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Zero-bit insertion is a bit-stuffing technique in which a zero bit is inserted after a series of one bits to highlight a sequence change or break. Zero-bit insertion is used with other bit-oriented protocols to prevent the accidental appearance of six consecutive one bits between the two framing flags indicating a transmission frame's beginning and end.
Zero-bit transmission is widely used in IBM’s High-Level Data Link Control (HDLC) - a data link format structure. The HDLC format requires that a frame's beginning and end be flagged. Zero-bit insertion is generally used to distinguish between the flag pattern and data of the same format. The flag byte normally contains the bit sequence "01111110."
To keep the flag byte bit sequence from occurring within the transmission frame, the HDLC transmitter inserts a zero after five consecutive bits. The only drawback of the zero-bit insertion technique is its irregular code or information rate.