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Glossary

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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.

AARP

AppleTalk Address Resolution Protocol: The protocol in an

AppleTalk 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.

ABM

Asynchronous Balanced Mode: When two stations can initiate a

transmission, ABM is an HDLC (or one of its derived protocols) communi-
cation technology that supports peer-oriented, point-to-point communica-
tions between both stations.

ABR

Area Border Router: An OSPF router that is located on the border of

one or more OSPF areas. ABRs are used to connect OSPF areas to the OSPF
backbone area.

access layer

One of the layers in Cisco's three-layer hierarchical model.

The access layer provides users with access to the internetwork.

access link

A link used with switches that is part of only one virtual LAN

(VLAN). Trunk links carry information from multiple VLANs.

access list

A set of test conditions kept by routers that determines "inter-

esting traffic" to and from the router for various services on the network.

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