3Com CoreBuilder 9000, User Manual

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442 A
PPENDIX
C: P
ROTOCOLS AND
I
NTERFACES
ATM Adaptation
Layer (AAL)
The role of the ATM Adaptation Layers (AALs) is to define how to deal
with the contents of cells. The AAL performs Segmentation and
Reassembly (SAR) on cells. It formats data into the 48 byte ATM cell
payload, in effect adapting applications to the cell-based ATM layer. After
the ATM cells reach their destination, they are reconstructed into
higher-level data and transmitted to the respective local devices. The AAL
runs from end-system to end-system and is transparent to the ATM
network.
Because ATM can carry multiple traffic types, several adaptation
protocols, all operating simultaneously, exist at the adapation layer.
AAL1 The Type 1 adaptation layer transmits applications with CBR via ATM
networks. The functions of the AAL1 layer include detecting lost cells and
providing time stamp to support a common clock between end stations.
The cells on the AAL1 frame are numbered to allow the reassembly
process. Like all AAL layers, this adaptation layer checks the frame
integrity. Some of these mechanisms involve lost or misinserted cells,
faulty headers, and so forth. An example for traffic handled by AAL1 is 64
Kbps digital voice.
AAL2 The Type 2 adaptation layer is designed for transmission of data with
Variable Bit Rate (VBR). It allows the support of bursty traffic to be
exploited. The AAL2 standard has not yet been finally defined, but will
cover error checking as well as delay monitoring and handling. An
example for traffic handled by AAL2 is VBR encoded video.
AAL3/4 The Type 3/4 adaptation layer specifies the connection-oriented and
non-connection-oriented transfer of data packets via ATM networks. Like
the other adaptation layers, it also deals with error checking and
correction. This protocol first puts error-checking functions before and
after the original data. The information is then chopped into 44-byte
chunks. The cell payloads include 2 bytes of header and 2 bytes of trailer.
A Cyclic Redundancy Check (CRC) is performed on each cell to check for
bit errors. There is also a Message Identification (MID) which allows
multiplexing and interleaving of large packets on a single virtual
connection. Two examples of traffic handled by AAL3/4 is frame relay
over ATM and CCITT 1.364 (SMDS) over ATM.