HDB3
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HDB3 (High-density bipolar-3 zeros) is a telecommunications line code mainly used in Japan,Europe and Australia (for example, in E-1 lines) and is based on AMI. It is also very similar to the B8ZS encoding used in T-1 lines.
Four Zero bits in a row are coded in the following way:
| Number of pulses since last replacement | ||
|---|---|---|
| last pulse | odd | even |
| negative | 000- | +00+ |
| positive | 000+ | -00- |
A little explanation:
Here B00V (+00+ or -00-) pattern is required for if preceding pulse is positive, there is even no of pulse since last substitution, then the last violating bit is positive, so this violating bit must be negative, now since the last but one is positive, it would not be possible if we don't introduce a negative bit, so B00V pattern is required to ensure this.de:HDB3-Code