nnection oriented or connectionless. Figure 6.63 shows the service classes, their basic characteristics, and corresponding AAL protocol. The timing relation characteristic tells if the information about timing has to be available for the receiver of ATM cells. This may be required in the case of the reconstruction of PCM-coded speech, which requires that samples arrive at regular intervals. The timing information is used to control playback. Four service classes support different applications: Data Communications Class A: Constant-bit-rate service for voice and video applications; Class B: Variable-bit-rate service with timing information for variable-bit-rate voice and video applications; Class C: Variable-bit-rate service for ordinary data applications; Class D: Variable-bit-rate service for connectionless transmission of very short data messages (no connection establishment). Figure 6.63 shows also which AAL is used to implement each service class. It also makes a conclusion about their main characteristics. 6.8.5.1 Class A/AAL1 AAL1 provides the support for traffic, which requires CBR service, and it is mainly used for voice and video applications. This AAL is quite simple because there is no requirement for error detection and recovery for this type of traffic. The transfer of timing information over a call is the major function that AAL1 has to provide to ensure high-quality playback of the data. This service simulates leased-line data or voice circuits and one important applica- tion could be PABX voice channels in integrated corporate networks that utilize ATM technology. Service classes A, B, C and D provide communication services for different applications. Adaptation layer defines the protocols for the mapping of particular types of traffic onto the cell layer. Protocol control information is transmitted in the information field of the cells. Cell header Service Class 5 octets Timing relation Isochronous Asynchronous AAL 1-5 Bit rate Const. Variable Connection oriented, CO, Connection oriented or conne