Glossary: Mostlyabreviations and acronyms:
Ask the Deacon                                        


AA: Access Adapter; device usedat the CO as part of an xDSL implementation.

AAL: ATMAdaptation Layer; One of three layers of the ATMprotocol reference model. It performs segmentation and reassembly (SAR),translates higher-layer data into ATMcell payloads, and translates incoming cells into a format readable bythe higher layers.

AAL-1: ATMAdaptation Layer type 1; relates to ATMservice class A; where a time relation exists between the source and thedestination, the bit rate is constant, and the service is connection-oriented(e.g., a voice channel).

AAL-2: ATMAdaptation Layer type 2; relates to ATMservice class B; where a time relation exists between the source and thedestination, the bit rate is variable, and the service is connection-oriented(e.g., a video or audio channel).

AAL-3: ATMAdaptation Layer type 3; relates to ATMservice class C; where no time relation exists between the source and thedestination, the bit rate is variable, and the service is connection-oriented(e.g., a connection-oriented file transfer).

AAL-4: ATMAdaptation Layer type 4; relates to ATMservice class D; where no time relation exists between the source and thedestination, the bit rate is variable, and the service is connectionless(e.g., LAN interconnection and electronic mail).

AAL-3/4: ATMAdaptation Layer type 3/4; since the differences between AAL-3and AAL-4were minor, the ITUmerged them into a single type called AAL-3/4.

A:B:C notation: The a:b:c notation for samplingratios, as found in the CCIR-601specifications, has the following meaning

ABCD parameters: A two-by-two matrix describinginput voltage and current-to-output voltage and the current relationshipof a two-port network.

ABR: Available Bit Rate; An ATMservice type in which the ATM network makes a "besteffort" to meet the transmitter's bandwidth requirements.

AC-3: Audio Compression - 3;

ACATS: Advisory Committee on Advanced TelevisionService.

Access Network: That portion ofa public switched network that connects access nodes to individual subscribers.The Access Network today is predominantly passive twisted pair copper wiring.

Access Nodes: Points on the edge ofthe Access Network that concentrate individual access lines into a smallernumber of feeder lines. Access Nodes may also perform various forms ofprotocol conversion. Typical Access Nodes are Digital Loop Carrier systemsconcentrating individual voice lines to T1 lines, cellularantenna sites, PBXs, and Optical Network Units (ONU).

Access unit: A coded representationof a presentation unit. In the case of audio, an access unit is the codedrepresentation of an audio frame. In the case of video, an access unitincludes all the coded data for a picture, and any stuffing that followsit, up to but not including the start of the next access unit. If a pictureis not preceded by a group_start_code or a sequence_header_code, the accessunit begins with a the picture start code. If a picture is preceded bya group_start_code and/or a sequence_header_code, the access unit beginswith the first byte of the first of these start codes. If it is the lastpicture preceding a sequence_end_code in the bit stream all bytes betweenthe last byte of the coded picture and the sequence_end_code (includingthe sequence_end_code) belong to the access unit.  (ATSC- Document A53; Digital Television Standard)

Ack: Acknowledgment

ACM: Association for Computing Machinery

ACS: Access Control System

ACTA: America's Carriers TelecommunicationAssociation; the group calling "foul", to the FCC forthe use of Internet phone software., amoung other things I'm sure.

A/D: Analog to Digital converter.

ADB: Apple Desktop Bus; That slow daisy-chainserial bus used to connect Apple/Mac peripherals.

ADC: Analog to Digital Converter; An analogintegrated circuit that samples, quantizes, and holds analog signals fordigital signal processing.

ADP: Application Distribution Protocol; ProprietaryPush protocol developed by "Marimba".

ADPCM: Adaptive Differential Pulse Code Modulation

ADSL: Asymmetric Digital Subscriber Line /Loop; Modems, or more accurately 'transceivers', attached to twisted paircopper wiring that transmit from 1.5 Mbps to 9 Mbps downstream (to thesubscriber) and from 16 Kbps to 640 Kbps upstream, depending on line distance.ADSL uses a modulation technology called discrete multitone, (DMT).For more information see the ADSLForum.

AEP: Application Environment Profile, (POSIX).

AES: Application Environment Specification,(OSF)

AES: Audio Engineering Society.

AFE: Analog Front End; The part that connectsdigital signal processing circuits to an analog signal.  AFE mainlyconsists of an ADC and a DAC

AFR: Available Frame Rate; one of Frame Relay'sanswers to QoS.

AFS: Andrews File System; A distributed filesystem built around a cell-based directory hierarchy. Originally developedout of Carneigie Mellon University, (CMU)then commercially launched by TransArc Corporation before being adoptedby the Open Software Foundation, (OSF) as part of theDCE.

AGC: Automatic Gain Control; A part of theAFE that adjusts the received signal level to that suitablefor the input of the ADC.

AGIS: Apex Global Information Services; Oneof the top 10 Network Service Providers, (NSP) along with MCI, Netcom,PSINet, Sprint, and UUnet.

AHM: ATMCircuit Handling Module

AI: Artificial Intelligence

AIFF: Audio? (Apple, SGI)

AM: Amplitude Modulation

AMC: ATMInter-Module Connector

AMI: Alternate Mark Inversion (line code);A simple baseband line code that altern ates the parity of data symbolsfor adjacent 1's.  For example, a sequence of 1011000 is encoded as+A 0 -A +A 0 0 0 for the AMI line code for which A is the magnitude ofthe line code.

Amplifier Nonlinearity:The nonlinear amplifier input-to-output relationship.

AMRC: Armerican Mobile Radio Corporation

AMSC: Armerican Mobile Satellite Corporation

Analog: The representation of numericalvalues by physical variables such as voltage, current, etc.; continuouslyvariable quantities whose values correspond to the quantitative magnitudeof the variables.

Anchor frame: A video frame that isused for prediction. I-frames and P-framesare generally used as anchor frames, but B-framesare never anchor frames.

ANN: Artificial Neural Network

ANS: Advanced Network Services

ANSI: AmericanNational Standards Institute.

ANT: ADSL Network Terminator

Anti-aliasing: Software adjustmentto make diagonal or curved lines appear smooth and continuous in computer-generatedimages. See also aliasing.

AO/DI: Always On / Dynamic ISDN.

API: Application Programming Interface

Applet: A dynamic and interactive programthat can run inside a web page.

ARP: Address Resolution Protocol

ARPA: Advanced Research Project Agency

ARPANET: Advanced Research Project AgencyNetwork; DoD sponsored agency along with BBNresponsible for laying the foundation of the Internet.

AS: Autonomous System; A collection of routersunder a single administrative authority using a common InteriorGateway Protocol for routing packets.

ASCI: Accelerated Strategic Computing Initiative;a $1 billion multivendor plan launched by the Department of Energy, (DOE).

ASIC: Application Specific Integrated Circuit; An integrated circuit (IC) that is designed for a specificapplication in contrast to some general purpose ICs thatare avialable from an IC catalog.

ASkyB: American Sky Broadcasting

ASN.1: Abstract Syntax Notation, version1; check out an ASN.1'homepage'! A standard, flexible method that (a) describes data structuresfor representing, encoding, transmitting, and decoding data, (b) providesa set of formal rules for describing the structure of objects independentof machine-specific encoding techniques, (c) is a formal network-management
Transmission Control Protocol/Internet Protocol (TCP/IP)language that uses human-readable notation and a compact, encoded representationof the same information used in communications protocols, and (d) is aprecise, formal notation
that removes ambiguities.

ASP: Active Server Page (Microsoft)

ASP: Authorized Service Provider

ASP: Association of Shareware Professionals

aspect ratio:
1. - of pictures. The ratio of length to height of pictures. Nearlyall TV screens are currently 4:3, ie four units across to three units inheight but there is a growing move towards widescreen 16:9. Pictures presentedthis way are believed to absorb more of our attention and have obviousadvantages in certain productions, such as sport. In the change towards16:9 some in-between ratios have been used, such as 14:9.

2. - of pixels. The aspect ratio of the area of a picture describedby one pixel.  The ITU-R 601 digital coding standard defines luminancepixels which are not square. In the 525/60 format there are 486 activelines each with 720 samples of which 711 may be viewable due to blanking. Therefore the pixel aspect ratio on a 4:3 screen is: 486/711 x 4/3 = 0.911(ie the pixels are 10% taller than they are wide)
For the 625/50 format there are 576 active lines each with 720 samplesof  which 702 are viewable so the pixel aspect ratio is:
576/702 x 4/3 = 1.094 (ie the pixels are 9% wider than they are tall)

Account must be taken of pixel aspect ratios - for example in executinga DVE move - when rotating a circle, the circle mustalways remain circular and not become elliptical. Another area where itis important is the movement of  images between (standard) computerplatforms and television systems.  Computers nearly always use squarepixels so their aspect ratio must be
adjusted to suit television. This process takes time and will not beperfect and the quality of the result will depend on the quality of theprocessing used.  See also: pixel
ASSP: Alogorithm Specific Signal Processor;An integrated circuit (IC) that is designed for a specificalgorithm for a category of general applications.  An example is anadaptive filter IC.

AT&T: American Telephone & Telegraph

ATEL: Advanced Television Evaluation Laboratory

ATG: Advanced Technology Group

Athena: Network project conducted at MIT.

ATIS: Alliance for Telecommunications IndustrySolutions

ATM: Asynchronous Transfer Mode; A digitalsignal protocol for efficient transport of both constant-rate and burstyinformation in broadband digital networks. The ATM digital stream consistsof fixed-length packets called "cells," each containing 53 8-bit bytes-a5-byte header and a 48-byte information payload.

Attenuation: The signal loss causedby a transmission channel at a particular frequency.

ATTC: Advanced Television Test Center.

ATU-C: ATM Terminal Unit - Central Office;ATU-C's perform the basic multiplexing, demultiplexing, transmitting, receivingand system control functions and provides interfaces to the loop, networktransport, and switching and operations systems. I've seen articles whereATU-C also refers to ADSL Transceiver Unit - Central Office, inwhich similarfunctions take place, but with the addition of combining and then at theATU-R, splitting out the telephony signal.

ATU-R: ADSL Terminal/Transceiver Unit - Remote

ATV: Advanced Television; refers to any televisiontechnology that provides improved audio and video quality or enhances thecurrent NTSC television system." As first used, ATVmeant an HDTV program compressed to fit the size ofa current NTSC broadcast channel. Now, the definitionis being broadened to include the concept of multiple video programs andother data simultaneously carried within one channel.

ATSC: Advanced Television Standards Committee,(US);  Established in 1982 to co-ordinate the development of voluntarynational technical standards for the generation, distribution and receptionof high definition television. In 1995 the ATSC published "The DigitalTelevision Standard" which describes the US Advanced Television System.This uses MPEG-2 compression for the video and AC-3for the audio and includes a wide range of video resolutions and audioservices. It uses 8 and 16 VSB modulation respectivelyfor terrestrial and cable transmission. (

           Aspect Ratio: 1= square samples, 2 = 4:3 display aspect ratio, 3 = 16:9ddisplay aspect ratio
           pFrame Rate: 1 = 23.976 Hz, 2 = 24 Hz, 4 = 29.97 Hz, 5 = 30 Hz, 7 = 59.94,8 = 60Hz
           Vertical Scan: 0 = interlaced scan 1 = progressive scana
*Note that 1088 lines are actually coded in order to satisfy the MPEG-2requirement that the coded vertical size be a multiple of 16 (progressivescan) or 32 (interlaced scan).a

AU: Access Unit

AVERT: Anti-Virus Emergency Response Team

Last Update:12/13/03
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