 above the ATM layer, which provided mapping of different types of applications to the ATM layer. This layer is thus service dependent. The basic functionality can be interpreted by having a look at the diagram below (Figure 3.34). Thus the use of the ATM makes it mandatory to require an adaption layer to support other information based protocols not based on the ATM. The various services provided by an AAL are: handling of transmission errors; segmentation and reassembly, to enable larger blocks of data to be carried in the information field of ATM cells; Digital Hierarchies - PDH and SDH Table 3.7 AAL protocol classification as given by the ITU-T Class A Class B Class C Class D Timing relation between the source Required Required Not required Not required and destination Bit rate Constant Variable Variable Variable Connection mode Connection Connection Connection Connectionless oriented oriented oriented AAL protocol Type 1 Type 2 Type 3/4, type 5 Type 3/4 handling of lost cell conditions; flow control and timing control. Different types and quality of services would require different AAL protocols to support the service. To minimise this wide variety of protocols, ITU-T has defined four classes of service that cover a broad range of applications. This classification is based on three categories, which are the timing relation between the source and the destination, the bit rate and finally the connection mode. Based on these properties different AAL protocols have been devised, which come under the four classes defined. Table 3.7 shows this service classification as given by the ITU-T. AAL protocols mentioned above work in two logical sublayers of the major AAL (shown in Fig- ure 3.35). These are the convergence sublayer (CS) and the segmentation and reassembly (SAR) sublayer. The convergence sublayer provides the functions needed to support specific applications using the AAL. Each application attaches to the AAL at a service access point, which is the address of the application. This sublayer is service depen