---
title: "Time division multiplexing explained"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:tdm-explain"
source: https://www.bctengineer.com/q/semester_5/data_communication/tdm-explain
site: BCT Engineering
---

# Time division multiplexing explained

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2079 Baisakh, 4 marks; 2072 Chaitra, 3 marks; 2071 Shrawan, 5 marks (asked 3 times)
- Video: [TDM, Statistical TDM & FDM](https://www.youtube.com/watch?v=fIZhDl35_XY) — Sunny Classroom
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/tdm-explain
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain the operation of TDM (time division multiplexing) with a diagram.

## Short answer

TDM shares a high-rate link by giving each input a non-overlapping time slot in a repeating frame. The multiplexer takes one unit (bit/byte) from each channel per frame, and the demultiplexer uses synchronization to separate them. Link rate equals n × R, where R is input rate.

## Answer

**Time division multiplexing (TDM)** is a digital multiplexing technique in which several low-rate digital signals share one high-rate link by occupying it in different, non-overlapping time slots. The whole bandwidth is used by one source at a time, in turn.

#### Operation
- Each input stream is divided into small units (a bit, byte or character).
- The multiplexer takes one unit from each input in turn and places it in a slot. One slot from each input makes one **frame**.
- Frames are sent one after another. The demultiplexer at the receiver uses a synchronised clock to deliver each slot to the correct output line.
- The link rate must be at least the sum of the input rates. With n inputs of rate R each, the link rate is n × R, and each slot is 1/n as long as the original unit duration.
- Extra **framing bit(s)** at the start of each frame keep sender and receiver synchronised.

```text
 A: A1 A2 A3 ..   --->|       |
 B: B1 B2 B3 ..   --->| MUX   |-- A1 B1 C1 | A2 B2 C2 | A3 ..
 C: C1 C2 C3 ..   --->|       |   [frame 1]  [frame 2]
                                  (slots)
                                      |
                              link --> DEMUX --> A, B, C
```

#### Types
- **Synchronous TDM**: fixed slot for every input in every frame, even if idle.
- **Asynchronous (statistical) TDM**: slots given only to active inputs, with addresses.

#### Other points
- Interleaving can be at bit level or byte level.
- Problems: unequal rates (solved by multilevel multiplexing, multiple-slot allocation or pulse stuffing/bit padding) and idle slots wasted.
- Example: T1 carries 24 voice channels, 8 bits each per frame, 8000 frames/s: (24×8+1) × 8000 = 1.544 Mbps.

## Check yourself

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

1. What makes one frame in TDM?
   A. One slot from each input
   B. All slots of one input
   C. A framing bit only
   D. Two slots
   Answer: A. One slot from each input makes a frame. (Key point: Frame)

2. With n inputs each of rate R, what is the minimum link rate?
   A. R
   B. R/n
   C. n x R
   D. R^n
   Answer: C. The link must carry the sum of the input rates. (Key point: Link rate)

3. What is the purpose of the framing bit?
   A. Keeps sender and receiver synchronised
   B. Carries data
   C. Detects errors
   D. Adds address
   Answer: A. Framing bits mark the frame start for synchronisation. (Key point: Framing bit)

4. T1 carries 24 channels of 8 bits plus 1 framing bit at 8000 frames/s. What rate?
   A. 1.536 Mbps
   B. 1.544 Mbps
   C. 2.048 Mbps
   D. 64 kbps
   Answer: B. (24 x 8 + 1) x 8000 = 1.544 Mbps. (Key point: T1 rate)

## Related in Data Communication

- [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3×
- [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×
- [Multiplexing techniques; analog one explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Alist-mux-techniques) — Data Communication, chapter 6

## Related in other subjects

- [TDM frame, time slot and 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-frame-24-channel) — Telecommunications, chapter 3
- [Multiplexing techniques and TDM standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-techniques-tdm-standards) — Telecommunications, chapter 3
- [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2×
- [Multiplexing display concept and operation](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amultiplexing-display) — Digital Logic, chapter 11, asked 5×