Protocol data unit

In order to do this, the protocol (lower) layer will add to the SDU certain data it needs to perform its function; which is called encapsulation.

The addition of addressing and control information (encapsulation) to an SDU to form a PDU and the passing of that PDU to the next lower layer as an SDU repeats until the lowest layer is reached and the data passes over some medium as a physical signal.

The sending post office might look only at the postcode and place the letter in a mailbag so that the address on the envelope can no longer be seen, making it now an SDU.

The letters themselves are SDUs when the bags are opened but become PDUs when the address is read for final delivery.

When the addressee finally opens the envelope, the top-level SDU, the letter itself, emerges.

The data link layer PDU in Asynchronous Transfer Mode (ATM) networks is called a cell.

A media access control protocol data unit (MAC PDU or MPDU) is a message that is exchanged between media access control (MAC) entities in a communication system based on the layered OSI model.