Skip to content

Framing Methods

The data data link layer receives from physical layer is just a stream of bits which doesn't have any meaning until it is framed, so that we can separate the header, body and trailer. This is done in data link layer and this process is called framing. This can be done in 3 ways - Byte count - Byte stuffing - Bit stuffing

Byte Count

the size of the frame is kept in the starting of the frame.

Example

Data = HELLO, WORLD!!
[5 H E L L O] [7 W O R L D ! !]

Warning

If the count byte itself gets corrupted, then the entire frame gets corrupted

Byte Stuffing

We use delimiters at starting and ending of the frame. Each delimiter is of size 8 bits (1 Byte). If the delimiter itself appear in the body, we escape it by escape character

  • DLE STX => Start of Frame
  • DLE ETX => End of Frame
  • DLE => Escape character

Example

Data = A B DLE C D
Frame = DLE STX A B DLE DLE C D DLE ETX
Receiver sees DLE STX and understands, it is the starting of the frame two DLE means => interprets it as a single one DLE ETX => it is the end of the frame

Disadvantage

  • Extra overhead due to escape characters
  • Can be used only for byte-based encodings, not efficient for arbitrary bit streams

Bit Stuffing

special flag sequence (e.g,. 01111110) is used to mark starting and ending of frames. If the same pattern appears in the data itself, the sender inserts a 0 after every sequence of five consecutive 1s. Receiver removes the stuffed bits.

Example

Flag = 01111110
Data = 01111101111110
Stuffed Data = 0111110 0 111110 10
Frame = Flag + Stuffed Data + Flag
     = 01111110 0111110011111010 01111110

Receiver gets the frame, finds, Flag in the start, removes the flag Scans the body, when it receives 5 consecutive 1s, it removes the next 0