ATM operates at Layer 1 and Layer 2. The structure is shown in Figure 20.4
The ATM data link layer (Layer 2) is called the ATM Adaptation Layer (AAL). The ATM protocol suite includes several protocols that can be used at Layer 2. The one that currently is used for data communications is called AAL5, and that is the one that will be discussed here.
AAL5 frames can be very big. The information field in an AAL5 frame can contain up to 65,535 bytes. However, real implementations generally stick to sizes of 9232 bytes or less.
A higher-layer protocol in an endpoint device passes data to the ATM Adaptation ...