4.4 Bipolar Encoding

4.4 Bipolar Encoding

Bipolar encoding utilises three voltage levels: positive, negative and neutral (zero). Zero is used to represent binary 0 and binary 1 is represented by alternating positive and negative voltages.

A common type of bipolar encoding is Alternate Mark Inversion (AMI).

Alternate Mark Inversion (AMI)

AMI is a bipolar encoding system where neutral (zero) voltage represents binary 0 and alternating positive and negative voltages represents binary 1.

With this line encoding it is the alternating voltages that determine the binary 1s.

Alternate Mark Inversion
Alternate Mark Inversion

Bipolar 8-Zero Substitution (B8ZS)

B8ZS works in a similar way to AMI by changing poles for each binary 1. However, there is a problem with synchronisation being lost when there is a stream of binary 0s being sent. B8ZS attempts to tackle this problem by making artificial signal changes. These signals are known as violations and occur when eight consecutive 0s occur in the bit stream.

The violation signal that takes place is based on the polarity of the last binary 1 before the 8 0s and will match this polarity. Therefore, the receiving end looking for an alternate polarity to the binary 1 will discover the same polarity and will thus determine that there have been a string of 8 0s.

B8ZS is a common method used in the US to avoid the synchronisation problem of long strings of binary 0s.

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




Alternate Mark Inversion (AMI)


000V (three 0′s and a violation)

0000 0000

B00V B00V

Binary 3 Ternary (4B3T) 

4B3T is an encoding technique used with Integrated Services Digital Network (ISDN) BRI equipment. It is a block encoding system where four binary digits can be output as 3 ternary digits. Using 4 bits gives 16 possible combinations with 3 ternary having 27 possible combinations. The ternary codes are:





+ + +



+ + 0


0 -+

-0 +


0 –

0 + +



+ + -


-+ -

+ -+


+ –

-+ +


-0 0

+ 0 0


0 -0

0 + 0


0 0 -

0 0 +


0 + -


0 -+


+ 0 -


-0 +


+ -0


-+ 0

4B3T requires less bandwidth than AMI because it makes better use of the 3 levels positive (+), negative (-) and zero (0).


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