Digital Subscriber Line is a fam-ily of two-way communications services that makes it possible for you to talk to your friend without hang-ing up the computer connection first.. A matri
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authentication, and/or signer authentication services
for creating independent, interoperable
implementa-tions The XML Signature is a method of
associat-ing a key with referenced data The XML namespace
Uniform Resource Identifier(URI)and prefix for
other sub-URIs for the W3C specification is
xmlns=/Ihttp://www.w3.org/2000/09/xmldsig#/1
See encryption, JEPI, Pretty Good Privacy, signature
Digital Signature StandardDSS A draft standard
to permit the creation and transmission of a secure
digital signature through a Digital Signature
Algo-rithm (DSA) to provide authentication ofdocuments
and transactions Web commerce is eagerly seeking
means by which documents can be electronically
se-cured in order to use them for trade, banking, stock
transactions, contract negotiations, etc and will
prob-ably quickly adopt this or another scheme when
suf-ficient confidence in its efficacy is attained See
Elec-tronic Certification
Digital Steppingstones projectA project~f t~e
Tomas Rivera Policy Institute (TRPI) that studies
IS-sues ofaccess to technology and telecommunications
networks and examines exemplary practical
imple-mentationsindiverse environments, including
librar-ies, schools, and community centers across the U.S
The Digital Steppingstones project focuses on~der
served communities such as low-income and
minor-ity communities.Animportant aspect of this project
is assessing access to the Internet and making
rec-ommendations for its broad and practical
implemen-tation in publicly accessible facilities
http://www.trpi.org/dss/
Digital Subscriber LineDSL, xDSL A data
trans-mission service operating over existing copper
pub-lic phone lines Imagine turningo~yourco~put.er,
connecting to the Internet, and findIng somethIng
In-teresting that you want to explain to abusin~ss
col-league or friend Ifyou have only one phoneh~e, ~~
you're using a modem to change the computer s
dIgi-tal signals into analog signals that can be sent over
the phone line, you would historically have to hang
up the modem, wait for a dial tone, and then call your
colleague or friend Digital Subscriber Line is a
fam-ily of two-way communications services that makes
it possible for you to talk to your friend without
hang-ing up the computer connection first You can do both
at the same time, which means you can talk through
the phone while you navigate the Nettoge~h~r,
dis-cussing the things that you both can see ThisIShow
it is done
Phone services historically have been analog
sys-tems, and there are millions ofmiles ofcopper wires
installed around the world to provide these services.
With the development ofcomputers, phone switching
centers began, in the late 1980s, to convert to digital
equipment and software This enabled vo!ce andd~ta
to be carried on one line at the same tzme, and
zn-stead ofusing a modem to change the computer
nal to analog, and leaving the voice as an analog
sig-nal, it could be done the other way around In other
words, now the voice call is changed to digital and
the computer signal remains digital This opens up a world ofpossibilities for faster transmission, better compression and security, and simultaneous datal voice communications, without having to replace those millions ofmiles ofcopper wires.
That sounds very practical, yet relatively few people have switched to DSL services One ofthe reasons is distance While most subscribers are within the
12 000 feet or so in which DSL services can operate
at their best speeds, about 20% of the population is not Crosstalk and other types of interference are problematic as well and are still being resolved For example, traditional phone lines have loading coils installed at intervals, to extend the signals on voice grade communications Unfortunately, at higher digi-tal data rates, these coils cause interference Perhaps more important is the way in which DSL ser-vices were deployed Originally, subscribing to DSL involved having the phone company install a special voice/data splitter on the subscriber premises and, further, installing a special peripheral device in the subscriber's computer This method was costly and not very practical, and most consumers are resistant
to having proprietary peripheral cards installed in their computers Most prefer the option of choosing
a vendor and interface, and also ofinstalling the hard-ware external to the computer, so things can be changed around as needed For this reason, a num-ber of commercial vendors have proposed several variations ofDSL services, such as DSL Lite DSL was first developed by Bell Communications Research Inc in 1987 to provide a means to deliver interactive TV and video-on-demand over copper wires The name is somewhat confusing, since it is
not the line that is installed, but rather the interfaces
at each end of the line The point of DSL was to cre-ate technology that would make use ofexisting lines
In fact, a DSL line typically consists oftwotelepho~e lines Since the introduction of DSL, further vana-tions have been adapted, as shown in the Digital Sub-scriber Line Services chart
Digital Subscriber Line coding and variations Since DSL is a multichannel service,it is necessary
to split the available bandwidth to utilize it efficiently This bandwidth splitting is typically done with echo cancellation (EC) or frequency division multiplexing (FDM)
There are two predominant schemes for subdividing available bandwidth into smaller units to individu-ally evaluate their transmission suitability This is useful over twisted-pair copper lines, which can vary widely in their characteristics The two most common are discrete multitone (DMT) and carrierless amplitude and phase modulation (CAP),~dothers are being developed Some ofthese modulatIontec~ niques have descended from the telegraph and radIO broadcast industries, and some, such as wavelet en-coding, are relatively new and still being explored Each ofthese has various trade-offs in terms ofavail-ability, cost, speed, and susceptibility to interference,
as shown in the Common Modulation Schemes chart
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Q.931 that is implemented over Layer 3 of the ISO
communication model It defines protocol for
estab-lishing, maintaining, and tearing down calls DSS 1
is commonly used in local loops providing Integrated
Services Digital Network (ISDN) services for
trans-mission over the D channel Once the DSS 1 signal
reaches the local telephone switching office, it is
usu-ally transmitted to external nodes using Signaling
System #7 (SS7) See D channel, Integrated Services
Digital Network
Digital Subtitle EncoderDSE A PoliStream
sec-ond-generation multimedia titling encoder that is
ca-pable of transmitting ideographic languages (e.g.,
Chinese) in varied font styles It is a Digital Video
Broadcasting (DVB) standards-compliant bitmap
imaging system designed to transmit multilanguage
subtitles It accepts subtitle data from traditional
con-trollers and generates realtime output ofcompressed
bitmaps The data may be sent through regular
broad-cast channels, satellite broadbroad-cast relays, or computer
network links (e.g., Ethernet)
Digital Supervisory Audio ToneDSAT A means of
signaling using audio tones on cellular data networks
In AMPS cellular signaling, there are three designated
supervisory audio tones (SATs) However, in
NAMPS, there are instead seven subaudible digital
vectors called DSATs SATs and DSATs are used to
verify the correct channel tuning after the channel has
been assigned The central office (CO) notifies as to
the new voice channel and vector
digital switching matrixDSM A matrix format for
digital switching that facilitates signal routing in high
bandwidth applications The device can be
concep-tualized as a grid ofchannels (e.g., 64¥64 data
chan-output to any other selected channel, or the channels may be grouped into frames for combined process-ing In commercial applications, the DSM may ac-cept both digital and analog signals, but the signals will be converted to digital signals before being switched (routed) Pulse code modulation (PCM) is typically used for digitization in telephony applica-tions A commercial DSM for high-capacity tele-phonic switching is microprocessor controlled and may also include memory, time base, speech, and control components
DSM is used in data switchers for realtime switch-ing between synchronous video sources.Itis also used in high-capacity telephone networks for rout-ing calls to various application processrout-ing units In a Datapath architecture digital telephone network, it is the switching unit between the data line card and the digital carrier module or trunk controller See Datapath Loop Extension
Digital Telephony and Communications Privacy Improvements ActAlso known as the Digital Tele-phony Bill, this law was passed by the U.S Congress
in October 1994 The terms of the bill, drafted to be supportive of the efforts of law enforcement agen-cies, would require common telecommunications car-riers to design networks in such a way that law en-forcement agencies could access,inrealtime, the con-tents ofcommunications on their networks and trans-actional signaling The Electronic Frontier Founda-tion came out strongly against the bill at the time, but after the events of 11 Sept 2001, the same issues have been raised again and proposed as changes to the Se-curity and Freedom through Encryption Act devel-oped in the late 1990s
When the Digital Telephony Bill was first introduced,
Varieties of Digital Subscriber Line Services
asymmetric DSL ADSL 6Mbps+ Twisted-pair copper phone wires The possible
maximum rate of transmissions is inversely proportional to distance Typically uses discrete multitone (DMT) line coding for data; frequency division multiplexing (PDM) or echo
cancellation is used to subdivide the bandwidth high bit-rate DSL HDSL TIlE1 speeds Symmetric Longer distances can be supported
through the use of repeaters See high bit-rate Digital Subscriber Line
single line DSL SDSL Still in development Can be used over a single
wire pair
rate adaptive DSL RADSL up to 8.7 Mbps Bandwidth can be tuned to subscriber needs It
works over longer transmission lines Rate and speed adjust to the line length and quality
very high rate DSL VDSL 13-60Mbps Used in conjunction with FTTC or FTTB
Different downstream and upstream speeds (Upstream speed is1.5to2.3Mbps.) Shorter maximum distance Still in development
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wiretapping proponents suggested that electronic
wiretapping was just an extension ofcurrent
wiretap-ping practices as applied to electronic
communica-tions, but technologists and privacy advocates argue
that the analogy is not tenable - the capabilities
in-herent in tapping into the National Information
In-frastructure through electronics opens up
far-reach-ing potentialities that are not safeguarded by current
or proposed legislation in a way that is comparable
to the safeguarding of traditional wiretaps
Asimportant as it is for law enforcement to find ways
to keep up with crimes that are increasingly
con-ducted through new electronic technologies, it is also
important to examine the implications of expanding
law enforcement tapping capabilities How is
elec-tronic wiretapping different from traditional
wiretap-ping? Here is a summary of just a few of the many
complex issues involved:
Visibility The establishment ofan electronic wiretap
can be remotely executed and is thus invisible, in
contrast to traditional wiretaps where a
telecommu-nications technician has to physically install and
deinstall a tap In a physical connection system,
ac-countability is more readily detennined and traced
Electronic wiretapping, on the other hand, is hard
to monitor A tap can potentially be engaged and
disengaged through software that cannot be seen by
anyone other than the user
Duration ofa Tap There are laws and warrants
de-termining the physical location and effective
du-ration oftraditional wiretaps The physical removal
of a tap or changing of a switch at a phone
com-pany ends the transaction when the court-assigned
permit expires Even with the same court order
re-quirements in place, there are currently few
safe-guards consistently integrated into the network
sys-tems of Internet Services Providers or the Internet
as a whole that can ensure that the process has been
ended as required Due to the nature ofsoftware and
network communications, an electronic tap could
be continued beyond the stated deadline with little
chance ofdiscovery Some proponents feel that
ac-countability needs to be ensured by agents outside
of the system requesting the tap ISPs might be a
natural choice, but then the time and economic
bur-den of compliance fall on vendors rather than on
law enforcement agencies and vendors would, in
essence, be entrusted with watching the police, a
situation whichhashistorically led to problems such
as deal-making and, in some cases, corrupt
alli-ances Before electronic wiretapping can be fully
endorsed and implemented, a new accountability
technology needs to be built into global networks
Perhaps commercial vendors looking for new
mar-kets can help resolve these issues with innovative
products
Eavesdropping on Computer Users Outside the
Pur-view of the Tap A physical wiretap is limited in
scope You must install the tap at or near the
pre-mises being tapped The tap may inadvertently pick
up conversations of innocent individuals in the
vi-cinity, but this is usually a local rather than a
large-scale problem With an electronic wiretap, the lo-cal physilo-cal limitation is gone Theoretilo-cally, com-munications and activities of hundreds of millions
of individuals could be monitored with the same mechanism used to monitor the alleged criminal, without any obvious sign that this is happening This aspect must be addressed before broad-rang-ing powers are given to officials to tap a medium like the Internet with technology that is exception-ally powerful and generalized in its capabilities See American Civil Liberties Association, Electronic Freedom Foundation, Security and Freedom through Encryption Act
Digital Television Standard DTS See ATSC Digi-tal Television Standard
Digital Television Station Project, Inc DTSP A collaborative project ofthe broadcast industry, includ-ing almost 300 broadcasters, computer manufactur-ers, content creators, and service providers Based in Washington, D.C., the DTSP originated as the Model HDTV Station Project(WHD- TV),established by the Consumer Electronics Manufacturers Association (CEMA) and the Association for Maximum Service Television (MSTV) in 1996, to provide a hands-on educational facility for the implementation ofdigital television (DTV) technologies DTSP is the January
2000 follow-up to this project, supported by the origi-nal sponsors and also by the Natioorigi-nal Association of Broadcasters (NAB) DTSP was established with an I8-month mandate to operate WHD-TV, a model digital television station, and provides continued op-portunities to develop and test interactive television, data broadcasting, and overlay services such as closed captioning, lip syncing, etc See Association for Maximum Service Television
digital trunked radio system DTRS A radio com-munications system in which comcom-munications chan-nels are shared overtrunks,as opposed to individual channels being shared by users as they become avail-able DTRS systems can be set up as individual or group channels (called talk groups) or combined as groups of groups, making it a useful system for gov-ernment, emergency/safety, and industrial communi-cations Priority and encryption can be incorporated into digital trunked systems and they can be config-ured to intelligently manage traffic over thetrunks
to optimize efficiency of the system
digital to analog conversion The conversion ofdata stored as discrete units, usually in ones and zeros on computer systems, to modulated analog wave pat-terns.Amodem is a common device which performs digital to analog conversion when changing computer signals to modulated analog signals that can be car-ried electrically through a phone line connection The process is reversed at the receiving end See D/A con-version, modem
Digital Versatile Disc DVD Also frequently referred
to as digital videodisc, when the content is primarily image-based, but the format is not limited to video images, and a large proportion of digital videodiscs also contain audio
DVD is a vendor consortium-developed
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store a much larger quantity of data Standardization
has not been a single process The DVD Forum, a
consortium of developers, and Philips and Sony,
in-dependent commercial developers, have provided
somewhat different versions ofDVD The Philips and
Sony technologies can be licensed on a royalty
ba-sis, while the DVD Forum specifications are shared
technologies The DVD fonnat is gradually
becom-ing standardized so that DVD consumer players and
DVD-enabled computers can interchangeably use the
discs DVD is quickly growing in support and
accep-tance by consumers, especially for entertainment and
computer data storage purposes One of the more
re-cent developments is the release of the DVD+RW
fonnat by the DVD+RW Alliance, which is not
di-rectly related to the DVD Forum
DVD physical discs are the same diameter as the CDs
popularly used for music (120 mm), but very slightly
thicker, bringing the recorded surface of the disc a
little closer to the laser pickup, pennitting a higher
resolution or areal density, i.e., smaller, more precise
pits can be used to store the information, depending
upon whether the disc is recorded single- or
double-layered, and single- or double-sided DVDs use
higher density storage and different modulation and
error-correction schemes than CDs Further
flexibil-ity is possible through the use of dual lens apertures
in the laser pickup to provide a dual CDIDVD player
While a DVD player can usually play CDs for
back-ward compatibility, a CD player does not inherently
have the capability to play DVDs
DVD specifies more than the compression and
play-back format, it also provides functionality to build
interactivity into the medium through menus,
mul-tiple languages, and other features This
functional-ity makes it attractive for educational software and
games programming and allows movie makers to
in-clude extra features
DVD designates MPEG-2 as the digital compression
standard for video recorded on DVDs MPEG-2 is a
fast digital motion recording and playback
specifi-cation A DVD can be recorded on both sides for up
to a total of about 18 Gbytes of data This capability
is very attractive to developers, as it means a
two-hour MPEG-2 encoded movie can fit on one side
Sound is encoded in either MPEG audio or Dolby
AC-3
DVD can be played on a stand-alone system similar
to a combination CD/laserdisc system; it works like
a laserdisc player, but is small like a CD player It
can also be played on a computer through a DVD
computer peripheral player
DVD can provide many types of audio storage and
playback but, because of its high storage capacity,
vendors are particularly interested in offering
mov-ies on DVD rather than on cassette tapes or the
larger-format videodisc larger-formats The DVD medium is more
robust and convenient than tapes or large discs,
pro-vides better sound than most tape technoloogies, and
can store motion pictures of up to 133 minutes in
cient to hold every type of movie, so longer films may be offered on two discs or are compressed with
a number of innovative predictive coding methods
It is probable that DVD, or something like it, will eventually supersede traditional video cassette tape movies DVD is also apromising technology for elec-tronic books (ebooks)
Digital Versatile Disc associationsMuch ofthe im-petus for DVD development has come from indus-try alliances to promote and develop the technology
See individual entries for organizations ofparticular interest, which include the DVD Forum and the DVD+RW Alliance
Digital Versatile Disc player, DVD playerA con-sumer electronics component for decompressing and outputting the digital data from a Digit8;1 Versatile Disc (DVD) The player uses a laser light beam to scan and read the infonnation coded into pits and grooves in the DVD medium DVD players may play
a single disc or may be capable of holding multiple discs (similar to CD disc changers and jukebox sys-tems)
Most DVD players will also play audio CDs but the DVDs themselves are not directly CD-compatible
The DVD fonnat uses a higher density of storage on the disc and different modulation and error correc-tion schemes than CD Consumer DVDs are designed
in much the same way as laserdisc players and VCRs
to play movies (multimedia) and audio recordings on
a television set or through a monitor and separate audio components DVD players are also available for personal computers and may be internal (similar
to a CD-ROM player) or external (similar to an ex-ternal CD-ROM or cartridge player) In the past, DVD players that worked on computers couldn't nec-essarily play the same DVDs as a separate consumer DVD player, but intercompatibility is now more common
DVD players are also being designed as small por-table entertainment devices (like small boom boxes
or portable television sets) Movies are one of the most popular types ofDVD products being sold and rented Typically movies played back on DVD play-ers with appropriate monitors support high quality component (YIC)video at about 500 horizontal lines
of resolution compared with only 210-225 horizon-tal lines ofresolution in VHS DVDs are mastered at CCIR6014:2:2 ratios DVD-Video supports multiple aspect ratios When movies are played back, the con-sumer is usually given a choice of aspect ratios, se-lected from letterbox (4:3), pan and scan (4:3), and anamorphic (wide screen 16:9) fonnats DVD play-ers sold in Europe often support both PAL and NTSC while those soldinNorth America may only support NTSC (this is changing as higher definition formats are finally beginning to catch on)
Audio frequency response is generally in the range
of 2 to 44,000Hzwith a signal-to-noise ratio of 110
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decibels or more The higher-end DVD players tend
to have added features, such as progressive scan,
vari-ous filters, digital video equalizers, surround sound,
and slow-motion effects, that are not available on the
less expensive models
DVD+RW, one ofthe newer versions ofDigital
Ver-satile Disc technology, is promoted by the DVD+RW
Alliance as a storage and digital video recording
for-mat that will be compatible with the majority of
ex-isting DVD players It is expected that DVD+RW
players/drives will be able to read CD-ROM,
CD-RW, DVD-R, DVD-RW, CD-Audio and other
similar discs See Digital Versatile Disc, DVD Forum
digital videoDV Technologies that enable the
re-cording and playback of digitally encoded moving
image infonnation and sound fall into the category
of digital video Some ofthe big barriers to
inexpen-sive digital video have been the large amount ofdata
that is required to record even small segments of
video and the wide bandwidth and processing that is
required to quickly display high resolution color
im-ages When still images from film frames using cell
animation techniques are individually digitized and stored, each frame may require up to 24 Mbytes, if it
is to approximate closely the image quality of35mm film Since each second ofanimation requires between
24 and 30 individual still frames, as much as 720 M-bytes may be needed to store a second of video A full-length movie is usually ca 7000 seconds or more, requiring more than 5,000,000 Mbytes of storage That's a lot, and that's not including sound or data that might be added to provide search and retrieval markers interspersed with the images
Digital video has been in development longer than most people realize Much ofthe research in this area originated in the Research Center in Yokohama, Ja-pan, in 1977 By 1979, Ampex, Bosch, and Sony were introducing technologies based on digital video con-cepts, and there was a move to introduce new stan-dards to encompass emerging digital technologies
In order to make digital video technology possible, a number of innovations and trade-offs have been implemented Data compression and decompression techniques are used to store images in less space, but
Digital Versatile Disc (DVD) Formats
Format Description
DVD-Audio A CD-like DVD with improvementsinaudio fidelity and higher capacity than
traditional CDs
DVD-R Primarily a read format.Itis a professional authoring fonnatinwhich the DVD
data, when ready to master, is recorded once to the DVD and then read as many times as desired
DVD-RAM A type of hard storage with random read-write access It is a medium that can be
written to, with an appropriate DVD drive, more than 100,000 times and functions much like a hard drive, except that the discs are easily swapped and stored (like cartridges) DVD-RAM discs can be used to store up to about4.7Gbytes of data
DVD-RW Similar to a DVD-RAM, in that it is a type of hard storage, but rather than
random-access, it uses sequential-access, similar to streaming tape storage for up
to about4.7Gbytes per side.Itcan be rewritten up to about 1000 times and thus is
an appropriate choice for data that doesn't often change, such as backup data, or database information that is read more often thanitis written
DVD+RW This rewritable DVD format is one of the more recent versions, promoted by the
DVD+RW Alliance Hewlett-Packard announced the release of its first DVD+RW drive in August 2001 DVD+RW is primarily intended for data storage and recording digital video onto4.7-Gbyte discs The Alliance plans to release software upgrades to support DVD+R as well This format has been developed without the endorsement of the DVD Forum and is competitive to current formats DVD-ROM Aformat for computer data storage drives that is generally available with SCSI
and ATAPI interfaces with capacities ranging from about2.xGbytes to over9 Gbytes Blank rewritable cartridges are available in both single- and double-sided optical cartridges and recordable discs DVD-ROMs may at some point supersede CD-ROMs
DVD Video Text Sometimes called DVD-Text, an optional means to store and access textual
information related to a DVD It can consist of consumer information for users of DVD players or DVD-ROM drives, text information for content providers or DVD authors, or textual supplements to a video or audio data stream to enhance the value of the main information on the DVD.Aguidebook forDVD-Text is available from the DVD Forum
Trang 6cessors and frame buffers are required to handle
play-back in realtime
In spite ofits limitations and technical requirements,
developers are forging ahead with digital video
prod-ucts partly because digital video can be edited and
manipulated in remarkable ways Special effects that
are impossible or difficult to achieve with analog film
are possible with digital video DV also has greatly
increased possibilities for interactivity and access
through the Internet Offurther importance is the fact
that it doesn't have to go through a chemical
photo-finishing process before it can be used See animate,
celluloid, D-l, Digital Versatile Disc, interactive
video, MPEG, video-on-demand
Digital Video BroadcastingDVB A family of
compatible television/media delivery standards
includ-ing cable and satellite technologies Digital Video
Broadcasting standards are supported by the Digital
Video Broadcasting Group and published by the
Eu-ropean Telecommunications Standards (ETSI) In
conjunction with the Centre for Electrotechnical
Stan-dards (CENELEC) and the European Broadcasting
Union (EBU), ETSI has formed ajoint technical
com-mittee to handle DVB standards DVB is gaining
ac-ceptance in Europe as a digital video infrastructure
and is gradually replacing the traditional analog
en-vironment There are a number of subdivisions of
DVB technologies
DVB Technology Subdivisions
Subdivision/Description
DVB-C cable transmission standard
DVB-S satellite transmission standard
DVB-T Digital Terrestrial Broadcasting
(DTTB) designed to be adapted to the
needs of local frequency and
geo-graphical environments It enables the
development of single-frequency
net-works (SFNs) Configurable to support
legacy 50- or 60-Hz systems
See Digital Video Broadcasting Group
Digital Video BroadcastingGroupDVBG A
Euro-pean trade consortium that provides support and
specifications for traditional and emerging broadcast
technologies, such as cable and satellites, and
schemes for the protection of commercial
program-ming See Digital Video Broadcasting
http://www.dvb.org/
digital video formatA series of digital video
stan-dards introduced since the 1980s, including 0-1,0-2,
0-3, etc See Digital Video Formats chart
Digital Video InteractiveDVI A digital recording
and playback chipset technology developed at the
David SamoffResearch Center The technology was
acquired by Intel Corporation which subsequently
developed it into Indeo 2 and Indeo 3.Itis now known
teractive
digital videodiscSee Digital Versatile Disc digital voice encodingA process of sampling and quantizing voice signals and storing them as digital data Since voice encoding can require much memory, the information is usually compressed for storage and decompressed when replayed Fractal and wavelet compression techniques are becoming popular choices Much of the technology for digital voice encoding has come from the music industry Research into the sampling and playback of synthesized music can be applied effectively in voice encoding and play-back applications
Digital voice encoding is used in speech and voice recognition systems and is used to send conversations over digital voice telephone channels, such as Inter-net telephone applications, ISDN, or digital PCS To save memory and cut down on transmission time, in-teresting algorithms for removing pauses and spaces are used in conjunction with digital voice Encryp-tion to ensure privacy is also possible with digital voice communications
Digital voice encoding can provide a data library of sounds, phonemes, words, or other units, for use in digital applications such as automated voice menu systems, speaking computer applications, digital talk-ing books, answertalk-ing machines, voice mail systems, and more Digitally encoded voice tends to be more pleasant and natural sounding than mathematically generated voice and, hence, is favored for applica-tions where instrucapplica-tions or responses are given to a human listener See quantize, silence suppression, speech recognition, voice recognition
Digital8A digital video format developed by Sony
Itrecords digital video data and stores it on Hi8/8mm tapes The recording time is about 45 minutes due to the faster rolling speed of the tape compared to Hi8/ 8mm (which records about two hours) See Hi8mm Digital-SSee digital video format (D-9)
digitizelJ.To convert from analog to digital, i.e., to take an analog signal and convert it into data that con-sists of discrete units, such as ones and zeros, usu-ally bysampling the analog signal at discrete points
and assigning a value to the data at that point Since most broadcast media are transmitted as waves, they are analog systems of communication However, in order to process the information or interface it with networks, it must be converted to digital format at some point in the transmission process Thus, digi-tizing a phone signal makes it possible to add fea-tures such as compression, encryption, and voice rec-ognition Digitizing a video signal allows processing
of the image: palette changes, overlays, image com-posites, split screen viewing, videoconferencing and more A desktop scanner is a type of digitizer, as is a digital camera See analog, digital, digitizer, pulse code modulation
digitizerAdevice that converts a signal from analog
to digital, usually by sampling the analog signal at discrete points over time and assigning a value to the measurement Up to a point, the more frequent the
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sampling, the better the encoding represents the
origi-nal Digitizers are commonly used in video and
au-dio applications The soundpatches used in electronic
music are digitized from analog sound samples
Digitizers are not constrained to only two dimensions
A pen or robot arm can trace contours on a physical
object and convert the spatial information into a 3D
coordinate system for rendering, ray tracing, or CAD
programs One ofthe first widely distributed 2D
digi-tizers for microcomputers (1986) was NewTek's
DigiView for the Amiga It is historically significant
as the forerunner to the Video Toaster, which
initi-ated the desktop video industry See sampling
DllSee defense information infrastructure
Dijkstra'sAlgorithmInATM networking, an
algo-rithm sometimes used in conjunction with link and
nodal state topology information to calculate routes
DIMSee document image management
dim fiberFiber optic cable in which the carrier
pro-vides the means to carry the signal through the fiber
but does not originate the signals at either end of the
circuit See dark fiber
dimmedLowered in illumination or visibility
Re-duced in intensity See ghosted
DIMS 1 Document Image Management System A
hypermedia-based document management system
jointly developed by HyperMedia and Aramco for the
purpose ofcapturing the geological, geophysical, and
reservoir engineering data at Aramco 2 document and image management system A system for auto-mating and integrating the management ofimages of text and graphical documents that are typically scanned and stored as digital data
DIN 1 Deutsches InstitutefUrNormung German in-stitute for standardization, one ofthe major standards bodies in Europe DIN is located in Berlin The DIN specification is a widely used standard for computer connectors 2 dual inline
DINA 1 Danish Informatics Network in the Agricul-tural Sciences 2 Distributed Intelligence Network Architecture 3 Dynamic Intelligent Network Archi-tecture A research project topic at Carnegie-Mellon University
dinosaurslangAlarge, obsolete, or aging system that
requires excessive resources to keep in operation Given the speed of technological obsolescence in computer technology, systems quickly become dino-saurs, but firms often hang on to them because the cost of installing and learning new technology may
be higher than continuing to use old, slow, but tried-and-true systems The term dinosaur applies espe-cially to old room-sized supercomputers that are now less powerful than many desktop systems
diode, Fleming tubeHistorically, an electron tube with only two electrodes, the cathode (electron-emitting) and the anode (electron-attracting) in which Digital Video Formats
D-l A component digital video format approved by SMPTE in 1985 and introduced
by Sony as the DVR-l 000 in 1987 It is favored by digital effects videographers D-l records uncompressed 8-bit video The cassette tapes are large, weighing up
to six pounds
D-2 A composite digital video format introduced by Ampex (1986) and Sony (1988)
that enjoyed broad popularity until the late 1990s, superseding composite video tape recorders(VTRs)using 1" tapes D-2 (CCIR 601) records uncompressed video The cassette tapes are large, weighing up to six pounds
D-3 A composite digital video format developed by Matsushita and introduced by
Pana-sonic as a competitor to Sony's D-2 format D-3 records uncompressed video D-5 A component digital video format developed by Matsushita and introduced by
Pana-sonic as a competitor to Sony's D-l format D-5 records uncompressed 10-bit video D-5 has caught on in High Definition Television (HDTV) Some D-5 systems can play D-3 tapes Panasonic has introduced a high definition 4: 1 compression ratio version ofD-5
D-6 A high-definition digital video format developed by Toshiba/BTS and introduced
by Philips D-6 records up to 64 minutes per 19 mm cassette through a system of helical scan tracks with a track pitch of 22 micrometers There is a longitudinal analog cue track, a control track, and a time code track
D-7,DVCPRO (DVCPRO is the more common name, D-7 is the SMPTE designation, but it is
placed here for easier comparison with other D- formats.) Acomponent digital video fonnat introduced by Panasonic D-7 records compressed 8-bit video at 34mmJ
second D-7 supports 16-bit, 48kHzdigital audio Streaming video is supported through SDTI and IEEE-1394 (FireWire) standards
D-9, Digital-S A component digital video format introduced byJve.D-9 records 3.3: 1ratio
com-pressed 1O-bit video Promoted as similar in quality to Digital Betacam
Trang 8wasn't very useful L de Forest developed a Fleming
tube into a triode, a three-electron tube, which enabled
control of the electron flow
In transistor electronics, a diode is a piece of
semi-conductor material, positive on one side and
nega-tive on the other, with a terminal at each end Where
the electrical positive and negative regions come
to-gether in the semiconductor, it is called a p-n
(posi-tive-negative) junction Like a two-element electron
tube, the electrons normally flow in one direction,
serving as a rectifier, enabling conversion of energy
from one fonn to another - an essential aspect
ofcom-munications technology An old crystal radio
recti-fier can change radio waves into audio waves that can
be heard through earphones A semiconductor
recti-fier can change alternating current (AC) to direct
cur-rent (DC) for powering electronic equipment
Forcing the flow of electrons to go opposite to the
natural direction can be accomplished in some
cir-cumstances with sufficient voltage, resulting in
re-verse bias This practice is sometimes useful in
semi-conductor technology for altering the information
stored in a chip See avalanche breakdown, erasable
programmable read-only memory, Zener diode
Fleming Valve - Two-Electron Tube
evacuated globe
metal sleeve
(anode) for
attracting electrons
filament
(cathode) for
emitting electrons
connecting leads - - -
The Fleming valve was an important historical
an-tecedent to three-element vacuum tubes but lacked the
controlling grid that made it possible to control
elec-tronjlow.
diode laser moduleDLM See laser diode
diode transistor logicDTL A circuit board
configu-ration wherein the logic is accomplished with diodes
as opposed to resistors, reducing interaction between
inputs and permitting many diodes to be used
How-ever, depending on the design and application, there
may be a trade-off in speed DTL calculators during
the 1960s can be found that include both transistors
and integrated circuits and represent some ofthe early
DTL devices Sometimes referred to as diode logic
(DL), the early implementations tended to be
discrete-seded by integrated circuits) As integrated circuits (ICs) became more commonly used, DTL became an
IC implementation ofthe basic diode logic See tran-sistor-transistor logic
DIP 1 document image processing 2 dual inline package See DIP switch
DIP switchdual inline package switch Avery small switch that is meant to be toggled to one side or the other (hence "dual").Itcan be changed with a pencil
or other pointed object Afew DIP switches are large enough to be toggled with the fingers The early desk-top microcomputers, such as the Kenbak-1 and the Altair, were programmed with DIP switches The smaller DIP switches are often found on SCSI devices, for setting SCSI ill numbers or for adjust-ing other settadjust-ings Graphics controller cards some-times have DIP switches to adjust resolution or scan rate settings
dipole antennaIn its basic sense, an antenna with two poles mounted horizontally to produce one long rod that is a subdivision of the length of the wave-length it is designed to receive Ideally the impedance (expressed in ohms) ofthe lead connecting the dipole antenna to the receiving equipment (radio, television, etc.) should match the impedance of the antenna at its strongest point (usually the centerina symmetri-cal antenna) The frequency response of a simple dipole antenna can be increased by placing the two rods proximate and parallel, and connecting them at
both ends to form afolded dipole antenna.
DirecPCA commercial service from Hughes Net-work Systems allowing data communications through Very Small Aperture Terminal (VSAT) satellite sys-tems with a personal computer The satellite signal
is received by a two-foot parabolic dish antenna that feeds the transmission to a peripheral card in a com-puter where the signal is demodulated, demultiplexed, decoded, and then sent to the software interface The software can also orient the satellite dish
An innovative version ofthis system for Internet use combines the satellite receiving system and a normal analog connection to an Internet Services Provider (ISP) as an upload/download hybrid system This service allows the user to connect to an ISP through the normal phone line with a conventional modem,
to interact through a Web browser When files are re-quested, rather than downloading through the ISP phone connection, the files are transferred from the DirecPC network operations to the VSAT satellite, and then to the user's DirecPC dish, thus providing downloads at 400 kbps compared to about 39.6 kbps direct access storage deviceDASD.Quick access computer storage devices such as hard drives and memory chips
direct broadcast satellite DBS Originally this phrase was intended to describe a particular service transmitting in the 12.2 to 12.7 GHz range, a fre-quency approved by the World Administrative Radio Conference (WARe) However, since the availabil-ity of smaller, more convenient receiving dishes, the term is also used in a broader context, to describe any
Trang 9Fiber Optics Illustrated Dictionary
satellite that transmits a signal that can be picked up
by individual home and business subscribers,
with-out going through an intermediary station
Since direct broadcasts are transmitted to small
con-sumer dishes, and the size ofthe dish is related to the
length ofthe radiant waves being received, the higher
wavelengths are used, such as Ku-band frequencies
This permits a dish as small as two or four feet in
di-ameter to be effective Further, as DBS systems are
transmitted to the small consumer dishes rather than
to a station with a powerful receiving antenna, it was
necessary for them to use high power transmissions,
usually around 40 to 160 watts, far more than was
being used for C-band communications in the 1980s
when DBS began to develop (Today satellites can
deliver almost ten times that power.)
DBS to the home presents a number ofthe same moral
challenges as widespread access to the Internet DBS
providers are concerned about illegal consumer
copy-ing and redistribution ofprogrammcopy-ing, and
consum-ers are concerned about program content These
is-sues are still being studied and resolved
DBS system guidelines for Europe were originally
established by segmenting the 11.7 to 12.5 GHz
fre-quency spectrum into 40 channels, to be shared
among the various European member nations
U.S DBS systems fall under the jurisdiction of the
Federal Communications Commission, through
guidelines established in the various
Telecommuni-cations Acts, and must conform to internationally
established technical standards See C-band,
Ku-band, microwave
direct connect modemA type of modem which
be-came popular on desktop systems in the early 1980s,
gradually superseding acoustic modems The
acous-tic modem was used to connect data and phone lines
by setting the handset of a telephone into two
suc-tion-cup style holders Room noise could be a
prob-lem and the holders didn't always provide a good
con-nection In contrast, the direct connect modem was
cabled electrically between the computer serial port
and the modem, if it was an external modem, or
be-tween the computer and the internal modem card to
the telephone line through the telephone jack that
normally connected to a phone This system offered
faster data rates and was more convenient and
effec-tive in reducing noise and ensuring a good
connec-tion See acoustic modem
direct currentDC, dc Amore-or-Iess constant
elec-trical current flowing in one direction DC current is
supplied to many small appliances by batteries
Much ofearly communications history was based on
direct current (DC) as a power source Telephones had
talking batteries and common batteries The
batter-ies tended to be large, leaky wet cells, that caused
inconveniences ifmoved or subjected to temperature
fluctuations More than fifty years after the
inven-tion ofthe telegraph, AC power for telegraph systems
was still considered a novel idea, but the shortage of
batteries, and their cost, provoked French and Swiss
engineers to experiment with AC generators, as
de-scribed in the Annales des Postes, Telegraphes et
Telephones in September 1919 Eventually the ad-vantages ofAC power were better understood, its use became common, and DC took its place as a current source for small, portable electric conveniences such
as calculators, radios, wristwatches, laptops, cameras, etc See AC to DC converter, alternating current; re-sistance; Tesla, Nicola
Direct Distance DialingDDD A commercial name
to indicate the capability of a network to connect a long-distance call without operator intervention Direct Inward DialingDID.Inthe past, calls going through a central office (CO) to a private branch ex-tension had to go through an attendant With increas-ing automation, this routincreas-ing is now rarely necessary With DID, the called digits are passed through cen-tral office DID lines directly into the private branch exchange (PBX) DID lines do not offer a dial tone and hence cannot be used for direct outgoing calls Direct Inward System AccessDISA A telephone setup in which outside callers can dial into a telephone system, usually a private branch exchange (PBX), and have the use of the system's services as though they were on the premises using the system from the inside Direct Marketing Association, Inc.DMA The larg-est trade association for users/vendors in the direct, database, and interactive marketing fields, founded
in 1917 Due to the significant impact ofthe Internet
on marketing technology and venues, the Association has also acquired two electronic commerce trade as-sociations, the Association for Interactive Media (AIM) and the Internet Alliance (IA) Telecommuni-cations are an integral aspect of marketing, and thus progress in electronics has shaped and been shaped
by the activities of the DMA
The DMA encourages and supports the growth and profitability of its membership and advocates adher-ence to high ethical standards It provides leadership
on behalfofthe membership in government and pub-lic affairs http://www.the-dma.org/
direct memory accessDMA Ameans to bypass the central processing unit (CPU) in a computer and in-teract directly with memory This access is used to reduce processing time and increase speed
direct outward dialing DOD The capability of a private branch exchange (PBX) to dial calls outside the exchange without first dialing an access code (typically "9"), or going through an operator Direct Print ProtocolDPP A''thin'' networking pro-tocol for data transfer with a pair of DPP command sets, Direct Print Command, and File Transfer Com-mand, for printing images over an IEEE 1394 serial bus, developed by HCL technologies As the result ofDPP interoperability events, a number ofcommer-cialfIrmS have implemented DPP There are other protocols that transmit data over IEEE 1394, includ-ing IP over 1394, SBP-2, and AV/C Each of these has its own strengths and limitations in terms of the types of data that are efficiently handled DPP has mainly been of interest to digital imaging and print-ing developers
direct sequencingA spread spectrum frequency-changing broadcast technology Spread spectrum
Trang 10of frequencies than is typically needed to contain the
broadcast A pseudorandom digital sequence directs
a phase modulator to distribute the original RF
trans-mission over a bandwidth that is proportional to the
clock frequency of that sequence The receiver must
then be synchronized to the same pattern as the
broad-cast generator in order to remove the phase
modula-tion and recreate the original signal
Although it's not commonly done,itis possible to
combine direct sequencing with frequency hopping
This combination would likely be used only in very
high security transmissions, as the synchronization
and receiving techniques are not trivial See
fre-quency hopping, spread spectrum
direct setInATM networking, a set ofhost interfaces
that can establish direct communications at layer two
for unicast
direct view storage tube DVST Acathode-ray
dis-play device introduced in the late 1960s to overcome
the slow refresh and large storage buffer needs of
early vector display monitors By employing a
slow-moving beam and a storage mesh, the DVST design
substantially decreased the cost of display devices
See vector display
directional antenna Aradio antenna that is designed
to concentrate its signal transmission or receiving
strength, resulting in a stronger signal, but one which
is not ofequal magnitude in all directions Commonly
found in AM radio transmissions
directory 1 A list, usually of names and associated
information, often sorted or organized and displayed
to enhance visual clarity These may be the names of
people, companies, institutions, files, or
sub-directories, etc 2 A table of organizational
identifi-ers that provides addresses to individual items within
the organizational path This path is frequently
hier-archical in structure
directory, file On a computer file system, an
organi-zational structure, comprising a file storage area,
un-der which there can be further files or directories A
directory listing typically includes other information
about the directory and its associated files or
subdirectories, such as creation date, permissions, file
type, and byte size The display may be in text or
graphical mode, and an icon resembling a file folder
is often used to symbolize the directory
Directory Agent DA 1 ANovell Directory Services
(NDS) database service that accepts advertisements
for Service Location Protocol (SLP) devices from
Service Agents, negotiates registration/deregistration,
and answers queries See Service Location Protocol,
Service Agent, User Agent.2.A database service for
negotiating links between buyer and seller agents on
a distributed network
Directory Assistance DA A telephone service in
which the caller dials directory assistance to request
the number ofa person outside the local calling area,
or within the local calling area if the number is not
listed The number may not be listed because the
in-dividual just moved in, just changed a phone
num-ber, or installed the phone just after the directory was
given out through Directory Assistance In most ar-eas, there is now about a $.75 charge associated with
a Directory Assistance request, and there is usually a two-number limit on each request
Directory Assistance used to be handled entirely by human operators, but there are now automated systems which will request the city and name of the person whose number is being sought and dispense the num-ber, or hand over the call to the operator to complete the transaction, or to clarify the information provided
by the caller These automated systems combine speech recognition and speech synthesis to carry out their tasks
directory caching A timesaving function of many operating systems that stores the local directory list-ing in memory so that each time the user accesses the directory information (for listing parts or all of the files), it will be displayed very quickly For small di-rectories there isn't much difference, but for very long directory listings it can be faster, especially if the in-formation is being output to a shell or MS-DOS text window Further, the information can be used to ac-cess more quickly the files in that directory Since the directory information is read from memory rather than from disk, transfer time can be faster
Directory Number Call Forwarding DNCF An interim Service Provider Number Portability (SPNP) which is provided through existing available tele-phone services, such as call routing and call forward-ing The DNCF is set up so that calls are forwarded
to a new number While it is called Directory Num-ber Call Forwarding, unlisted numNum-bers can also be set up with the service
Directory System Agent DSA A directory applica-tion process in the Open Systems Interconnecapplica-tion (OSI) system OSI utilizes a system ofagents (helper applications) that can query a local database, com-municate with other agents, or hand off requests to other agents when appropriate The DSA specifically provides associated Directory User Agents with ac-cess to the directory information base(Om) directory tree The name given to a hierarchical di-rectory file structure The "tree" includes the current directory and subdirectories and files associated with the current directory both above it and below it in the hierarchy Directory tree commands are used for file creation, deletion, renaming, protection, or for search-ing a series of files or all the files located in the tree Directory UserAgent DUA Adirectory application process in the Open Systems Interconnection (OSI) system OSI utilizes a system of agents (helper ap-plications) that can query a local database, commu-nicate with other agents, or hand offrequests to other agents when appropriate The DSU specifically as-sists and represents the user in accessing a database through a Directory System Agent (DSA) See Di-rectory System Agent
dirty power Electrical power that is spiky, bursty, noisy, or otherwise unreliable Dirty power can be dangerous to delicate electronic components Lap-tops plugged in on ferries and trains should be used