ATM Adaptation Layers
ATM Adaptation Layer (AAL) interprets data from higher levels and deals with segmentation and reassembly.
AAL sits between ATM and variable length packet protocols such as IP.
Different AAL are provided for different services. Adaptation layer of interest to computer and multimedia applications is AAL5 which can pass large variable size packets.
AAL5 encapsulates ATM data payload into Protocol Data Units (PDUs) in the convergence sublayer and segments PDUs into cells in the segmentation and reassembly (SAR) sublayer. Dedicated SAR hardware (needed to do this efficiently) increases the cost of network adapters. AAL5 reduces overhead by providing error cheking on complete packet.