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ATM Adaptation Layer Service-dependent sublayer of the data link layer. The AAL accepts data 

from different applications and presents it to the 

 layer in the form of 48-byte ATM payload segments. 

AALs consist of two sublayers: 

. AALs differ on the basis of the source-destination timing used, 

whether they use 

 or 

, and whether they are used for connection-oriented or connectionless mode 

data transfer. At present, the four types of AAL recommended by the ITU-T are 

, 

, and 

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ATM Adaptation Layer 1One of four AALs recommended by the ITU-T. AAL1 is used for con-

nection-oriented, delay-sensitive services requiring constant bit rates, such as uncompressed video and other 
isochronous traffic.

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ATM Adaptation Layer 2 One of four AALs recommended by the ITU-T. AAL2 is used for con-

nection-oriented services that support a variable bit rate, such as some isochronous video and voice traffic.

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ATM Adaptation Layer 3/4 One of four AALs (merged from two initially distinct adaptation 

layers) recommended by the ITU-T. AAL3/4 supports both connectionless and connection-oriented links, but 
is primarily used for the transmission of 

 packets over 

 networks.

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ATM Adaptation Layer 5 One of four AALs recommended by the ITU-T. AAL5 supports con-

nection-oriented 

 services and is used predominantly for the transfer of classical 

 over 

 and 

traffic. AAL5 uses 

 and is the least complex of the current AAL recommendations. It offers low band-

width overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-
recovery capability. 

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Advanced Intelligent Network In 

, an expanded set of network services made available to the 

user, and under user control, that requires improvement in network switch architecture, signaling capabilities, 
and peripherals.

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Alternate Mark Inversion Line-code type used on 

 and 

 circuits. In AMI, zeros are represented 

by 01 during each bit cell, and ones are represented by 11 or 00, alternately, during each bit cell. AMI requires 
that the sending device maintain ones density. Ones density is not maintained independently of the data 
stream. Sometimes called binary coded alternate mark inversion.

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Application Programming Interface (1) The interface to a library of language-specific subroutines 

(such as a graphics library) that implement higher-level functions. (2) A set of calling conventions defining 
how a service is invoked through a software package.

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American Standard Code for Information Interchange The standard binary encoding of alpha-

betical characters, numbers, and other keyboard symbols.

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Asynchronous Transfer Mode International standard for cell relay in which multiple service types 

(such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell pro-
cessing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-
speed transmission media such as 

, and 

.

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Binary 8-Zero Substitution Line-code type, used on 

 and 

 circuits, in which a special code is 

substituted whenever 8 consecutive zeros are sent over the link. This code is then interpreted at the remote 
end of the connection. This technique guarantees ones density independent of the data stream. Sometimes 
called bipolar 8-zero substitution.

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Bearer Channel In ISDN, a full-duplex, 64-kbps channel used to send user data.

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Basic Input/Output System The built-in program that controls the basic functions of communica-

tions between the processor and the Input/Output (I/O) devices of a computer.

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Broadband ISDN 

 communication standards designed to handle high-bandwidth applica-

tions such as video. BISDN currently uses 

 technology over 

-based transmission circuits to pro-

vide data rates from 155 to 622 Mbps and beyond.

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boot Read-Only Memory Chip mounted on the printed circuit board used to provide execut-

able boot instructions to a computer device.