---
title: "B8ZS line coding"
subject: "Data Communication"
chapter: 5
key: "semester_5/data_communication:b8zs-coding"
source: https://www.bctengineer.com/q/semester_5/data_communication/b8zs-coding
site: BCT Engineering
---

# B8ZS line coding

Data Communication · Chapter 5: Data Encoding and Modulation

- Asked in: 2075 Chaitra, 4 marks; 2074 Chaitra, 5 marks (asked 2 times)
- Video: [SCRAMBLING| B8ZS](https://www.youtube.com/watch?v=DtfUPa7RG9s) — Prachi Surlakar
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/b8zs-coding
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-5

## Question

Explain Bipolar 8-zero substitution (B8ZS) line coding with example.

## Short answer

B8ZS is a scrambling technique on AMI; eight consecutive zeros are replaced by 000VB0VB (V = same polarity as previous pulse, B = normal alternation). For data 11000000001 with prior pulse negative, the encoded signal is + - 000 - + 0 + - +.

## Answer

**B8ZS (Bipolar with 8-zero substitution)** is a scrambling technique built on bipolar AMI. It is used in North America (T1 lines) to prevent long strings of zeros, which would cause loss of synchronisation and clock recovery.

#### Rule
Normal AMI coding applies (0 = 0 V; 1 alternates +/-). If **eight consecutive zeros** occur, they are replaced by the pattern **000VB0VB**:
- **V** (violation): a pulse with the **same polarity** as the previous pulse, which violates the AMI alternation.
- **B** (bipolar): a pulse that follows the normal alternation (opposite to the previous pulse).

| Previous pulse | Substitution |
|---|---|
| Positive (+) | 0 0 0 + - 0 - + |
| Negative (-) | 0 0 0 - + 0 + - |

The receiver recognises the two violations and replaces the pattern by eight zeros again. The DC level stays zero because the V and B pulses cancel.

#### Example
Data: 1 1 0 0 0 0 0 0 0 0 1 (the pulse before the data is taken as negative, so the first 1 is +).

```text
Data  : 1 1 0 0 0 0 0 0 0 0 1
B8ZS  : + - 0 0 0 - + 0 + - +
            0 0 0 V B 0 V B
```
The second 1 is -, so the previous pulse is - and the substitution is 000-+0+-: V = - (same as the previous pulse), B = + (opposite), V = + (same as B), B = - (opposite). The final 1 then follows B (-), so it is +.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Eight zeros are replaced by:
   A. B00V
   B. 000VB0VB
   C. 000V
   D. 0B0VB0V
   Answer: B. B8ZS pattern. (Key point: Substitution)

2. A V pulse has:
   A. Opposite polarity
   B. Zero volts
   C. Same polarity as the previous pulse
   D. Alternating polarity always
   Answer: C. It violates AMI. (Key point: V pulse)

3. B8ZS is used to:
   A. Increase bandwidth
   B. Prevent long zero strings that lose synchronisation
   C. Remove error detection
   D. Convert analog to digital
   Answer: B. Keeps clock recovery. (Key point: Purpose)

4. Substitution after a positive pulse is:
   A. 0 0 0 - + 0 + -
   B. + - 0 0 0 - + 0
   C. 0 0 0 + - 0 - +
   D. 0 0 0 + + 0 - -
   Answer: C. V same as previous. (Key point: After + pulse)

## Related in Data Communication

- [Line coding: polar RZ and bipolar AMI](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-polar-rz-ami) — Data Communication, chapter 5
- [HDB3 (HD3S) line coding](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ahdb3-coding) — Data Communication, chapter 5, asked 2×
- [Line coding waveforms for 1101110111](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-1101110111) — Data Communication, chapter 5
- [Line coding: properties; encode 01001100011](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-01001100011) — Data Communication, chapter 5
- [Polar, unipolar, bipolar, Manchester; encode 101000111011](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-101000111011) — Data Communication, chapter 5
- [List digital-to-digital line coding techniques](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-techniques-list) — Data Communication, chapter 5

## Related in other subjects

- [Line coding 100000000001101 with HDB3 and B8ZS](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aline-codes-polarrz-diffmanch-hdb3-b8zs) — Communication Systems, chapter 6
- [Line coding 100111010 (Polar RZ, bipolar NRZ, AMI, Manchester)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aline-codes-100111010) — Communication Systems, chapter 6
- [Line coding and NRZ, RZ, AMI](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aline-coding-nrz-rz-ami) — Telecommunications, chapter 3
- [Line coding in PCM and the right technique](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aline-coding-pcm-solution) — Telecommunications, chapter 3