---
title: "Transmission channels; sync vs async"
subject: "Data Communication"
chapter: 1
key: "semester_5/data_communication:transmission-modes"
source: https://www.bctengineer.com/q/semester_5/data_communication/transmission-modes
site: BCT Engineering
---

# Transmission channels; sync vs async

Data Communication · Chapter 1: Introduction

- Asked in: 2073 Shrawan, 4+4 marks
- Video: [Synchronous and Asynchronous Transmission](https://www.youtube.com/watch?v=SLjjgjp2bAA) — MrBrownCS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/transmission-modes
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-1

## Question

Identify and discuss different data transmission channels. How does synchronous transmission differ from asynchronous transmission?

## Short answer

Transmission channels are simplex (one-way), half-duplex (bidirectional but not simultaneously), full-duplex (simultaneous), serial or parallel. Asynchronous transmission sends characters framed by start/stop bits with gaps; synchronous transmission uses continuous blocks with sync patterns and recovered clock, providing higher efficiency and speed.

## Answer

#### Data transmission channels (modes)
**By direction**
- **Simplex**: one-way only; the receiver cannot reply. Example: TV broadcast, keyboard to computer.
- **Half duplex**: both ways, but only one at a time, using the full bandwidth in each direction. Example: walkie-talkie.
- **Full duplex**: both ways simultaneously, by two paths or by splitting the bandwidth. Example: telephone, Ethernet switched links.

**By the number of bits sent together**
- **Serial**: one bit at a time on one line; used for long distance.
- **Parallel**: n bits at a time on n lines; fast but short range.

#### Synchronous vs asynchronous transmission
```text
Async: | S | 8 data bits | P | St |  gap  | S | data ...
Sync : | SYN SYN | data block ... | CRC | SYN |
```

| Point | Asynchronous | Synchronous |
|---|---|---|
| Unit sent | One character (5-8 bits) | Block/frame of many characters |
| Framing | Start and stop bits on every character | Header/trailer, sync flags per frame |
| Clock | Separate clocks, resynchronised at each start bit | Common clock, recovered from the data |
| Overhead | High (about 20-30%) | Low |
| Speed | Slow | Fast |
| Gaps | Irregular gaps allowed | Continuous stream without gaps |
| Cost | Cheap and simple | Complex and costly |
| Example | Keyboard, RS-232 | Ethernet, HDLC, leased lines |

## Check yourself

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

1. Full duplex means
   A. both directions, one at a time
   B. one direction only
   C. both directions simultaneously
   D. bits sent on n lines
   Answer: C. Telephone and switched Ethernet are examples. (Key point: Full duplex)

2. In asynchronous transmission
   A. a common clock is used for blocks
   B. frames have no gaps
   C. overhead is low
   D. start and stop bits frame every character
   Answer: D. Separate clocks resynchronise at each start bit. (Key point: Async framing)

3. Compared with asynchronous, synchronous transmission has
   A. lower overhead and higher speed
   B. higher overhead
   C. slower speed
   D. simpler cheaper hardware
   Answer: A. Sync sends blocks with a common clock, but is more complex and costly. (Key point: Overhead)

4. Parallel transmission is
   A. one bit at a time, for long distance
   B. n bits at a time on n lines, fast but short range
   C. used for long distance
   D. slower than serial always
   Answer: B. Serial is used for long distance. (Key point: Parallel)

## Related in Data Communication

- [Multiplexing; synchronous vs asynchronous TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-compare-sync-async-tdm) — Data Communication, chapter 6, asked 6×
- [Types of data transmission technology](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adata-transmission-technologies) — Data Communication, chapter 1
- [Guided vs unguided transmission media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-vs-unguided-media) — Data Communication, chapter 4
- [Optical fiber vs satellite transmission](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aoptical-fiber-vs-satellite) — Data Communication, chapter 4
- [FDM bandwidth for 10 voice channels](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-10-voice-channels) — Data Communication, chapter 6
- [Data representation; analog vs digital system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adata-representation-analog-digital) — Data Communication, chapter 1

## Related in other subjects

- [Sync vs async; mod-12 synchronous counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amod12-synchronous-counter) — Digital Logic, chapter 8, asked 3×
- [Sync vs async; MOD-10 synchronous counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amod10-synchronous-counter) — Digital Logic, chapter 8, asked 2×
- [Sync vs async; 3-bit synchronous down counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Async-down-counter-3bit) — Digital Logic, chapter 8
- [Sync vs async; 3-bit synchronous up counter (T)](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Async-up-counter-3bit-t) — Digital Logic, chapter 8