Data Link Layer Frame

In networking, a frame is layer 2 (data link layer of the OSI model) data unit that is transmitted between network points. This frame contains complete addressing, necessary protocol, and control information. The data link layer received layer 3 (Network layer of the OSI model) PDU from layer 3 within the data field and then prepare this PDU as layer 2 frame for carrying across network media. Before transmission, data link layer protocol encapsulates this with trailer and header. There are a lot of different data link layer protocols that describe data link layer frames. At the receiving end again data link layer protocols explain and de-encapsulate this frame. Figure 3.24 Illustrate frame which has three basic parts:-

  • Header

  • Data

  • Trailer

Figure 3.24 Illustrate the layer 2 frame general structure.

The frame fields

The general frame as shown in above mention image but, the structure of the frame contains fields in the header and trailer varies according to the protocol. There is no frame structure that meets the requirements of all data transportation across all type of media. Depending on the background, the size of control information needed in the frame varies to match the access control requirements of the media and logical topology. So frame has no standard size its varies according to the media and environment.


Typical frame fields are following:-

  • Start and stop indicator fields – These fields  explain the start and end restrictions of the frame

  • Addressing fields- indicate the source nodes and destination nodes.

  • Type  – this field explains layer3 protocols in the data field.

  • Control – identify special flow control services like QoS (Quality of Service).

  • Data -The frame payload (Network layer packet)

  • Error Detection- These frame fields are used for error detection and are included after the data to form the trailer.

Figure 3.25 Illustrate the frame fields