In the ATM reference model, a layer that comprises two sublayers.The Convergence Sublayer (CS) functions are determined by the specifics of the service class supported by that particular AAL.The Segmentation and Reassembly (SAR) sublayer functions to segment the user data into payloads for insertion into cells, on the transmit side. On the receive side, the SAR extracts the payload from the cells and reassembles the data into the information stream as originally transmitted. There exist defined AAL Types 1, 2, 3/4, and 5, each of which supports a specific class of traffic. See also ATM, ATM reference model, CS, and SAR.