Encapsulated PDU Fragment A PDU (Protocol Data Unit) is typically to large to transmit across the radio link. Consequently it must be segmented. This function is performed by the SAR (Segmentation And Reassembly) Sublayer. At this point the segment is termed an Encapsulated PDU Fragment. It is passed from the SAR to the MAC (Medium Access Control) function for coding prior to transmission across the radio link.