B3ZS vs B8ZS: Understanding the Difference in Line Coding Techniques

This page compares B3ZS vs B8ZS and mentions the difference between B3ZS and B8ZS line coding.

What is Binary N-Zero Substitution (BNZS)?

• Bipolar signaling offers several advantages:

  • Its spectrum has a DC null.
  • Its bandwidth is not excessive.
  • It has single-error-detection capability because a single detection error will violate the alternating pulse rule.

• However, bipolar signaling also has disadvantages:

  • It requires twice as much power (3 dB) as a polar signal.
  • It is not transparent. We need a minimum density of 1’s in the source to maintain timing at the regenerative repeaters. Low density of pulses increases timing jitter.

• Solution: Binary N-zero substitution (BNZS) augments a basic bipolar code by replacing all strings of N 0’s with a special N-length code containing several pulses that purposely produce bipolar violations.

BNZS Line Codes

• High Density Bipolar (HDB) coding is an example of BNZS coding format. It is used in E1 primary digital signal.

• HDB coding replaces strings of four 0’s with sequences containing a bipolar violation in the last bit position. Since this coding format precludes strings of 0’s greater than three, it is referred to as HDB3 coding.

• 000V and B00V, where B=1 conforms to the bipolar rule and V=1 violates the bipolar rule. The choice of sequence 000V or B00V is made in such a way that consecutive V pulses alternate signs in order to maintain the dc null in PSD.

• B00V is used when there is an even number of 1’s following the last special sequence.

• 000V is used where there is an odd number of 1’s following the last sequence.

B3ZS Line Coding

• B3ZS stands for Bipolar 3-Zero Substitution.

• It is a line coding/data transmission format used on DS-3 carrier systems.

• Each string of three 0’s in the source data is encoded with either 00v or B0V.

• It is more involved than B6ZS coding and is used to prevent too many consecutive zeros from being transmitted.

• If too many zeros go down the line in a row, the transmission line effectively becomes a flat line, with no timing.

B8ZS Line Coding

• B8ZS stands for Bipolar 8-Zero Substitution.

• It is a line coding/data transmission format used for T1 (i.e. DS1 signals) lines.

• This transmission format is used to prevent too many consecutive zeros from being transmitted.

• If a sequence of 8 bits are detected prior to being transmitted, they are replaced with a different pre-determined byte that is not all zeros.

• It replaces any string of 8 zeros in length with a sequence of 1’s and 0’s containing two bipolar violations. There are two bipolar violations in every substitution.

B6ZS Coding Example

Above is the B6ZS coding example. Similarly, in B6ZS code used in DS2 signals, a string of six zeros is replaced with 0VB0VB.