ATM Adaptation Layer The ATM Adaptation Layer AAL

ATM Adaptation Layer • 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 • Segmentation and reassembly (SAR) • Convergence sublayer: adds sequencing and control information to aid SAR process 1

Example: ATM Adaptation Layer TCP IP AAL ATM NETWORK 2 ATM

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 3

AAL Protocols (Cont’d) • 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!!! 4

Adaptation Layer : A detailed view Higher layer PDU 5

Adaptation Layer : A detailed view Higher layer CS layer PDU CS-header PDU 6 Pad CS-trailer

Adaptation Layer : A detailed view Higher layer CS layer PDU CS-header PDU SAR-PDUSAR-PDU layer header payload trailer 7 Pad CS-trailer SAR-PDUSAR-PDU header payload trailer

Adaptation Layer : A detailed view Higher layer CS layer PDU CS-header PDU SAR-PDUSAR-PDU layer header payload trailer ATM layer Cell header payload 8 Pad CS-trailer SAR-PDUSAR-PDU header payload trailer Cell header payload

AAL Protocols (Cont’d) • Example: AAL 5 – an adaptation layer for data traffic designed by data neworking researchers – Simple and Efficient Adaptation Layer (SEAL) – much lower overhead – 8 byte trailer per PDU (i. e. , last cell of a PDU) – 1 bit of header in last cell of a PDU – 48 bytes of data in each ATM cell 9

AAL 5 User data 5 byte header flag 48 bytes of data 10 8 byte trailer

AAL 5 User data Pad 0 -47 bytes 11 Control field Length field CRC 32 2 bytes 4 bytes

Summary • 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 • Several AAL’s to choose from • AAL 5 is the best for data traffic 12