In Asynchronous Transfer Mode (ATM) networks, the ATM Adaptation Layer (AAL) provides facilities for non-ATM based networks to connect to ATM network and use its services.
AAL is basically a software layer that accepts user data, which may be digitized voice, video or computer data, and makes them suitable for transmission over an ATM network. The transmissions can be of fixed or variable data rate. AAL accepts higher layer packets and segments them into fixed sized ATM cells before transmission via ATM. It also reassembles the received segments to the higher layer packets.
The following diagram illustrates the function of AAL −
This layer has two sub layers −
Convergence sub layer
Segmentation and Reassembly sub layer.
Some networks that need AAL services are Gigabit Ethernet, IP, Frame Relay, SONET/SDH and UMTS/Wireless.
International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has defined five AAL protocols to provide the range of services.
AAL Type 0 − This is the simplest service that provides direct interface to ATM services without any restrictions. These cells are called raw cells that contain 48-byte payload field without any special fields. It lacks guaranteed delivery and interoperability.
AAL Type 1 − This service provides interface for synchronous, connection oriented traffic. It supports constant rate bit stream between the two ends of an ATM link. An AAL 1 cell contains a 4-bit sequence number, a 4-bit sequence number protection and a 47-byte payload field.
AAL Type 2 − This service also provides interface for synchronous, connection oriented traffic. However, this is for variable rate bit stream between the two ends of an ATM link. It is used in wireless applications.
AAL Type 3/4 − This includes a range of services for variable rate data or bit stream. It is suitable for both connection – oriented, asynchronous traffic as well as connectionless traffic. These ATM cells contain a 4-byte header.
AAL Type 5 − AAL 5 provides the similar services as AAL 3/4, but with simplified header information. It was originally named Simple and Efficient Adaptation Layer (SEAL). It is used in a number of areas like Internet Protocol (IP) over ATM, Ethernet over ATM and Switched Multimegabit Data Service (SMDS).