For example, a very tiny icon file, when en-coded with an image or general purpose data com-pression program, may actually be larger than the original by the time the header, decompressi
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D'Arsonval current Ahigh-frequency, somewhat
high-amperage, low-voltage current
D' Arsonval galvanometer See galvanometer,
D'Arsonval
D'Arsonval movementA description used in
con-texts where a pointer associated with a dial moves to
show a reading when stimulated by direct current
DIA digital to analog SeeDIA conversion
D/Aconversiondigital to analog conversion In the
general sense, a process wherebyanysystem or device
converts a signal with discrete states (e.g., binary ones
and zeroes) into a signal with theoretically infinite
states (e.g., aradio frequency signal).Indata
transmis-sions, a process or device to convert discrete digital
information to a continuous form for transmission
over analog circuits, usually through one or more
modulation processes Thus, infonnation from a
com-puter can be converted by a comcom-puter modem to
analog audio signals that are sent through a phone line, or digital signals from an Internet phone can be converted to analog pulses that can be heard over an analog headset See AID conversion, modem D/CLEC, D-CLECCompetitive local exchange car-riers (CLECs) that specialize indatadelivery services
D/CLEC services mainly arose as a competitive
DSL-based option to expensiveTI services for data
net-working As such, D/CLEC services have been of
particular interest to small businesses
D4 In TI digital transmission lines, D4 is a type of channel bank Channel banks carry out a variety of interface tasks, including time slot framing and de-tecting and transmitting signaling information See SuperFrame
DA I See desk accessory 2 See destination address
3 See Directory Agent 4 See Directory Assistance
5 discontinued availability
Trang 2DAB 1 See digital audio broadcasting 2
dynami-cally allocatable bandwidth
DACS See Digital Access and Cross-connect System
daemon A computer process that lurks in the
back-ground to handle low priority or intermittent tasks,
especially in Unix environments Daemons carry out
many tasks on computer networks, including
low-level operating tasks, to automate some aspect of a
system administrator's responsibility, and are
trans-parent to most users A daemon may be a continuous
background process or intermittent, as needed
Dae-mons are useful as print spoolers, mail message
man-agers, and general resource allocators, especially for
client/server requests that are invoked irregularly
DAF 1 See Data Administration Forum 2 See
Den-ver AdDen-vertising Federation 3 See Destination
Ad-dress Field
Daguerre, Louis Jacques Mande (1789-1851) A
French artist and inventor who made significant
im-provements in photographic imagery technology in
1839 His early photos, calleddaguerreotypes,were
impressed in silver plated onto copper They have a
very soft, low contrast quality to them, and the
clar-ity of the image is affected by the angle at which the
plate is held when viewed, due to the reflectivity of
the metallic medium They are fade-resistant, and
many of the original daguerreotype images that still
survive retain their images See Talbot, Fox
daisy chainlJ.To connect items individually, one to
another in a series, usually through cable or
connec-tor hookups Communication through a daisy chain
of electronic units may be unidirectional or
bidirec-tional SCSI devices such as hard drives, scanners,
cartridge drives, and CD-ROM drives are frequently
daisy-chained to one another and to one controller on
the logic board When chaining SCSI devices, care
must be taken to assure that each device has a unique
illnumber (usually from 0 to 7), and that the last
member of the series (the one farthest from the SCSI
controller) orchainis terminated, either with a
physi-cal connector attached to the outside, or by setting
external or internal switches accordingly Depending
on the types ofdevices in the chain, it may be
neces-sary for all devices to be turned on for other devices
in the chain to function correctly, especially if the
chain has been established for transferring electrical
power as opposed to data signals In other situations,
devices may be individually turned on or offwithout
interrupting the flow of data through the chain
DAL See Dedicated Access Line
Dall testA variation on the historic Foucault test for
assessing the optical quality of lens components that
includes a test lens placed in the path of the incident
light beam The test was described by Horace Dall
in 1947 It is similar to the Foucault arrangement, but
includes a lens placed between a point light source
and the paraboloid surface under test to reveal any
spherical aberrations opposite in sign to the surface
at the radius of curvature Thus, by determining the
index of refraction of a lens that is flat on one side
and convex on the other and setting the lens at a
source and the reflecting mirror or lens under test, the two spherical components can be made to cancel one another out and reveal other characteristics ofthe re-flecting component This is a type of single reflec-tion null test
When a similar configuration is combined with at least two traversals of light through the lens, it is
Dalton, OrvA prominent amateur radio enthusiast who contributed substantially to the design and con-struction ofthe first three OSCAR satellites Callsign K6UEY See OSCAR
DAMA See Demand Assigned Multiple Access
damped waveRadiant wave oscillations that gradu-ally diminish in amplitude or that are being deliber-ately suppressed so that the amplitude diminishes
dampingThe process ofdecreasing the amplitude of wave oscillations The term is often used in reference
to progressively suppressing sound waves (sound damping),though it can generally be used to indicate the suppression ofa variety oftypes ofenergy, as elec-trical oscillations in a circuit
Daniell batteryA historic, fairly simple chemical battery, providing approximately 1.1 volts per cell which was suitable for providing constant, depolar-izing current for early telegraphic systems, such as the Morse system This battery tended to last longer than others in a closed circuit and was widely used during the first three decades ofcommercial telegra-phy The cells were usually housed in a box that was placed near a telegraphic station (often under a table
or desk)
The Daniell battery was comprised of a copper elec-trode in a copper-sulfate solution on one side of a porous separator, and a zinc electrode in a diluted sulfuric acid or zinc-sulfate solution on the other side housed within glass or earthenware This two-fluid battery was an important improvement over the vol-taic pile It is named after J.F Daniell See cell
Daniell, John Frederic(1790-1845) An English chemist and colleague of Michael Faraday who ap-plied his talents, in the mid-1830s, to the develop-ment of batteries that could last longer than those in use at the time He invented a chemical battery known
as the Daniell cell which was used in early telegraph systems See Daniell battery
DAP See Directory Access Protocol
DAQ 1 data acquisition 2 delivered audio quality
DAR See digital audio radio
dark conductionThe property of a substance, such
as a photosensitive material, to retain electrical con-ductance in darkness It is usually a residual effect and tends to diminish over time until restimulated by light
dark fiber, dry fiberPlain, unconnected fiber optic cable, not currently carrying a signal Since fiber is often sold as the hardware portion ofa subscriber ser-vice, this phrase was coined to indicate fiber that is sold just as fiber, with the purchaser doing the wiring
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of the components and transmitters See dim fiber
DARPA See Defense Advanced Research Projects
Agency
DARPANET A distributed network of the u.S
De-fense Advanced Research Projects Agency,
origi-nating in 1969, from a desire on the part of theu.s.
military to exchange information among different
sites and to provide redundancy in the event of an
at-tack This project grew to become ARPANET by
1972 In 1983, ARPANET had grown so large that it
was split into MILNET, specifically for U.S military
use, and NSFNET (National Science Foundation
Net-work), which opened it up to researchers and
scien-tists See ARPANET, Internet, RFC 791
DARS See Digital Audio Radio Service
Darwin See Mac OS X
DAS See Dial Access Switching
DASD See direct access storage device
DAT See digital audio tape
data 1 Constituent basic elements ofinformation that
can be formally organized and combined to provide
communication, most commonly through written
means, though the term is not restricted to written
communications 2 Building blocks that can be
ma-nipulated and presented by electronic means, or
which are used, interpreted, and organized by human
perception and thinking
data access arrangement DAA Asystem for
inter-facing a communications device to the public
switched telephone system Amodem interface is one
example of a DAA that enables computers to
trans-mit and receive data through the telephone system
A DAA may be customer supplied or carrier supplied
and may require approval on the part of the common
carrier providing the telephone line access
Data Administration Forum OAF Adivision ofthe
British Columbia Advisory Council on Information
Management, the OAF encourages corporate data
administration and information management
activi-ties throughout the Canadian province and is active
in the establishment ofdata administration standards
data awareness Mechanisms for aggregating and
switching network data traffic in the appropriate layer
data base See database
data bus See bus
data carrier detect DCD In telephony, DCD is a
nal from the DCE to the DTE, indicating a valid
sig-nal between the DTE and DCE devices It is typically
used to set port status for a connection and to
gener-ate a signal indicating the loss of a connection The
DeEis commonly a modem or serially connected
printer, and the DTE is the terminal or computer
data circuitAcircuit that uses transmission wires and
components suitable for the fast transmission
ofdigi-tal information
Data Communications Channel DCC In SONET
networking, a channel related to the OAM&P which
includes security and performance information
asso-ciated with facility and network elements (NEs) Both
generic and vendor-specific information can be
in-cluded The DCC is incorporated into both the
sec-tion and line overhead
data communications equipment DeE Acategory
of devices specified by the Electronic Industries As-sociation (EIA) that typically includes common se-rial communications peripherals, such as modems and printers These inturninterface with data terminal equipment (DTE) In Frame Relay networking, DCE has a more specific meaning, as switching equipment that is separate from the various peripheral devices that are connected to a network or workstation See data terminal equipment
data compression The process of encoding data to store it in a smaller amount of space Data compres-sion is typically achieved with specialized software tools or with software built into data transmissions hardware Data compression may be done in advance,
if files are to be stored or transmitted later, or it may
be done dynamically at the time it is needed, some-times calledrealtimeoron-the-jly,a capability that
is built into some modems There are many different general-purpose and specialized means ofcompress-ing data Some data compression algorithms are paired with data decompression algorithms, for ar-chiving search and retrieval and audio/video record-ing and playback
Data compression tools do not always make the data smaller For example, a very tiny icon file, when en-coded with an image or general purpose data com-pression program, may actually be larger than the original by the time the header, decompression, or statistical information about the file is inserted by the compression program Yet the same tool may quite effectively achieve as much as 60% compression on large images, so the selection of data compression technologies depends on finding the right tool for the job To overcome this problem, three developers at Western DataCom have developed Ardire-Stratigakis-Hayduk (ASH), a compression scheme that incorporates some of the pattern-matching, and predictive concepts associated with artificial intelli-gence programming This scheme attempts to broaden the scope ofcompression to handle many dif-ferent types of data in the increasingly media-rich communications that are evolving See Ardire-Stratigakis-Hayduk, Lempel-Ziv
data compression approaches There are many prac-tical approaches to data compression One ofthe sim-plest is to remove redundant data, such as gaps, spaces, or repetitions This approach is used in en-coding voice conversations, which typically have many pauses It is also useful for compressing graph-ics that have large areas of similar colors and text documents with repetition and blank spaces Another means for compressing information is expressing it
in a different way For example, a bitmap image of a large letter0may require 15 kilobytes, whereas the mathematical definition for ellipses that can define the letter0may require only 5 kilobytes Athird way
to compress data is totryto match the human per-ceptual recognition of the data image ratherlhan the structural data characteristics of the original presen-tation In other words, there are ways to display graphic images or to play sound files so that they look
Trang 4listener, even though the construction and dynamic
range of the information may have been altered
Hu-mans have a remarkable ability to conceptually add
information or construct a view from a few clues If
you've eVer watched a black and white TV show and
"could have sworn" you had seen it in color, you've
experienced one aspect of this phenomenon
Data compression can belossy or lossless, that is, it
can retain most of the information in a file or all of
the information in a file Acommonly used lossy
im-age format in which most of the information is
re-tained is JPEG, often for displaying Web graphics and
videoconferencing images
With the ever-growing volume of data unleashed by
the capabilities of computer technology, and greater
demands for perceptually rich multiple media, the
demands for data compression to reduce file space,
transmission time, and costs are very high Some of
the most promising recent data compression
pro-grams incorporate fractal and wavelet theories into
their encoding techniques See
Ardire-Stratigakis-Hayduk, JPEG, Lempel-Ziv, PNG, wavelet theory
data conversionThe process ofconverting computer
data stored in one format to another The three most
common reasons for converting data are achieving
compatibility (upward, downward, and
inter-applica-tion), saving space and/or saving time (compression!
decompression), and needing to convert between
digi-tal and analog forms of information
When software applications are upgraded, they
of-ten incorporate new features that are not available in
the older versions Data conversion may be
neces-sary to store information in the older or newer file
format, and some of the information may be lost in
the conversion process
Computer data conversions tend to happen within
families of data Graphics formats are frequently
in-terchanged, audio formats are frequently
inter-changed, but there isn't much need to convert audio
data into visual data, except for experimental
appli-cations That is not to say computer data has to be
rigidly defined; it doesn't For example, the
Inter-change File Format (IFF) developed jointly in the
mid-1980s by Electronic Arts and Commodore
Busi-ness Machines is a broad specification for data
defi-nition that can be generically applied to text, sound,
and graphics Similarly, Adobe PostScript fonts, while
following specific guidelines, are not just fonts, but
rather are shapes that fit in the context ofa larger
pic-ture, that ofa page description language which is
ca-pable ofdescribing many types ofgraphical elements
besides fonts
Many shareware and commercial data conversion
utilities are available, especially for converting
among the myriad graphics formats such as PNG,
JPEG, Compuserve GIF (which now uses the PNG
specification), BMP, ILBM, and TIFF PNG, JPEG,
and GIF are the most commonly used raster
graph-ics formats on the Web, and TIFF is the most widely
used graphics format in the publishing and document
industry (faxes are also definedwithin the TIFF
specifi-sions, and Microsoft's Interchange Format (Rich Text Format or RTF) can be used for text conversions that retain formatting such as bold, indents, fonts, etc For database information, dBASE formats are often used for converting between one program and another See data compression, digital to analog conversion
Data Country CodeDCC In networking, the DCC
~ a a~~~~~~~~;i~~::eSI~~~fiae~~~~i~O:~Z~~1~~~.
data country code is in Binary Coded Decimal (BCD) format, contained in two octets, in ISO 3166 format
See Data Network Identification Code
data descriptionIn a data dictionary, a unit or group
of information which may comprise one or more of the following: a definition ofmeaning and usage, at-tributes or characteristics, and category or classifi-cation information
data dictionary1 A reference set of data descrip-tions that can be machine-processed, and shared by
a variety ofapplications 2 In database mal;1agement,
a lookup reference ofdata descriptions with a format
or relationship such that the database engine can ef-ficiently save, extract, or scan information to/from the dictionary according to the needs of the database
data elementA basic unit of information defined generically, or for a specific application For example,
a data element in an employee database might con-sist of a name or job category A data element may
be further defined as including data items components
or subcategories
Data Encryption StandardDES A cryptographic system consisting of an algorithm and a key com-prised ofa long series ofnumbers which are used to-gether to transform data into information which ap-pears unintelligible, and back into data by the per-son for whom the information is intended DES was developed by the National Bureau ofStandards (now the National Institute of Standards and Technology (NIST)) and is intended for public use and for gov-ernment protection of certain federal unclassified data See Clipper Chip, Pretty Good Privacy
data entryThe act of using a hardware interface to input data to a computing device Data entry is com-monly accomplished through a keyboard and mouse, but voice recognition systems, touch screens, and pen computers are broadening the choice ofinput devices
Typically, data entry is used to describe repetitive, discrete types of data, like database entries (names, addresses, order numbers, etc.), spreadsheet entries, etc When the data is more fluid and conceptual and less repetitive, it is still, in its broadest sense, data entry but is more likely to be described in terms of the type ofapplication being used, such as word pro-cessing
data exchange interfaceDXI Alayer 2, frame-based interface installed between a packet-based router and
a SMDS or ATM CSU/DSU The DXI performs as-sembly and reasas-sembly tasks on behalfofarouter that may not have these capabilities Since most routers now can handle these tasks, use ofthis particular type
of interface is diminishing
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Data General Corporation DG One of the better
known computer companies in the 1970s, Data
Gen-eral was founded in 1968 to develop minicomputers
and became a Fortune 500 firm a decade later With
almost half a million systems installed worldwide,
Data General targeted high-performance computing
environments, including scientific, technical, and
in-dustrial sites By the late 1990s, DG was beginning
to support server applications for Pentium II
proces-sors as well, a strong sign of the convergence of the
workstation and personal computer markets Data
General became a division of EMC Corporation
af-ter an announced stock swap in 1999
data grade circuit Adistinction made to indicate the
more stringent needs of computer data transmission,
as compared to voice grade transmissions Data is
transmitted at different frequencies and is more
pre-cise and easily interrupted than a phone conversation
Phone conversations use a narrow frequency range
and have a great tolerance for pauses, spaces, and
extraneous noise, particularly since part of the
pro-cessing equipment in a voice conversation is the
hu-man brain, which understands context and innuendo,
as well as just the words Data, on the other hand,
re-quires a cleaner line, less interference and a greater
frequency range and has low tolerance for pauses and
spaces if they affect the integrity of the information
that is being transmitted
Voice grade circuits over phone lines are improved
by load coils, a system of looping the wires that are
strung along utility poles Data grade circuits built
in the same basic way are hindered by load coils, as
they introduce noise at the higher frequencies used
data line card DLC In a digital telephone network,
a link between the transmissions wire connecting the
subscriber unit (e.g., modem) on one side and the
digital switching matrix (DSM) on the other
Data Link Connection Identifier DLCI A means
of assigning logical connections within a shared
physical transmissions path In Frame Relay
net-works, the DLCI is a unique 10-bit identification
number (address) assigned to a virtual circuit (VC)
endpoint that identifies the endpoint within a local
access channel It is a mechanism for keeping track
of endpoint devices (e.g., routers) and is used for
switching and multiplexing When a network is
con-figured, addresses are assigned and entry tables are
created to map the DLCIs to one another for routing
data over the network If the Frame Relay is linked
with a larger network, such as a global public
net-work, the Frame Relay handles the routing offrames
through the DLCIs
A router may have multiple DLCIs If there are
mul-tiple ports, identification numbers can be used again
If frame traffic is forwarded across the Frame Relay
cloud (shared connectionless resources), the address
ID number may change, as the mapping is handled
by the Frame Relay switch (note that global IDs are
optional Local Management Interface (LM!)
exten-sions and their implementation is not widespread)
ISDN frames are similar to Frame Relay frames, and
Frame Relay networks may be hooked into ISDN
networks Commercial products exist to interpret ISDN messages and route them through Frame Re-lay networks In ISDN, the address is a unique 13-bit identifier See Frame Relay
Data Link Control DLC A layer in the Open Sys-tems Interconnection (OSI) model, DLC is respon-sible for a number ofadministrative and error-check-ing functions In satellite communications, some spe-cial adaptations are needed at this level to accommo-date the high bandwidth/delay characteristics of these transmissions
data link layer DLL In the Open Systems Intercon-nection (OSI) reference model, the layer that ensures transmission ofdata between adjacent network nodes Bridges work at the data link layer See Open Sys-tems Interconnection
Data Link Switching DLS, DLSw Originally devel-oped by International Business Machines (ffiM), in
1993, DLS was submitted to the IETF as an infor-mational Request for Comments (RFC) DLS defines
a reliable means of transmitting SNA and NetBIOS
TCP!IP traffic using IP encapsulation through
multi-protocol router networks See RFC 1795
Data Link Switching Workgroup DLSW The tech-nical group that worked on the development ofa new switching standard for integrating networks over
TCP/IP See Data Link Switching.
Data Link Switching Special Interest Group DLSw SIG Avendor implementation group created
in 1993 to address some of the issues raised in re-gard to RFC 1434 in which International Business Machines (ffiM) provided preliminary information
on Data Link Switching This activity resulted in a new RFC being submitted to the IETF as RFC 1795 which obsoleted RFC 1434 See Data Link Switching data lump cable A data communications cable that includes integrated circuits (ICs) or other components underneath the cable shielding such that they produce
a lump on the profile of the cable These are popular for interconnecting small handheld devices to com-puters or computerized components for two reasons The first is that an added battery source or conver-sion or translation software or hardware may be needed to make the handheld device operable with other devices The second is that the shielding from the cable helps protect the components from short circuits from other objects that might be carried in a handbag, briefcase, or pocket
A data lump cable may connect directly to a compo-nent or may interface with a cradle into which the component is coupled
data mining The process of seeking out relevant in-formation from a large storehouse of electronic data that may be on many different systems in many dif-ferent formats Data mining involves using intelligent strategies and algorithmstosearch for relevant ma-terials based upon various parameters such as previ-ous search history, data patterns, information corre-lations, preferences of individual users, keywords, and other triggers implemented to maximize the relevance of the information retrieved or flagged Data mining has become a topic of substantial interest
Trang 6tion that is flowing onto the Internet Anyone who has
used a search engine and received 300,000 hits, after
narrowing a search a couple of times, can see the
value in data mining algorithms that can carry out
some of the work in advance Data mining is of
par-ticular interest to researchers and marketing
profes-sionals and can be an important and legitimate way
of searching, sifting, and sorting information for
medical research, astronomy, inventing, investing,
and information technology (IT) research
Data mining on public networks has become the
sub-ject of controversy and political discussions due to
the amount of personal information on the Web that
is accessible to a wide audience and thus subject to
misuse or abuse by individuals with questionable
eth-ics or illegal intentions Many children put personal
information on the Web in the process of developing
school projects Much genealogical information is
posted on the Web, and there is a great deal of
infor-mation about people who don't even own or have
access to computers, who thus cannot track or trace
information that is being distributed about them by
third parties While individual pieces of information
may not be harmful in themselves, data mining
al-gorithms have been devised to develop profiles
ofin-dividuals that indicate where they live, work, and
shop, what they buy, how they spend their leisure
time, and with whom they associate These
sophisti-cated profiles have never before in history been
pos-sible and there is currently very little legislation to
protect individuals from their misuse by thieves,
stalkers, or manipulative political or marketing
bod-ies operating outside ethical constraints See
Ameri-can Civil Liberties Union, Ask Jeeves, Electronic
Frontier Foundation, cybrarian
data multiplexingSee multiplexing
Data Network Identification CodeDNIC An
ITV-T internationally specified system of network host
identification that permits individual local networks,
tied to public networks, to be located and recognized
for internetwork communication, much as a country
code and local phone number identifies a phone line
This data network identification scheme, somewhat
analogous to a phone number, is used to locate hosts
on interconnected public networks by means ofX.75
The DNIC is the first four digits of a longer 14-digit
international code The first three digits are assigned
by the lTV-T to specify a data country code (DCC)
and the fourth digit is assigned by the national
ad-ministration to specify the public data network within
that country Network Terminal Numbers (NTNs) are
the responsibility of the administrators of the public
network See X Series Recommendations
Data Numbering Plan AreaDNPA An ITU-T
X.25-specified system of endpoint terminal identification
implemented in the U.S using the first three digits
of a 1O-digit network terminal number (NTN)
Data Over Cable Service Interface Specification
DOCSIS MCNS/DOCSIS. An interoperable cable
service delivery specification developed jointly by the
Multimedia Cable Network System partners (Cox,
MediaOne, and Rogers Cable Numerous other ven-dors have contributed to the DOCSIS specifications process The standards facilitate the provision ofcable services through intercompatible hardware and busi-ness systems Thus, DOCSIS provides an industry standard for cable Internet access and possible future services The Motorola SURFboard cable modems installed by AT&T for their cable modem data ser-vices are DOCSIS-compliant, for example
DOCSIS 1.0 supports the deployment ofhigh-speed data services through various standards and protocols
An update incorporating Quality of Service (QoS) extensions for improved security and realtime deliv-ery was released as DOCSIS 1.1 The DOCSIS de-scriptions also encompass existing ratified and de facto standards in the multimedia industry, including some of the ITU-T Series Recommendations, IEEE
802.x, MPEG-2 transport (downstream framing), and
DES encryption schemes
DOCSIS is an asymmetric specification, supporting data rates of 27 or 36 Mbps in the downstream tion and 320 Kbps to 10 Mbps in the upstream direc-tion Modulation is through QAM (downstream) and QPSK and QAM (upstream) Since USB and IEEE
1394 (Firewire) computer interfaces have become popular and are much faster than traditional serial ports, USB and Firewire 1OBase-T subscriber inter-faces are planned
In DOCSIS 1.0, Baseline Privacy is a scheme for encrypting user data using the Cipher Block Chain-ing mode ofDES with a 56-bit encryption key (it may
be less for international locations) Regular key changes are specified for management purposes RSA public and private key pairs are installed into cable modems in the manufacturing process Data is en-crypted only for transmission on the cable network and is not intended for high security in other envi-ronments Authentication is not directly supported by DOCSIS 1.0 specifications DOCSIS 1.1 extends Baseline Privacy with longer RSA keys (1024 bits) and the association of a digital certificate with each cable modem for authentication The public and pri-vate key pair and digital certificate are installed in the cable modem dwing manufacture
To aid implementors, Kinetic Strategies has prepared
a research report, DOCSISInfrastructure Deployment
Forecast: The North American Marketfor Standards-Based Cable Modem Products and Services
2000-20004 which is available for a fee See Multimedia
Cable Network System
data over voiceAmeans ofincluding data on a trans-missions line carrying voice signals by using fre-quency division multiplexing (FDM) to secure and utilize the remaining available bandwidth for the data signals Thus, for example, a telephony device can
be equipped with readouts for data about the call
data processingA broad category of activities en-compassing the manipulation of digital data, as in word processors, spreadsheets, paint programs, etc
In database systems, data processing has a more spe-cific meaning, referring to the creation, access,
,
•:.)
;.:· ••.: :!.:.'.·~.:t.·; ,
~f:f
Trang 7Fiber Optics Illustrated Dictionary
retrieval, manipulation, and analysis of textual and
fmancial data Data processing is commonly used in
payroll accounting, statistical analysis and reporting,
customer profiling, and many other common tasks
re-lated to commerce and business management
data protection Abroad category of actions and
sys-tems that are designed to protect data There are two
general categories ofdata protection: keeping the data
available and in its desired form (uncorrupted), and
keeping the data safe from unauthorized use
In the first category, data backups, archiving,
mirror-ing, and other means are used to protect data from
being lost or corrupted This can occur at the local
applications level, in the hardware, and at the
over-all systems level Many file system directories are
duplicated to provide access if corruption occurs in
one Some operating systems allow multiple versions
of a file to be saved automatically, so that there is
al-ways a history of recent changes and a previous
ver-sion that can be used if needed Backup hardware in
the form of tapes, cartridges, optical media, and
re-dundant drives are used by many for scheduled or
dynamic backups
In the second category, passwords, digital encoding/
encryption, secure channels, proprietary formats,
vaults, safe-deposit boxes, data certificates, digital
signatures, etc., are all used to protect the data from
unauthorized access, use, or abuse See backup,
backup file, encryption, mirroring, Pretty Good
Pri-vacy,RAID
data rate A quantification of the input or
transmis-sion of computer data Data rates are very situation
specific For example, in data entry jobs, the data rate
may be the number of fields filled per minute or the
number of customer orders entered per hour In
net-work communications, it may be the number of bits
or packets transmitted per second See baud rate
data service unit DSU A device used in ISDN
sys-temstointerconnect computers with digital phone
services for end-to-end digital communications It is
similar to a modem in the sense that it fits between
the computer and phone line service, but it differs in
that it does not perform analog to digital and digital
to analog conversions The DSU is installed in the
customer's premises and connects the synchronous
communications system through a four-wire line
(usually a leased line) to the local central office The
DSU is used in conjunction with a Channel Service
Unit (CSU) which is installed at the central office
data set ready DSR Acontrol signal commonly used
in serial network communications and included in the
pinout specifications for the ubiquitous RS-232
elec-trical connector The DSR indicates whether the
com-munications device is connected and ready to begin
handshaking For example, assume the user has
di-aled a BBS or Internet Access Provider (lAP), and
the called modem has just picked up the line The
DSR senses the connection and provides a signal that
lets the hardware/software know that it can continue
to the next step of negotiating a connect speed and
beginning the communications See data terminal
ready, RS-232
data striping A means of distributing data across drives in an array A fault tolerant means of
provid-ing data security that is incorporated into redundant
array ofinexpensive disks(RAID) systems data terminal equipment DTE A communications data terminal hardware specification See data com-munications equipment
data terminal ready DTR A control signal com-monly used in serial communications and included
in the pinout specifications for the ubiquitous RS-232 electrical connector The DTR signals whether the communications device is connected and ready after
it has successfully begun handshaking For example, assume the user has connected with a BBS The DSR verifies the connection, a connect speed is negotiated, handshaking begins, and the terminal is ready to con-tinue communicating The DTR signals this state of readiness DTR serves an output for DTE devices and
an input for DCE devices See data set ready, RS-232 data typing The process of specifying or determin-ing the format of a variable, file, or block of data data unit DU A generic term for any modular or connecting unit in a data path, although the abbre-viation is most commonly applied to small, limited-function digital or digitaUanalog units as opposed to more complex systems Examples include computer modems and passive switchers
data warehousing A term used primarily by large corporations with very large databases until hard drives became bigger and less expensive and the Web made huge databases easily accessible through the In-ternet Data warehousing is a system of information storage and retrieval for vast databases that are com-prised of smaller storehouses of databases Large da-tabases present unique logistical and programming challenges Many database storage and retrieval sys-tems are limited in the number ofrecords or files they
can handle; in other words, many ofthem are not
scal-able. Consequently, various new strategies for data warehousing are being developed
The Internet has made accessibility, through a local Internet Access Provider (lAP), so easy and inexpen-sive that companies are demanding increased access
to databases not only online, but at branch offices in other states and countries Thus, data warehousing through Web browsers is developing, even though the suitability ofHTML to this task is somewhat limited With one of the Web-friendly programming environ-ments, such as Sun's Java, the job becomes easier, but the logistical demands of taking geographically divergent databases that may be in a variety of for-mats to meet local needs, and accessing them all as a conceptual unit over the Net, is an ongoing program-ming challenge that will probably continue to evolve for some time Efforts to promote open systems and object-oriented programming strategies may contrib-ute to streamlining the process of data warehousing See COREA, Open Systems Interconnect
database Any collection of data organized in some form for storage or for storage and retrieval A data-base can be as simple as a list of names or as com-plex as a relational, distributed, multisite archive of
Trang 8A file system hierarchy on a computer storage device
is a type of database, as is an employee file that
in-cludes pictures, birthdates, addresses, and social
se-curity numbers
Database creation and management programs
typi-cally have text-based interfaces, graphics interfaces,
or both With text-based interfaces, information is
organized into lines and fields and usually listed
se-quentially from top to bottom In more flexible
graphical databases, a screen mask or input template
can be created almost as though using a paint
pro-gram to draw the input screen Lines, boxes, colors,
and other visual elements can be used to make the
database appealing and its functions and input actions
apparent to the user Fields are then assigned to the
graphical elements, and the order of input is defined
More sophisticated databases include scripting or
symbolic programming languages to allow
automa-tion ofthe database so error messages, prompts, help
windows, and other applications elements can be
pre-sented when appropriate
There are many ways to store data in a database:
com-pressed or uncomcom-pressed, encrypted, encoded, or
plain ASCII The format ofthe data isn't usually what
creates compatibility problems More often the
orga-nization of the data, which can vary widely, is the
hurdle that must be overcome when interchanging
data among applications or systems See data
ware-housing, expert system
database reportsCharts, graphs, lists, and other
sta-tistical reports that can be selected or computed from
information in a database or more than one database
Reports are widely used for financial statements,
business plans, demographics, research, etc
database serverA system, computer, or application
specifically designed to provide database capabilities,
security, and file access to multiple users on a
sys-tem There are two aspects to a database usually
in-corporated into a client/server model: the application
that generates, searches, and retrieves the data and
the data itself Sometimes the data is on the server,
and the application is on the individual user's
ma-chine Sometimes it's the other way around, and
sometimes all aspects of the database system are
handled by the server It depends on the
sophistica-tion of the system and the needs of the users In high
security situations, the server usually handles
every-thing In smaller networks, where security is less of
an issue, the applications may be installed on
indi-vidual machines to run faster, while the data is banked
on the server, with password access, number ofuser
restrictions, etc., centrally handled by the server
software
datagramThis term is used in a general sense to
mean a unit ofinformation in a packet-switched
net-work without regard to previous or following
pack-ets Depending upon the network architecture, it may
also have a defined format within that system In
lay-ered architectures, the datagram may be associated
with a specific layer or layers A datagram may be
ceiving end, for example when tunnelling through dif-ferent systems Internet Protocol (IP) datagram trans-mission over connectionless X.25-based public net-works has been defined by a variety oforganizations Source and destination information are typically en-codedinassociation with the datagram See Point-to-Point Protocol, RFC 877, RFC 998
DataSPAN Frame Relay ServiceA Nortel Frame Relay digital telephony service aimed at users with virtual private line networks (corporations, educa-tional institutions, etc.) and high-speed interconnec-tions The DataSPAN service provides bandwidth on demand through space-saving multiple virtual circuits for each access port that connects to the user's equip-ment DataSPAN services can transmit over perma-nent connections at DS-O, DS-I (fractional or full) rates It can support switched access from circuit-switched services atOs-orates See Dialable
date and time stampA common function of com-puter applications and operating systems (OSs) that records when some event occurred For example, files are usually date- and time-stamped as to the time of their creation or the time they were last updated (or both) Entries to databases are frequently date- and time-stamped, as are computerized physical premises access systems, electronic timecards, and many more The only problem with date and time systems on com-puters is that not all comcom-puters take the time or date from a reliable source Some have lithium-battery powered realtime clocks, but many do not It may be
up to the user to set the date and time manually, and
a power outage can change the settings
To improve time-stamping, many operating systems now have a utility that can automatically access a computer network and take the time from a reliable source on a network This is useful for synchroniz-ing with time-sensitive sites (auctions, stock ex-changes) and for general file management
Datapath Loop Extension, Datapath Extension DPX.Intelephony networks, a commercial loop ex-tension capability and card manufactured and li-censed by Nortel Datapath Services enable digital full-duplex synchronous and asynchronous fast trans-mission rates over standard twisted-pair wiring The importance of Datapath Services is that they allow smaller, local exchange carriers (LECs) to provide services previously accessible only to bypass and in-terexchange carriers (IECs) Thus, high-bandwidth telephony applications such as multimedia services (e.g., videoconferencing) can be made more readily available to a broader range of subscribers
The DPX may terminate either a channel bank or digi-tal loop carrier, forming a link between the subscriber's connecting device and DS-O channels
on DS-I network facilities on a digital public switched telephone network (PSTN), now also known as a public switched digital service (PSDS) The device that connects the user's computer to the wire loop and the DPX unit may be a modem or other similar data unit, a cluster control unit, or a terminal
Trang 9Fiber Optics Illustrated Dictionary
25-pin male D connector used primarily for serial and SCSI data communications.
DBD 1 database drivers 2 See digital bearing dis-criminator
DBMS See Database Management System DBS See direct broadcast satellite
DDT 1.database thang.Atongue-in-cheek name for
a Berkeley database data structure 2 defect-based testing 3 Deutsche Bundespost Telecom 4 DBT A versatile, extendible MATLAB Toolbox for radar sig-nal processing suitable for sigsig-nal intelligence and communications applications, developed by the Swedish Defence Research Establishment DBT sup-ports modeling of array antennas and simulation of many situations associated with these antennas
DC 1 In telephone communications, Delayed Call
2 See direct current 3 disconnect conform In Open Systems Interconnection (OSI), a transport protocol
source, and in 1808 he invented the arc lamp by con-necting the terminals of a voltaic cell to a piece of charcoal, resulting in a brilliant light now known as
an arc light, or electric arc Michael Faraday became his laboratory assistant in 1813
dB abbrev See decibel.
DB 1 data bus 2 See database
DB-9 A common designation for a 9-pin D-shaped computer connector, used on many laptops and desk-top computers, especially for serial connections through an RS-232 cable DB-9 simply describes the physical connecting portion and does not define the electrical relationships of the pins to the wires in the cable to which the connector attaches RS-232, on the other hand, defines specific pinouts and pathways for various types of signal and information data DB-IS Acommon designation for a 15-pin D-shaped computer connector most often used for monitor cables and Ethernet transceivers DB-15 simply de-scribes the physical connecting portion and does not define the electrical relationships of the pins to the wires in the cable to which the connector attaches DB-2S A designation for a 25-pin D connecter very widely used for computer data transfer, especially serial cables, and one end ofmany parallel and SCSI cables DB-25 describes the physical connecting por-tion and does not define the electrical relapor-tionships
of the pins to the wires in the cable connector Many ofthe common, inexpensiveAlBswitchboxes are in-stalled with DB-25 female connectors
25 14
D-Style Connector - 25-Pin
interface unit (e.g., for ISDN connections)
Depend-ing on the wire gauge, the distance between the data
unit and the DPX may be up to about 18,000 feet In
some situations, the DPX may replace a DLC in a
channel bank
Datum Corporation Manufacturers oftime and
fre-quency technologies used in computer networks,
land-based wireline communications, and wireless
satellite and cellular communications Datum is a
world leader in the manufacture of cesium atomic
clocks, supplying Europe and most of the American
Global Positioning System (GPS)
daughterboard, daughtercardjargon Aprinted
cir-cuit board that piggybacks onto a motherboard (which
contains the main processing circuitry) in an
elec-tronic system The daughterboard is frequently,
though not necessarily, smaller than the motherboard
and usually adds some specific type offunctionality:
more memory, acceleration to the main CPU, a
de-vice interface, etc A fatherboard has further been
described as a connection to a motherboard that
pro-vides a series of connectors, into which several
daughterboards can be connected
DAV See digital audio video
DAVIC See Digital Audio-Video Council
Davies, Donald (1924-2000) Aresearcher at the
Brit-ish National Physical laboratory, Davies developed
concepts for distributed digital networks
contempo-raneously with Paul Baran in the mid-1960s It is
be-lieved by Baran that each conceived the idea
inde-pendently This important new mode of data
trans-mission became known as packet-switching and was
incorporated into the ARPANET which evolved into
the Internet See Baran, Paul
Davisson, Clinton Joseph (1881-1958)An
Ameri-can physicist, Bell Laboratories researcher, and
win-ner of the Nobel Prize in physics in 1937 along with
George Paget Thomson, Davisson studied electron
diffraction and theories of electron optics During
World War IT, he researched crystal physics and the
theory of electronic devices
Davy, Edward (1806-1885)AnEnglish physician
who invented the electromagnetic repeater (electric
renewer) and created one of the early copper
wire-based telegraph systems in the mid-1830s He
dem-onstrated a needle telegraph in 1837 and received a
telegraph patent in December 1837, half a year after
Wheatstone and Cooke received their telegraphic
patent Later, after emigrating to Australia, he
devel-oped a process for refining copper Thus, along with
Morse, Cooke, and Wheatstone, he was one of the
pioneers of telegraphic equipment and perhaps the
first to develop a telegraphic relay See telegraph
history
Davy, Humphry (1778-1829)AnEnglish scientist
and educator who passed a current through potash in
1807, decomposing it and discovering a new element
(potassium) Davy subsequently discovered more
el-ements and clarified that some substances considered
elements actually were not He also proposed a theory
of electrolysis In the early 1800s, he observed the
properties of carbon when connected to an electrical
Trang 10DCA 1 Defense Communications Agency A U.S.
Government agency involved in military standards
development 2 Document Content Architecture An
International Business Machines (IBM) system of
specifying a document series from draft to final
docu-ment 3 Dynamic Channels Allocation A wireless
concept that is used in DECT PCS services
DCC 1 See data communications channel 2 See
Data Country Code 3 digital command center 4 See
Digital Command Control Standards 5 digital
com-munications center 6 See Digital Comcom-munications
Conference 7 See Distributed Computing&
Com-munications Laboratory
dcdThe name of a freely distributable (under the
GNU General Public License) Linux CD player,
de-veloped by David E Smith, that allows CDs to be
played on an appropriately configured computer.It
provides a means to enter playlists, etc
DCD 1 See Data Carrier Detect 2 See Document
Content Description
DCE 1 See data communications equipment 2
digi-tal communications media 3 See Distributed
Com-puting Environment
DCM1 See digital switching matrix 2 See
dynami-cally controllable magnetic
DCP 1 Department of Consumer Protection 2 See
Digital Communications Protocol
DCS 1 digital communications system 2 digital
cross-connect system 3 distributed computing system
DCT 1 digital carrier trunk 2 See discrete cosine
transform
DCTISee desktop computer telephony integration
DDB1 device-dependent bitmap 2 digital databank
See data warehousing
DDCMPSee Digital Data Communications Message
Protocol
DDDSee Direct Distance Dialing
DDESee dynamic data exchange
DDNSee Defense Data Network
DDOSdistributed Denial of Service See Denial of
Service
DDS 1 digital data service 2 digital data storage
3 distributed data system
DESee Discard Eligibility
de Coulomb, Charles A.See Coulomb, Charles A
de facto standardAformat, specification, or design,
usually from a self-interested commercial source, that
has become widespread This often confers a large
degree of industry control to the major stakeholders
Occasionally de facto standards are good, if a
pub-lic-service body hasn't provided a standard, and ifthe
standard brings down the cost ofgoods to make them
more widely available to the general public
Some-times de facto standards are bad, since the
specifica-tion or products themselves may be of poor quality
and may only have become widespread through
ag-gressive advertising, consumer trust, or lack of
un-derstanding of the technology By the time the
con-sumer learning curve catches up with the technology,
the standard may be too entrenched to change
Very frequently, the cheapest product becomes the de
on good design principles, but because it's more af-fordable Sometimes the most convenient product be-comes the de facto standard If a product is easier to use, or more portable, it may outsell more flexible or powerful designs
Sometimes the first product to hit the market becomes the de facto standard, and manufacturers are
some-E!:B!ll:~~~~;'r;~~~ff:~2~;a:~!~ 1~~·'.
dem on the shelves often had a big say in specifying the format for communications and, consequently, a short-term de facto monopoly
de Ferranti, Sebastian(1864-1930)Aninventor who collaborated with Elihu Thomson and William Stanley in the development of the transformer He was also responsible for developing the first high voltage alternating current (AC) distribution system
at a time when direct current (DC) distribution sys-tems were prevalent
de Forest, Lee (1873-1961) A highly ambitious American inventor who harnessed the power ofelec-trons by inventing the Audion vacuum tube, which was granted a patent in January 1907 (U.S #841,387)
This very significant technology has been used in electronics in many industries for decades, though eventually transistors superseded vacuum tubes, ex-cept for some specialized high frequency applica-tions Although he was loathe to acknowledge his pre-decessors, de Forest's invention stemmed from the work ofT Edison and J.A Fleming However, he is
to be credited with the introduction of the electron tubegridunit, creating atriode,which was a signifi-cant advance over the design of the Fleming tube
Lee de Forest's three-electrode vacuum tube made transcontinental communication possible, and the proliferation of vacuum tubes for radio wave detec-tion caused the decline of crystal detector radio sets
In the 1920s, de Forest contested E Armstrong for the invention ofregeneration and won (Lee was born
"De Forest, "but later in life is said to have preferred
"de Forest "His wireless company was spelled
"DeForest.") See Audion, DeForest Wireless Tele-graph Company, Edison effect, transistor
de-encapsulationAn important aspect of packet-switched networking in whichuser data fielddata in
an encapsulated packet is extracted upon receipt
dead bandIn guidance systems, a means of intro-ducing hysteresis by preventing errors from being corrected until they have exceeded a certain speci-fied magnitude, a safety precaution against the guided object reacting prematurely to interference or spuri-ous signals
Dead Link CheckDLC A Perl program to check HTTP links, with reporting capabilities, DLC was released in 1999 by Martial Michel Since 'dead' links, those that no longer point to a valid address on the World Wide Web, can be a problem for users ofa site, a utility that alerts the Webmaster to problems
is a welcome tool It can be used by itself or as an extension to Public Bookmark Generator