The ATM Adaptation Layer
The ATM Adaptation layer (AAL) represents the top layer in the ATM Protocol model. This layer is responsible for providing an interface between higher-layer protocols and the ATM layer. Because this interface normally occurs based on a voice, data, or video application accessing an ATM network, the operations performed by the AAL occur at endpoints and not at ATM switches. Thus, the AAL is shown in Figure to reside at ATM endpoints.
The primary function of the ATM Adaptation layer is format conversion. That is, the AAL maps the data stream originated by the higher-layer protocol into the 48-byte payload of ATM cells, with the header placement being assigned by the ATM layer. In the reverse direction, the AAL receives the payload of ATM cells in 48-byte increments from the ATM layer and maps those increments into the format recognized by the higher-layer protocol.
Because it is not possible to address the requirements of the diverse set of applications designed to use ATM within a single AAL, the ITU-T classified the functions required by different applications based on their traffic and service requirements.
This classification scheme defined four classes of applications based on whether a timing relationship is required between end stations, the type of bit rate (variable or constant), and the type of connection (connection-oriented or connectionless) required.
A constant bit-rate application represents an application that requires an unvarying amount of bandwidth, such as voice or real-time video. In comparison, a variable bit-rate application represents "explosive" traffic, such as LAN data or transmission via a packet network.
The capability to support connection-oriented or connectionless applications enables ATM to support various existing higher-layer protocols. For example, Frame Relay is a connection-oriented protocol, whereas IP is a connectionless protocol. Through the use of different AALs, both can be transported by ATM.
Based on the four application classes, four different types of AALs were defined: AAL1, 2, 3/4, and 5. At one time, AAL3 and AAL4 were separate types; however, they had a sufficient degree of commonality to be merged. Figure illustrates the relationship between application classes and ATM Adaptation layers with respect to the different parameters used to classify the application classes.
Although several aspects of different AAL operations remain to be specified, the use of different AALs provides the mechanism for the cell-based switching technology on which ATM is based to transport different types of information using a common cell structure.