HDB3

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HDB3 (High Density Bipolar of order 3 code) 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:

000-

+00+

000+

-00-

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 0 "

encoded 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 0 "

(the corresponding encoding using AMI is " + 0 - 0 0 0 0 0 + - 0 0 0 0+ - 0 0 0 0 0 0 ").