High Density Bipolar 3 (HDB3)
HDB3 is the European equivalent of B8ZS. It works in a similar way to HDB3 but uses 4 zeroes rather than 8. After the 4 zeros a violation will occur and the polarity is set to be the same as the previous positive bit, e.g. 000V. The use of violations in the signal give it extra 'edges' which makes synchronisation possible and data retrieval more accurate.
An additional technique is used to stop DC voltage being introduced by having too many zeros. This works by adding a balancing pulse to any pattern of more than 4 bits the same, i.e. B00V. The value of B will be changed to positive or negative as required to make alternate violations of opposite polarity. The balancing pulse and the violation can then be removed at the receiver to create the original data.
The following table shows the encoding rules for HDB3:
|Transmitted Data||HDB3 Encoded Pattern|
|1||Alternate Mark Inversion (AMI)|
|0000||000V (three 0's and a violation)|
|0000 0000||B00V B00V|
Click on this link to review the wikipedia resource on HDB3.