Computer Networks Topic Bit Stuffing and Byte Stuffing

Computer Networks Topic Bit Stuffing and Byte Stuffing

While sending data over network, the data link layer divide into frames. Framing have several advantages than send raw very large data. It reduces the probability of error and reduces the amount of retransmission needed.

There exist two general methods for framing: fixed size framing and variable size framing. In fixed size framing, the data divided into fixed size frames and send over the transmission media. In fixed-size framing, there is no need for defining the boundaries of the frames; the size itself can be used as a delimiter. ATM network use fixed size packets called cells.

In variable size framing, the data divided into variable size frames. Here the network system needs a mechanism to distinguish the end of a packet and beginning of another one. Two protocols are used for this purpose: character oriented protocol and bit oriented protocol.

Character-Oriented Protocols

In character-oriented protocol, we add special characters (called flag) to distinguish beginning and end of a frame. Usually flag has 8-bit length. The character-oriented protocols are popular only with text data. While using character–oriented protocol another problem is arises, pattern used for the flag may also part of the data to send. If this happens, the destination node, when it encounters this pattern in the middle of the data, assumes it has reached the end of the frame. To deal with this problem, a byte stuffing (also known as
character stuffing) approach was included to character-oriented protocol. In byte stuffing a special byte is add to the data part, this is known as escape character (ESC). The escape characters have a predefined pattern. The receiver removes the escape character and keeps the data part. It cause to another problem, if the text contains escape characters as part of data. To deal with this, an escape character is prefixed with another escape character. The following figure explains everything we discussed about character stuffing.

Byte stuffing

Bit-Oriented Protocols

In a bit-oriented protocol, the data to send is a series of bits. In order to distinguish frames, most protocols use a bit pattern of 8-bit length (01111110) as flag at the beginning and end of each frame. Here also cause the problem of appearance of flag in the data part to deal with this an extra bit added. This method is called bit stuffing. In bit stuffing, if a 0 and five successive 1 bits are encountered, an extra 0 is added. The receiver node removes the extra-added zero. This process is illustrate below,

Bit stuffing

Simply, Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data. Byte stuffing is the method of adding 1 extra byte if there is a flag or escape character in the text.