HDLC is a bit oriented data link protocol, and it satisfies a variety of data control requirements. The data field of HDLC frame is capable of carrying text and non-text information. But a problem is introduced for some message types during the transmission. If the data field of an HDLC frame contains a pattern 01111110 which is reserved for the flag field, then the receiver will treat that sequence as the end flag. The remaining bits are interpreted as the bits from the next frame. This is called as lack of data transparency. To overcome this problem bit stuffing is carried out.

Bit stuffing:

In a bit-oriented protocol, the data section of a frame is a sequence of bits to be interpreted by the upper layer as text, graphic, audio, video, and so on. However, in addition to headers (and possible trailers), we still need a delimiter to separate one frame from the other. Most protocols use a special 8-bit pattern flag 01111110 as the delimiter to define the beginning and the end of the frame. This flag can create the same type of problem we saw in the byte-oriented protocols. That is, if the flag pattern appears in the data, we need to somehow inform the receiver that this is not the end of the frame. We do this by stuffing 1 single bit (instead of I byte) to prevent the pattern from looking like a flag. The strategy is called bit stuffing. In bit stuffing, if a 0 and five consecutive I bits are encountered, an extra 0 is added. This extra stuffed bit is eventually removed from the data by the receiver. Note that the extra bit is added after one 0 followed by five 1s regardless of the value of the next bit. This guarantees that the flag field sequence does not inadvertently appear in the frame. Bit stuffing is the process of adding one extra 0 whenever five consecutive 18 follow a 0 in the data, so that the receiver does not mistake the pattern 0111110 for a flag. Figure 11.4 shows bit stuffing at the sender and bit removal at the receiver.

Note that even if we have a 0 after five 1s, we still stuff a O. The 0 will be removed by the receiver. This means that if the flag like pattern 01111110 appears in the data, it will change to 011111010 (stuffed) and is not mistaken as a flag by the receiver. The real flag 01111110 is not stuffed by the sender and is recognized by the receiver.