envelope

In the seven- layer OSI reference model, when a data packet is passed from the application layer to the physical layer, a protocol data unit( PDU) is added with the header in each layer.

The application data is first provided with the application protocol data unit, the Application Protocol Data Unit( APDU). This is followed by the protocol data unit for the presentation layer, which is the Presentation Layer Protocol Data Unit, followed by the Session header, the Transport header and Network header. Finally comes the Data Link header, Data Link Protocol Data Unit (DPDU) and lastly the Physical Layer header, the Physical Protocol Data Unit( PPDU). The final character after the payload is a single trailer.

Interleaving of the PDUs

At the receiving end, the headers are processed one after the other before the data packet is processed, e.g. CRC check for data transmission errorsin the link layer.