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Glossary
Glossary
AAL—ATM Adaptation Layer. Service-dependent sublayer of the data link layer. The AAL accepts
data from different applications and presents it to the ATM layer in the form of 48-byte ATM
payload segments. AALs consist of two sublayers: convergence sublayer (CS) and segmentation and
reassembly (SAR). AALs differ on the basis of the source-destination timing used, whether they use
constant bit rate (CBR) or variable bit rate (VBR), 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 AAL1, AAL2, AAL3/4, and AAL5.
AAL1—ATM adaptation layer 1. One of four AALs recommended by the ITU-T. AAL1 is used for
connection-oriented, delay-sensitive services requiring constant bit rates, such as uncompressed
video and other isochronous traffic.
AAL5—ATM adaptation layer 5. One of four AALs recommended by the ITU-T. AAL5 supports
connection-oriented VBR services and is used predominantly for the transfer of classical IP over
ATM and LANE traffic. AAL5 uses simple and efficient AAL (SEAL) and is the least complex of
the current AAL recommendations. It offers low bandwidth overhead and simpler processing
requirements in exchange for reduced bandwidth capacity and error-recovery capability.
ABR—available bit rate. QoS class defined by the ATM Forum for ATM networks. ABR is used for
connections that do not require timing relationships between source and destination. ABR provides
no guarantees in terms of cell loss or delay, providing only best-effort service. Traffic sources adjust
their transmission rate in response to information they receive describing the status of the network
and its capability to successfully deliver data.
AESA—ATM End System Address
ATM—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 processing to occur in hardware, thereby reducing transit delays. ATM is designed to take
advantage of high-speed transmission media such as E3, SONET, and T3.
CAS—channel-associated signaling
CBR—constant bit rate. QoS class defined by the ATM Forum for ATM networks. CBR is used for
connections that depend on precise clocking to ensure undistorted delivery.
CCS—common-channel signaling
CES—circuit emulation service. Enables users to multiplex or concentrate multiple circuit
emulation streams for voice and video with packet data on a single high-speed ATM link without a
separate ATM access multiplexer.
E1—European digital carrier facility used for transmitting data through the telephone hierarchy. The
transmission rate for E1 is 2.048 megabits per second (Mbps).
E3—Wide-area digital transmission scheme used predominantly in Europe that carries data at a rate
of 34.368 Mbps. E3 lines can be leased for private use from common carriers.
ESI—end station identifier
ILMI—Interim Local Management Interface. Specification developed by the ATM Forum for
incorporating network-management capabilities into the ATM User-Network Interface (UNI).
ISDN—Integrated Services Digital Network. Communication protocol, offered by telephone
companies, that permits telephone networks to carry data, voice, and other source traffic.
NM—Network module
POTS—Plain Old Telephone Service. Basic telephone service supplying standard single-line
telephones, telephone lines, and access to the public switched telephone network.
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Cisco IOS Release 12.0(7)T
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