Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations.
To prevent loss of synchronization when a long string of zeros is present in the payload, deliberate bipolar violations are inserted into the line code, to create a sufficient number of transitions to maintain synchronization; this is a form of run length limited coding.
When voice signals are digitized for transmission via T-carrier, the data stream always includes ample 1 bits to maintain synchronization.
However, when used for the transmission of digital data, the conventional AMI line code may fail to have sufficient marks to permit recovery of the incoming clock, and synchronization is lost.
Increased demand for bandwidth, and compatibility with the G.703 and ISDN PRI standards which called for 64,000 bits per second, led to this system being superseded by B8ZS.
Commonly used in the North American T1 (Digital Signal 1) 1.544 Mbit/s line code, bipolar with eight-zero substitution (B8ZS) replaces each string of 8 consecutive zeros with the special pattern "000VB0VB".
Used in all levels of the European E-carrier system, the high density bipolar of order 3 (HDB3) code replaces any instance of 4 consecutive 0 bits with one of the patterns "000V" or "B00V".
The choice is made to ensure that consecutive violations are of differing polarity; i.e., separated by an odd number of normal + or − marks.
The choice is made to ensure that consecutive violations are of differing polarity, i.e. separated by an odd number of normal B marks.