B8ZS

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

Coding rules

Same as with Bipolar AMI plus:

  1. +00000000 is coded as + 000 + −0− +
  2. −00000000 is encoded as −000− + 0 + -

These two rules prevent a constant zero signal. These are code violations, because in both replacements two positive or negative levels occur one after the other. Bipolar AMI requires exactly one low level between two high levels and, conversely, exactly one high level between two low levels. These complex code rule violations are very unlikely to occur due to disruption of the connection.

example

The example uses both of the above rules. The second rule is applied to bit position 2 because bit position 1 was coded negatively. The first rule is applied to bit position 13 because bit position 12 was positively coded. 

Daten :     110000000011100000000001111
bipolarAMI: +-00000000+-+0000000000-+-+
B8ZS :      +-000-+0+-+-+000+-0-+00-+-+
              ^^^^^^^^   ^^^^^^^^