---
title: "Multiplexing; synchronous vs asynchronous TDM"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:mux-compare-sync-async-tdm"
source: https://www.bctengineer.com/q/semester_5/data_communication/mux-compare-sync-async-tdm
site: BCT Engineering
---

# Multiplexing; synchronous vs asynchronous TDM

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2076 Chaitra, 2+4 marks; 2074 Asoj, 3+3 marks; 2073 Shrawan, 2+6 marks; 2071 Chaitra, 2+2 marks; 2070 Asar, 2+4 marks; 2069 Asar, 3 marks (asked 6 times)
- Video: [Time Division Multiplexing (TDM) | Synchronous, Asynchronous, Statistical TDM | Computer Networks](https://www.youtube.com/watch?v=qCOLTYVhNXs) — Dr. Ankit Verma
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/mux-compare-sync-async-tdm
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Define multiplexing with example. Compare synchronous and asynchronous (statistical) TDM.

## Short answer

Multiplexing combines several signals into one shared link using a MUX, with demultiplexing at the receiver (e.g., telephone T1 trunks). Synchronous TDM assigns fixed time slots, wasting capacity when sources are idle, and needs no addressing. Statistical TDM dynamically allocates slots only to active inputs, adding address overhead but greatly improving efficiency for bursty traffic.

## Answer

#### Multiplexing
**Multiplexing** is the technique of combining signals from several sources into one signal that is sent over a single shared link (channel), and separating them again at the receiver. A **MUX** at the sender and a **DEMUX** at the receiver do this.

```text
 A -->|     |            |     |--> A
 B -->| MUX |=== link ===|DEMUX|--> B
 C -->|     |            |     |--> C
```

Example: FM radio stations share the air using FDM; the telephone trunk network carries thousands of calls on one fibre using TDM. The main benefit is that a high-capacity link is shared, so cost is reduced.

#### Synchronous TDM
Each input is given a fixed time slot in every frame, whether or not it has data. If a source is idle, its slot goes empty.

#### Asynchronous (statistical) TDM
Slots are given only to inputs that have data. Because slots are not fixed, every slot carries an **address** (header) identifying its source.

| Point | Synchronous TDM | Asynchronous (statistical) TDM |
|---|---|---|
| Slot allocation | Fixed, one per input per frame | Dynamic, only for active inputs |
| Empty slots | Wasted when input idle | Almost none |
| Addressing | Not needed (position gives source) | Needed (address in each slot) |
| Link capacity | Sum of input rates | Less than sum of input rates |
| Efficiency | Low for bursty traffic | High for bursty traffic |
| Overhead | Only framing bits | Address bits per slot |
| Complexity / cost | Simple | Complex, needs buffers |
| Used in | T1/E1, SONET | ATM, frame relay, packet networks |

Example: with 4 inputs where only 2 are active, synchronous TDM sends 2 empty slots per frame; statistical TDM sends a frame of 2 slots only.

## Check yourself

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

1. What does multiplexing do?
   A. Splits one signal into many links
   B. Combines signals from several sources into one shared link and separates them at the receiver
   C. Encrypts data
   D. Amplifies the signal
   Answer: B. MUX combines and DEMUX separates. (Key point: Multiplexing definition)

2. How does statistical TDM allocate slots?
   A. Fixed slot per input in every frame
   B. Slots only to inputs with data
   C. Slots by frequency
   D. Slots by code
   Answer: B. Only active inputs get slots. (Key point: Slot allocation)

3. Why does statistical TDM need an address in each slot?
   A. Slot position no longer identifies the source
   B. To save bandwidth
   C. To detect errors
   D. To fix frame size
   Answer: A. Because slots are dynamic, position doesn't identify the source. (Key point: Addressing)

4. With 4 inputs and only 2 active, what does synchronous TDM send per frame?
   A. 2 slots only
   B. 4 slots, 2 empty
   C. 8 slots
   D. 1 slot
   Answer: B. Fixed slots go empty when idle, wasting capacity. (Key point: Efficiency)

## Related in Data Communication

- [Compare FDM, WDM and TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acompare-fdm-wdm-tdm) — Data Communication, chapter 6
- [Multiplexing vs multiple access](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-vs-multiple-access) — Data Communication, chapter 6
- [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3×
- [Transmission channels; sync vs async](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atransmission-modes) — Data Communication, chapter 1

## Related in other subjects

- [TDM vs statistical TDM and STDM frame](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-statistical-tdm) — Telecommunications, chapter 3, asked 2×
- [Synchronous vs asynchronous serial transmission](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Async-async-serial) — Microprocessor, chapter 4, asked 3×
- [Synchronous vs asynchronous transmission](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Async-vs-async) — Instrumentation II, chapter 3
- [Synchronous vs asynchronous circuits; SR to JK](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Async-async-sr-to-jk) — Digital Logic, chapter 6