Framing in Data Link Layer – Exclusive Explanation

/ /

Data-link layer framing is an important subject in the network. The data-link layer takes the packets from the Network Layer and encapsulates them into frames. If the frame size becomes too large, then the packet may be divided into small frames to make efficient flow control and error control.

Frames are data-link layer data units transmitted between network points. They contain complete addressing, necessary protocol, and control information. Before transmission, the data link layer protocol encapsulates the frame with a 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. The figure below illustrates a frame which has three basic parts:-

  • Header
  • Data
  • Trailer
Framing

The frame fields

The general frame is shown in the above-mentioned image, but the structure of the frame, which contains fields in the header and trailer, varies according to the protocol. For example, if you examine the fields of PPP, which are different from the fields of HDLC. There is no framing structure that meets the requirements of all data transportation across all types of media.

Depending on the background, the size of control information in the data link layer framing 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 framing fields are also illustrated in the image below:-

  • Start and stop indicator fields – These fields  explain the start and end restrictions of the datalink framing
  • Addressing fields– indicate the source nodes and destination nodes.
  • Type  – this field explains layer 3 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- This frame detects errors and includes the data to form the trailer.

[qsm quiz=3]