2 octets 2 octets SAR-PDU header SAR-PDU trailer SAR-PDU Segment type (2 bits) Sequence number (4 bits) Multiplexing identification (10 bits) Length indicator (6 bits) Cyclic redundancy check code (10 bits) Figure 20.10 SAR-PDU format for AAL-3/4. [From ITU-T Rec. 1.363, page 13, Figure 6/1.363 (Ref. 10).] 9 Trailer consists of overhead fields added to the end of a data frame or cell. A typical trailer is the CRC parity field appended at the end of a frame. ASYNCHRONOUS TRANSFER MODE can be delineated. The header also includes a sequence number for protection against misordered delivery. There is the MID (multiplexing identification) subfield which is used to identify the CPCS (common part convergence sublayer) connection on a single ATM layer connection. This allows for more than one CPCS connection for a single ATM-layer connection. The SAR sublayer, therefore, provides the means for the transfer of multiple, variable-length CS-PDUs concurrently over a single ATM layer connection between AAL entities. The SAR_PDU trailer contains a length indicator (LI) to identify how much of the cell payload is filled. The CRC field is a 10-bit sequence used to detect errors across the whole SAR_PDU. A complete CS_PDU message is broken down into one BOM cell, a number of COM cells and one EOM cell. If an entire message can fit into one cell, it is called a single segment message (SSM), where the CS_PDU is 44 or less octets long. AAL-3/4 has several measures to ensure the integrity of the data which has been segmented and transmitted as cells. The contents of the cell are protected by the CRC- 10; sequence numbers protect against misordering. Still another measure to ensure against corrupted PDUs being delivered is EOM/BOM protection. If the EOM of one CPCS_PDU and the BOM of the next are dropped for some reason, the resulting cell stream could be interpreted as a valid PDU. To protect against these kinds of errors, the BEtag numeric values in the CPCS_PDU headers and trailers are compared, to ensure that they match. Two modes