HDB3 ( High Density Bipolar 3) is used primarily in Europe for 2.048MHz (E1) carriers. This code is similar to BNZS in that it substitutes bipolar code for 4 consecutive zeros according to the following rules:
If the polarity of the immediate preceding pulse is (-) and there have been an odd (even) number of logic 1 pulses since the last substitution, each group of 4 consecutive zeros is coded as 000-(+00+).
If the polarity of the immediate preceding pulse is (+) then the substitution is 000+(-00-) for odd (even) number of logic 1 pulses since the last substitution.