FDM, TDM, WDM & Circuit/Packet/Message Switching
---
1. Frequency Division Multiplexing (FDM)
FDM divides the total available bandwidth into separate frequency sub-bands, each carrying a different signal simultaneously. Designed for analog signals.
How FDM Works
- Each input signal modulates a distinct carrier frequency
- Modulated signals are combined into one composite signal
- Guard bands (unused frequency gaps) between channels prevent interference
- At the receiver, bandpass filters extract each channel and demodulate
FDM Characteristics
- All channels transmit simultaneously — no time sharing
- Guard bands waste some bandwidth but are necessary to prevent inter-channel interference
- Applications: AM/FM radio, cable TV (each channel occupies a 6 MHz band), ADSL broadband, OFDM in Wi-Fi 6 and 5G
---
2. Time Division Multiplexing (TDM)
TDM divides time into fixed time slots and assigns each channel a slot in a repeating cycle. Designed for digital signals.
Synchronous TDM
- Each channel gets a fixed slot regardless of whether it has data to send
- Empty slots waste bandwidth but the scheme is simple — no per-slot overhead
- Used in T1 (24 channels × 64 kbps = 1.544 Mbps) and E1 (32 channels × 64 kbps = 2.048 Mbps)
Statistical (Asynchronous) TDM
- Slots are allocated dynamically only to channels with data ready
- More efficient — no wasted slots
- Each slot includes a channel identifier so the receiver knows the source
- Used in ATM (Asynchronous Transfer Mode) and cable modem DOCSIS
| Feature | Synchronous TDM | Statistical TDM |
|---|---|---|
| Slot assignment | Fixed, round-robin | Dynamic, demand-based |
| Idle channel behaviour | Empty slot transmitted (wasteful) | Slot not used |
| Per-slot overhead | None | Channel identifier |
| Efficiency | Low when channels are idle | High |
| Complexity | Simple | More complex |
---
3. Wavelength Division Multiplexing (WDM)
WDM is the optical equivalent of FDM — it transmits multiple data streams on the same fibre using different wavelengths (colours) of light simultaneously.
| Type | Channels | Wavelength Spacing | Distance | Use |
|---|---|---|---|---|
| CWDM (Coarse) | 8–18 | 20 nm | < 100 km | Metro access, shorter haul |
| DWDM (Dense) | 40–160+ | 0.8 nm or less | 1000+ km | WAN backbone, undersea cables |
Applications: Undersea cables (dozens of DWDM channels × 100+ Gbps each), ISP backbone, data centre interconnects.
---
4. Multiplexing Comparison Table
| Feature | FDM | TDM | WDM |
|---|---|---|---|
| Signal type | Analog | Digital | Optical (light) |
| Division basis | Frequency bands | Time slots | Wavelengths |
| Medium | Copper, coaxial, radio | Any digital medium | Fibre optic only |
| Guard separation | Guard bands required | Frame synchronisation | Wavelength spacing |
| Application | Radio, ADSL, cable TV | T1/E1, SONET, ATM | DWDM backbone |
| Simultaneous TX | Yes (all at once) | No (interleaved) | Yes (all at once) |
Exam Tip: FDM = analog + frequency bands + guard bands. TDM = digital + time slots. WDM = fibre + wavelengths. Only FDM uses guard bands — this distinction is commonly tested.
---
5. Circuit Switching
A dedicated, end-to-end physical path is established before any data can be sent. Resources along the entire path are reserved for the duration of the communication session.
Three Phases
- Circuit establishment (setup phase): Resources reserved hop-by-hop along the entire path — this takes time
- Data transfer: Continuous data stream with very low latency (dedicated path, no queuing)
- Circuit teardown: Reserved resources released after communication ends
| Aspect | Detail |
|---|---|
| Latency (post-setup) | Very low — dedicated path, no store-and-forward |
| Bandwidth efficiency | Poor — reserved bandwidth is wasted during silences |
| Setup time | High — must reserve path before first bit is sent |
| Reliability | High — dedicated resources, no contention |
| Example | PSTN telephone network, ISDN |
---
6. Packet Switching
Data is divided into fixed-size packets, each independently routed through the network. Each switching node uses store-and-forward: receive complete packet, check for errors, route to next hop.
| Aspect | Detail |
|---|---|
| Path | No dedicated path — packets may take different routes |
| Bandwidth | Shared and efficient — other packets use idle capacity |
| Latency | Variable — depends on queuing delay at each node |
| Setup | None — packets sent immediately without connection setup |
| Reliability | Best-effort (IP); TCP adds reliability at Layer 4 |
| Example | Internet (IP networks), Ethernet |
---
7. Message Switching
The entire message is transmitted node-to-node using store-and-forward. Unlike packet switching, there is no fragmentation.
| Aspect | Detail |
|---|---|
| Storage | Each intermediate node stores the entire message |
| Latency | Very high — large messages take long to store and forward |
| Suitable for | Asynchronous, non-real-time traffic only |
| Example | Early telegraph, telex; conceptually similar to email delivery |
---
8. Switching Comparison Table
| Feature | Circuit Switching | Packet Switching | Message Switching |
|---|---|---|---|
| Dedicated path | Yes | No | No |
| Bandwidth usage | Wasteful (reserved) | Efficient (shared) | Efficient |
| Data transfer latency | Low | Medium | High |
| Connection setup | Required (high delay) | None | None |
| Store & forward | No (continuous stream) | Yes (per packet) | Yes (entire message) |
| Real-time capable | Yes | Yes (with QoS) | No |
| Example | PSTN telephone | Internet IP | Early telegraph |
Study Deep: Why the Internet Uses Packet Switching
- Resilience: If a router fails, packets automatically route around it. Circuit switching would require re-establishing the entire circuit.
- Efficiency: In circuit switching, a silent phone call still holds its reserved bandwidth. In packet switching, silence uses zero bandwidth.
- Multiplexing: Many users share the same physical links simultaneously, reducing cost dramatically.
- Statistical multiplexing: Not all users are active simultaneously — packet switching takes advantage of this "burstiness" to serve more users than circuit switching ever could on the same infrastructure.