ATM adaptation layer

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The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM. This adaptation layer defines how to segment higher-layer packets into cells and the reassembly of these packets. Additionally, it defines how to handle various transmission aspects in the ATM layer.

Examples of services that need adaptations are Gigabit Ethernet, IP, Frame Relay, SONET/SDH, UMTS/Wireless, etc.

The main services provided by AAL (ATM Adaptation Layer) are:

• Segmentation and reassembly
• Handling of transmission errors
• Handling of lost and misinserted cell conditions
• Timing and flow control

The following ATM Adaptation Layer protocols (AALs) have been defined by the ITU-T.^[1] It is meant that these AALs will meet a variety of needs. The classification is based on whether a timing relationship must be maintained between source and destination, whether the application requires a constant bit rate, and whether the transfer is connection oriented or connectionless.