Fiber Optics Illustrated Dictionary - Part 27 pdf

PCM is a very common means of converting analog to digital sig-nals and is widely used in telecommunications.In dif-ferential PCM, a transmitted digital signal is used to represent the d

Fiber Optics Illustrated Dictionary

serve as wavepass filters, bandpass filters that pass a

range of wavelengths, or notch filters that reflect a

range of wavelengths

Dichroic filter arrays can be assembled by lining up

multiple components on a flat or curved base

sub-strate and selectively coating the surface with

dich-roic reflective materials Thus, the surface could be

patterned to selectively filter not only certain

frequen-cies, but wavelengths coming from certain directions

over a specified area

Dicke, Robert H (1916-1997)Aninventor and

edu-cator, Dicke is known for his modeling of

gravita-tional forces, his contributions to aerospace research,

and his pioneer work in maser/laser technology

In1941, he joined MIT's Radiation Laboratory,

be-coming a Princeton professor a few years later.In

May 1956, Dicke submitted a patent application for

"Molecular Amplification and Generation Systems

and Methods" describing ways to generate and

am-plify electromagnetic waves, such as those in the mi-crowave frequencies, through the activity ofresonant gases (e.g., ammonia) Dicke's patent anticipates Fabry-Perot interferometers and cavity-resonating gas lasers Since an infrared light source is described

in some of the drawings, it is a pioneer laser device (U.S #2,851,652) Dicke's invention sought to estab-lish more efficient ways to generate coherent micro-waves or infrared micro-waves and to improve the cavity-resonating (amplification) effect

Dicke was a member ofprominent astronomical and physics societies and won numerous awards for his work See lavan, Ali; laser history; Townes, Charles Dicke radiometer Adevice developed by Robert H Dicke to detect the very subtle radiation residual from the Big Bang, the theoretical cataclysmic expansion of our universe

DID See Direct Inward Dialing

dielectric Nonconducting material that provides an Microwave (Maser) and Optical (Infrared) Patent Application - 1956

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These are excerptsfrom the diagrams that accompany Robert DickesMay1956patent submissionfor "Molecular Amplification and Generation Systems and Methods" describing a means to generate and amplify electromagnetic waves, both in the microwave (maser) and infrared (laser) frequencies through resonant gases such as ammonia acting in an open cavity resonator The patent is a milestone in the sense that it was rapidly followed by a number of interferometers and optical laser and gas resonating inventions The Dicke patent was published in September 1958 and may have inspired more scientists than have given Dicke credit for the concepts described •

On the left is an example ofa pumping system On the right, a resonating cavity with coherent infrared radiation aimed into the reflective cavity through switching valves (forerunner to Q switches) [U.S patent #2,851,652.]

of current On an atomic level, a dielectric exposed

to an electric field experiences slight changes that

result in a bound charge at the surface of the material

through polarization (as opposed to movement as

occurs in conducting materials)

Dielectric materials are often used as cable

shield-ings or applied in layers between sheets of

conduct-ing materials in condensers Common dielectrics used

over the decades include paper, cloth, air, Bakelite,

glass, ceramic, and certain synthetics Glass and

ce-ramic are the ones used in Leyden jars and utility pole

insulators See air dielectric, insulator

a dielectric (insulating) material will show

conduct-ing/sparking characteristics, usually expressed in volts

constant is the degree to which a material can be

po-larized which determines the bound charge

associ-ated with the material It describes how much an

elec-tric field is reduced within a dielecelec-tric material The

dielectric constant is related to the properties of a

material such as composition, density, homogeneity,

and temperature The bound charge of a dielectric

surface can be calculated by using Gauss's law

The dielectric or permitivity property of a material

influences the relative speed at which an electrical

signal will propagate The speed of the signal is

roughly inversely proportional to the square root of

the dielectric constant Thus, low dielectric constants

are associated with higher signal speeds and vice

versa As networking systems become faster and

more sophisticated, the dielectric constants of

fabri-cations materials becomes more important See air

dielectric, dielectric, Gauss's law

is a type of microwave lens that fits into the mouth

of an antenna waveguide It provides a wideband

al-ternative to scalar feeds

presence or proximity ofdielectric materials relating

to the storage and discharge ofenergy that may cause

interference or distortion to signal transmission

systems One solution to reducing undesired

dielec-tric interaction is to use air dielecdielec-tric cables in which

energy-interacting materials are held away from

con-ducting materials by a cushion ofair

as a twisted-pair wireline with two wires

transmit-ting the same data at the same time, except that one

is transmitted as a positive(+)signal and one is

trans-mitted as a negative (-) signal It is sometimes also

called a balanced cable Since noise is typically

in-troduced along the transmissions path, especially over

longer distances, there will be slight changes in the

signal at the receiving end The "difference" is then

taken between the two signals on the two wires in

order to eliminate the portion associated with the

noise Twisted pair is used to cause the two separate

lines to occupy, as nearly as possible, the same

physi-cal space along the transmissions path This system

enables longer cable lengths to be used (e.g., RS-422

(e.g., RS-232 serial specification)

Global Positioning Service designed to improve lo-cal accuracy of the data One or more high-end GPS receivers are placed at known locations where they receive GPS signals These become reference sta-tions, which estimate the variations of the satellite range measurements, forming corrections for GPS satellites within current view and then broadcasting the correction information to local users See local differential GPS

modu-lation based upon the detected state of the previous instant, rather than on an absolute predefined param-eter See delta modulation for an example of a com-monly used, simple type of differential modulation

relative modulation in which the previous state ofthe carrier signal phase is detected, and the subsequent state is based on the previous, rather than on an ab-solute predefined parameter See phase shift keying

relay in which the armature is polarized by contact with a permanent magnet and is operated by the dif-ference in the strength of the currents The direction

of the currents constantly changes and can be con-trolled with a pole changer

ana-lytical tool for extracting secret keys from crypto-graphic devices using statistical analysis and error correction techniques Since a large percentage of sensitive computer communications is stored on me-dia that may be carried about (portable computers, smart cards, etc.) or are transmitted over wireless networks, which can be intercepted by eavesdroppers, analytical tools for evaluating cryptographic integ-rity are important for developing and testing new technologies

DPA can be used to attack a system to try to discern encoded information using hardware that is readily available While many technologies are resistant to Simple Power Analysis (SPA), Differential Power Analysis can break many systems that are immune

to SPA attacks As is often the case, there may be a trade-off in time and power; a DPA attack may take longer than a SPA attack DPA has been put into prac-tical application by Paul Kocher and Cryptography Research

of sampling a signal, subdividing it, and assigning values to the individual parts (quantization) in order

to add this information to a carrier signal This modu-lation can be done in a number of ways, and not all PCM transmissions are compatible PCM is a very common means of converting analog to digital sig-nals and is widely used in telecommunications.In dif-ferential PCM, a transmitted digital signal is used to represent the difference between consecutive analog signals These differences are obtained by using a fixed quantization step size See quantization, pulse code modulation

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Fiber Optics Illustrated Dictionary

differential quadrature phase shift keying

DQPSK In general, quadrature phase shift keying

(QPSK) is a modulation scheme in which four

sig-nals are used, each shifted by 90 degrees, with each

phase representing two data bits per symbol, in

or-der to carry twice as much information as binary

phase shift keying DQPSK is a subclass in which the

difference between the current value ofthe phase and

the previous value of the phase are used instead of

the absolute value of the phase See modulation,

quadrature phase shift keying

differential Ziv-LempeldifiZL A text compressor

combining Lempel-Ziv compression and arithmetic

coding with a form ofvector quantization, described

in 1995 by Peter Fenwick This compression scheme

is similar to Lempel and Ziv's original LZ77 scheme

but without explicit phrase lengths or coding for

lit-erals It combines dictionary compression and

vec-tor quantization by using a standard scan to detennine

the longest earlier phrase to match ensuing text to

create a reference phrase A phrase includes a

posi-tion code, sequence of zero symbols, and

terminat-ing nonzero symbol which is processed through

arith-metic coders for displacement and data Coding

pro-ceeds until an unexpected character is encoded;

dis-placement coding accounts for most of the

com-pressed output stream

Performance of diftzL compares reasonably well

against LZB and LZ3VL The unique characteristic

of diffZL is that it has no explicit phrase length or

literal encoding The development of diffZL leads,

in part, to the suggestion that the limits ofLZ77

com-pression may be about 3.0 bitslbyte, a limit that has

very nearly been reached See Lempel-Ziv

Diffie-HellmanA fairly fast public key encryption

system described by Whitfield Diffie and Martin

Hellman in a 1976 IEEE issue ofTransactions on

Information Theory entitled "New Directions in

Cryptography." This concept has since been

incor-porated into many encryption schemes, including

some Cellular Digital Packet Data (CDPD) systems

and the well-known Pretty Good Privacy (PGP)

program developed by Philip Zimmermann While

the inventors patented the system, it came under

dis-pute because ofits public disclosure prior to the patent

application See Hellman-Merkle, Pretty Good Privacy

diffractionThe reflection ofwave or wavelike

phe-nomena as they encounter an obstacle that is in their

line of travel can cause a complex interaction called

diffraction as the incoming waves are reflected away

from the obstacle and somewhat towards the

incom-ing wave (dependincom-ing upon the angle of incidence)

If the obstacle causing the diffraction is fixed and

ordered and the diffraction phenomenon is narrow

and homogenous, the diffraction pattern may be more

easily studied and exploited

As an example, some ofthe sound waves emanating

from an audio speaker component may hit the

inte-rior ofthe speaker cabinet and reflect back across the

original sound waves, causing a complex pattern in

the sound waves where they interact Some of the

waves hitting the cabinet interior will be absorbed and

some will be diffracted, depending upon the shape and composition ofthe cabinet, the distance from the speaker to the cabinet interior, and the reflectivity/ absorbancy of its construction materials

Diffraction may have both positive and negative con-sequences In acoustics, diffraction can sometimes create a more complex or interesting sound, but it may also cause unwanted interference, depending upon the nature and magnitude of the diffraction

Similar effects occur in the movement and interac-tion of electromagnetic phenomena When light en-ergy encounters an obstacle, a fringe pattern may re-sult from the light diffracting from the obstacle as it reflects back from the reflecting obstacle This fringe pattern may be seen with scientific instruments and can provide information about the character of the light and the obstacle When viewed with optical in-struments, diffraction patterns tend to appear ellipti-cal

In Fresnel lenses, diffraction through successively angled prism-shaped projections serves to concen-trate light.Inrear projection systems, diffraction helps display an image over a large surface

Diffraction patterns in crystal structures have inter-esting properties due to the lattice arrangement ofthe atoms When hit with a stimulus (e.g., collimated X-rays), some of the atoms in the upper surface will re-flect back the beams, while some beams travel through the upper surface to reflect from the next level in the lattice, etc., creating a complex but pre-dictable diffraction pattern that was first described mathematically by W Lawrence Bragg in the early 1900s

In some cases, diffraction can be acoustically or op-tically controlled to produce complex effects or fil-tering mechanisms See Bragg's law, diffusion, dif-fraction grating, dispersion, Fresnel lens, spot of Arago, Wood anomaly

diffraction grating Acomponent designed to diffract electromagnetic or acoustic phenomena as they im-pact a grating component, which is often a corrugated structure In optics, a diffraction grating can help reveal some of the characteristics oflight and can be harnessed to provide some control over the direction

of diffracted light and the wavelengths affected by its shape, its period (distance between corrugations), and its materials This is useful in filtering wave-lengths and is important in the development of nar-row-wavelength lasers and fiber optic filament switching assemblies See Bragg grating, diffraction, dispersion, grating equation

diffraction ordersAnordered set of reflection pat-terns associated with an ideal diffraction grating that diffracts incident radiant energy in discrete directions

at angles that can be calculated See grating equation

diffraction, FraunhoferDiffraction in situations where the factors that influence the complexity ofthe diffraction patterns are somewhat controlled For ex-ample, a light source should be a sufficient distance from the reflecting obstacle and the beams monochro-matic and parallel in order to create planar waves that can be easily observed (somewhat like surface waves

known asfar-field diffraction.Because laser beams

can be collimated more readily than other wave

sources, they are favored for this type of study

Diffraction Types and Light Sources

B I~~§:::::::=:::::::::=::::::::::~"o/

Several basic concepts in diffraction light sources

are illustrated here Imagine light beams generated

by lasers that are aimed at obstacles set at distances

that arefar or near related to the breadth ofthe beam.

A collimated laser beam can provide planar light

suit-able for diffraction studies (A and B) When the

par-allel beams strike a planar obstacle sufficiently

dis-tant, they are reflected back in ways that are relatively

straightforward to observe and predict with

math-ematical calculations This is called far-field or

Fraunhofer diffraction (As with most things, its

ac-tually a little more complicated Since laser light is

readily planar, you can place the obstruction closer

and still get good results, but you will probably have

to compensate by putting the viewing optics farther

away.)

Laser light can also be used to provide a point light

source that creates a spherical light beam when a lens

is interposed to focus the beam on a small hole(C)

through which it exits When the spherical light beams

encounter an obstacle, especially one close to the light

source, the reflection pattern and its interaction with

incoming lightwaves is more complex (D), as is the

resulting diffraction pattern This situation is called

near-field or Fresnel diffraction.

chal-lenging to differentiate and calculate than basic

Fraunhofer diffraction due to factors that complicate

the interactions ofthe incoming waves and those that

are reflected by an obstacle, such as nonparallel

waves, or the close proximity of source and refracted

waves is called Fresnel diffraction ornear-jield dif

fraction.

view-ing ofdiffraction patterns in a selected area ofa

speci-men, usually as viewed through an aperture for this

purpose This is a means to narrow the field of

inter-est, for example, to view a single-crystal pattern For

even smaller views, such as a single particle, it may

be necessary to use microdiffraction, with tradeoffs

in precision due to optical limitations See diffraction

uniform lossless reflectance over a wide spectrum equal to unity (one) While no physical diffuser has perfect diffusing characteristics, there are some ma-terials that have excellent diffusing properties and thus are useful as references for calculations, lab work, etc Barium sulfate (BaS04)is a material with excellent diffusing properties useful as a reference

~:::g ;~~~~~~~a~:~~~~~~o~~;~~:ei~~~~~11 Lambertian diffusers to study their influences on sur-rounding objects or to create certain lighting effects

See diffusion, isotropic antenna

laminated with plastic film, or sand blasted, to "break up" the incoming light in terms of its direction of travel such that the exiting light has a smoothly vary-ing quality Perceptually, diffused light looks "soft" and even rather than bright and narrowly focused

Sometimes a second type of clear or colored glass is used as a laminating layer in the fabrication of dif-fusing glass Thus, the exiting light emanates in many directions as it passes through the diffusing layer as-sociated with the glass

Small-diameter glass fiber filament arrays may be used in place of a sheet of glass, the filaments acting

as prisms to diffuse light evenly

Diffusing glass is useful for scientific instruments, lab experiments, projectors, interior lighting, windows (especially skylights, which are typically small), and other applications where a more uniform distribution oflight is desired Diffusing plastic is sometimes used

in place of diffusing glass (e.g., in photo finishing)

to reduce cost or weight Light diffusion tends to in-crease scattering loss and glare Scattering losses can

be minimized by placing a reflector around the dif-fused light to reflect back up to about 90% ofthe light

in the desired direction, as in a car headlight

Materials can be tested for their diffusion character-istics with a spectrophotometer or modeled with ray-tracing computer algorithms Either way has benefits

Physical testing gives a real-life measurement but may be cumbersome in terms oftime and assessment, especially when evaluating unevenly diffusing ma-terials over a wide surface area Algorithmic testing can save time if many measurements are required or the surface area area is large or uneven, but is only

as accurate as the theory and programming inherent

in the software

The fabrication ofdiffusing glass must be carried out such that laminated structures or coatings are tightly bonded to the glass with materials with matched (or othelWise appropriate) refractive indexes to minimize loss at laminating seams Bonding materials must also

be transparent, to let the light pass into the next layer ofthe laminate, and must be appliedinaway that keeps out air bubbles or particles

Since loss at joints and through materials with vary-ing refractive indexes is characteristic oflayered com-ponents, there have been suggestions for ways to in-corporate the diffusing structures into the lens itself, whether this be a glass lens or a fiber optic lightguide

Fiber Optics Illustrated Dictionary

Monolithic Lens/Diffusing Components Diffusion Configurations

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Some examples ofmonolithic diffusion lenses, that

is, lenses that incorporate the diffusing microsurfaces

in the lenses themselves rather than relying upon

lami-nated coatings or plastics to provide a separate

dif-fUsing layer The advantages ofmonolithic components

include longer lifespan (coatings can peel away) and

lower light losses from bubbles, particles, or

differ-ing defractive indexes [Shie et al., U.S patent

#6,266,476, July 2001.}

Shie et al have developed a number of designs for

embodiments of monolithic diffusing elements that

overcome some ofproblems oflayered fabrications

diffusion At the molecular level, a net transfer of

mass due to random molecular motion caused by

con-centration gradients in the diffusing material

Mol-ecules will move from regions ofhigh concentration

to low, a process that continues (unless interrupted)

until the concentration reaches a state ofequilibrium

As an example, sugar stirred into a hot cup of coffee

diffuses through a process called osmosis until there

is an even suspension ofthe sugar dissolved

through-out the liquid Certain ions are known to have higher

diffusing properties than others due to their

molecu-lar motion These properties can be exploited to

cre-ate mcre-aterials for facilitating diffusion in scientific

experiments or commercial products

Anisotropic diffusing medium is one that is not

con-sidered dependent upon the direction of motion or

Some example configurations for incorporating a specially fabricated lens with a diffusing microstruc-ture as it might be used in the path ofa light beam to homogenize (diffuse) and propagate the beam in the desired direction(s) In this case the lens and diffu-sion surface are integrated, but the general concepts may apply to certain laminated structures, as well [Shie et al., U.S patent#6,259,562,July 2001.J

orientation of its motion and thus would diffuse evenly in all directions from the point of reference (see isotropic antenna) An anisotropic diffusing medium is one in which direction or orientation would influence its diffusion properties

In the broader sense, diffusion refers to changes in the direction of travel of a constrained or point radi-ant energy source (e.g., a beam of light) such that it spreads over a wider area The amount of diffusion that occurs in any given situation is dependent upon the angle of incidence of the radiant energy source,

Laminated Diffusing Glass

Digital and Analog Clock Examples

On the alarm clock on the left, a digital readout shows the time incremented in minutes On the right,

an analog display with a 'sweep hand'travels through the minutes and seconds in its arc in a continuous movement.

ing, and variously encoding this digital information Most communications systems prior to the 1970s were analog, but the trend is strongly towards con-verting analog signals into digital signals Digital systems allow a far greater degree of control, secu-rity, compression, noise control, and modifiability compared to analog systems

A simplistic explanation of the difference between digital and analog is often illustrated with watches

An analog watch has a hand that sweeps around in a

3600 arc, showing hours and minutes and the posi-tions in between A digital watch has a readout that displays the time incrementally, usually in one sec-ond or one minute increments

Another example is an analog dial on an AM radio that allows the tuner to be gradually adjusted through adjacent stations As the dial moves, the signal vol-ume increases and decreases, and there may even be periods where multiple radio signals overlap This type of radio dial does not allow the listener to jump directly from a low frequency station to a high fre-quency station With digital pushbuttons on AM and

FM radios, the tuner can be set to jump to a specific frequency, and stations can be selected in any order, even if they are not in adjacent frequency ranges

Inan analog phone system, the phone equipment con-verts sound waves into electrical signals that are sent through the lines between the conversing parties, but

it is also possible to encode the conversation as a digi-tal signal and send it through a computer network or computerized phone system Digital encoding allows the information to be compressed, modified, stored for later retrieval, or sent in conjunction with other data signals (such as a computer data file transfer) at the same time See analog, ISDN, quantization, voice over ATM

devel-oped by Sony It is a hybrid transitional format that records digital video signals onto Hi8mm tapes See D-VHS, Hi8mm

A technology for reconfiguring a circuit, without

coating

glass

ant energy, and ambient environmental conditions

(e.g., humidity)

Diffusion is characteristic of other propagating

sources besides radiant acoustic or electromagnetic

energy A drop ofwater hitting a flat surface straight

on will diffuse in a more or less even pattern, as it

spreads from the initial point of contact See

diffrac-tion, diffusing glass, Fick's first law

By coating a glass surface with chemicals or

plas-tic film laminates, it is possible to alter the course of

travel ofa light beam to spread it over a wider area,

thus providing more even illumination over a broader

area This is usefUlfor microscope stages, for example,

where even lighting aids in reducing contrasty

point-light shadows, thusfacilitating sample observation and

imaging.

ex-pressing the transfer rate of diffusing atoms through

molecular movement from the point at which they

diffuse This provides a way to evaluate or model the

different diffusing characteristics of media and

en-ergy stimuli See Fick's first law

at-oms in a semiconductor upon stimulation by heat 2

The process by which dopants are introduced into a

semiconductor medium to create p-n junctions,

con-duction channels, and other useful structures

di-gest access

digA Unix command providing information about

computer network domain names

dis-crete units, rather than as a continuous stream In

communications technologies, information is

typi-cally represented in terms of binary units There are

many ways to represent information in a binary

sys-tem: on/off, high/low, large/small, changed from

pre-vious state, loud/soft, fast/slow, lit/unlit, up/down,

present/absent, etc In digital computing, the binary

units are usually ones and zeros.Inelectronic circuits,

the units are often represented electrically by on/off,

high/low, or change from previous state Despite the

simplicity ofa binary system, it is powerful and

flex-ible, and extremely sophisticated processes and

Fiber Optics Illustrated Dictionary

manually changing the interconnections It is

simi-lar to a multiplexer, except that changes can be made

with software, rather than through physical rewiring

Digital Advanced Mobile Phone Service See

DAMPS

digital audio broadcastingDAB A transmission

modulation technique that sends digital rather than

analog audio signals One of the more interesting

developments in DAB is that impediments that cause

interference to analog signal transmissions (e.g.,

mountains) are used as reflectors in digital signal

transmissions to improve reception In the process,

the best regional transmitter is automatically selected

to forward the signal to local listeners Thus, a

regu-lar antenna, like the kind commonly mounted on a

car, can be used to receive distortion-free

program-ming with excellent sound quality

DAB receivers have been commercially available

since 1998, and as oftheturnofthe century there were

more than 200 million DAB users worldwide, a

num-ber that is expected to grow substantially

DAB signals are individually coded at the source,

error protected, and time-interleaved in the channel

coder The services are then multiplexed in the Main

Service Channel (MSC), combined with Multiplex

Control and Service information, and transmitted

through the Fast Information Channel (FIC)

Trans-mission frames are formed in the TransTrans-mission

Mul-tiplexer The DAB signal is shaped through

Orthogo-nal Frequency Division Multiplexing (OFDM) and

transposed to the selected radio frequency, where it

is amplified and transmitted At the receiving end, the

digital signal is demodulated and channel decoded to

process and error-correct the signal and then fed to

the listener's radio device Left and right audio

chan-nels are produced by processing through an audio

decoder, data information is processed as is

appro-priate if textual or other enhanced features are

in-cluded, and the final product presented to the user

DAB is not restricted to audio broadcasting; the

con-cept is applicable to a variety of types of

program-ming, which means car radios, for example, could

display images and textual information (song titles,

artist names, lyrics, etc.) in conjunction with the

ra-dio programming Given these characteristics, it's not

surprising that there is much excitement in the radio

industry about the potential ofthis new digital

broad-casting technology Within the next few years, it is

likely to revolutionize the face and form ofradio

pro-gramming Currently DAB is established as a

stan-dard for terrestrial broadcasting, but it has been

de-veloped with an eye toward future satellite

broadcast-ing as well See Digital Audio Broadcastbroadcast-ing Forum

Digital Audio Broadcasting Forum, DAB Forum,

World DAB ForumA consortium dedicated to the

commercial implementation of the DAB Standard

through international coordination and cooperation

among official governmental and regulatory

authori-ties, manufacturers, network providers, and data

broadcasters

World DAB was initiated in Europe, but the

technol-ogy developed by the consortium is being

imple-mented worldwide In June 2000, the Asian DAB Committee was established during a conference in Singapore, and a cooperative Memorandum of Understanding was signed between the WorldDAB and the Singapore Broadcasting Authority

http://www.worlddab.org/ http://www.asiadab.org/

Digital Audio Broadcasting Standard, DAB Stan-dardAnITU standard for digital broadcasting de-veloped by the Eureka 147 Consortium, an interna-tional group of broadcasters, consumer electronics developers and scientists, and radio network opera-tors The work on the DAB Standard culminated in January 2000 and went onto the next stage, commer-cial implementation with the Technical and Commer-cial Committee of the Digital Audio Broadcasting Forum See digital audio broadcasting

http://www.eurekadab.org/

digital audio radioDAR A new audio broadcast technology that provides high quality sound over the airwaves and a wider selection of regional program-ming.Italso integrates with various news, paging, and email services There has been talk of putting DAR in the S-band, but a number of technical char-acteristics of DAR indicate this may not be the best choice.In1995, the Federal Communications Com-mission (FCC) assigned a frequency spectrum for DAR use See digital audio broadcasting

Digital Audio Radio ServiceDARS A high-qual-ity-audio satellite-delivered radio programming ser-vice Thus, listeners in remote areas who are without terrestrial broadcasting services or those who wish

to receive higher-quality audio than might be avail-able through a local analog-based station can receive high-quality radio transmissions

Technologies change, so these numbers may change

as capabilities improve, but during the mid-1990s, at least 12.5 MHz of radio frequency spectrum was needed to support a commercial DARS system Since only 25 MHz of spectrum had been allocated to DARS in 1997 (2320 to 2345MHz),when licenses were auctioned by the Wireless Telecommunications Bureau(WTB)ofthe Federal Communications Com-mission (FCC), only two licenses were made avail-able to four applicants that filed by 1992 The auc-tion bids for these licenses approached $90 million each and were finally awarded to Satellite CD Ra-dio, Inc and American Mobile Radio Corporation The FCC stipulated that winning applicants must have a satellite operational within four years, with the system as a whole operational within six years

digital audio tapeDAT Ahigh-quality, high-capac-ity digital audio recording format suitable for high quality digital audio recordings and computer data storage For audio recordings, the sound is sampled, quantized, and converted to a specified encoded for-mat The encoding includes error checking mecha-nisms and tracking information to facilitate search-ing for a particular location on the tape DAT became popular in Europe in the early 1990s, but American vendors were so concerned about audio piracy on DATs that they effectively blocked the spread of the technology in the United States DAT is now used to

Some technologies incorporated into DAVIC are the

intellectual property of the contributors; they have

agreed to make the technology available to anyone

for free or for reasonable royalty fees

http://www.davic.org/

digital bank, digital channel bankSee D bank

digital bearing discriminatorDBD A digital

RF-based system used in direction-finding applications,

often with microwave frequencies In conjunction

with a radio-frequency receiver, it enables the

direc-tion of arrival of incoming signals to be estimated

A DBD can be configured by creating a circular

ar-ray ofopen direction finding receivers, thus enabling

a full 360-degree field of view and a high

probabil-ity ofintercept, without the need to rotate the antenna

or to guess at the incoming direction in order to

ori-ent the antenna correctly Each elemori-ent in the array

feeds individually into the network where the

incom-ing data can be processed for spatial modelincom-ing

digital camcorderA digital camera capable of

cap-turing and storing information at a rate that is fast

enough to create a series of digital frames which,

when played back, show full motion video See

cam-corder, digital video,drycamera

digital audio-visualDAV Digitized audio/video data

that typically bypasses a computer's main bus

Digital Audio-Video CouncilDAVIe A nonprofit

association established in 1994 in Geneva,

Switzer-land, to promote global open interfaces and protocol

specifications (DAVIC specifications) in audio-visual

applications and services There are over 200

mem-ber companies from more than 25 countries

world-wide, representing manufacturing, service, research,

and government agencies

DAVIC concerns itself with the specification and

development of tools rather than systems, with a

fo-cus on identifying and specifying components which

are relocatable on a specific platfonn, and which are

also cross-platform

The DAVIC 1995 specification recommends SDH!

SONET as the core network physical layer to which

ATM cells, as standardized by various international

bodies, can be mapped Timing involves the use of a

transmit clock derived from the network Jitter is also

managed with the network clock as the reference

clock There are five main entities within the

speci-fication, as shown in the chart

Abbrev Notes

cellular telephone communications in which the voice conversations are sampled, quantized, and encoded for transmission This permits increased security, pri-vacy, capacity, better handling of noise interference, and corrective processes when roaming across cells digital certificateSee certificate

Digital Command Control Standards, DeC Stan-dards Aset ofNational Model Railroad Association (NMRA) standards for packet-based digital com-mand control The DCC standards encompass the format ofinformation that is sent via digital command stations to digital encoders using NMRA digital signals Digital Communications ConferenceDCC.An an-nual communications conference jointly held by the

ARRLand TAPR radio associations It isan interna-tional forum for beginning and expert amateur radio operations to discuss digital communications tech-nologies and their applications

digital cross-connectDXC A centralized network component for aggregating and interconnecting a variety of digital signal links, ranging from a few to several thousand For example, a multibank cross-connect running at44Mbps can carry almost 700 tele-phone voice channels per line DXCs are used in wired, fiber optic, and digital radio systems SONET ring-based optical interfaces for DXCs were intro-duced in the mid-1990s, by Lucent Technologies DXCs typically have interface ports for remote ac-cess and monitoring Smaller units are often rack mountable Larger units may be floorstanding and may include cable management enclosures and prewired fuse panels See add/drop multiplexer Digital Data Communications Message Protocol DDCMP A station-to-station, byte-oriented, link-layer protocol developed by Digital Equipment Cor-poration (DEC) in the early 1970s that was used to develop DEC's network architecture as a processor-to-processor system (DECnet) DDCMP continued to evolve and had reached PhaseIVby the mid-1980s DDCMP functions over half- and full-duplex syn-chronous and asynsyn-chronous channels in point-to-point and multipoint-to-point modes It provides management

of the physical channel, message sequencing, and data integrity DDCMP has been implemented on a wide variety of types of systems, including worksta-tions, personal computers, and robots

Digital Equipment Corporation DEC A well-known computer hardware/software/services com-panywhich was established in the 1950s by Kenneth

H Olsen DEC is perhaps best known for its PDP minicomputer series, the subsequent VAX series (VMS and UNIX operating systems), and the DEC Alpha Many universities are equipped with VAX machines In 1998, DEC was bought by Compaq, one

of the leading makers of desktop computers See Compaq

Digital European Cordless Telecommunications DECT Now called Digital Enhanced Cordless Telecommunications.Anorganization and wireless standard developed in Europe and adopted by the Eu-ropean Telecommunication Standard Institute (ETSI)

Connects CPS

to SPS Connects SPS

to SCS

Content Provider System CPS

Service Provider System SPS

Service Consumer System SCS

CPSSPS Delivery System

SPSSCS Delivery System

-Entity

Fiber Optics Illustrated Dictionanj

in 1992.Itwas originally proposed as a unifying

digi-tal radio standard for European cordless phones It

has since been adopted by other countries, including

Britain and some Asian countries The DECT

stan-dard improves on previous technologies by

support-ing two-way callsupport-ing as well as better mobility

Open Systems Interconnection (OSI) principles have

been incorporated into DECT in the sense that it

con-sists of a physical layer, a data link layer, and a

net-work layer

DECT is implemented with transceiving base stations

and mobile handsets As it is optimized for

capabili-ties different from those developed for cellular, it

re-quires more cells to be used in a manner similar to

cellular, due to the low power signals of DECT, but

higher densities are then also possible

DECT incorporates handover capabilities and

Dy-namic Channels Allocation (DCA) instead of fixed

channels, with the hand unit scanning for the best

signals

Digital HDTV Grand AllianceA consortium of

major U.S and European entities with a stake in

con-sumer electronics and broadcast television

technolo-gies The Alliance was founded in May 1993 as a

re-sult ofwork by the Advisory Committee on Advanced

Television Service (ACATS) In order to streamline

the process of development and testing of advanced

television (ATV) systems, the Alliance was tasked

with taking the best of the best proposals evaluated

and researched by ACATS and combining them into

one superior technology, without making the

speci-fication too rigid or industry-centric ACATS

contin-ued to advise the Alliance through the ACATS

Tech-nical Subgroup which was divided into six Expert

Groups

Based on the work of the Alliance and ACATS, an

advanced system proposal was tendered in late 1993

and early 1994 which was subsequently approved for

prototyping, construction, and laboratory and field

testing in 1995 The results were communicated in

the ACAT Final Technical Report Requests from the

Federal Communications Commission resulted in the

addition of standard definition television (SDTV)

multiple stream scanning formats being adopted for

inclusion in the ATSC Digital Television Standard

See Advisory Committee on Advanced Television

Service, ATSC Digital Television Standard

Digital Loop CarrierDLC Similar to a Local Loop

Carrier, which provides a physical connection

be-tween subscribers and a main distribution switching

frame, except that the DLC is committed todigital

services over twisted-pair copper phone wires The

DLC is a system of switches and multiplexers which

concentrates low-speed services prior to distribution

through a local central switching office or controlled

environment vault (CEV) By multiplexing signals

up to a local terminal where it then splits to provide

service to subscriber pairs, the cost of wiring can be

reduced DLC systems were developed in the early

1970s See Next Generation Digital Loop Carrier

Digital Micromirror DeviceDMD A spatial light

modulator semiconductor technology that has been

incorporated into high-resolution Digital Light Pro-cessing (DLP) display devices Tiny mirrors, only 16 microns square, can be organized in rows and col-urnns to form the basis for a high-resolution display DMD displays from Texas Instruments combine a CMOS SRAM with a movable micromirror mounted over each memory cell corresponding to a pixel on the display The light is pulse-width modulated inci-dent to the mirror by electrostatic forces controlled

by the cell data Filters or color wheels can be used

to create color

DMD was developed by Texas Instruments (II) and first demonstrated by II and Sony in the mid-1990s

as a digital high-definition display system DMD-based projection technologies are designed to be scal-able and intended to provide images superior to cath-ode-ray tube (CRT) and liquid crystal diode (LCD) DMD technology also has applications in the digital color printing field

Digital Millennium Copyright ActDMCA AU.S act enacted in 1998 through a comprehensive reform ofU.S copyright law to encompass changes brought about by evolutions in electronics that effect the cre-ation of original digital works The DMCA was also

a step in the preparation for ratification of the inter-national World Intellectual Property Organization (WIPO) treaties

digital multiplexerA system for aggregating or in-terleaving two or more digital signals, so they can be carried over fewer transmission lines, and sometimes also to aid in synchronization of multimedia appli-cations that may require more than one signal (e.g., audio and video for videoconferencing) The signal

is frequently demultiplexed at the receiving end in order to separately handle the various component sig-nals See digital cross-connect

DigitalNetwork Architecture DNA.I An architec-ture that incorporates many aspects of the Open Sys-tems Interconnection (OSI) model used by Digital Equipment Corporation (DEC) to develop applica-tions 2 Acommercial network system from Network Development Corporation

Digital Performance ArchiveDPA A means for tracing the rapid developments in digital technologies

as they pertain to performance arts activities The ar-chive is managed within the Arts and Humanities Research Board of the University of Salford, Manchester, U.K

Digital Private Network Signaling SystemDPNSS Astandard for integrating private branch systems with E1lines DPNSS was originally developed as an open standard by British Telecom pic and U.K PINX manufacturers, in the 1980s In the early 1990s, DPNSS and ISDN internetworking was described, followed by DPNSS and Signaling System No l.

Open documents are available from British Telecom

Digital Research Inc.DR Originally called Inter-Galactic Digital Research, Digital Research was founded by Gary Kildall and his wife at the time, Dorothy McEwen Gary was the developer of CP/M (Control Program for Microcomputers), a popular text-based operating system for microcomputers DR

CP/M-80 GEM, the DR graphical operating system

predated working versions ofMicrosoft Windows by

several years, and DR-DOS was often described by

reviewers and users as superior to MS-DOS

DR's efforts were not limited to software In 1984,

the company released an expansion board for Intel

8088-based personal computers that allowed four

ter-minals to be networked to a PC using standard

RS-232 With Concurrent PC-DOS, it provided the user

the ability to run up to four MS-DOS or CP/M-86

applications concurrently, along with the program

running on each individual terminal

Over the years, Digital Research introduced many

basic desktop computing and networking tools that

have become intrinsic to the industry The company

was purchased in the 1990s by the Novell

Corpora-tion, who subsequently transferred DR-DOS to

Caldera who released it as OpenDOS Unfortunately,

Kildall, who pioneered so many fundamental

contri-butions to the microcomputer industry, was found

dead at the age of 52 See CP/M; Graphics

Environ-ment Manager; Kildall, Gary

digital selective callingDSC A synchronous

trans-missions system developed by the International

Ra-dio Consultative Committee (CCIR

Recommenda-tion 493) DSC is the basis for the Digital Selective

Calling (DSC) communications service that provides

automated access to coastal stations and marine craft

Four priority levels have been established, from

rou-tine to distress, with distress calls receiving priority

handling and, in the U.K., routing to a Rescue

Coor-dination Centre (RCC) Alarms may be associated

with incoming distress calls received on marine craft

DSC calls include the caller identity, phasing signals,

and error-checking signals A dot pattern alerts

scan-ning receivers of a call about to be received Ships at

sea are required to maintain continuous DSC watches

at designated frequencies The GMDSS Master Plan

and the lTU List of Coast Stations list the DSC

dis-tress and safety call frequencies

Digital Short-Range RadioDSRR Initially

per-ceived as an easy-to-license evolutionary heir to

pub-lic domain Citizen's Band radio services, DSRR has

since become a commodity for offering commercial

services within the business community in North

America and a venue for offering remote public

ser-vices in Europe It incorporates digital radio

commu-nications technologies for small low-range portable

radio sets and has become an ETSI and TETRA

stan-dard for low-cost radio communications DSSR

op-erates in single- and double-frequency repeater modes

in 933-935-MHz and 880-890-MHz bands In North

America and Australia, the double-frequency band is

reserved for AMPS cellular services

Enhanced Digital Short-Range Radio E-DSRR was

introduced through the RACE MOEBIUS project,

designed to make use of the INMARSAT HSD

Sat-ellite mode Initially the system is being tested in

re-mote European sites for applications such as the

edu-cation ofthe children of itinerant travelers or

person-nel stationed in inaccessible areas It is also being

digital signal hierarchyDS- ANorth American time division multiplex (TDM) signal hierarchy, which is used in connection with data communications proto-cols See DS-O through DS-4, TI

digital signal processorDSP Aspecialized computer processor designed to work with digitized wave-forms, often audio and video samples, in order to speed execution and provide more complex opera-tions Their computing power and flexibility allow them to be used for a wide variety of applications, such as the compression of voice and video signals, multimedia applications, medical imagery, combina-tion phone/fax/modem devices, etc

digital signal cross-connect panelDSX panel A type of electrical cross-connect wiring bay or closet

to facilitate the interconnection or patching of digi-tal telecommunication facilities and equipment This facilitates rearrangement, restoration, or monitoring ofcircuits Bantamjacks are common in DSX panels DSX panels connect a wide variety of equipment, depending upon the type of service (e.g.,' Tl) Cir-cuit connections may include channel banks, multi-plexers, switches, repeater bays, and terminating con-nectors or circuits See digital cross-connect, tap digital signatureA type of digital identification as-sociated with an individual or association that is suf-ficiently unique, secure, and resistant to forgery that

it can be used for confidential and commerce-related online messages and transactions A digital signature

is essentially the electronic equivalent ofa handwrit-ten signature that canbetraced to the person who cre-ated it Adigital signature was initially seen as a digi-tized version ofthis handwritten signature (somewhat like a rubber stamp signature), but it was quickly re-alized that a digitized version did not have the same verification characteristics (pressure, direction, speed, etc.) that were inherent in a handwritten signature, and other more abstract versions of the digital signa-ture were developed that not only were more unique but were amenable to strong encryption techniques

to ensure security (The author feels there may still

be some merit in developing algorithms that actually encode the pressure, speed, and other characteristics that can be measured in a person's signature through

a special pressusensitive pad There is more re-search that can be done in this area and specialized circumstances in which it could be applied, as in en-coding signatures for local legal transactions to help prevent fraud.)

In a more technical sense, for the purpose of imple-menting software, a digital signature has been de-scribed as a value generated from an application via

a cryptographic algorithm that embodies data integ-rity, message authentication, and/or signer authenti-cation A number ofdigital signature schemes are al-ready in use for stock-related transactions, contracts, and general messaging Digital signatures typically employ key encryption methods

In Aug 2001, the W3C described a Proposed Rec-ommendation for XML digital signature processing rules and syntax to provide integrity, message