ATM maps to the Data Link and Physical layers of the OSI model, but uses its own reference model to describe its functions. This model consists of three layers, which map to the OSI model as shown in the figure below.
At the lowest layer of the ATM reference model is its Physical layer, which serves the same function as the corresponding layer in the OSI model – defining interfaces, supported media, and so forth. Both the ATM and ATM adaptation layers map to Data Link layer. The lower layer of the two (the ATM layer) is responsible for managing virtual circuits and cell relay functions. The ATM adaptation layer is somewhat more complex – not only does it encapsulate upper-layer data into a cell, but also defines the different service classes associated with ATM traffic.
One of the main benefits of ATM as a network transmission technology is its support for Quality of Service (QoS). Different types of ATM traffic are categorized according to their required bandwidth and the types of connections that they require. The ATM adaptation layer (AAL) supports 4 main types of service for ATM cells, as outlined below:
AAL1. AAL1 is a connection-oriented service that provides a constant bit rate (CBR) for the purpose of transporting very time-sensitive data such as voice or video traffic. AAL1 traffic requires that timing be synchronized between the ultimate source and destination endpoints.
AAL2. AAL2 is also a connection-oriented service that requires clocking between a sender and receiver, but is meant for traffic that is more intermittent or “variable” in nature. This makes it highly applicable to voice traffic, as it usually doesn’t have a constant data flow.
AAL3/4. AAL3/4 provides both connection-oriented and connectionless delivery services. Formerly two separate services (AAL3 is connection-oriented and AAL4 is connectionless), they have been merged into a single service. AAL3/4 provides a variable bit-rate service suitable for traffic that is not time-sensitive.
AAL5. AAL5 is the service most commonly implemented for the purpose of transferring data over ATM networks, such as standard IP traffic. It supports both connection-oriented and connectionless services, and is best suited to traffic that is not delay-sensitive.
Remembering the purpose of the ATM adaptation layers can be tough. Just remember that time-sensitive traffic will generally use a lower ATM AAL number, while traditional data traffic is typically implemented over AAL 5.