Explain AAL 5 layer and bring out advantages of this layer with respect to other layers.

Mumbai University > Electronics Engineering > Sem 8 > Advanced Networking Technologies

Marks: 10M

Year: May 2014

1 Answer
  1. Figure below shows operation of AAL5. A PDU is accepted by The Convergence Sublayer which itself is divided into Common Part (CPCS) and Service Specific Part (SSCS). The SSCS processes the PDU if necessary which is then passed to CPCS which attaches pads of 0-47 bytes and 1-byte trailer to produce a CPCS PDU that is a multiple of 48 bytes.
  2. SAR sublayer divides the received PDU into blocks of 48 bytes which is passed to ATM layer which adds its own header and cells are formed. The SAR layer also directs the ATM layer to set the PTI field in the header of the last cell of CPCS PDU. This allows boundary between groups of cells corresponding to different messages to be distinguished at the destination. These cells are transmitted over the network.
  3. Maximum length allowed for CPCS PDU is of 65,535 bytes. In AAL 5 header is not added by CPCS while in other AAL services header is added.
  4. Fig shows CPCS PDU for AAL5. The trailer contains 1 byte of UU which is User to User identifier. This byte is passed transparently between end system entities.
  5. There is Common Part Identifier (CPI) which aligns the trailer to 8 bytes
  6. There is two-byte LI (Length Indicator) which specifies the number of bytes of user data in the CPCS PDU payload. 4 byte of CRC checks to detect the errors in the PDU.


  1. AAL 5 are much more efficient that AAL 3/4 as AAL5 does not add any overhead in the SAR sublayer.
  2. AAL 5 does not include sequence numbers for the SAR PDUs and instead relies on its more powerful CRC checksum to detect lost, disinserted, or out of sequence cells. This makes AAL 5 most widely implemented AAL.

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