ATM Cell Types The ATM Layer

1. Timing relationship between source and destination 2. Bit rate

3. Connection Mode

Figure 5-15 illustrates the four ITU-T defined service classes labeled A through D. Class A is constant bit rate CBR while C through D are variable bit rate VBR. Classes X and Y are the additional two proposed by the ATM forum. In class X cells are transported as they are presented to the ATM layer; thus it is defined as an Unassigned Bit Rate UBR Service. In the case of class Y the cells are transported across the ATM network if there is capacity available for them. As a result, class Y is defined as Available Bit Rate ABR Service. The various service classes shown in Figure 5-15 result in six types of services. The null type of AAL AAL-0 is used for cell relay services that are inherently cell based and need no adaptation. The rest are labeled AAL-1, AAL-2, and so forth.

C. ATM Adaptation Layer Functions

ATM cells consist of 5 byte headers and 48 byte payloads. ATM cell headers are used for multiplexing and demultiplexing as well as switching based on the connection number at the ATM layer. The AAL only deals with payloads so there are no cells at this layer. The cell header as discussed in Section II of this chapter is generated at the transmitter to the ATM layer and stripped at the receiver. As seen in Figure 5-15 ATM supports various service classes. Thus AAL is composed of service classes. Thus AAL is composed of a series of compartments and it is modular in structure. Each compartment contains a service dependent function and some other functions that are common to all ATM network applications. At the transmitter, based on the required user service the corresponding active AAL passes on the payloads to the ATM layer. At the receiver the ATM layer is responsible for demultiplexing the cells based on the connection number VPIVCI and passing on the payloads to the proper AALs. Figure 5-16 General AAL functions. At the transmitter the user data flowing into the ATM network through the AAL are often in variable