2.5 The ATM Adaptation Layer

As illustrated in Figure 2.15, the ATM adaptation layer consists of two sublayers : a convergence sublayer and a segmentation and reassembly sublayer. The function of the AAL is to adapt higher-level data into formats compatible with ATM layer requirements. To accomplish this task, the ATM adaptation layer subdivides user information into segments suitable for encapsulation into the 48-byte information fields of cells. The actual adaptation process depends on the type of traffic to be transmitted, although all traffic winds up in similar cells . Currently, there are four different AALs defined, referred to as AAL classes, which are described later in this section.

When receiving information, the ATM adaptation layer performs a reverse process. That is, it takes cells received from the network and reassembles them into a format the higher layers in the protocol stack understand. This process is known as reassembly. Thus, the segmentation and reassembly processes result in the name of the sublayer that performs those processes.