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B8ZS line coding

Short answer

B8ZS is a scrambling technique on AMI; eight consecutive zeros are replaced by 000VB0VB (V = same polarity as previous pulse, B = normal alternation). For data 11000000001 with prior pulse negative, the encoded signal is + - 000 - + 0 + - +.

Explain Bipolar 8-zero substitution (B8ZS) line coding with example.

B8ZS (Bipolar with 8-zero substitution) is a scrambling technique built on bipolar AMI. It is used in North America (T1 lines) to prevent long strings of zeros, which would cause loss of synchronisation and clock recovery.

Rule

Normal AMI coding applies (0 = 0 V; 1 alternates +/-). If eight consecutive zeros occur, they are replaced by the pattern 000VB0VB:

  • V (violation): a pulse with the same polarity as the previous pulse, which violates the AMI alternation.
  • B (bipolar): a pulse that follows the normal alternation (opposite to the previous pulse).
Previous pulseSubstitution
Positive (+)0 0 0 + - 0 - +
Negative (-)0 0 0 - + 0 + -

The receiver recognises the two violations and replaces the pattern by eight zeros again. The DC level stays zero because the V and B pulses cancel.

Example

Data: 1 1 0 0 0 0 0 0 0 0 1 (the pulse before the data is taken as negative, so the first 1 is +).

Data  : 1 1 0 0 0 0 0 0 0 0 1
B8ZS  : + - 0 0 0 - + 0 + - +
            0 0 0 V B 0 V B

The second 1 is -, so the previous pulse is - and the substitution is 000-+0+-: V = - (same as the previous pulse), B = + (opposite), V = + (same as B), B = - (opposite). The final 1 then follows B (-), so it is +.

✓ Answer checked by Claude Sonnet 5.5

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