dent. The segmentation and reassembly sublayer is responsible for packing the information received from the CS into cells for transmission and unpacking the information at the other end. The SAR sublayer Audio Circuit Emulation 48 bytes Video Audio / Video 3/4 Data Transfer ATM Layer : Lower Overhead AAL for Data Figure 3.34 The ATM adaptation layer Transmission Network Planning and Optimisation AAL User Convergence CS PDU Sublayer(SC) SAR sublayer SAR PDU SAR PDU SAR PDU ATM layer ATM Cell ATM Cell ATM Cell Physical layer Figure 3.35 Working of the ATM model hierarchy must pack any SAR headers and trailers plus the CS information into the 48-byte blocks and send them to the ATM layer. A closer look at the packing will make the process and the AAL working clearer. When a higher layer application block of user data is encapsulated in a single protocol data unit (PDU) consisting of the higher layer data and a header and trailer containing protocol information at the CS layer, this CS PDU is then passed down to the SAR sublayer and segmented into a number of blocks. Each of these blocks is encapsulated in a single 48-byte SAR PDU, which may include a header and a trailer in addition to the block of data passed down from the CS. Finally, each SAR PDU forms the payload of a single ATM cell. AAL Type 1 The AAL type 1, a connection oriented service, is suitable for handling circuit emulation services, such as voice and video conferencing. AAL1 requires timing synchronisation between the source and destination. For this reason, AAL1 depends on a medium, such as SONET, that supports clocking. The AAL1 process prepares a cell for transmission in three steps. Firstly, synchronous samples (e.g. 1 byte of data at a sampling rate of 125 us) are inserted into the payload field. Secondly, sequence number (SN) and sequence number protection (SNP) fields are added to provide information that the receiving AAL1 uses to verify that it has received cells in the correct order. Thirdly, the remainder of the payload field is filled with