Dedicated lines can be installed on various types of circuits, ranging from copper phone lines to fiber optic.. In the future, it is hoped that DSRC can provide a consistent framework fo
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dead spot Aphrase often used in broadcast
commu-nications (radio, TV, cellular) to describe a region in
which there are no signals, due to terrain or other
obstructions Cellular customers in particular are
sus-ceptible to dead spots, because they may be constantly
moving between buildings, boulders, mountains, etc
deadlock In competition for resources such as
print-ers on a network, a route on the Internet, or space for
a car on a road, deadlock occurs when there is more
demand than there is supply and the contention is
un-resolved Contention is commonplace on networks
and there are many ways to handle the situation, but
sometimes a system lacks a mechanism for
resolv-ing unexpected contention which may result in the
system "freezing," looping (making repeated,
unful-filled requests), or losing the directive for resources
(e.g., a document disappearing from a print queue)
The term deadlock is usually reserved for contentious
situations where all the parties involved in the
com-petition for resources experience some type of
pro-tracted slowdown or lockup which is difficult or
im-possible to resolve except by extraordinary measures
(e.g., a reboot) See contention
debug v To rid a system systematically of problems
orbugs, especiallyincomputer software In software,
bugs include syntax errors, looping errors, logical
errors, and user interface design ergonomics
prob-lems This process has become easier with the
avail-ability of debugging software and higher level
pro-gramming languages, but it is still an arduous,
exact-ing, painstaking activity In the programming
com-munity, not all good programmers are good
debuggers, and code is sometimes passed on to a
pro-grammer who has a particular mindset and talent for
this exacting, detail-oriented work Software
data-bases for tracking and reporting bugs are becoming
popular and automated programs to test software, in
order to find problems and report bugs, may be
in-cluded in the debugging arsenal
decay Reduction through processes such as
absorp-tion, attenuaabsorp-tion, erosion, or corrosion Light or sound
signals will decay over time and space due to
inter-action with the environment as energy is converted
into other forms (e.g., heat) Physical structures, such
as cables, will decay from abrasion and chemical
in-teraction from heat, humidity, acidic or base solutions,
or ultraviolet radiation from the Sun
The term tends to be associated with conditions or
phenomena that decline gradually, often in a
some-what smooth manner, as opposed to those that decline
very quickly or in ways that would be graphed as
spikes rather than a smooth curve The measure of
the speed of decay is somewhat related to the
phe-nomenon itself Light is a fast-moving phephe-nomenon
and thus decay patternsinlight are much faster than
those in a corroding cable
In a fiber optic cable, dopants may be incorporated
into the filament to influence the reflection ofthe light
and wavelengths that are allowed to pass through the
lightguide The tradeoff is that the signal will decay
more rapidly over any given distance as compared
to an undoped cable with the same characteristics
Since decay is related to distance, and transmission over distance is important in telecommunications, many methods to minimize decay and amplify sig-nals have been devised, including electrical and op-tical amplifiers See absorption, amplifier, attenua-tion
DECCa Defense Commercial Communications Of-fice (U.S.)
decibel dB One tenth ofa bel A dimensionless ratio
of two powers, which, in electricity related to tele-communications circuits, may be referenced to mil-liwatts More familiarly in acoustics, a decibel may
be calculated and expressed inpascals as a ratio of
the sound pressure to a reference pressure
Since that's a little difficult to understand without knowing the formulas and individual frames of ref-erences, it may be easier to understand decibels in terms ofexamples in acoustics Sound volume is typi-cally expressed in decibels, on a logarithmic scale (the bigger it gets, the proportionally louder it sounds) with lower numbers representing lower volumes At the high end of the scale, around I75±25 dB, perma-nent hearing damage occurs Even at sustained lev-els of 100 to 150 dB (and sometimes lower), loss may occur Sudden contrasts from low to high volume sounds can be especially harmful to human hearing Environmental noises tend to range from about 5 to
100 decibels, with low volume sounds emanating from appliances and traffic, and high volume sounds coming from horns, explosions, collisions, etc See bel, neper
decimal system Based on a base 10 numbering sys-tem, the most common one in human culture, prob-ably due to the fact that we have 10 digits each on our hands and feet Computers, on the other hand, are commonly based on binary systems, base 2, due to the fact that electrically powered computers are easy
to design around systems that use two states: on or off, high or low, etc
decimetric wave An ultra-high frequency (UHF) electromagnetic wave in the approx 300- to 3000-MHz (3- to 30-GHz) range Waves in this range are detected during solar flareups and those at the lower end of the scale are used in certain wireless voice and data communications Wireless decimetric systems are used in military applications, aerospace, and com-munication systems of developing nations
In fiber optic cables, amplitude modulation of laser light in the decimetric range may be used for testing embedded optical fibers (e.g., for strain tests) with resulting reflection from the fiber analyzed by a UHF-capable network analyzer
DECNet Digital Equipment Corporation's propri-etary Ethernet-based local area network (LAN) See Digital Equipment Corporation
decollimate To cause to diverge from a parallel di-rection For example, if a collimated laser light hits the endface of a fiber optic cable at the wrong angle
or encounters dopants within the material, the light may be deflected from its original parallel path and
be reflected in many directions Lenses can be used
to spread or concentrate a collimated !,Jeam For
Trang 2beam, while a Fresnel lens could concentrate a beam
by focusing it over an area that is smaller than the
source beam See collimate
decompressionThe process ofdecoding, expanding,
and otherwise reverse-engineering the information in
a compressed file Decompression is usually done to
restore a file to its original state, or to restore the
in-formation to a form that is comprehensible to a
viewer/listener/reader, but which may approximate,
rather than duplicate the original state
Decompres-sion may happen in advance of using the
informa-tion or may be carried out as the data is being read,
as in MPEG animation playback systems See
com-pression, data compression
DECTSee Digital European Cordless
Telecommu-nications
Dedicated Access Line, Dedicated LineDAL, DL
A private network connection between a business or
individual and a phone carrier or network service
pro-vider Calls through a dedicated line are automatically
routed to the phone carrier or other service provider
Dedicated lines can be installed on various types of
circuits, ranging from copper phone lines to fiber
optic
dedicated array processorDAP A processor in a
redundant array of inexpensive disks (RAID) system
that specifies various array-specific tasks related to
management of the multiple disks and the
informa-tion and organizainforma-tion ofthe informainforma-tion on the disks
This information is particularly important if a
prob-lem has occurred with the data, and the disk array
needs to be adjusted or rebuilt
Dedicated Short-Range CommunicationsDSRC
Aninternational standards effort for short-range
com-munications that is ambitious in the sense that it must
contend with a heterogenous installed base,
compet-ing commercial interests, and varycompet-ing levels of
fed-eral support The standards effort has produced a
number ofdrafts for two independent but related
stan-dards, including DSRC using microwave frequencies
and a DSRC data link standard
The long-term goal is to support an interoperable
North American transportation system, and there is
still discussion as to whether interoperability should
be mandated or promoted In the future, it is hoped
that DSRC can provide a consistent framework for
communications, including travel information
sys-tems, navigation aids, tracking and routing, and a
wide variety of service-to-vehicle services that are
possible through wireless data information services
(maps, trip planners, emergency locators, etc.)
DSRC has applications for a wide range of
transpor-tation users, including casual travelers, professional
travelers, trucking companies, emergency services,
and transit authorities The Transit Standards
Consor-tium, for example, has investigated the use of a
se-lected set ofradio frequencies for DSRC applications
for transit vehicles See intelligent vehicle highway
systems
Deep Space NetworkDSN Around-the-clock
com-munications network that facilitates communication
interplanetary probes Originally established in 1958,
it is managed and operated by the Jet Propulsion Laboratory (JPL) with treaty agreements for foreign complexes The three complexes that comprise the network are located in Goldstone, California;
Robledo, Spain; and Canberra, Australia See Gold-stone Deep Space Communication Complex, NASA
~;:~~~o~tiliC;:i:e~:~r:/~~:~:~:t1~:e~f ·1. remote electron microscopy by Berkeley Lab
re-searchers Parvin, O'Keefe et a1 in August 1995 This historic achievement is important because electron microscopy is very sensitive to changes that can oc-cur in a split second and thus is not tolerant oftrans-mission delays while the viewer makes adjustments, waits for images to display, etc The solution to this problem was to have a system onsite with the micro-scope that can monitor the environment and make ad-justments as needed to take some of the burden off the remote viewer and reduce the time it takes to com-municate commands and data offsite However, this first successful experiment was a single-user solution
What if numerous users wanted to view and control the same microscope over the Web, for example in the same manner as many pan-and-tilt Web cams?
Parvin, O'Keefe, and Taylor subsequently developed
a system that came to be called DeepView, wherein multiple microscopes with different characteristics could be accessed by multiple remote participants.It
is a scalable, distributed network concept for a com-mon interface for accessing multiple microscopes that may change from time-to-time The system also fa-cilitates the exchange ofinformation and data analy-sis Combined with image processing that enables images to be improved, augmented, and combined, Deep View is a powerful system for coordinated mi-croscope-based research
Deep Space Network Parabolic Antenna
The 70-meter parabolic antenna for space commu-nications located at the Canberra Deep Space Net-work site in Australia, Jan 1990 [NASAIJPL image.]
defaultThe initial setting, factory setting, reset setting,
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Satellite Sensor Sensitivity Regions
The Defense Meteorological Satellite Program (DMSP) has been serving weather data collection needs for the U.S military operations for several de-cades through 2.3-ton satellites operating in polar orbits at about458nautical milesfrom the Earth This diagram illustrates the two regions ofsensitiv-ity of the two onboard sensors of the Operational Linescan System on board the satellite.
in monitoring and predicting weather patterns, espe-cially potentially dangerous weather systems such as thunderstonns and typhoons It also provides infor-mation on electromagnetic fields that could influence long-range communications systems and ballistic-missile early warning radar systems based upon elec-tromagnetic sensing systems
Data are relayed from tracking stations in New Hamp-shire, Greenland, and Hawaii to the military meteo-rological center in Nebraska The data can also be relayed directly from the satellites to appropriately-equipped transportable military tactical units See De-fense Support Program, Milstar
Electro-optical
i
1
11
-2 (microns)
I
4
I
6
I
8
Infrared
12 10
Defense Satellite Communications System DSCS
A global military communications satellite network operating in super high frequency bands DSCS is administrated by the DISA
Defense Support Program DSP Asatellite data and defense support program operated by the U.S Air Force Space Command as part of North America's early warning system DSP satellites are intended to detect missile and space launches and nuclear deto-nations See Defense Meteorological Satellite Pro-gram, Milstar
Deflate Protocol DP A means of compressing PPP encapsulated packets that was proposed in the mid-1990s DP is based on thedeflatecompression for-mat which was, in tum, based on Lempel-Ziv LZ77 compression Deflate has a good compression ratio,
is openly specified as zlib source code by Gailly and Adler, and is widely implemented in such popular compression programs as PKZIP and gzip See Point-to-Point Protocol, RFC 1979
deflectvt. To divert something from its original course, preferred direction, or current position The term is most often used to describe the redirection of something that is moving, such as radiant energy or
an object Deflection is intrinsic to many semicon-ductor assembly processes and the functioning of communication systems
Reflective shielding is used to deflect electromagnetic
parameters, or configuration; the state or parameters
that are in effect at the start of a program, available
in a dialog box, or stored in a file Sometimes the user
can modify the default settings Many customizable
software applications or hardware devices have a
button that the user can press to restore default
settings in case erroneous or confusing settings have
been subsequently established.Insome systems,
in-terrupting power to the system (e.g., removing a
PRAM battery) can restore system defaults
default carrier In telephone communications, the
long-distance carrier assigned to handle calls for
cus-tomers who have not specified an alternate carrier
Defence Science and Technology Organisation
DSTO A division of the Australian Department of
Defence, the DSTO has been investigating future
technologies for use in defence applications, along
with aiding in procurement, developing new
capabili-ties, and enhancing existing capabilities since 1910
The DSTO has developed a trial high-frequency
(HF) radio data network, as well as virtual reality
simulations of naval and aeronautic systems
http://www.dsto.defence.gov.au
Defense Advanced Research Projects Agency
DARPA Fonnerly ARPA, an agency ofthe U.S
De-partment ofDefense (DoD) that handles research and
developmentinbasic and applied research, and which
takes on imaginative, high-risk/high-payoffprojects
which may result in dramatic technological advances
on behalf of the DoD
DARPA was established in 1958 to assure U.S
glo-bal technological advancement in coordination with,
but independent of, the U.S military research and
development establishment DARPA has, as part of
its mandate, a design that is a deliberate counterpoint
to traditional research ideas and approaches It
man-ages a budget of about $2 billion
DARPA's technical staff is rotated every few years
to encourage new perspectives and is drawn from
in-dustry, academia, and government laboratories
Defense Data Network DDN The packet switching
network of the U.S Department of Defense,
estab-lished in 1982, which later became MILNET DDN
was separated from ARPANET (which evolved into
the Internet) It includes classified and unclassified
sections (DISNETx and MILNET)
defense information infrastructure DII The
facili-ties, networks, software and information that
com-prise both peacetime and wartime resources of the
u.S Department of Defense
Defense Meteorological Satellite ProgramDMSP
A program initiated by the u.S Department of
De-fense (DoD) in the mid-1960s, DMSP satellites
sur-vey environmental features such as pollution, clouds,
water, snow, and fIre, in the visual and infrared
spec-trums This data is downloaded to ground stations and
processed for U.S military operations
The primary sensor on the DMSP is the Operational
Linescan System (OLS), which provides continuous
visual and infrared spectra images ofcloud cover over
a sensing breadth of about 1,600 nautical miles The
data from the system aids military weather forecasters
Trang 4components are designed to deflect some
wave-lengths and allow others to pass, providing filtering
capabilities
Light travels in a straight line unless other forces act
upon it to change its direction Thus, the cladding in
a fiber optic cable, which has a slightly lower
refrac-tive index than the conducting core, deflects/reflects
the light back into the conducting core, enabling it to
continue along the core and to follow paths that may
curve See reflect, refraction
deflection modulationOn a scanning cathode-ray
tube (CRT), a system of deflecting the scan vertically
from a base horizontal path This makes the signal
appear like a series ofpeaks above the baseline
deformerA tool for defonning the shape of an
ob-ject It may be a physical tool that uses pressure or
heat or some other mechanism for defonning an
ob-ject, or it may be an algorithmic or virtual tool for
deforming a digital or conceptual object
degaussTo demagnetize; to bulk erase; to provide
electrical current-carrying coils to neutralize
magne-tism Cathode-ray tube (CRT) monitors are equipped
with magnets to influence the direction of the
elec-tron beams Some monitors will have a degaussing
button to reset the magnetic environment in the
moni-tor Named after Karl F Gauss
Delaney lamp detectorA novel solution to creating
a radio wave detector is the Delaney lamp detector,
devised by u.S Naval electrician Delaney It was an
electrolytic detector that consisted ofan incandescent
lamp bulb with the top broken off, the filament
re-moved, and a 20% solution of nitric acid poured into
the globe This detector was found to respond well
to signals originating nearby, without burning out
from the oscillations of a strong signal coming from
a nearby wireless station, a problem that was
com-mon to other types of electrolytic detectors See
de-tector
delayLag, hysteresis, gap, retardation of a signal or
phenomenon, time lapse Delay is a relative tenn in
that it implies a lag as compared to other processes
or compared to what was expected
Delay is inherent in many types of
telecommunica-tions technologies, and systems design and operation
must take into consideration a variety oftypes of
de-lay Transatlantic phone callers used to experience
ap-preciable delays between the time one side ofthe
con-versation was heard and understood and the other side
responded This type of delay is now rare, or within
acceptable levels.Inthe 1980s, the delay between
sending and receiving email correspondence was
typically about a week, now the delay is often only
seconds or minutes
Delay can be a serious problem in systems that use
more than one line to transmit associated
phenom-ena, as in many videoconferencing systems that
trans-mit graphics and sound over separate
communica-tions lines.Inpacket switched networks in which
packets are split, sent through different routes, and
reassembled at the receiving end, portions of the
transmission might delayed, affect the reconstitution
In systems where synchronization ofpulses is impor-tant, delay must be taken into consideration or may
be deliberately introduced into faster aspects of the system to align the pulse timing oftwo or more com-ponents
Delay is sometimes deliberately introduced in data transmission systems to prevent overly quick reac-tions to situareac-tions that might be nonmeaningful (as
in guidance systems or briefpower brownouts) De-lay is inherent in store-and-forward systems that may collect email messages, or other types ofnetwork traf-fic, and dispatch them in batches or when CPU time
is at a low ebb See cell rate, hysteresis, jitter
delay distortionIn transmission technologies, dis-tortion ofthe expected shape or characteristics ofthe signal as some aspects of it are delayed over other component aspects Delay distortion tends to be cu-mulative over time and distance In mathematical tenns, it can be described as the different Fourier components of a signal traveling at different speeds relative to one another
Delay distortion is more prevalent at certain wave-lengths and with waves comprised of multiple fre-quencies.Itmay also be more significant at higher data rate speeds where processing ofthe signal at the destination must be handled more quickly and where there may not be as much time to respond to and cor-rect distortion
In fiber optic cables, delay distortion arises from light dispersing within the confines ofthe lightguide cable such that different wavelengths arive at the destina-tion at slightly different times See chromatic disper-sion
delay encodingA means of encoding within the bit period of a signal, a method that is more prevalent in radio signaling A signal level transition can occur in the middle of the bit period or may be delayed to the end of the bit period Binary data is represented by two signal levels representing zero (0) or one (1) If
a zero is initially transmitted, the signal level does not change However, if a second zero follows a pre-vious zero, the signal changes at the end of the bit period Ifit changes in the middle ofthe bit period, it represents a one See time-delay modulation
delimiterAny symbol, code, character, or other data used to signal a gap, break, boundary, or stopping/ starting point Delimiters are very important in both human and computer communications People use spaces, commas, paragraphs, and other punctuation and symbology to serve as delimiters in written text
Itgreatly enhances the ease with which the infonna-tion can be understood Computer programs use sym-bols, punctuation, and spaces as common delimiters for arrays, lists, paths, and other types of informa-tion Radio broadcast systems use spaces as delimit-ers between songs, or tones as delimitdelimit-ers before and after emergency announcements
Dellinger fade-outIn radio communications, sun spot activity may be associated with highly absorb-ing areas in the ionosphere which can impair short wave transmissions
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delta channelSee D channel
delta matched antennaSee Y antenna
delta modulation, difference modulationA
com-mon method for converting/encoding analog signals
into digital signals (AID conversion) for storage on
digital media or transmission over data networks
This scheme was developed during the 1940s for
te-lephonyapplications
In delta modulation, a scanned value is compared to
the previously scanned value, to see if it is greater or
less than the previous value, sending a one (1) if it is
greater, and zero (0) ifit is less (Actually the one and
zero values can be reversed, just as long as the sender
and receiver agree on which is assigned to the greater
or lesser result.) Described another way, the
sinusoi-dal analog wave is sampled, each sample is compared
to the previous sample, and the modulated signal
re-flects the relationship by encoding it as a one or zero
Thus, the frequency of the sampling will influence
the encoded signal in terms of how much
informa-tion can be conveyed and how true it is to the
origi-nal sigorigi-nal This scheme is relatively simple and
re-sults in very fast encoding
There are many different versions of delta
modula-tion (e.g., linear and adaptive), and many are not
com-patible Pulse code modulation (PCM) is the other
common method for converting from analog to
digi-tal Because more information is encoded, it's not as
fast as delta modulation, but it can be more readily
converted between different versions of PCM
Un-like delta modulation, PCM doesn't require an
inter-mediary analog stage to convert between different
versions, and thus it is more popular than delta
modu-lation See AID conversion, delay encoding,
sigma-delta modulation, pulse code modulation, sampling
delta testingFinal stage of testing a finished
prod-uct on endusers who would normally be customers
or users of the product This testing is more
com-monly known as User Acceptance Testing See alpha
testing, beta testing, gamma testing, User Acceptance
Testing
Demand Assigned Multiple AccessDAMA.In
wire-less communications, on-demand channel sharing It
is accomplished by assigning a call to a channel that
is currently idle, or to an unused time slot, sometimes
called Bandwidth On Demand Thus, a single
tran-sponder can support hundreds or thousands of
sub-scribers, though actual capacity depends upon the
DAMA implementation DAMA is common on mesh
communications satellite networks
Prototype OAMA systems were developed in the
mid-1990s by the U.S Defense Information Systems
Agency (DISA) DAMA is a newer technology
in-tended to increase channel capacity over methods
common in the 1990s.Itis typically used in
single-hop satellite connections for a variety of
point-to-point or multipoint-to-point voice, facsimile, or data services
DAMA assigns channels automatically, rather than
manually or by establishing fixed channels When a
specific OAMA transmission is terminated, the
chan-nel is returned to a pool of available resources
DAMA is used in commercial satellite systems and
on the U.S Navy's UFO communications satellites (designed to replace lower capacity FLTSAT sys-tems) Mini-DAMA is a type of U.S Navy DAMA system that includes encryption and data transfer ca-pabilities previously handled by devices outside the DAMA system
demand circuitsData network segments whose costs are related to usage
Demand Priority ProtocolDPP Aclient-server pro-tocol mechanism that handles message exchange and sensing by multiple devices in IEEE 802.12 (1OOV G-AnyLAN) networks The process is governed by the Media Access Control (MAC) states The client re-quests access to the communications medium and the server evaluates the request and returns permission
to the client, if the request is granted Once permis-sion is granted, the requester goes into a transmit frame mode and sends the data to the network demarcation pointThe point at which a telecommu-nications carrier's equipment or responsibility ends, and the subscriber's equipment or responsibility be-gins This point may be at a junction box on the side ofa building or a patch panel within a building Resi-dentiallines tend to be demarcated outside, while multiline phone systems in businesses tend to be de-marcated at a patch panel or terminal block inside the premises The block itself is sometimes called a de-marcation strip
demodulationThe process oftaking a signal that has been manipulated to carry information, and extract-ing that information from a carrier wave or math-ematically recreating that information from sidebands
or other parts of the original transmission
A crystal detector uses a crystal as a type ofone-way valve to detect and demodulate radio waves A mo-dem is a device that mo-demodulates an analog phone signal to convert it to digital information that can be understood by a computer Abroadcast receiving sta-tion takes modulated airwaves and demodulates them for local transmission through cable or aiIwaves See modulation
demon dialer, war dialerA function of a program, usually for telecommunications or telemarketing, that automatically calls a phone number repeatedly or calls down through a list of phone numbers, cycling back to the top until one answers
DEMS 1 digital electronic management system
2 digital electronic message service
demultiplexing 1 Ina multiplexed signal, a process for recovering signals combined within it, usually to restore distinct channels contained within the trans-mission See carrier wave, band, channel 2 In ATM networking, a function performed by a layer entity that identifies and separates Service Data Units (SDUs) into the individual connections ofwhich they are comprised
DENet Denmark's Ethernet network which intercon-nects academic institutions
Denial of ServiceDoS A network administration term for situations in which users are unable to ac-cess the system DoS may be intentionally imple-mented by an administrator or may be the result of
Trang 6ism, or security breaches such as hacking or
flood-ing With the Internet being increasingly used for
sen-sitive government and financial transactions
(espe-cially time-sensitive stock and auction transactions),
DoS situations arising from deliberate tampering have
become a significant security concern Safeguards
against unauthorized DoS are difficult to implement
without also affecting legitimate users; sometimes
security measures to prevent one type of abuse (e.g.,
excessive junk email) can leave a system more
vul-nerable to other types of abuse (e.g., DoS attacks),
making it a challenge to configure a system for
opti-mum use while keeping it secure See Denial of
Ser-vice attack
Denial of Service attack DoS attack Anetwork
ser-vice security breach in which access to legitimate
users is blocked or significantly impeded by
unau-thorized users Sometimes DoS attacks are launched
to crash a system in order to take it offline or to
re-veal other security weaknesses through which an
in-vader might further infiltrate the system
Denial of Service attacks can be directed toward
dif-ferent aspects of a computer network Filling up the
available disk space, planting a program that
greed-ily consumes system resources, or overwhelming the
connection algorithms are various strategies that can
cause DoS problems
SYN-flooding is the transmission of a barrage
ofre-quests for a network connection to deliberately
com-promise normal use When the server unsuccessfully
tries to connect to an overwhelming number of
un-reachable return addresses (those that don't return a
connection acknowledgment), a DoS condition
re-sults
Denial ofService attacks have existed as long as there
have been computer networks, but they achieved
prominence in the media and in industry and
govern-ment discussion groups in February 2000 when an
ordered series of attacks was launched against some
of the most prominent commerce sites on the World
Wide Web See Land attack, Trojan horse, virus,
RFC 2505, RFC 2827
dense wavelength division multiplexing DWDM
A transmission system capable of producing and
si-multaneously transmitting many wavelengths over a
single fiber optic cable On DWDM multiple
chan-nels can be carried on the same transmissions
me-dium, in essence a virtual "fiber bundle" within a
single conducting core This substantially increases
broadband capacity and is of particular interest to
firms installing long-distance cables over difficult
terrain such as mountains or ocean bottoms
(subma-rine cables)
DWDM is speed and protocol independent, meaning
it can carry a wide variety oftransmission protocols,
including SONET, Internet Protocol (IP), Ethernet,
and SDH Different types ofcommunications can be
carried at different speeds over the same physical
network
DWDM was commercially introduced in the
mid-1990s and Sprint was one of the first high-profile
long-distance telephone selVices
Due to cost and implementation, DWDM was origi-nally favored for long-haul networks, but was be-ginning to be used in metropolitan area installations
in 2000 DWDM can increase capacity over time di-vision multiplexed (TDM) systems by a factor offour
to eight times Metro 1500 systems from Cisco, for example, can accommodate up to 32 wavelengths (channels) per strand of optical fiber Gigabit Ether-net transmission speeds are readily implemented over DWDM optical networks
By 2001, it was possible to transmit 1.60 Tbps over
a distance of 2000 kilometers By March 2002, Lu-cent Technologies had demonstrated a 64-channel differential phase shift keying (DPSK) system that could transmit over a long haul of 4000Ian (2500 miles) with each channel transmitting at 40 Gbps through 100kmspans
Testing and maintenance technologies for DWDM were also evolving in the late 1990s, with equipment capable of measuring wavelength, frequency, chan-nel spacing, signal-to-noise ratio, and power being offered by companies such as Wavetek Wandel Goltermann in 2000 See lambda switching densitometer Aphotoelectric instrument for measur-ing the opacity or relative degree of light absorption ofa material Darker materials absorb more light and thus have a higher optical density There are differ-ent types ofdensitometers, some that measure trans-mission and some that measure reflection They are commonly used in the printing industry to monitor consistency and quality See hygroscope
density A general descriptive or comparative term describing the proximity of individual elements, hence the total number that can be fit within a speci-fied area Density is used to describe optical and mag-netic storage capacities, screen and printer resolu-tions, compression efficiencies As a general rule of thumb, especially for storage media, the denser the information capacity, the higher the cost
Denver Telephone Dispatch Company One of the earliest phone companies in the United States, estab-lished by Frederick O Vaille in early 1879
Department ofHealth DOH State, federal, and cor-porate health agencies have been devoting substan-tial resources to evaluating various distance educa-tion and physician support technologies and databases that can aid in health and safety With telemedicine, for example, a surgeon in one region can oversee and direct an operation in a remote location With shared databases, diagnosis and treatment research and in-formation can be collectively used and managed The applications oftelecommunications to medical man-agement are far-reaching, and we've currently only begun to implement the technologies
depletion layer A barrier region in a semiconductor
in which the mobile carrier charge density is not suf-ficient to neutralize the charge density ofdonors and acceptors The donors contain impurities to facilitate electron activity, and the acceptors include trivalent impurities, forming "holes" to accept the electrons
Trang 7Fiber Optics Illustrated Dictionary
depolarization Demagnetization or the counteraction
of polarization In many transmission systems, the
radiant wave signal may be horizontally or vertically
polarized and may gradually lose that polarization as
it passes through various media (moisture, particles,
terrain obstructions, etc.) The wave may also be
de-liberately depolarized at the reception point One of
the important evolutionary steps in the history ofthe
battery is the depolarizing characteristics of the
Daniell battery
Polarization and depolarization are significant in
many technologies and are of particular interest to
quantum physicists, antenna manufacturers,
astrono-mers, and photographers When light waves vibrate
in more than one plane (e.g., sunlight), they are
con-sidered to be unpolarized When the vibrations
oc-cur in only one plane, the light is considered to be
polarized This can happen through scatter, reflection,
or refraction or can occur or be deliberately effected
in a transmitter Reflection offa highly reflective
sur-face can yield unpolarized light The angle
ofreflec-tion offa less reflective surface will affect the amount
of polarization to a different extent
deregistration The process ofdissociating two
enti-ties, which may be names, processes, objects, etc
Deregistration is a process that happens often in
com-puter applications, particularly those associated with
networks The process of creating virtual links
be-tween applications, icons, and processes allows for
keeping track or creating shortcuts Often these
vir-tuallinks are removed or reorganized, particularly in
a dynamic network environment
In public or pay systems, deregistration may be
as-sociated with billing or security, in that a subscriber,
cellular handset, computer, or other entity is
regis-tered in order to assure service to the subscriber, or
to provide privacy and authentication In these
sys-tems, registration is often with a unique identifier,
which must be deregistered before services can be
discontinued, or before the identifier can be assigned
to another
deregulation A process of removing authority from
a governing body in general, or ofremoving
author-ity ofthe government body over specific jurisdictions
As examples, control over banking,
telecommunica-tions, or other large public or commercial systems
have at various times been removed or reduced in
or-der to stimulate competition and innovation
Some-times deregulation has the intended positive effect
Sometimes deregulation causes a change in the
sys-tem that attracts unscrupulous members of society,
who seek new loopholes that can be used to take
ad-vantage of the system to the detriment of others
DES 1 See Data Encryption Standard 2 destination
end station
descramble Scrambling is mixing up a signal or
digi-tal data file so that it cannot be used without fIrst
be-ing processed Descramblbe-ing is recreatbe-ing the
origi-nal in lossless or lossy format so thatitis
human-read-able or viewhuman-read-able Scrambling provides security and
protection from unauthorized viewing or copying and
is a common way to commercially protect cable or
satellite TV programs from being viewed by those who haven't paid for the service In most regions, un-authorized descrambling devices are illegal See encyrption
Design Layout Record DLR A database record of telephony design layout ADLR may be searched by
a circuit type with additional parameters supplied such as the U.S state in which the circuit is located
A DLR search yields administrative and design in-formation specific to the queried circuit
Design Structure Matrix, Dependency Structure Matrix DSM Asystem and project analysis tool for modeling information flow, as in the product devel-opment process Parameters are charted in a matrix representation containing subsystems or activities and corresponding information exchange and dependency patterns Research on DSM is ongoing at MIT Design System Language DSL Apredecessor to the PostScript page definition language, DSL originated
in the mid-1970s at Evans & Sutherland Computer Corporation Evans & Sutherland (E&S) became well known for their pioneering work in flight simulation and other 3D graphics software, and the Design Sys-tem was one of the outcomes of a research project using an interpretive language to build complex graphics databases The Design System Language was later put to use in CAD applications See Post-Script
Designated Transit List DTL In ATM networking, DTL is network routing policy data consisting of a list ofnodes and any associated link identifiers, which specifies the path across a PNNI peer group desktop A broad term in the computer industry de-scribing any common workspace found in a home or office (or garage) where the various user tools and appliances are contained Desktop computers are those that can fit comfortably on a desk (as opposed
to dishwasher-sized minicomputers or room-sized supercomputers) See desktop metaphor
Desktop Management Interface DMI Aspecifica-tion developed by the Desktop Management Task Force (DMTF) to allow easier, more consistent ac-cess to management database information and appli-cations through Management Interfaces (MIs) The DMTF is a consortium ofhardware and software ven-dors
desktop metaphor Aphrase to describe the concep-tual rationale for the design and display of informa-tion on a computer output device, usually a monitor The idea is to take objects and actions that are famil-iar to workers and home users and represent them in
an easily recognizable form on the computer desktop publishing DTP Using desktop computers
to design, layout, and print files It is now straight-forward to create a file on a home computer and send
it directly, via FTP, to a commercial printer Many of the pioneer tools for the evolution of desk-top publishing were developed at Xerox PARC, sub-sequently implemented and popularized on the Macintosh computer in the mid-1980s, and enhanced through the development of laser printers DTP has had a resounding impact on the publishing industry
Trang 8nies and individual publishers.
Quark XPress, Adobe PageMaker, Adobe
Frame-Maker, and TeX are DTP software applications
widely supported by users, service bureaus, and
pub-lishers Ventura Publisher was popular in the 1980s
Quark is favored for ad layout, posters, and other
large-format or complex color projects and, to a lesser
extent for books, mainly those that have many
illus-trations and those that don't require a lot of
exten-sive indexing and cross-referencing Design
profes-sionals like Quark
TeX is preferred for sophisticated technical symbols
as are found in mathematics and physics treatises and
textbooks and, although its interface is less intuitive,
it has capabilities in symbol representation and
for-matting beyond most general purpose desktop
pub-lishing programs Corel has been promoting its
prod-ucts as desktop publishing tools, and for illustrations
it can work well; however, many service bureaus and
professionals prefer some of the other products, at
least at the present time For creating and inserting
PostScript EPS (encapsulated PostScript) illustrations
into other desktop publishing programs, many people
swear by Corel Draw! There are also a number of
other publishing programs available and word
pro-cessors that are sometimes stretched to perform
ba-sic desktop layout tasks
desktop videoJust as the Macintosh computer
facili-tated the development of a new mass market field
called desktop publishing, the Amiga computer
fa-cilitated the development ofa new mass market field
called desktop video Within two years of its debut
in 1985, many video genlock and broadcast video
products were introduced for the Amiga, the most
sig-nificant being the NewTek Video Toaster, which is
still used today by many local cable companies and
some of the large broadcast networks A desktop
video-equipped computer enables the development,
editing, and merging ofcomputer images, video taped
footage, and live broadcasts at a fraction of the cost
of using older video equipment
desktop computer telephony integrationDCTI
The integration ofcomputer processing and database
search and retrieval functions, especially, with
tele-phone communications equipment and procedures
For example, a database that is used to qualify and
select potential or existing customers can be used to
automatically dial on the behalfofsales agents Ifthe
call is successfully connected, the next available agent
can be notified and the information in the database
can be displayed on the sales agent's terminal This
type of integration and automation is becoming
in-creasingly prevalent Videoconferencing is another
example ofDCTI
desktop videoconferencingVideo phone
capabili-ties provided on a small desktop dedicated system or
microcomputer equipped with a microphone, video
camera, and fast network or phone access Businesses
are considering videoconferencing as an option to
expensive travels, especially for meetings involving
participants who are widely distributed
geographi-but the speed of the line greatly affects the refresh rate ofthe image If the transmission rate is slow, the image will be blurry and slow to update, and it will look more like a series of still shots than natural movement End-to-end fiber optics networks are well suited for the transmission of audio/video confer-ences
~~:au:s::~~nS~~el~~l~~f~:~~O~~~~~s~~:~~nge: [.
in a specified direction, usually a region on the Earth
There were early electronically despun antenna ex-periments on u.S military satellites in the late 1960s which met only limited success Later, mechanically despun antennas were also tried
destination addressDA A flag, field, or other indi-cator commonly used to designate the receiving point for a data transmission, call, or physical correspon-dence In Token-Ring, Ethernet, and Fiber Distrib-uted Data Interface (FDDI) networks, the DA is a data field sent in the direction of the recipient that de-scribes a unique Media Access Control (MAC) ad-dress See address, Destination Address Field, domain name, Media Access Control
Destination Address FieldDAF In frame-based networking, the DAF specifies the intended destina-tion for the frame, which may be one or more sta-tions (singlecast or multicast) In Ethernetworks, the DAF is usually implemented as a six-byte address, and, if the DAF contains all ones, it is considered to
be an all-stations broadcast In the IEEE 802.3 Ether-net standard, the Destination MAC Address follows the Start Frame Delimiter and precedes the Source MAC Address See destination address
destination end stationDES In ATM networking, the end or termination point of a transmission It is used as a reference point for available bit rate (ABR) services Since ATM has a number of traffic flow control mechanisms, which often depend on cells ar-riving at the destination at a certain time or in a cer-tain manner, ABR and constant bit rate (CBR) dis-tinctions are important with relation to the defined end station See cell rate
Destriau effectWhen exposed to an alternating elec-tric field, certain phosphorescent inorganic materials suspended in a dielectric medium will luminesce,
emit electroluminescent light rather than
incandes-cent light Certain semiconductors can be designed
and excited to emit carrier injection luminescence.
detectSense, become aware of, register a reading as
a result of some influence, respond to
detector1 In radio electronics, a device to appre-hend or detect radio waves.Inearly schemes, many materials were tried, including barretters, natural rec-tifying crystals, synthetic crystals, and electrolytic cups The challenge with most detectors was solving the problem of amplifying the signal, or shifting the range of frequencies into the audible range Current electronics have sophisticated and effective ways of achieving it, but in the late 1800s and early 1900s, ways ofcapturing and amplifying the waves were just being developed 2 Asubstance or circuit that reacts
Trang 9Fiber Optics Illustrated Dictionary
when exposed to electromagnetic waves such as
ra-dio waves or light The detector may also convert or
rectify the electromagnetic oscillations so they can
be incorporated into a circuit with practical
applica-tions A crystal detector works as a type of one-way
valve to rectify radio waves, without outside power
sources Later, vacuum tubes were designed that
could accomplish this task in ways that allowed great
selectivity over the frequencies detected (tuner
cir-cuits) and that could amplify the signal so the
broad-cast could be played through a speaker rather than
through small headphones
Fiber optics are now incorporated into many
detec-tors They are useful as detecting and imaging probes
and as lightguides to draw a light signal from one part
ofthe system to another See crystal detector, Delaney
lamp detector, electrolytic detector, electron tube,
Lodge-Muirhead detector, magnetic detector, Massie
Oscillaphone, optical detector, photodetector,
Shoe-maker detector, silicon detector
Deutsch, Peter Along with Alan Emtage, one ofthe
developers ofthe Archie system created originally at
McGill University and a cofounder of "the Archie
group," a group of volunteers dedicated to
support-ing and enhancsupport-ing the Archie project Archie is very
widely used on the Internet
Deutsche Telekom The German telephone services
authority DT includes a Technology Center, which
engages in research, development, and
demonstra-tions of innovative telecommunicademonstra-tions systems
http://www.telekom.de/
device configuration management In networks, the
configuration, tracking, and management of various
devices, ports, and interface cards These devices are
increasingly software configurable, and the software
used to manage them will often show actual images
of the physical switches through a graphical user
in-terface
Device Control Protocol DCP.Anassigned
well-known port 93 on TCP/UDP systems.
device driver Computer software that controls, or
provides an interface to, one of a variety of devices
such as hard drives, printers, video cards, CD-ROM
players A device driver is a software interface
through which the operating system interacts with
various peripherals attached to the system See
dis-play driver
DFA See doped fiber amplifier
DGEP See dynamic gain equalization processor
DGPS Differential Global Positioning System See
Differential GPS, Global Positioning System
DGPTDepartment General of Posts and
Telecom-munications, Viet Nam
DGTDireccion General de Telecommunicaciones
Spanish telecommunications authority
DBCP See Dynamic Host Configuration Protocol
Dhrystone A relative performance test intended to
evaluate system program execution other than
float-ing point and input/output operations As with many
benchmarks, the information is useful only within a
narrow viewpoint, as in controlled experiments The
Dhrystone was developed in the Ada programming
language by R.P Weicker ofSiemens and is described
in overview with other benchmarks in IEEE
Com-puter, Dec 1990 See benchmark, Rhealstone,
Whetstone
diagnostic programs Application programs used to run hardware test suites, to evaluate processor func-tioning, or both Most computers will run a systems check of the hardware before booting up the higher level operating functions Network servers now of-ten have diagnostic tools with graphical user inter-faces to show virtual and physical connections, traf-fic flow, routers, switches, and various configuration settings Diagnostic software is available for many computers to self-check and indicate possible prob-lems in processing rates or connections Many diag-nostic routines are intended to run on a regular sched-ule, to locate potential problems before they become
serious Engine diagnostics used to mean getting
grandpa to stick his head under the hood ofthe car to listen to the engine Now it means taking the car into
a shop and getting it hooked up to a computerized engine evaluation system that displays various sys-tem parameters on a computer monitor with charts and graphs
diagnostic techniques Various means of ascertain-ing and measurascertain-ing the properties of a system or cir-cuit in order to understand its characteristics, moni-tor its behavior, or detect any problems or anomalies Diagnostic techniques form a part of troubleshoot-ing a system and are often part of or combined with installation and testing Some systems incorporate au-tomated diagnostics that will check systems on startup, at random intervals, or at scheduled intervals See ammeter, tap, voltmeter, Wheatstone bridge dial A circular, movable mechanism for entering a number or other code While dials are used in many electromechanical devices, they are most commonly associated with rotary telephone sets.Ina telephone system, turning the dial causes a pulse of a prede-termined length to be sent along the line to indicate the desired destination address This process allows the system to set up an end-to-end connection for a conversation or data communication The dial was
also historically known as afinger wheel.
The dial superseded the hook on old telephones and effectively obsoleted human operators for local calls within a few years Dials were commonly used until the 1970s, when touchtone phones began to become prevalent in North America (many other countries still use dial phones) See Strowger switch
Dial Access Switching DAS A Cisco commercial network switching system that provides dial services (and dial backup services) for Frame Relay perma-nent virtual circuits (PVCs) in Cisco wide area net-works (WANs) The PVCs are activated by ISDN calls to the switching network DAS is comprised of
the DAS Server Shelfhardware and Dial-Up Frame
Relay software.
dial toneAnaudible signal that indicates that a phone line is active and ready to be dialed The dial may be provided by the local phone carrier or by a private branch system A different type of dial tone, called a
Trang 10phone companies offering voice messaging services
to their subscribers, to indicate that messages are
available to be retrieved
dial-around serviceAcompetitive telephone service
in which the user dials an access code prefix in order
to hook into a third-party service by bypassing the
nonnallocallong-distance carrier The 1010 (ten ten)
long-distance service marketed through television
advertising is an example
Dialable Wideband Service DWS.A sophisticated
digital telephony circuit-switched data service from
Nortel that enables public telephone networks to
of-fer a wide variety ofdigital services to their
subscrib-ers DWS provides on-demand, trackable variable
bandwidth connections which can be implemented
over public telephone networks within the
interna-tional number plan See DataSPAN Frame Relay
Ser-vice
Dialed Number Identification ServiceDNIS
Aser-vice to display the number the caller dialed on an
in-coming phone call This service is useful for regional
numbers that are rerouted to a central administration
or sales system For example, an economic region
may have several area codes, but there may be one
sales office handling all the calls The DNIS allows
the person answering the call to see whether the caller
dialed the local number, the toll-free number, or a
long-distance number, giving some feedback as to
which lines are being used
DNIS does not provide the number of the calling
party; a service like Callerillis needed to provide
this additional information
dialing parityDefined in the Telecommunications
Act of 1996 and published by the Federal
Commu-nications Commission (FCC), as
" a person that is not an affiliate of a local
ex-change carrier is able to provide
telecommunica-tions services in such a manner that customers have
the ability to route automatically, without the use
of any access code, their telecommunications to
the telecommunications services provider of the
customer's designation from among 2 or more
tele-communications services providers (including
such local exchange carrier)."
This measure was designed to level the playing field
and encourage fair practices See Federal
Commu-nications Commission, TelecommuCommu-nications Act of
1996
dialup networkingDUN In the days before Tl and
cable modem access to the Internet, most access to
computer networks and bulletin board systems
(BBSs) was through a modem and a phone line
Be-cause modems and computer time were both
expen-sive until the late 1980s, dialup networking was
usu-ally done on an as-needed basis, with the phone
con-nection live only as long as was necessary to carry
out the necessary data transfers (e.g., file uploads and
downloads, reading mail, leaving messages) The
ear-liest dialup networks were available only to
corpo-rations and government officials but, by the late
1970s, 300 baud modems and desktop
computer-lar with hobbyists Passwords were rare, courtesy was common, and various software programs were devel-oped to facilitate the process and keep phone connec-tions to a minimum As modem technology improved,
1200 baud and 2400 baud modems were sold in the 1980s and, by the 1990s when Internet access was growing by leaps and bounds, 33 Kbps and 56 Kbps modems were generally available at reasonable prices At this point many people added extra phone lines so they could stay online longer without tying
up their voice lines, and service providers added ex-tra modems to support the high demand
With the availability of cable modem, T1, and other 24-hour access technologies in the late 1990s, dialup networking is becoming less prevalent, although it will probably still be available for a number ofyears
At some point, 24-hour fiber optic connections or satellite connections to every home and business might be standard, just as it is in some motels and educational institutions now
DUN connecations are facilitated through scripts and files that store information for initiating a dialup con-nection DUN files include phone numbers, modem settings, and other relevant parameters
diaphragmA thin, flexible sheet or membrane, usu-ally with a curved surface In electronics, a diaphragm usually is designed to respond to vibrations and has
an interface to a device that can interpret those vi-brations into electrical signals Or conversely, it may take electrical pulses and interpret them into vibra-tions Diaphragms are common in microphones and speakers
dichroicLiterally, two-colored, though the term is often applied to phenomena with multiple-color in-teractions Adichroic substance can cause an incident electromagnetic wave to pass some frequencies and reflect others - a very useful and often very beauti-ful characteristic The iridescence associated with a hummingbird's feathers, when viewed from the right angle, gives some idea ofthe type ofcolor effects that can be deliberately created with dichroic materials Light propagated from a dichroic interaction has its associated electric field vector vibrating in a single direction See absorption, anisotropic, dichroic filter, polarization
dichroic filter/reflectorThere are a variety ofreflec-tors that exploit dichroic phenomena to reflect some wavelengths and allow others to pass through For example, a reflector may reflect only light in the vis-ible range and filter out other frequencies by allow-ing them to continue on through the reflector Dich-roic filters differ from gel filters in that they reflect rather than absorb undesired wavelengths This gives dichroic filters low susceptibility to heat degradation Dichroic reflectors can be fabricated through vapor deposition processes in which fine layers of differ-ent materials are deposited within a thermal cham-ber The layered composite may subsequently be stamped or cleaved into smaller components Depending upon their construction and the frequen-cies with which they will be interacting, they can