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A&B bit signaling

Used in T1 transmission facilities and sometimes called

"24th channel signaling." Each of the 24 T1 subchannels in this procedure
uses one bit of every sixth frame to send supervisory signaling information.

AAA

Authentication, authorization, and accounting: A Cisco description

of the processes that are required to provide a remote access security solu-
tion. Each is implemented separately, but each can rely on the others for
functionality.

AAL

ATM Adaptation Layer: A service-dependent sublayer of the Data

Link layer, which accepts data from other applications and brings it to the
ATM layer in 48-byte ATM payload segments. CS and SAR are the two sub-
layers that form AALs. Currently, the four types of AAL recommended by
the ITU-T are AAL1, AAL2, AAL3/4, and AAL5. AALs are differentiated by
the source-destination timing they use, whether they are CBR or VBR, and
whether they are used for connection-oriented or connectionless mode data
transmission. See also: AAL1, AAL2, AAL3/4, AAL5, ATM, and ATM
layer.

AAL1

ATM Adaptation Layer 1: One of four AALs recommended by the

ITU-T, it is used for connection-oriented, time-sensitive services that need
constant bit rates, such as isochronous traffic and uncompressed video.
See also: AAL.

AAL2

ATM Adaptation Layer 2: One of four AALs recommended by the

ITU-T, it is used for connection-oriented services that support a variable bit
rate, such as voice traffic. See also: AAL.

AAL3/4

ATM Adaptation Layer 3/4: One of four AALs (a product of two

initially distinct layers) recommended by the ITU-T, supporting both con-
nectionless and connection-oriented links. Its primary use is in sending
SMDS packets over ATM networks. See also: AAL.

AAL5

ATM Adaptation Layer 5: One of four AALs recommended by the

ITU-T, it is used to support connection-oriented VBR services primarily to
transfer classical IP over ATM and LANE traffic. This least complex of the
AAL recommendations uses SEAL, offering lower bandwidth costs and sim-
pler processing requirements but also providing reduced bandwidth and
error-recovery capacities. See also: AAL.

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