- Multiplexing lets many signals share one link: FDM (by frequency), TDM (by time slot), WDM (by light wavelength on fibre).
- Circuit switching: a dedicated path is set up for the whole call (traditional telephone).
- Packet switching: data is split into packets that travel independently and are reassembled (the Internet). It uses links efficiently and tolerates failures.
Why multiplexing is needed
Laying a separate cable for every phone call, TV channel or computer connection would be impossibly expensive. A single fibre can carry far more capacity than any one user needs, so it makes sense to share it. A multiplexer (MUX) combines several signals onto one link, and a demultiplexer (DEMUX) at the other end separates them again.
The three methods explained
- FDM (frequency division): each signal is given its own frequency band, with small gaps between bands. All signals travel at the same time. Radio and TV stations work this way: each station has its own frequency, and your radio tunes to one of them.
- TDM (time division): each signal gets the whole link, but only for a short time slot, in turn: A, B, C, A, B, C... The slots are so short and repeat so quickly that every signal seems to have a continuous link. Traditional digital telephone systems use TDM.
- WDM (wavelength division): the fibre version of FDM. Each signal uses a different colour (wavelength) of light, so one fibre can carry dozens of channels, each running at very high speed. Undersea and ISP backbone cables rely on it.
Wi-Fi and 4G/5G use advanced methods (such as OFDMA) that combine frequency and time sharing among many users.
Circuit switching in detail
Before any data is sent, a complete path is set up from end to end and reserved for this one conversation. It stays reserved until the call ends, even during silences.
- Advantages: guaranteed capacity and constant, predictable delay, which is good for voice.
- Disadvantages: time is needed to set up the call; capacity is wasted during silences; and if any link on the path fails, the call drops.
Packet switching in detail
The data is split into packets. Each packet carries the destination address in its header, and each router on the way decides where to send it next. Packets from many users share the same links, and they may take different routes and arrive out of order; the receiver puts them back in order.
- Advantages: links are used efficiently, because capacity is only used when there is data to send; if a link fails, packets are sent another way; and many users can share the network.
- Disadvantages: delay can vary (jitter) when links are busy, and packets can be lost when buffers overflow, so protocols such as TCP must detect losses and resend.
| Feature | Circuit switching | Packet switching |
|---|---|---|
| Path | Reserved for the whole call | Shared; each packet is routed separately |
| Setup | Needed before sending | None |
| Use of capacity | Wasted during silences | Used only when data is sent |
| Delay | Fixed | Varies with load |
| If a link fails | The call drops | Packets are rerouted |
| Example | Traditional telephone network | The Internet, including voice calls over IP |
Remember
- Multiplexing shares one link: by frequency (FDM), by time slot (TDM) or by light wavelength (WDM).
- Circuit switching reserves a path; packet switching shares links among independent packets.
- The Internet is a packet-switched network.
Check yourself. When you have finished this unit, try the 1.10 hands-on lab and the 1.11 review questions.