Explain in short different framing Methods.

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written 8.5 years ago by

teamques10 ★ 68k

The DLL takes packets from the Network Layer and converts them into frames. The components of a frame are:

(i)Header (ii) Payload Field (iii) Trailer.

DLL divides the stream of bits received from the network layer into manageable data units called frames.
Frames can be of fixed or variable sizes:

(i)Fixed Size Framing: Frames are of a fixed size. E.g.: ATM Wide Area Network.

(ii) Variable Size Framing: Here the size of the frame is variable thus we need a method to specify the start and end of a frame.
Techniques of Variable Size Framing:

(i) Character Count: The first field in the header specifies the number of characters in the frame.

Disadvantage: Count may get garbled by a transmission error.

(ii) Character Stuffing (Character Oriented Protocol):
- Each frame starts and ends with a FLAG byte. Thus adjacent frames are separated by two flag bytes.
- Problem: It is possible that FLAG is actually a part of the data.
- Solution: At the sender an ESC character is inserted just before the FLAG byte present in the data. At the receiver the ESC is removed from the data.
  
  Now if an ESC is present in the data then an extra ESC is inserted before it in the data. This extra ESC is removed at the receiver.
(iii) Bit Stuffing (Bit Oriented Protocol):
Each frame begins and ends with a special bit pattern 01111110 called the flag byte.
When sender’s DLL encounters 5 consecutive 1’s in the data it automatically stuffs a ‘0’ bit in the outgoing data stream.
When the receiver encounters 5 consecutive 1’s followed by a ‘0’ it removes the ‘0’ bit.

(a) 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 0

(b) 0 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 0 0 1 0

(c) 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 0

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