ATM Adaptation Layer

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• The ATM Adaptation Layer (AAL) defines the rules for breaking up a higher level protocol data unit (PDU) into ATM cells for transmission on the network, and for reassembling the PDU at the other end
AAL Services
• Handle transmission errors
• Segmentation/reassembly (SAR)
• Handle lost and misinserted cell conditions
• Flow control and timing control
AAL Protocols
• AAL layer has 2 sublayers:
o Convergence Sublayer (CS)
Supports specific applications using AAL
o Segmentation and Reassembly Layer (SAR)
Packages data from CS into cells and unpacks at other end


AAL Protocols
• There are several different proposed AAL protocols for different traffic types
• Example: AAL 1
– for real time or continuous bit rate services, such as video traffic
– requires 1 byte of overhead per ATM cell (i.e., within the 48 byte payload)
– 1 bit CSI, 3 bit sequence count, 3 bit CRC, and 1 bit parity
• Example: AAL 3/4
– an adaptation layer for connectionless or connection-oriented data traffic
– 4 bytes overhead per ATM cell
– 16 bit header (2 bit type, 4 bit seq, 10 bit MID)
– 16 bit trailer (6 bit length, 10 bit CRC)
– Very high overhead!!!