The Data Encapsulation Process at protocol data units (PDUs)

As data is passed from the Application Layer to the Physical layer, a layer-specific header is added at each layer. The information contained in the header is specific to the protocol and the layer that added it. These headers are called protocol data units (PDUs).

Tracing the process from the top of the OSI model down reveals what PDUs are used at each layer.

1. At the top three layers (Application, Presentation, and Session), the data is referred to as data.
2. This data stream is broken up at the Transport layer. A Transport layer-specific header is added to each piece of the broken-up stream, and the result is a data segment.
3. The data segments are then sent to the Network layer for Layer 3 routing. A Layer 3-specific header is added, creating a packet.
4. The packets are sent to the Data Link layer. The Data Link layer will encapsulate each packet into a frame. The frame header identifies the source and destination hardware address, the MAC address.
5. The physical layer will convert these frames into bits that can then be transmitted on the physical wire.