AAL is an additional layers to the ATM protocol for data synchronization or error correction.
ATM adaptive layers are responsible for segmenting and reassembling cells from applications. ATM has been designed to carry various data streams, video, voice or data. But the transport of these different types of data flows requires different types of services. For example, the constraints on the data are not the same for the transport of the voice. To cope with these various application needs, various AAL layers have been defined:

AAL1: Suitable for video and audio applications with constant bit rate, such as voice transport.
AAL2: suitable for variable speed video and audio applications.
AAL3 / 4: suitable for secure data transfer.
AAL5: suitable for data transport.

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