1 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

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

3 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

4 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!!!

5 PDU Higher layer Adaptation Layer : A detailed view

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

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

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

9 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

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

11 AAL 5 CRC 32 Control field PadUser data Length field 0-47 bytes2 bytes4 bytes2 bytes

12 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 AAL5 is the best for data traffic