AAL-2 provides error recovery and indicates errored information which could not be recovered. As the source genurates a vairable bit rate some of teh cells transfered maybe unfull and therefore aditional features are required at the segmentation and recovery layer. This is achieved by inserting time stamps or timing information into the convergence sublayer - protocol data unit. The CS-PDU is segmented at this layer and a header and trailer added to each peice. The header contains two fields.
Sequence number is used to detect inserted or lost cells.
The trailer also contains two fields length indicator indicated the number of true data bytes in a partially full cell. CRC is a cyclic redundancy check used by the segmentation and reassembly sublayer to correct errors. AAL-3 is designed to transfer variable rate data which is time independent. It supports both message mode and streaming mode services. Message mode services are transported in a single ATM adaption layer interface data unit , while streaming mode services require one or more AAL-IDUs.