Phone and ADSL signals coming from the subscriber premises are sent to the central office where a splitter sends phone signals to the voice switching mechanisms and ADSL signals to the d
Trang 1Fiber Optics Illustrated Dictionary
that supports mid-range speeds and the transmission
ofboth voice and data over existing copper-pair wires
ADSL is becoming apopular cost-effective option for
faster computer network access (e.g., Internet) in
households and small businesses It enables users to
be online virtually all the time without interfering with
the use of the phone for voice messages or having to
dial the service provider each time access is desired
ADSL services are competitive with cable modem
"Asymmetric" refers to the discrepancy between the
upstream and downstream transmission speeds Most
users download far more than they upload, so a
com-promise is established to balance cost and speed
Thus, download speeds are configured to be faster
than upload speeds (e.g., 640 kbps versus 8 Mbps)
Installation of ADSL service involves installing a
network interface device (NID) that houses a spliner
(a low-pass filter) at the subscriber premises
Sepa-rate lines run from the splitter to the phone and from
the spliner to the ADSL remote transceiver
(some-what like a traditional modem) which, in turn,
con-nects to the computer Software to handle
communi-cations is installed on the computer The combined
telephone and ADSL signals travel from the loop to
the subscriber NID where the signal is split The
tele-phone signal is then directed to the tele-phone set and the
ADSL signal is directed to the terminal unit (the
re-mote transceiver) where it is processed and
transmit-ted to a network interface card (NIC) installed in the
computer Much the same happens in the reverse
di-rection Phone and ADSL signals coming from the
subscriber premises are sent to the central office
where a splitter sends phone signals to the voice
switching mechanisms and ADSL signals to the data
network where an Internet Services Provider handles
user requests such as Internet access, Web hosting, etc
Copper wire lines are not optimal for ADSL, as bridge
taps and load coils on utility poles can interfere with
ADSL signals See Digital Subscriber Line for a fuller
explanation See cable modem, discrete multitone,
G.lite, UAWG
asymmetric transmissionAtransmission channel in
which information flows more readily (faster) in one
direction than the other, or moves primarily in one
direction or the other at anyone time, or in which a
greater volume of information flows in one direction
or the other There are many instances in which
in-formation typically flows more in one direction than
another, as in interactive TV, where most of the time
the user is observing and not transmining but may
make an occasional request for a specific movie or
file The medium itself may not be inherently
asym-metric For example, a data upload over a modem is
primarily one-way, but the line capacity is two-way,
and the direction can be easily switched when
upload-ing The slower channel, or the one with a lower
vol-ume capacity, may be called theback channel.
asymmetrical compression In data compression
techniques, some types of files can be compressed
faster than they can be decompressed and some work
the other way around.Indesigning compression
al-gorithms, sometimes optimization in one direction or
the other is preferred In creating animation se-quences, it is usually very important that they decom-press and play quickly; otherwise the illusion ofmo-tion is lost However, it is usually not a problem if the compression takes longer than the decompression because the computer can handle that while the user
is working on other projects
asymmetrical modemA modem designed to favor the transmission of the bulk of the data in one direc-tion over the other This is appropriate in situadirec-tions where most of the communication is one-way, as in managing an archive site, where downloads typically outnumber uploads thousands-to-one See Asymmet-ric Digital Subscriber Line
asynchronousNot synchronous A concept that ap-plies across many areas of telecommunications, in which the timing of the information being received and transmined is not predefined and may be unpre-dictable, as in many modem communications and in-teractive radio communications This type ofcommu-nication typically requires some means of indicating the starting and stopping points of the transmission There are various schemes for handling this, from verbal cues ("Roger"), to start/stop bits, and various handshaking signals
asynchronous balanced modeABM.Inan Interna-tional Business Machines (IBM) Token-Ring net-work, a service in the logical link control (LLC) at the SNA data link control level that allows devices
to send and respond to data link commands
asynchronous communications interface adapter
AClA A data formaning device that translates sig-nals between the computer and a peripheral such as a modem
asynchronous packet assembler/disassembler
APAD A mechanismtoassemble a stream of bytes from an asynchronous source (e.g., a computer) into packets and transmit them to a network, and vice versa In terms of serving a translation function, it can
be thought ofas loosely working like a traditional mo-dem, which takes asynchronous digital signals from
a computer and modulates and demodulates them for compatibility with an analog phone system In the case of a PAD, however, the data is being packetized and sent over an X.25 network from one or more de-vices that are not directly X.25 compatible The
ITU-T has defined more than one standard for perform-ing these translation functions to facilitate connec-tions with X.25 Examples include:
X,3 basic packet data network assembly/disas-sembly
X.5 packet data network assembly/disassembly for facsimiles
X.29 packet data network assembly/disassembly control information and user data exchange procedures
X,39 packet data network assembly/disassembly control information and user data exchange procedures for facsimiles See X Series Recommendations
asynchronous transfer modeATM A high-speed, cell-based, connection-oriented, packet transmission
Trang 2rates ATM is a commercially significant protocol due
to its flexibility and widespread use for Internet
con-nectivity ATM evolved from standardization efforts
by the CCITT (nowlTU-T) for Broadband ISDN
(B-ISDN)i~the mid-1980s It was originally related to
Synchronous Digital Hierarchy (SDH) standards
ATM allows integration of LAN and WAN
environ-ments under a single protocol, with reduced
encap-sulation.Itdoes not require a specific physical
trans-port and thus can be integrated with current physical
networks It provides Virtual Connection (VC)
switching and multiplexing for Broadband ISDN, to
enable the uniform transmission ofvoice, data, video
and other multimedia communications See
Anchor-age Accord for information on acquiring ATM
tech-nical specifications See the Appendix for details and
diagrams on ATM and ATM adaptation layers
AT 1 See access tandem 2 advanced technology 3
AudioTex A commercial telephony-based
informa-tion service, offering announcements, messages,
music, meeting schedules, etc
AT, PC/AT Advanced Technology The common
name for a series of 80286-based personal computers
introduced by International Business Machines
(IBM) in the mid-1980s This model was released
about a year later than the Apple Lisa, at about the
same time as the Apple Macintosh, and about a year
before the Amiga 1000, Apple IIGS, and Atari ST
computers This is historically significant in the
de-velopment ofuser interfaces, as most of the
compet-ing computers were evolvcompet-ing graphical user
inter-faces (GUIs) and included built-in serial ports and
sound cards, while most ofthe AT systems were
text-oriented (primarily MS-DOS), with sound and
vari-ous interface cards optional The IBM AT and
li-censed clones from other manufacturers were
pur-chased primarily by business users, in part because
the IBM name was well known in the business
in-dustry, and also because IBM had a decades-old
tradition at the time of providing service and repair
options to business owners Two ofthe chiefsoftware
products used on the AT were spreadsheets and word
processors
AT commands, Hayes Standard AT Commands A
very simple control and reporting language built into
Hayes Microcomputer Products, Inc modems, and
Hayes-command-compatible modems from other
manufacturers Originally modems were "dumb"
devices; they had no significant memory or
algo-rithms incorporated into the device to process
com-mands or data from the computer Hayes introduced
"smart" modems in the early 1980s that could
pro-cess a limited command set and enhance the utility
of modems This instruction set has since been
in-corporated into almost every make and model
ofcom-puter modem, usually with enhancements by
indi-vidual manufacturers
The AT command set allows computer control of a
modem and provides a way for the modem to report
information back to the computer software The AT
stands for "attention" and is a way of alerting the
"AT" which is to be acknowledged or executed When you run a telecommunications program through your modem, the software is talking to the modem with
AT commands along the path provided by the serial cable that typically connects the modem to the com-puter Ifyour software can be set to interactive mode, you can type the AT commands directly to your mo-dem and see what happens The AT commands are usually listed at the back of the manual that comes with a modem
Many modem manufacturers have included supersets
of the basic Hayes command set to provide control
of proprietary or enhanced features specific to their products, so AT commands usually include most or all of the Hayes commands, and additional ones as well
AT commands fall into a number ofcategories There are commands for querying the status of the phone line, for querying the status of the modem, and for carrying out operations such as dialing, setting the transmission speed, setting the number of redials, setting the length of wait periods, etc
Modems contain a number of registers in which in-formation is stored, often in the form ofa toggle (true
or false) or integer setting Thus, setting the register
tozero signifies one thing, and setting it tooneor another integer, when appropriate, signifies another Thus, AT 80=0 sets the "8' register to zero Since register SO determines how many rings to AutoAnswer, setting it to zero effectively turns off AutoAnswer AT SO=l instructs the modem to AutoAnswer after it detects one ring If you are run-ning a computer bulletin board, or a friend is calling
to send you a file over the phone line, AutoAnswer can be turnedon(or you can type ATA "attention, answer" when you hear the phone ring) Remember
to set AutoAnsweroffwhen you are finished trans-mitting, or the next voice caller may get a nasty mo-dem-blast in the ear Some modems have enhance-ments that allow them to autodetect whether the in-coming call is voice or data and to react accordingly
so this doesn't happen
AT 80=0 M1 DT 555-4321 W DT 123 attention; set autoanswer to zero rings set speaker to be on(M!)during establishment
of call (so you can hear dial tone and dialing) and off during connection (so you don't have to hear the modem sounds)
dial tone mode555-4321 wait for dial tone dial tone mode123(to dial an inner extension, for example)
AT commands can be combined You needn't type AT
in front of each individual instruction For example, you might wish to initialize your modem, and dial out
as a single string of commands
Trang 3Fiber Optics Illustrated Dictionary
AT&TAmerican Telephone and Telegraph Company
A company established almost 150 years ago to
cre-ate practical commercial applications from the early
telegraph and telephone patents filed in the 1870s,
primarily those ofAlexander Graham Bell and Elisha
Gray Some ofthe patents became the property ofthe
Bell System, and some served simply as competitive
motivation to implement the new ideas and technologies
The American Telephone and Telegraph Company
(AT&T) began as a long-distance subsidiary of the
American Bell Telephone Company in 1885.In1899
the two companies were again merged into one
un-der the AT&T name In the 1900s, AT&T was
reor-ganized, becoming a holding company, the parent of
the Bell companies and Western Electric In the
en-suing years, several additional reorganizations
oc-curred, some voluntary, some mandated by u.S
jus-tice authorities
In the early 1900s, there was a period of substantial
change in the phone industry, since the original Bell
patents, protected for a term of 17 years, were
expir-ing and independent companies were enterexpir-ing the
phone market in substantial numbers This situation
resulted in independents collectively holding almost
half of the phones until, by 1913, AT&T was again
the majority holder, due to mergers and acquisitions,
and was legally restrained from acquiring any more
independents AT&T was also mandated to permit
in-dependents to use the AT&T toll lines
The Communications Act of 1934 further regulated
the industry and established the Federal
Communi-cations Commission (FCC), which was given
jwis-diction over the telephone and broadcast industries,
a responsibility it still holds In 1956, the u.s
gov-ernment and AT&T entered into an agreement that
AT&T would offer only phone-related services and
not engage in common carrier communications such
as computer network services AT&T was further
re-quired to license Bell patents for royalties to
inter-ested applicants
Anumber ofantitrust suits ensuedinthe 1970s
charg-ing AT&T with monopolistic practices, and there
were calls for divestiture resulting in divestiture
pro-ceedings in the 1980s During this same period,
AT&T was granted limited permission to engage in
computer-related services
While its political history was undergoing many ups
and downs, the researchers in the Bell Laboratories
provided an enormous amount ofresearch and
devel-opment in telephone technologies, beginning in the
late 1800s and early 1900s AT&T researchers
devel-oped the first two-wire telephone circuit, which is still
in use today, the first practical transistor, and many
other inventions that are in broad use See Bell,
Alex-ander; Bell Laboratories; Bell System; Carty, John
1.; Kingsbury Commitment; Modified Final
Judg-ment; Vail, Theodore
AT&T TeleMedia ConnectionA Microsoft
Win-dows-based videoconferencing product from AT&T
Global Information Solutions providing audio/video,
file transfer, and application-sharing utilities over
ISDN through ITU-T H Series andQSeries
Recom-mendations standards and encoding techniques Atanasoff, John Vincent(1903-1995)AnAmerican physicist and inventor who developed a vacuum tube calculating device in the mid-1930s that foreshad-owed the famous ENIAC computer that was opera-tional in the post-World War II years.In1939, with a small grant from Iowa State College, he pioneered the development of a binary logic computer called the ABC, or AtanasofI-Berry Computer, with assis-tance from Clifford Berry a recent graduate in elec-trical engineering Unfortunately, the War brought the project to a halt He left his academic position to be-come Chief of the Acoustics Division of the u.S Naval Ordnance Laboratory where he worked on computing devices for the Navy along with atomic testing.Inthe 1950s, Atanasofftook a number ofcor-porate positions and retired in 1961 See Atanasoff-Berry Computer; Atanasoff-Berry, Clifford E
Atanasoff-Berry ComputerABC Apioneering bi-nary, direct logic computer with a regenerative memory, designed and built by J V Atanasoffwith assistance from Clifford E Berry After two years on the drawing boards, it was prototyped in 1939 It is significant not only for its historic place in the early history ofcomputers, but also becauseitwas designed with a separation between memory and data process-ing functions The electricity needed to keep the memory refreshed, so the information wasn't lost, was provided by rotatingdrumcapacitors
Atanasoff had been working on the ideas that led up
to the ABC since 1935 and related that the idea for the ABC came to him in a roadhouse in 1937 after he and his graduate students had developed a calculator for complex mathematics manipulation Punch cards, which had been developed to store information for electromechanical devices in the late 1800s, were used to enter data into the ABC In 1940, Atanasoff and Berry authoredComputing Machines for the So-lution ofLarge Systems o/LinearAlgebraic Equations
with illustrations, in preparation for a patent appli-cation that was never completed due to circumstances associated with World War ll
Much ofthe information about Atanasoff's invention did not come to light until a long court battle in the 1970s between Sperry Rand and Honeywell Unfor-tunately, after Atanasoff left for a position with the u.S Navy, the computer was dismantled, without notifying the inventors The Ames Laboratory is building a working replica of this historic invention See ENIAC; Zuse, Konrad
Atari CorporationA historically significant games and computer company established in 1972 by Nolan Bushnell Atari shipped the first computer game to achieve wide commercial acceptance "Pong" was a simple monochrome game with a ball and two paddles, a form ofelectronic table tennis that became wildly popular Atari continued developing games but also subsequently introduced a number ofmicrocom-puters, including the Atari 800 and the Atari 520ST The 520ST had a graphical interface and built-in MIDI and was competitive with the Amiga for the home market in the mid-1980s
Trang 4ATCASee Antique Telephone Collectors Association.
ATCPSee AppleTalk Control Protocol
ATCRBSair traffic control radar beacon system
ATD1 asynchronous time division 2 Attention Dial
A modem command from the Hayes set that instructs
a modem to dial the number following the command
Often aTorPwill precede the number to indicate
whether to dial as a tone orpulse signal For example,
advanced technology demonstration
ATDRSSSee Advanced Tracking and Data Relay
Satellite System
ATELSee Advanced Television Evaluation
Labora-tory
ATG1 See address translation gateway 2.Art
Tech-nology Group Inc.Ane-commerce platform
devel-opment vendor
Athena project, Project AthenaA project of the
Massachusetts Institute of Technology (MIT)
Com-puter Science Lab begun in 1984 The goal was to
take various incompatible computer systems, and
develop a teaching network that could utilize the
dif-ferent resources of each in a consistent manner The
development of The X Window System originated
from efforts to provide a graphical user interface
(GUI) for Athena See X Window System
ATI1 Accelerated Technology Incorporated
Acom-mercial supplier ofrealtime operating system (RTOS)
source code for embedded systems, based upon the
Nucleus PLUS multitasking kernel 2 Advanced
Telecommunications, Inc A commercial vendor of
telecommunications-related system design,
installa-tion, and training ATI is one of several firms
associ-ated with Applied Cellular Technology 3 See
Ad-vanced Telecommunications Institute
ATIS1 Advanced Travelers Information System A
forum ofSAE International, an engineering society for
advancing transportation mobility 2 See Alliance for
Telecommunications Industry Solutions
ATM1 See asynchronous transfer mode 2 See
Au-tomated Teller Machine
ATM-PONasynchronous transfer mode passive
op-tical networks Atype ofopop-tical distribution network,
promoted as a means to implement large-scale,
full-service subscriber telecommunications full-services See
fiber to the home
ATM Adaptation LayerAAL A layer in an ATM
network See asynchronous transfer mode in the
Ap-pendix for extended information and diagrams
ATM cellThe basic unit of infonnation transmitted
through an ATM network.AnATM cell has a fixed
length of53 bytes, consisting ofa 44- or 48- byte
pay-load (the information transmitted), and a 5-byte
header (addressing information) with optional4-byte
adaptation layer information Interpretation ofsignals
from different types ofmedia into a fixed length unit
of data makes it possible to accommodate different
types oftransmissions over one type ofnetwork See
asynchronous transfer mode; see the Appendix for
details and diagrams
ATM cell rate In ATM networks, a concept that
convey data, signals, and priorities Common cell rate concepts include leaky bucket and cell rate margin See ATM Cell Rate Concepts Table
ATM endpointIn an ATM network, the point at which a connection is initiated or terminated See asynchronous transfer mode
ATM endpoint addressA location identifier func-tionally similar to a hardware address in an ATMARP environment, although it need not be tied to hardware See asynchronous transfer mode, ATM endpoint ATM Forum, TheAn international nonprofit orga-nization founded in 1991 to further the evolution and implementation of asynchronous transfer mode (ATM) technology as a global standard The Forum provides educational information onATM and speci-fications and recommendations to the lTU-T based
on standards of interoperability between vendors, with consideration to the needs ofthe end-user com-munity The ATM Forum is a membership-by-fee group which includes a number oftechnical commit-tees to discuss and report on specific issues such as signalling, traffic management, emulation, security, testing, and interfacing See asynchronous transfer mode, UNI http:/www.atmforum.com/
ATM hardware addressThe individual IP station address See asynchronous transfer mode, ATM end-point address, Internet Protocol
ATM line interfaceALI A device at the physical layer enabling connection to a variety ofphysical me-dia allowing, for example, the accommodation ofdif-ferent line speeds
ATM Link EnhancerALE A commercial error-cor-recting mechanism for satellite communications de-veloped by COMSAT The Header Error Control (HEC) specified for asynchronous transfer mode (ATM) is suitable for transmissions carried through low error rate media such as fiber optic cables It be-comes inadequate, however, in bursty transmissions environments such as wireless networks, particularly those that are satellite-based To compensate for this limitation, COMSAT developed an ALE module which is inserted in the data paths before and after the satellite modems, to isolate ATM cells from burst errors This module allows selective interleaving of ATM cells before they are transmitted through the satellite link, thus providing a lower bit error rate (BER) and an improved cell loss ratio (CLR) See asynchronous transfer mode, cell rate
ATM modelsThere are a variety oftypes and imple-mentations ofATM networks, including Classical IP, LANE, IP Broadcast over ATM, and others See asyn-chronous transfer mode, ATM Transition Model, Classical IP Model, Conventional Model, Integrated Model, Peer Model See Appendix B for details and diagrams
ATM slotAtime indicator for the duration ofone cell, usually described in microseconds It will vary de-pending upon the cell-carrying medium.InATM, one
use of the term slot is to describe delay in switch
performance See asynchronous transfer mode, ATM cell
Trang 5Fiber Optics Illustrated Dictionary
AIM switch processor ASP A modular component
from Cisco Systems that provides cell relay,
signal-ing, and management processing functions It
in-cludes an imbedded IOO-MHz MultiChannel
Inter-face Processor (MIP) R4600 RISC processor, with
ATM access to the switch fabric, to provide high call
setup rates and low call setup latencies It includes
an Ethernet port and dual serial ports
The ASP works in conjunction with a
field-replace-able feature daughtercard which supports advanced
ATM switch functions, including intelligent packet
discard, dual leaky bucket traffic policing, and
avail-able bit rate (ARR) congestion control mechanisms
See asynchronous transfer mode
AIM traffic descriptor A list of network traffic
pa-rameters, such as cell rates and burst sizes, and,
op-tionally, a Best Efforts (BE) indicator, within an
asyn-chronous transfer mode (ATM) virtual connection
This information is used to determine traffic
charac-teristics and to allocate resources See asynchronous
transfer mode, BEC, cell rate, PCR, SCR
AIM Transition Model A model lying between the Classical IP Models and the Peer and Integrated Mod-els See ATM modMod-els
AIM Wireless Access Communication System AWACS.AnACTS project to support and influence emerging ATM wireless standards The project con-sidered various link level and system level simula-tions in order to research system concepts based on a 19-GHz air interface It also addressed the feasibil-ity ofdifferent modulation schemes and ofdirectional antenna technology See Advanced Communications Technologies and Services
AIMARP ATM Address Resolution Protocol ATMARP is the ATM Address Resolution Protocol (ARP) with extensions to support address resolution
in a unicast server environment ATMARP provides
a means of resolving Internet Protocol (LP) addresses
to ATM addresses ATMARP use of public UNI ad-dresses or ATM endpoint adad-dresses is similar to Ethernet addressing; ATM addresses need not be tied
to hardware.lnATMARP (Inverse ATMARP) is used
AIM Cell Rate Concepts
ACR allowed cell rate A traffic management parameter dynamically managed by
congestion control mechanisms ACR varies between the minimum cell rate (MCR) and the peak cell rate (PCR) CCR current cell rate A traffic flow control concept that aids in the calculation
ofER The CCR may not be changed by the network elements (NEs) CCR is set by the source to the available cell rate (ACR) when generating a forward RM-cell
CDF cutoff decrease factor Controls the decrease in the allowed cell rate (ACR)
associated with the cell rate margin (CRM)
CIV cell interarrival variation Changes in arrival times of cells nearing the receiver If
the cells are carrying information that must be synchronized, asinconstant bit rate (CBR) traffic, then latency and other delays that cause interarrival variation can interfere with the output
GCRA generic cell rate algorithm A conformance enforcing algorithm which evaluates
arriving cells See leaky bucket
ICR initial cell rate A traffic flow available bit rate (ARR) service parameter
The ICR is the rate at which the source should be sending the data
MCR minimum cell rate Available bit rate (ABR) service traffic descriptor The
MCR is the transmission rate in cells per second at which the source may always send
PCR peak cell rate The PCR is the transmission rate in cells per second that
may never be exceeded, which characterizes the constant bit rate (CBR)
RDF rate decrease factor Anavailable bit rate (ABR) flow control service
parameter that controls the decrease in the transmission rate of cells when it is needed See cell rate
SCR sustainable cell rate The upper measure of a computed average rate of cell
transmission over time
Trang 6nections (PVCs) Inverse Address Resolution
Proto-col (InARP) supports dynamic address resolution
en-abling a protocol address corresponding to a given
hardware address to be requested See asynchronous
transfer mode and the Appendix for a fuller
explana-tion ofATM See RFC 1293, RFC 1577
atmosphere1 Ambience, mood, feeling about a
lo-cation or room 2 A gaseous mass enveloping a
ce-lestial body See atmosphere, Earth's
atmosphere, Earth'sThe gaseous envelope
sur-rounding the Earth which provides breathable air,
moisture, weather variations, protection from the
sun's radiation, especially the ultraviolet rays, and
particles which deflect radiant energy that can be
har-nessed for telecommunications Atmospheric pressure
at sea level is approx 14.7 pounds per square inch,
with local weather variations, and decreases
some-what uniformly as altitude increases Barometers are
used to measure atmospheric weather, and
baromet-ric altimeters indicate altitude through changes in
pressures in the atmosphere
The atmosphere has been divided into three main
re-gions From the surface going away from the Earth,
they are the troposphere, stratosphere, and
iono-sphere See ionoiono-sphere
ATN See Aeronautical Telecommunications
Net-work
atomAfundamental unit ofenergy or matter
(depend-ing upon how you look at it) that is the essential
build-ing block ofmolecules which, intum,are
fundamen-tal building blocks of elements.Anatom is
chemi-cally indivisible However, from a physics point of
view, atoms are described in terms of even smaller
components, including protons, neutrons and their
associated electrons.Inthe weird and wild world of
quantum physics, there are even smaller units of
en-ergy called quarks and other atomic interactions yet
to be fully understood
atomic clockAn instrument devised in the 1940s for
precise timing and synchronization, it is now
particu-larly important in the u.S Global Positioning
Sys-tem (GPS) and many scientific research applications
An atomic clock uses the frequency associated with
a quantum transition between two energy levels in an
atom as its reference It exploits the unique frequency
characteristics of photons in a given transition
Atomic clock is actually a general category name for
oscillators whose characteristics are based on
quan-tum mechanical energy state transitions Advanced
atomic clocks can be accurate to within fractions of
a second over hundreds ofthousands ofyears
Space-based atomic clocks can be designed to be more
ac-curate than Earth-based atomic clocks due to the
lesser influence of the Earth's gravity
Coordinated Universal Time (UTC) reporting centers
make use of atomic clocks for establishing an
inter-national time reference Atomic clocks can be used
to validate satellite data for integrity and accuracy for
use in navigational applications Synchronization
between transmitting and receiving
telecommunica-tions statelecommunica-tions can be maintained by means of atomic
which, in tum, can derive their timing from the more expensive atomic clocks
Inthe early 1990s, atomic clocks were improved with the introduction of a Hewlett-Packard cesium-beam clock which was more rugged and more stable than previous models
Europe's Geostationary Navigation Overlay System (EGNOS) is being designed to use cesium and ru-bidium atomic clocks to provide System Time (ST) for calculating precise navigation infonnation for GPS-based land, marine, and air transportation sys-tems By using atomic clocks and GPS data from sat-ellites, rather than traditional beacon-based naviga-tional methods, accuracy can be established within seven meters
Computers that are permanently or frequently con-nected to the Internet will sometimes be configured
to poll the U.S Naval Observatory atomic clock to synchronize their system clocks with the time on the Navy's clock This synchronization is useful for sensitive file and database management and for time-critical e-commerce applications such as stock and auction transactions Accurate time-stamp informa-tion is also useful for computer-based legal transac-tions and event tracking
The National Institute of Standards and Technology (NIST) sponsors a Web site called "A Walk through Time" that features infonnation and illustrations on time-keeping through the ages and the development
of atomic clocks See Coordinated Universal Time, Datum Corporation, Global Positioning System http://physics.nist.gov/GenInt/Time/time.html atomic laserA device or process that emits matter
in the same general sense as an optical laser stimu-lates the emission of coherent pulses of light Such a laser was developed out ofresearch on Bose-Einstein condensates in the mid-1990s by W Ketterle et al at MIT Rather than using mirrors to deflect light within
an optical cavity, they used magnetic fields to deflect matter in a magnetic cavity, using sodium atoms, which are magnetically sensitive, as the "ammuni-tion" for the atomic laser"gun."As energy is built
up, a pulse of coherent matter manages to break through the magnetic barrier in much the same way
as pulses of light break through the semitransparent mirrors ofan optical cavity The system was success-fully demonstrated and described in 1997 By Spring
2002, the scientists had found a way to emit a con-tinuous stream of atoms
The potential for this technology in terms ofdesign-ing complex crystal lattices, diffraction gratofdesign-ings, cir-cuit boards and other fabrications important to the semiconductor industry may be very great The sys-tem works in the environment of a vacuum, so it's not quite as easy to set up, but it is fascinating tech-nology which will no doubt by harnessed in exciting ways
For their discoveries, the inventors received a Nobel Prize in physics in 2001 See laser
atomic numberAnumber characteristic determined
by experimentation, since the atom is far too small
Trang 7Fiber Optics Illustrated Dictionary
to be seen by any natural or microscopic means This
number is used to represent an element in a periodic
table and describes electrons in relation to the
pro-tons in a neutral atom
ATS 1 See Applications Technology Satellite
pro-gram 2 Automation Tooling Systems
ATSC See Advanced Television Systems Committee
ATSC Digital TV Standard - Examples
Doc Date Description
Al80 Jul 99 Modulation and Coding
Requirements for Digital TV
(DTV) Applications over Satellite
Modulation and data coding for
satellite communications are
defmed for a variety of
programming types, including
video, audio, data, multimedia, or
others It includes multiplexed bit
streams such as MPEG-2
Al64AMay 00 Transmission Measurement and
Compliance for Digital Television
describing test, monitoring, and
measuring methods
Al65AMay 00 The Program and System
Information Protocol for
Terrestrial Broadcast and Cable,
providing a methodology for
transporting digital television data
and electronic program guide data
with an amendment on Directed
Channel Change (DCC) for
program tailoring
Al90 JulOl The Data Broadcast Standard
defining protocols for data
transmission compatible with
digital multiplex bit streams
according to ISOIIEC 13818-1
(MPEG-2 systems) standards The
standard encompasses both
non-TV and non-TV programming,
including Webcasting, streaming
video, etc
Al53BAug 01 The Digital Television Standard
for advanced television (ATV)
systems The document specifies
the parameters and video encoding
input scanning format, along with
preprocessing and compression
parameters of the video encoding
Itdescribes audio encoder signal
formats, preprocessing, and
compression, as well as the service
multiplex and transport layer
characteristics and specifications
ATSC Digital Television Standard ATSC DTS The
Advanced Television Systems Committee (ATSC) Technology Group on Distribution released the ATSC
Digital Television Standardin September 1995
(Document AJ53) along with Document A/54 which
describes use ofthe DTS The DTS is based upon the ISO/IEC MPEG-2 Video Standard, the Digital Au-dio Compression (AC-3) Standard, and the ISOIIEC MPEG-2 Systems Standard It was part of an ongo-ing effort to upgrade consumer broadcastongo-ing pro-gramming and equipment to reflect improvements in technology
The Digital Television Standard was, in large part, adopted by the Federal Communications Commis-sion (FCC) in December 1996 as well as by Canada and some Asian and South American countries This influential document was revised by the ATSC and released asAJ53AinApri12001 It specifies the tech-nical parameters ofadvanced TV systems Examples ofstandards include those listed below See Advanced Television Standards Committee
ATSE Academy of Technological Sciences and En-gineering http://www.atse.org.au/
ATT See Automatic Toll Ticketing
attachment Something connected to or with In data communications, a note or file that is attached to the end of an existing file or other electronic communi-cation Commonly, binary files are sent with email messages as attachments because the message text part ofmany email systems cannot transcribe or trans-mit 8-bit binary code.The system will convert the bi-nary attachment to a compatible mode (e.g., 7-bit text) and reconvert it back to binary at the destina-tion.Anemail binary attachment allows you to send
a picture, sound file, Adobe PostScript document, or other nontext transmission in conjunction with regu-lar text
attachment unit interface, autonomous unit inter-face AUI Certain cables and connectors used to at-tach equipment to Ethernet transceivers Commonly Ethernet connections are made via a printed circuit board installed in a slot in a computer with a BNC or RJ-45 connector protruding from the computer for making the connection to the Ethernet transceiver and cable The ANSI/IEEE standard 802.3 (originally re-leased as Document 1802.3-1991) defines an AUI physical layer interface called DB-15 or DIX attack time The time it takes for a signal or sound to
go from its initiation to its full volume or power On
a violin, for example, it's the time interval from the moment the bow begins to move and a consistent note achieves itsfullvolume and tone The attack time on
an electronic system is the time it takes from the ini-tiation ofa pulse, signal, or power-on action until the system reaches its intended activation threshold, out-put, or throughput level See decay time
ATTC See Advanced Television Test Center attemptAneffort to initiate or establish a commu-nications connection.Insome systems that are billed
on a flat rate or per-call basis, attempts are not billed
In other systems, such as those that bill by air time, the attempts are charged by the minute or second whether or not the call is connected
Trang 8initial transmission and its power when received or
measured at specified points, usually expressed as a
ratio in decibels Loss in power can result from
dis-tance, transmission lines, configurations, faults, and
weather See absorption, contrast with gain
attitude and articulation control subsystemAACS
A spacecraft guidance system employed on the
Cassini spacecraft mission to permit dynamic
con-trol ofrotation and translation maneuvers The AACS
uses star and sun sensors to establish reference points
for determining its position The main engine and
smaller engines are used for propulsive maneuvers
Sensors estimate attitude and rate of both the base
body and the articulated platforms A series of
vec-tors, kinematically propagated in time, aids the
sys-tem in detennining motion ofvarious bodies in
rela-tion to the base frame The AACS works in
conjunc-tion with the command and data subsystem (CDS),
which is the main processor on the craft The CDS
receives RF signals from Earth and sends
informa-tion and control parameters to other systems, such as
the AACS, accordingly See Cassini
atto- (symb - a) Used as a prefix to represent a very
minute quantity, one quintillionth of, 10-18
•See femto-
ATUSee African Telecommunications Union
ATU-CADSL transceiver unit-central office
Amo-dem-like device installed at telephony central offices
to process data communications received from the
subscriber that are then fOlWarded through a data
net-work to a subscriber-chosen data services provider
See Asymmetric Digital Subscriber Line
ATU-RADSL transceiver unit-remote A
modem-like device installed on the subscriber premises to
process data communications It typically interfaces
between the computer and a network interface device
splitter See Asymmetric Digital Subscriber Line
ATV1 See advanced TV 2 See amateur television
ATVEFSee Advanced Television Enhancement
Fo-rum
ATVEF Enhanced Content SpecificationA
foun-dation specification developed by the Advanced
Tele-vision Enhancement Forum for the creation of
HTML-enhanced television content It focuses
mainly on existing technologies rather than
promot-ing new ones, laypromot-ing out a means for providpromot-ing and
viewing broadcast programming on the World Wide
Web or through specialized viewing software
simi-lar to Web browsers It promotes the reliable
trans-mission of a variety of types of broadcasts through
Internet, cable, and land-based networks to
comput-ers, enhanced televisions, and dedicated enhanced TV
consumer appliances The specification is intended
to encompass both one-way and two-way systems
and both analog and digital systems See Advanced
Television Enhancement Forum, broadcast data
trig-ger
audible ringing toneAnaudible signal transmitted
to the calling party to let the caller know that the
called number is ringing See busy signal
audible soundSound waves that are perceived by
the ear/brain of a particular species Audible sound
hearing generally between the ranges of20 to 20,000 hertz The upper ranges tend to drop off during the teenage years and decline gradually throughout a person's lifetime Illnesses, very sudden loud noises, protracted loud noises, and sustained low level noises can have profound negative effects on a person's hear-ing See audio, sound
audioPertaining to sounds, primarily those within range ofhuman perception, from frequencies ofabout
20 to 20,000 hertz (the upper range especially tends
to diminish as people get older) The comfortable hearing range varies in loudness from a few decibels
to about 80 decibels At volumes near and above 160 decibels, permanent hearing damage is almost cer-tain Sudden loud sounds, frequent exposure to loud sounds, or even long-term exposure to medium level sounds can damage the sensitive structures associated with hearing
The types of sounds most commonly used for com-munication are speech and music Most hearing is done with the ears, although some people augment their understanding ofauditory information by read-ing lips or sensread-ing physical vibrations through their fingers or bodies Helen Keller was known for "lis-tening" to symphonies through a sensitive sound board placed in the symphony hall under her chair Many deaf or hard-of-hearing people use their fin-gertips pressed against the larynx of a speaker to aid
in sensing auditory vibrations
While humans can hear a broad range offrequencies, not all these frequencies are used in human speech
We can detect pitches up to about 18,000 to 20,000 hertz but don't utter sounds that high in conversation Thus, telephone and other speech circuits typically are not designed to transmit the full hearing range of frequencies and will be optimized for the frequencies associated with the information being transmitted See acoustic
AudioMsual Service Specific Convergence Sub-layerAVSSCS A multimedia convergence protocol for transmitting video over AAL5 using available bit rate (ABR) services There is a particular focus on supporting MPEG over ATM as ATM has become a dominant networking medium and MPEG is a widely supported video format ATM is capable of support-ing simultaneous video and other data transmissions See asynchronous transfer mode, MPOA
Audio Applications Programming InterfaceAAPI
A library of functions designed to facilitate the de-sign ofaudio applications The functions can be called
by compatible applications programs in order to in-teract with audio servers Thus, the conversion, play-back, or recording of audio can be accomplished without each aspect of the application being written from the ground up (Le., without reinventing the wheel) Most operating systems now have AAPIs available for developers
Audio Engineering SocietyAES Since 1948, the AES has been promoting and fostering the develop-ment and advancedevelop-ment of audio technologies The AES Standards Committee contributes information
Trang 9Fiber Optics Illustrated Dictionary
and technical expertise that supports national and
international audio standards development The
Au-dio Engineering Society Historical Committee (AES
HC) researches, collects, and preserves historical
in-formation and artifacts related to audio history AES
publishes the Journal ofthe Audio Engineering
So-ciety and various papers and conference proceedings.
http://www.aes.org/
audio frequency AF Aspectrum ofwavelengths that
can be heard For humans this is from about 30 hertz
up to about 20 kilohertz, although the upper level
de-clines to about 16 to 18 kilohertz by adulthood
Audio Interchange File Format AlFF, Audio IFF
AIFF is a widely used audio file storage and exchange
format descended from Interchange File Format
(IFF) IFF is a flexible, multiplatform means of
digi-tally encoding a variety of types of media-related
in-formation (not just sound) IFF was developed in the
mid-1980s by Electronic Arts and
Commodore-Amiga In 1985, the format quickly became standard
on the Amiga computer Later variations of the
con-cept and the file format were ported to other platforms
toprovide compatibility with Amiga files
While TIFF, JPEG, and BMP have now superseded
IFF for the exchange of image files, the Audio
Inter-change File Format concept has survived and
mi-grated to other platforms and has been adapted by
Apple Computer, Inc as the standard audio file
for-mat for Macintosh systems
AlFF facilitates the data storage and transmission of
monaural (mono) and multichannel sound samples
using a chunky format On the Macintosh, it is stored
in the data fork AlFF is also supported on a number
of professional workstations, including Avid
Tech-nology and Silicon Graphics (SGl), and has further
been adopted as a standard audio format by the Open
Media Format Interchange (OMPI) group The
origi-nal IFF format is documented in the Amiga ROM
Kernel manual, Appendix H Details of Apple
Computer's AIFF are available from Apple
Devel-operTechrUcalSupport
Audio Messaging Interchange Specification
AMIS.Ananalog telephony protocol that facilitates
the exchange of voice mail messages among users
on voice mail systemsfromdifferentvendors.AMIS
specifications were released in the early 1990s Not
all the features ofcommercial voice mail systems can
be directed through AMIS Depending upon the
im-plementation, AMIS may not permit broadcasting to
multiple users on another system, and there may not
be a full complement ofconfirmation messages
avail-able from the other systems A number of
commer-cial products implementing AMIS systems or
inter-facing with AMIS systems have been developed by
large vendors such as Toshiba and Lucent Active
Voice Corporation claims to be the first voice
pro-cessing systems manufacturer to incorporate AMIS
standards into its products See Voice Profile for
In-ternet Mail
audio tape Atype ofmagnetic storage medium used
for audio recordings Most audio tapes are small, so
they can be used in portable tape decks or car stereos,
with playing times ranging from 10 to 120 minutes Common music tapes are 30, 45, or 60 minutes per side for a double-sided tape Some audio tapes are designed as a continuous loop with the tape ends fused for continuous playing Video tapes are some-times used as high quality audio tapes
Eight-track tape cartridges were introduced in the early 1960s and were popular for a few years Cas-sette tapes were introduced soon after eight-tracks and eventually superseded them By the 1980s, video tape-based audio recording approached CD sound quality
Computers in the late 1970s and early 1980s used large magnetic tape spools and small audio tapes (e.g., cassette tapes) for recording data Various types
of magnetic tape systems (e.g., DAT) are still used for data backups As CD players become less expen-sive and more prevalent they provide higher quality sound reproduction than most tape systems and a less volatile alternative to data tape backups
audio-follow-video AFV In many broadcast sys-tems, audio and video are recorded and/or transmit-ted separately In AFV, the audio signals are automati-cally routed together with their associated video signals
audio-on-demand AoD Audio services provided to
a user on request AoD is one ofthe earliest services-on-demand (SoD) systems implemented in the tele-communications industry In the days of operator-managed telephone services, imaginative service pro-viders realized they could place a phone at the switch-board center near a radio or gramophone player and play music for the subscriber on request It was an unsophisticated system, but the concept was timely, and the idea is now implemented with digitally au-tomated technologies in the form of video-on-de-mand, and other custom request services See ser-vices-on-demand, video-on-demand
AudioGram Delivery Services ADS ANortel sub-scriber telephone service option that enables callers who get a busy signal or no answer to their ring to leave a message that will be delivered to the callee at
a later scheduled time Essentially it's a phone line answering machine service
audiographics A multimedia network communica-tions system suitable for distance learning, in which remote computer screens are shared as a conference and lecture interactive medium for dynamically shar-ing images, video, and text Electronic Classroom, written by Robert Crago for the Macintosh, is an ex-ample of this type of application, designed to work over public switched telephone networks (PSTNs)
Audiographics is sometimes called telematics Some
people like to make a distinction between audiograph-ics, which is the transmission of still images and sound, and videoconferencing, the transmission of motion video and sound With improvements in tech-nology, the distinction is blurring See whiteboard-ing, electronic; videoconferencing audiometer, sonometer An instrument for measur-ing hearmeasur-ing acuity invented by Alexander Graham Bell Bell's use ofthe Audiometer to test hearing was
Trang 10At the April 1885 meeting of the National Academy
of Sciences, Bell is reported to have demonstrated his
Audiometer devised from two flat coils of insulated
wire adjusted with graduated distances such that
elec-trical cutrent from an armature between magnetic
poles was passed through one coil (interrupted by a
rotating disk), while a phone was attached to the other
coil Thus, the current could be used to control the
intensity of the sound, and the responses of the
chil-dren being tested could be recorded and analyzed
About 10% of the students tested with this early
in-strument were found by Bell to be hard ofhearing in
their better ear
While the Audiometer was initially used to test
hu-man hearing, hu-many ofthe components developed for
the audiometric industry (speakers, jacks, amplifiers,
tone generators, transmission components, etc.) have
since been adapted for telecommunications devices
and testing systems (P.A systems, telephony
com-ponents, transmission line testing equipment, etc.)
Audiometers have become very sophisticated since
their invention by Bell, and the term has become
ge-neric to a wide variety of audiometric instruments
Commercial audiometers now commonly include
keyboards, internal digital storage for saving
hun-dreds of audiograms, programmable functions, and
serial interfaces for connecting to computers
Audion&Electron Tube Controlling Grid
grid
The triode electron tube (left), developed from the
Fleming valve by Lee de Forest, is one of the most
significant inventions in electronics history The third
element, a controlling grid, added by de Forest to the
two-element Fleming valve, enabled the flow
ofelec-trons, from the cathode to the anode, to be controlled.
Electron tubes are now more streamlined and
so-phisticated than the original Audion electron tube
which resembled a lightbulb with a plate, filament,
and grid inside and two wires running out the top.
AudionAnextremely significant invention of the
early 1900s, evolutionarily descended from simple
flame detectors,that led to the three-element vacuum
electron tube patented by American inventor Lee de
Forest The Audion was a tantalum lamp with a
and plate A simple wire bent in a zigzag pattern be-came a grid, providing control over the flow of the electrons from the filament to the plateina way that had not been previously possible Thus, electron tubes could be used to amplify signals, notjust rectify them,
as in the Fleming oscillation valve upon which de Forest's Audion was based
Thistriodeelectron tube's control grid represented breakthrough technology which de Forest sold to AT&T at the bargain price of $50,000.Itwas used for decades throughout the electronics industry until
it was superseded by the transistor for most consumer applications Repeater devices based on the Audion enabled long-distance telephony
Interestingly, like many inventions through history, the inventor himself didn't understand the detailed mathematics/physics behindwhythe Audion worked, creating problemsinmanufacturing The only way
to know if the tube was good was to test it, and the sensitivity varied from tube to tube Edwin 4rmstrong was one of the few early scientists to grasp some of the physics associated with the Audion's functioning
He authored an article inElectrical Worldin Decem-ber 1914, explaining the action of the Audion and how it could be more effective if more gases were removed from the bulb in manufacturing
The term Audion was originally trademarked but has become generic for three-element tubes See de For-est, Lee; Edison effect; electron tube; flame detec-tor; Fleming oscillation valve
Auditory Research LaboratoryARL A lab at McGill University in Montreal, Canada specializing
in the study of the perceptual organization of sound augerAtool designed for boring, or a bit that fits into
a drill designed to make large bore holes, which can
be used for wiring installations
Augustine, SaintA philosopher who authoredDe civitate Dei(The city of God) in 428 AD This im-portant record ofwestern knowledge includes historic observations of magnetic phenomena
AUISee Attachment Unit Interface
AUPSee Acceptable Use Policy
auralHeard or perceived through the ear; auditory See acoustic, sound
auroraSolar flare, a nuclear effect from the sun that can sometimes be seen by its influence on the Earth's upper atmosphere The ionization that results causes the undulating light shows we know as the aurora bo-realis and aurora australis
Aurora 1Aregional communications satellite in geo-stationary orbit over Alaska
AUSEAnetThe Australian-Asian network which supports multination VLSI project communications
of the Assocation of Southeast Asian Nations (ASEAN) for sharing project information among par-ticipating countries
Australian Communications AuthorityACA The governing body ofAustralia responsible for regulat-ing telecommunications and radiocommunications, including the management of the radio frequency spectra and the National Numbering Plan ACA also