What is Protocol Data Unit?
The data unit in the LLC level is known as the protocol data unit (PDU). The PDU having of four fields a destination service access point (DSAP), a source service access points (SSAP), a control field and an information field. DSAP, SSAP are addresses used by the LLC to recognize the protocol stacks on the receiving and sending machines that are produce and using the data. The control field specifies whether the PDU frame is an information frame (I - frame) or a supervisory frame (S - frame) or a unnumbered frame (U - frame).