The ATM Adaptation Layer

Because most applications do not work directly with cells, a layer above the ATM layer has been defined that allows the user to send packets larger than a cell. The ATM interface segments these packets, transmits the cells individually, and reassembles them at the other end. This layer is the AAL. It is split into two sublayers: SAR (Segmentation and Reassembly) sublayer and CS (Convergence Sublayer). The lower sublayer breaks packets up into cells on the transmission side and puts them back together again at the destination. The upper sublayer makes it possible to have ATM systems offer different kinds of services to different applications.