wo fields is to check that mis-sequencing of information does not occur by verifying a 3-bit sequence counter. It also allows for the original clock timing of the data received at the far end of the link. The SAR-PDU format of AAL-1 is shown in Figure 20.9. 8 The 4-bit sequence number (SN) is broken down into a 1-bit CSI (convergence sublayer indicator) and a sequence count. The SNP contains a 3-bit CRC and a parity bit. End-to-end syn- chronization is an important function for the type of traffic carried on AAL-1. With one mode of operation, clock recovery is via a synchronous residual time stamp (SRTS) and common network clock by means of a 4-bit residual time stamp extracted from CSI from cells with odd sequence numbers. The residual time stamp is transmitted over eight cells. It supports DS1, DS3, and E1 digital streams. Another mode of operation is structured data transfer (SDT). SDT supports an octet-structured nXDSO service. AAL-2. AAL-2 handles the variable bit rate (VBR) scenario such as MPEG (motion picture experts group) video. It is still in the ITU-T organization definitive stages. 8 SAR-PDU stands for segmentation and reassembly-protocol data unit. 20.6 ATM LAYERING AND B-ISDN Cell header SAR-PDU payload 4 bits 4 bits 47 octets SAR-PDU header SAR-PDU Sequence number (4 bits); to detect lost or misinserted cells. A specific value of the sequence number may indicate a special purpose, e.g. the existence of convergence sublayer functions. The exact counting scheme is for further study. Sequence number protection (4 bits). The SNP field may provide error detection and correction capabilities. The polynomial to be used is for further study. Figure 20.9 SAR-PDU format for AAL-1. This figure shows the content of a cell that contains an SAR-PDU. [From CCITT Rec. 1.363, Figure 1/1.363, page 3 (Ref. 10).] AAL-3/4. Initially, in ITU-T Rec I.363 (Ref. 10), there were two separate AALs, one for connection-oriented variable bit rate data services (AAL-3) and one for connectionless service. As the specifications e