Framing in Data Link Layer – Exclusive Explanation

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

• Header
• Data
• Trailer

The frame fields

The general frame is shown in the image mentioned above, 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 PPP fields, which are different from the HDLC fields, No framing structure meets the requirements of all data transportation across all media types.

Depending on the background, the size of control information in the data link layer framing varies to match the media’s access control requirements and logical topology. So, the frame has no standard size; it varies according to the media and environment.

 Typical frame 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.