The ATM AAL 3/4 protocol provides for reliable end-to-end transmission of SAR an
ID: 665484 • Letter: T
Question
The ATM AAL 3/4 protocol provides for reliable end-to-end transmission of SAR and CPCS PDUs, as well as multiplexing of multiple data streams on the same virtual connection, in ATM networks.
1) Assume that AAL 3/4 is being used and that the receiver is in the idle state. A block of user data is then transmitted as a sequence of SAR PDUs. Given that sixteen consecutive COM PDUs are lost, will this error be detected at the receiving end and, if so, how will it be detected? If this error can be detected in more than one way, be sure to enumerate each detection method.
2) Assume that AAL 3/4 is being used and that the receiver is in the idle state. Two blocks of user data are then transmitted as two separate sequences of SAR PDUs. Given that the EOM SAR PDU of the first sequence and the BOM SAR PDU of the second sequence are both lost, will this error be detected at the receiving end and, if so, how will it be detected? If this error can be detected in more than one way, be sure to enumerate each detection method.
Explanation / Answer
1)
When EOM SAR-PDU is received, the CS-PDU will be discarded because it is shorter than the buffer allocation size indication field. SAR does not recognize that SAR-PDUs were lost because it uses sequential number mod 16 and hence after 16 data units, the SN is repeated.
However, when EOM is delivered to the CPCS, the CPCS will check the length field in the trailer of CPCS-PDU that it has assembled and will detect the assembled data is shorter than the length field. CPCS will discard it.
2)
This might be detected by the SN mechanism. If not, the loss will be detected when the Btag and Etag fields fail to match. The Length indication may fail to pick up this error if the cell bust loses as many cells as are added by concatenation the two CPCS-PDU fragments. In this case only the first BOM may be legitimately retrieved.
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