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ATM Adaptation Layer

ATM Adaptation Layer. Rizwan Rehman, CCS. 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

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ATM Adaptation Layer

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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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