Data Encapsulation

The process by which data is handled layer-to-layer through the OSI model is referred to as data encapsulation. The following list shows the five steps of encapsulating data:

1. End-user information creates the data (from an OSI Model standpoint). This occurs at OSI Layers 5 7.

2. Data is converted to segments. This occurs at OSI Layer-4.

3. The segments are converted to packets or datagrams. This conversion occurs at OSI Layer-3.

4. Packets (or datagrams) are converted to frames (OSI Layer-2).

5. Frames are converted to bits, which occurs at OSI Layer-1.

NOTE

Headers are placed in "front" of the datagram and trailers are placed at the "end" of the datagram. The addition of headers and trailers wraps, or encapsulates, the data with protocol-specific information (pertinent to the protocol's respective OSI layer). OSI Layer-1 (Physical) does not encapsulate data because it does not use headers or trailers.

Common terminology is necessary to discuss the data that a particular layer is processing. Layer-N protocol data unit (PDU) is a term used to describe a set of bytes that includes the Layer-N header and trailer, all headers encapsulated, and the user data. For example, the Layer 2 PDU (including the data-link header and trailer) is called a frame; the Layer-3 PDU is called a packet, or sometimes a datagram; the Layer-4 PDU is called a segment.