The Bipolar With 8 Zeros Substitution Encoding Scheme is a format of the line coding for computer networks and is based on bipolar AMI for the OSI layer 1 (physical layer ). B8ZS prevents synchronization loss in a long stream of zeros. This represents the single but significant advantage over bipolar AMI dar. B8ZS is used in the European E1 carriers.

Coding Rules

Same as in Bipolar AMI and in addition:

These two rules prevent a constant zero signal. These are code violations (Code Violations ), because in both two substitutions each positive or negative levels occur in succession. Bipolar AMI requires between two high- levels exactly a low level and vice versa between two low levels exactly a high level. These complex code violations occur only with low probability of occurrence by disrupting the connection.


In the example above two rules are used. At bit position 2, the second rule is applied because the bit position 1 was negatively coded. At bit position 13, the first rule is applied because the bit position 12 was positively coded.