Framing in Data Link Layer

Data-link layer framing is an important subject in the network. 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-sized frames to make efficient flow control and error control.

Frames are data-link layer data unit that is transmitted between network points. It contains complete addressing, necessary protocol, and control information. Before transmission of the frame, the data link layer protocol encapsulates the frame 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. The figure below Illustrate frame which has three basic parts:-

• Header
• Data
• Trailer

The frame fields

The general frame as shown in the above mention image but, the structure of the frame contains fields in the header and trailer varies according to the protocol. For example, if examine the fields of PPP which is different from the field of HDLC. There is no framing structure that meets the requirements of all data transportation across all type 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 following 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 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– This frame detects error and has included after the data to form the trailer.

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