CCNA Cisco Certified Network Associate Study Guide - FTP Server

650 Glossary
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.
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 sublayers
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 connectionless
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 simpler
processing requirements but also providing reduced bandwidth and
error-recovery capacities. See also: AAL.
AARP AppleTalk Address Resolution Protocol: The protocol in an Apple-
Talk stack that maps data-link addresses to network addresses.
AARP probe packets Packets sent by the AARP to determine whether a
given node ID is being used by another node in a nonextended AppleTalk
network. If the node ID is not in use, the sending node appropriates that
node’s ID. If the node ID is in use, the sending node will select a different ID
and then send out more AARP probe packets. See also: AARP.