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## B3ZS vs B8ZS | Difference between B3ZS and B8ZS line coding

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

### What is Binary N-Zero Substitution (BNZS) ?

• Bipolar signaling has several advantages as follows:
(1) its spectrum has a dc null. (2) its bandwidth is not excessive. (3) it has single-error-detection capability. This is a due to the fact that if a single detection error is made, it will violate the alternating pulse rule.
• Disadvantages of bipolar signaling are as follows:
it requires twice as much power (3 dB) as a polar signal. It is not transparent, i.e, 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 trings 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 refereed 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. It 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.