

frequently carried out, yet they can share common

files, applications, and devices distributed around the system This system permits more open and efficient use of resources

A render farm is another example ofdistributed com-puting in which a centralized system parcels out in-dividual tasks related to constructing 3D models and images These are assigned to various machines, and the completed processes or objects are reintegrated

to create a completed rendering, thus potentially speeding up processing time The Internet represents

an example of distributed computing wherein rout-ing and transmittrout-ing ofmessages occur through a co-operative network of many interconnected systems, and many common resources (Web sites, archives, applications, search utilities, online chats) are shared among users

Distributed Computing & Communications Labo-ratoryDCC.Anexperimental research lab at Colum-bia University that develops fundamental, novel net-working technologies and exports them to academic and industrial organizations Projects include mobile agents; programmable, active network systems; adap-tive, self-managed network systems; and manage-ment technologies for Quality of Service (QoS) The DCC is under the umbrella ofthe Columbia Network-ing Research Center (CNRC)

http://www.cs.columbia.edu/dcc/

Distributed Computing EnvironmentDCE A middleware set ofcomponents from The Open Group that includes procedure, directory, file, and security services which may be bundled into a vendor's oper-ating system or integrated into a system by a third-party developer to fulfil the above-listed functions

Distributed Computing EnvironmentDCE A commercially developed set ofservices from Digital Equipment Corporation (DEC) that supports the de-velopment, maintenance, and use ofdistributed com-puter applications It is based on the Open Software Foundation's DCE (The Open Software Foundation

is now The Open Group.) Distributed-Feedback Laser Diode

DFB lasers have a layered, corrugated structure above the active lasing region The spacing ofthe cor-rugations is related to the refractive index ofthe ma-terials, allowing the corrugations to serve as a grat-ing filter that selectively reflects the desired wave-length into the cavity from which it is emitted The component is distributed in the sense that the light reflection andfiltering process occurs across the length ofthe component.

conductor laser component used in lightwave

trans-mission systems DFB lasers are based upon

Fabry-Perot (FP) lasers, but have an extra grating filter that

provides a sharply concentrated wavelength output

without the lower powered sidewaves that are

char-acteristic ofFPs This makes DFBs suitable for tasks

such as optical amplification and dense wavelength

division multiplexing (DWDM) applications where

finely concentrated signals allow closer proximity of

C-band transmission applications are common DFBs are sold in various configurations in terms of power requirements, connector options, and the optical wavelength emitted See Distributed-Feedback laser Fundamentals diagram See Fabry-Perot laser

distributed file systemA file system that is distrib-uted across more than one partition, more than one

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Distributed-Feedback Laser Fundamentals

pump

A distributedfeedback (DFB) semiconductor laser is constnlcted out ofa number ofcarefully organized active and

passive layers, depending upon the intended size and function of the component DFB lasers can be housed inside

traditional computer chip packaging or may be fabricated of thin film layers for insertion into other assemblies.

Silicon-based compounds are commonly used under the grating layer and as base substrates Whatever shape or

materials are selected, the basic concepts are the same - the organization of the layers and the grating period

(distance from one corrugation to the next) serves as a reflective filtering component, causing some wavelengths to

be reflected back and very specific tuned wavelengths to pass through to be output at the other end of the assembly.

This is a highly simplified cross-section of the grating filter within a semiconductor component, illustrating the

layered composition and corrugated structure of the grating The grating period would be much finer than shown

here and the number and thickness ofthe layers vary with the materials andfabrication techniques selected Energy

applied to one end ofthe component bounces along within the conductive layers and is selectively back-reflected and

filtered as it passes across the length of the component (hence the name distributed-feedback laser) leaving only a

very na1TOW wavelength range by the time it exits the end as laser light Altering the grating period allows the

wavelength to be tuned to a specific frequency and the signal is very stable, qualities that are extremely useful for

communications applications and the fabrication ofscientific instruments.

Fiber Optics Illustrated Dictionary

uted file system across partitions or across disks, in

such a way that a directory or set of files appears to

the user as though it were all in one place, thus

mak-ing it transparent, appearmak-ing as a smak-ingle logical unit,

even though the physical underpinnings are different

devices See redundant array of inexpensive disks

(RAID).

Distributed Management Task Force, Inc.DMTF

A trade organization promoting the development,

unification, and adoption of management standards

and initiatives for desktop, enterprise, and Internet

environments The organization seeks to enable

in-tegrated, cost-effective technologies to be used

effi-ciently rather than in a crisis mode

http://www.dmtf.org/

distributed network administrationDNA 1 A

wiring and services distribution scheme that divides

up the administration and actual physical connections

into smaller units, which may be one per department,

one per floor, or some other arrangement This is

con-venient for systems where there are significant

physi-cal limits on the length of individual transmissions

cables (as in some fiber optic installations), where

operations or maintenance personnel are assigned to

different sections of the building, or where different

types ofwiring are used for different departments or

areas of the premises See distribution, distribution

frame 2 Awireless services system in which

admin-istration is subdivided into particular regions or

sec-tions of the service (a local area, city, or state) This

is a common administrative system for nationwide

wireless systems that cover a large area of territory,

but through which many users roam from region to

region

distributed programming platformDPP A

pro-gramming platfonn connected to a network such that

the programming process is carried out transparently

over a number of machines or CPUs to whatever

ex-tent is desired Traditionally, programming has been

done on discrete platfonns or on connected platfonns

whereupon the programming process itselfis not

sig-nificantly different because of the connectivity A

DPP, on the other hand, is one in which the

connec-tivity to other processors, in a distributed fashion, is

exploited to take advantage of the increased power

and redundancy supplied by the distribution Thus,

attributes such as parallel processing, background

processing, fault-compensation, and other factors can

aid in the programming DPP also implies there is a

means to evaluate the performance in terms of

la-tency, reliability, and other functional trade-offs and

priorities

Distributed Queue Dual BusDQDB

Aconnection-less packet-switched network protocol with a

53-oc-tet cell and header/information structure somewhat

like ATM, used for telecommunications services in

Metropolitan Area Networks (MANs) DQDB is

de-scribed in IEEE 802.6 It is descended from Queued

Packet and Synchronous Exchange (QPSX) DQDB

supports isochronous and nonisochronous

commu-nications

DQDB employs theslotas the basic unit of data

transfer A slot is further subdivided into a one-octet Access Control Field (ACF) and a 52-octet segment Distributed Single-Layer Embedded(Test Method) DSE.InATM networking, an abstract method for test-ing a protocol layer, or sublayer, which is part of a multiprotocol Implementation Under Test (IUT) distribution1.Acabling tenn referring to the deliv-ery of services through cables and/or wires A distri-bution system is the combined media, connections, and topology that provide services through wires or cables, usually consisting ofelectricity, voice, or data network services Many different types of distribu-tion arrangements exist and are described through-out this reference See distribution frame, horizontal distribution 2 Apportioning, assigning, sending out, delivering of products, services, or computing pro-cesses

distribution frameAcentralized circuit management structure for creating, troubleshooting, and accessing

a variety of incoming and/or outgoing lines, some-times in the thousands if it is a commercial switch-ing system A distribution frame may be for supply-ing electrical power or may organize lines and con-nections for data communications Frames are often built into closets, floors, or ceilings, depending upon available space and whether frequent access is re-quired The frame may include blocks, conduits, and other physical structures to facilitate cabling distribution panelA grid-like frame, usually of metal, with rows and columns of punched out holes through which cables can be threaded and mounted These are often designed to fit standard 19-in rack mounts

distribution rollerSee stringing roller

District Telephone CompanyHistorically, one ofthe first entrepreneurial efforts to incorporate A Graham Bell's telephone invention into a commercial prod-uct and service George W Coy, Herrick P Frost, and Walter Lewis opened the District Telephone Com-pany in New Haven, CT in 1878 Initially there were only eight conducting lines strung up wherever it was possible, and fewer than two dozen subscribers The simple switchboard, designed and built by George Coy, was mounted on a table District Telephone be-came Connecticut Telephone and then Southern New England Telephone, in 1882

ditheringA term that the imaging field has adapted from physical appliances in scientific labs where dith-ering devices are used to shake up items to minimize clogging, settling, or the effects of friction In com-puter imaging, dithering is the blending and combin-ing ofpixels to simulate the effect of a shade ofgray

or a color that may not be available on the system divestTo dispossess of authority, property, or juris-diction; to take away, to deprive, to disinherit; to dis-mantle

divestitureThe act ofbreaking up, dispossessing, or otherwise dismantling an institution In the U.S le-gal system, divestiture is a process wherebylhe U.S Justice Department oversees the alteration of a company's organizational and asset structure in or-der to deconcentrate power or enforce fair competitive

curs when a company's activities are monopolistic in

nature or when a company has engaged in unethical

or illegal practices that provide the company with an

advantage The process ofdivestiture thus is intended

to re-establish safeguards to competitive access by

new or smaller contenders In the telecommunications

industries, the three most high profile Justice

Depart-ment actions against major vendors include the AT&T

divestiture in the mid-1980s and the investigations

into the competitive policies and practices

ofMicro-soft Incorporated and Intel Corporation At the time

of writing, no conclusions have been drawn, and no

divestiture proceedings have taken place, but

dives-titure is one of the possible steps the Justice

Depart-ment may follow if any of the allegations of unfair

practices are found to be true For infonnation on the

AT&T divestiture, see Consent Decree of 1982, Judge

Green, Kingsbury Commitment, Modified Final

Judgment

DL1 See distant learning, distance learning 2

dis-tribution list A list of parties receiving

correspon-dence, items, or processes Email and discussion

fo-rums are often handled through distribution lists

DLC1 Danish Lithosphere Centre 2 See data line

card 3 See Data Link Control 4 See Dead Link

Check 5 See Digital Loop Carrier.6.Disability Law

Center http://dlc-ma.orgl

DLCISee Data Link Connection Identifier

DLL1 See data link layer 2 See Dynamic Link

Li-brary

DLMdiode laser module See distributed-feedback

laser, laser diode

DLP Digital Light Processing See Digital

Micromirror Device

DLR1 See Design Layout Record 2 Deutschen

ZentrumfUrLuft- und Raumfahrt Gennan Air and

Space Center involved in planetary exploration and

other space sciences DLR supports educational

pro-grams, space forums, asteroid research, satellite

re-search, robotics, and more The DLR Asteroid

Re-search Team is known as DART

DLS1 See Data Link Switching 2 Distance

Learn-ing Section of the American Library Association &

Association of College and Research Libraries

3 Distributed Learning Subcommittee ofthe

Univer-sity of British Columbia within the Centre for

Edu-cational Technology 4 Division of Laboratory

Sys-tems ofthe U.S Centers for Disease Control and

Pre-vention

DLSwSee Data Link Switching

DLSWSee Data Link Switching Workgroup

DM See delta modulation

DMA 1 Dayton Microcomputer Association

http://www.dma.org/ 2 See Direct Marketing

Asso-ciation, Inc 3 See direct memory access 4 See

Document Management Alliance

DMCASee Digital Millennium Copyright Act

DMD 1 differential mode delay 2 See Digital

Micromirror Device

DMI 1 See Desktop Management Interface 2

Digi-tal Multiplexed Interface An AT&T interface that

Tl trunks and private branch exchanges

DML Development Markup Language See Inter-national Development Markup Language

DMS 1.data management system 2 decision man-agement systems 3 Digital Multiplex System A se-ries of programmable communications switches Northern Telecom (NTI) provides one ofthe switches

in common use in telecommunications, called the DMS-250 (there are other models as well) 4 dimin-ishing manufacturing sources 5 Division of Math-ematical Sciences of the National Science Founda-tion The DMS supports small-group projects aimed

at exploring and developing the properties and applications of mathematical structures http://www.nsf.gov/mps/divisions/dms/6. Dutch Meteor Society

DMSPSee Defense Meteorological Satellite Program DMTSee discrete multitone

DMTF1.Desktop Management Task Force 2 See Distributed Management Task Force, Inc.I

DMUXdigital multiplexer See multiplexer

DNSee Directory Number

DNA1.See Digital Network Architecture 2 See dis-tributed network administration

DNC1 direct numerical control Data connectivity between a computer processor and a computer-com-patible numerical control (NC) device 2 distributed networking computing See distributed computing

3.dynamic network control DNCFSee Directory Number Call Forwarding DNICSee Data Network Identification Code DNISSee Dialed Number Identification Service DNRDynamic Network Reconfiguration A feature

on commercial International Business Machines (mM)network that allows network address designa-tions to be reorganized without closing down the system

DNS See Domain Name System

DNSSE See Domain Name System Security Exten-sions

DOCSISSee Data Over Cable Service Interface Specifications

documentcolloq.In computerese, a text file in a for-mat specific to the word processor that created the file The.docfile name extension is often used to identify this type offile Document files in this sense

of the word are not the same as standardized ASCII text files Doc files provide fonnatting, font, and color information specific to the applications program that created the file One ofthe best formats for exchang-ing files between applications that are not intercom-patible is the Interchange format, also known as Rich Text Format (RTF) It holds more information than ASCII text files (font names and sizes, and text at-tributes such as bold and underline are retained), and

it is widely supported on many platforms and by ev-ery major word processor and desktop publishing pro-gram In fact, it would be great ifmore people would use RTF files on the Web for document downloads, instead of using word processor-specific document files that can't be loaded by many users

Fiber Optics Illustrated Dictionary

document camera A camera specially adapted for

photographing documents, usually for historical

pres-ervation, archiving, or replication This type

ofcam-era is mounted and optimized for a suitable focal

length and artificial lighting conditions It may be

combined with a bank oflights to illuminate the

docu-ments evenly It is typically attached to a stand so that

it can be readily raised or lowered like an enlarger

More recently, document cameras have been

equipped with electronics that enble the signal from

the camera to be fed into a computer system The

camera may be a digital still camera, digital

cam-corder, or analog camcorder.Inthis way, documents

can be digitized, stored directly on networks, or used

in videoconferencing sessions

Document Content Description DCD A structural

schema facility proposed in 1998 to the World Wide

Web Consortium for specifying rules for the

struc-ture and content ofXML documents DCD utilizes a

subset of the XML-Data Submission DCD defines

document constraints in XML syntax, using an RDF

vocabulary, much the same as traditional CML DTDs

HTML coders will recognize the basic angle-bracket

nested tag structure of a DCD However, rather than

the common paragraph, anchor, and image tags found

in HTML, the contents of the DCD tags include

ele-ment definition types and parameters The DCD

se-mantics are intended to be a superset ofXML DTDs

while maintaining conformance with RDF

document database Adigital repository ofdocument

information, usually consisting ofdocument images

which show documents as individual pages as they

might be viewed in print form, or individually

stor-ing the elements of the documents (pictures, text,

mathematical formulas, etc.), in such a way that the

information can be manipulated, stored, and retrieved

with word processors, drawing programs, etc The

design and programming of document databases

is a great challenge, as it is not always known how

the information might be used in the future It is also

becoming imperative to design systems that can

handle vast amounts of information yet still search

and retrieve the information with reasonable speed

document image management DIM Electronic

storage, access, processing, and retrieval ofdocuments

stored in image format Image format is a very

com-mon way to archive information that would othelWise

require massive space It means simply that text,

graphics, and everything else is stored together as a

picture, with no explicit differentiation oftextual

con-tent Microfiche is one of the systems designed to

store image directory lists and image "snapshots" of

newspapers,journals, documents, certificates, etc.,in

order to provide reasonably quick lookup and to save

storage space

document image processing DIP The process of

taking printed documents and converting them to a

visual digital form to be manipulated and/or viewed

electronically

Inthe past, converting text information to image

for-mat somewhat limited what could be done with the

images.Itwasn't possible to apply search/retrieval,

and editing to the text itself, because it was not in a form amenable to database software That is no longer true While work still needs to be done on document recognition systems, there are now programs which can take information stored in image format and process the characters into text; some can even rec-ognize which parts ofthe page are images, and which parts are text, in order to intelligently handle the con-tent of the page See document image management; zone, optical recognition

Document Management Alliance DMA Acompre-hensive standard for interoperability among elec-tronic document management systems, sponsored by the Association for Information and Image Manage-ment.Itis related to the ODMA, a standard for inter-operability between desktop applications and elec-tronic document management systems

Document Object Module An interface model for allowing programs and scripts to dynamically access and process documents independent of language or platform

Document Printing Application DPA A standard for printing within an open, distributed network en-vironment (ISO/IEC 10175) originally published in

1996 The Internet Printing Protocol (IPP) is a

"lighter" version with some ofthe functionality ofthe comprehensive DPAstandard

documentation Text and/or images that, taken to-gether, describe how a system works and how to use

it Good documentation goes beyond simply describ-ing system features and functions; it describes them

in such a way as to provide a context as towhenand

whythey should be used Documentation is often sub-divided into tutorial and reference sections This is generally a good idea, as tutorials are useful when you are first learning a system, and references are useful once you have grasped how to use a system The worst computer documentation is a manual that simply repeats and describes the contents of the menus The better ones tell youwhatthe program does, andhow, why,andwhenyou should do it DoD See Department ofDefense

DOD See direct outward dialing

DOH I See Department ofHealth 2 See Document Object Module

Dolbear, Amos Emerson (1837-1910)A Tufts Col-lege professor for almost three decades, beginning in

1874, Dolbear was an associate of Thomas Edison Dolbear was awarded a patent for an induction-based wireless telegraph in 1886 (filed in 1882) He also developed an electrostatic version of the telephone,

in the early 1880s, apparently improving upon the concepts of Philip Reis, butitlost out to the Bell phone in both utility and recognition

In the 1890s, Dolbear described discoveries related

to the effects oftemperature on the properties ofmet-als and their relationship to conductivity and stated

" that at absolute zero their electrical conductiv-ity becomes infinite, or, as it is more generally stated, the electrical resistance ofmetals becomes zero so it seems altogether probable that the qualities and states of matter so familiar to us as

temperature and that at absolute zero there would

be neither solid, nor liquid, nor gas, and that

elec-trical and magnetic qualities would be at a

maxi-"

mum

Dolbear wrote many magazine articles and books,

in-cluding The Art ofProjecting: A Manual

ofExperi-mentation in Physics, Chemistry, and Natural

His-tory, with the Porte Lumiere and Magic Lantern, in

1883, and Matter, Ether, and Motion, in 1894 The

Amos Emerson Dolbear Scholarship was established

in 1947 to aid students ofexceptional promise in

elec-trical engineering and physics The New England

Wireless and Steam Museum, Inc includes wireless

artifacts from Benjamin Dolbear, Amos Dolbear's son

Amos Dolbear - Physicist, Writer, Educator

Dolbear was an associate ofThomas Edison and a

talented inventor in his own right He described

im-portant aspects ofconductiVity and developed a

num-ber oftelephone devices.

Dolby-NR Dolby Noise Reduction A system

devel-oped by Ray Dolby to improve sound quality by

re-ducing noise Quality levels are designated with

let-ters (Dolby SR is professional level quality)

Dolby-NR is widely applied in the sound portion ofthe

mo-tion picture industry See THX

domain Sphere of influence or activity, province,

dominion; the set ofprocesses, items, or actions that

constitute a sphere of influence or activity There are

two aspects to the concept of domain in

telecommu-nications The first is recognition and access to and

from the domain How does the domain provide a

presence to other systems, and how do they

recog-nize and acknowledge its presence? The second is

what constitutes a domain, what is included within

the organizational grouping that is called a domain?

See domain identification and domain organization

domain identification In most data network

archi-tectures, a domain is perceived from outside a

sys-tem as a unique address or identifier describing the

logical and sometimes physical access points to a

system, since systems external to the domain frequently

elements within the domain

Since computers work with binary digits, the domain

on a computer network that allows it to be identified

and signaled by other domains is usually an address

that can be expressed as a numeral or alphanumeric series This address will often have a name or sym-bol associated with it, to make it easier for humans

to recognize and remember it Information about the nature of the domain, such as its basic function or geography, are sometimes expressed in the name For example, on the Internet the alphanumeric expression

of a domain name typically indicates the type of do-main (commercial, not-for-profit, educational, etc.)

and the location (.ca [Canada], au [Australia], etc.).

This naming scheme is not completely consistent, but

it is helpful in most circumstances It also is not a guarantee oflocation, as a system may remotely dial into a domain in another country See domain name, for further details on Internet naming, and domain name server, Domain Name System, firewall, host, Internet, server

domain name Aunique identifier, usually expressed

in alphanumeric characters, to a computer domain Local networks that are not connected to outside en-tities can setup this domain name in any way they choose or according to the domain naming param-eters of the network software that is running on the system On the Internet, however, which is a global distributed network, it is necessary to maintain an extensive database ofdomains in order that each one

is uniquely identified Thus, a domain name on the Internet must be registered with one of the assigned registration entities, the oldest ofwhich is InterNIC See domain name, Internet

domain name, Internet Aglobally unique identifier for a domain that is continually or occasionally online

on the Internet through an Internet Access Provider (lAP), which may provide additional services as an Internet Services Provider (ISP) This identifier is used by the system to locate a domain in order to send and receive files, email, messages, routing informa-tion, and other network traffic A certain flexibility

is inherent in this scheme in that levels below the lAP can be rearranged according to the needs ofthe local network

The domain name is actually a name resolved to an Internet Protocol (IP) address, which is composed of numbers, but the name is what is familiar to most users, and it is automatically converted by the sys-tem Once an Internet domain name has been

as-signed, such as ourdomain.org, subdomains can be

locally assigned, such as accounting.ourdomain.org administration.ourdomain.org sales.ourdomain.org

Similarly, usernames associated with email addresses can be expressed in a variety ofways:

max@sales.ourdomain.org bighoncho@ourdomain.org