Multiplexing Techniques: FDM, TDM, STDM, and WDM
Laying high-speed optical fiber cables or copper wires between cities costs crores of rupees. It is financially impossible to run a separate physical cable for every single telephone call or internet user.
1. What is Multiplexing? (In Simple Words)
Multiplexing is the technique of combining multiple low-speed data signals from different users into one single high-speed composite signal that travels over a shared transmission cable.
[ Channel 1 ] ──┐
[ Channel 2 ] ──┼──▶ [ MULTIPLEXER (MUX) ] ════ Shared Fast Cable ════▶ [ DEMUX ] ──┬──▶ [ Receiver 1 ]
[ Channel 3 ] ──┘ ├──▶ [ Receiver 2 ]
└──▶ [ Receiver 3 ]- Multiplexer (MUX): Combines N input lines into 1 physical transmission line at the sender side.
- Demultiplexer (DEMUX): Splits the 1 line back into N separate output lines at the receiver side.
2. The Four Major Multiplexing Techniques
Main Multiplexing Techniques
1. FDM (Frequency Division)
Total bandwidth split into distinct frequency channels (FM radio, TV broadcast)
2. WDM (Wavelength Division)
FDM applied to optical fiber: different colors (wavelengths) of laser light
3. Synchronous TDM
Time divided into fixed round-robin slots; slots wasted if user is idle
4. Statistical TDM (STDM)
Time slots allocated dynamically on demand; zero wasted bandwidth!
Detailed Comparison: Synchronous TDM vs. Statistical TDM (STDM)
This is a favorite 5-mark question in DAVV exams:
| Feature | Synchronous TDM | Statistical TDM (STDM / Asynchronous) |
|---|---|---|
| Slot Allocation | Fixed, predetermined round-robin time slots. | Dynamic allocation based on demand. |
| Idle User Handling | If User B has no data to send, their time slot travels completely empty (wasted capacity). | If User B is idle, their time slot is immediately given to another active user. |
| Addressing Needed? | No addressing needed: Position of the slot indicates which user it belongs to. | Yes: Each data packet must carry a small header indicating sender and receiver ID. |
| Bandwidth Efficiency | Low (wastes line capacity on idle channels). | Maximum efficiency: Line capacity is never wasted. |
| Buffer Memory | Requires minimal buffer memory. | Requires input buffers to queue packets during traffic surges. |
💡 DAVV Exam Tip
Question (10 Marks): “What is Multiplexing? Explain FDM and TDM. Differentiate between Synchronous and Statistical TDM.”
- Draw the MUX / DEMUX block diagram.
- Explain FDM: Dividing total bandwidth into frequency slices separated by Guard Bands (to prevent signal overlap).
- Include the Synchronous vs Statistical TDM comparison table above.