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Understanding ATM Adaptation Layer (AAL) Protocols and Processes

Learn about the essential rules, processes, and protocols involved in the ATM Adaptation Layer (AAL) for efficient data transmission. Explore AAL types, SAR, and convergence sublayer details to enhance network understanding.

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Understanding ATM Adaptation Layer (AAL) Protocols and Processes

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  1. ATM Adaptation Layer Rizwan Rehman, CCS

  2. 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

  3. TCP TCP IP IP AAL AAL ATM ATM Example: ATM Adaptation Layer ATM NETWORK

  4. 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

  5. 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!!!

  6. Adaptation Layer : A detailed view PDU Higher layer

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

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

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

  10. 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

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

  12. 0-47 bytes 2 bytes 2 bytes 4 bytes AAL 5 User data Pad Control field Length field CRC 32

  13. 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

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