- HDB3
HDB3 (High Density Bipolar of order 3 code) is a
telecommunications line code mainly used inJapan ,Europe andAustralia (for example, in E-1 lines) and is based on AMI. It is also very similar to theB8ZS encoding used in T-1 lines.One refinement is necessary, to prevent a dc voltage being introduced by excessive runs of zeros. This refinement is to encode any pattern of more than four bits as B00V, where B is a balancing pulse. The value of B is assigned as + or - , so as to make alternate "V"s of opposite polarity.Four Zero bits in a row are coded in the following way:
A little explanation: Here B00V (+00+ or -00-) pattern is required for the following reason: If the preceding pulse is positive and there is an even number of pulses since the last substitution, then the last violating bit is positive. This violating bit must be negative (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.
Example of HDB3 encoding
The pattern of bits
1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0Encoded in HDB3 is:
+ 0 - 0 0 0 V 0 + - B 0 0 V - + B 0 0 V 0 0, which is:+ 0 - 0 0 0 - 0 + - + 0 0 + - + - 0 0 - 0 0The corresponding encoding using AMI is:
+ 0 - 0 0 0 0 0 + - 0 0 0 0 + - 0 0 0 0 0 0ee also
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