segmentation and reassembly One of the two sublayers of the ATM adaptation layer (AAL) common part convergence sublayer (CPCS), responsible for dividing (at the source) and reassembling (at the destination) the protocol data units (PDUs) passed from the CS. The SAR sublayer takes the PDUs processed by the CS and, after dividing them into 48-byte pieces of payload data, passes them to the ATM layer for further processing. See also AAL, ATM layer, CPCS, CS, and SSCS.