How can many messages share one communication link?

A communication link can carry several distinguishable messages if each is assigned a separable part of the link's available resources. Multiplexing combines those messages for shared transmission, and demultiplexing separates them at the receiving end. The separation can be based on frequency, time or other controlled properties of the signals.

In frequency-division multiplexing, different messages occupy different frequency bands at the same time. Each message modulates a carrier or set of carriers in its allocated band. The combined waveform is the sum of those signals. Receiver filters and demodulators extract the intended band. Guard regions or carefully designed signal shapes help prevent neighboring channels from interfering. The drawing shows three separated bands to make the allocation visible.

In time-division multiplexing, messages share a frequency channel but take different time intervals. A frame may contain a slot for source A, then B, then C, followed by another frame. The receiver needs timing information to know which source each slot belongs to. If a source is sampled or sends a stream, buffers temporarily hold data until its opportunity arrives. Waiting and synchronization are therefore part of the system's operation.

A shared link does not acquire unlimited capacity simply because more sources are connected. Fixed slots may waste time when a source has nothing to send. Statistical multiplexing allows active sources to use available capacity more flexibly, but queues and contention can create variable delays. Headers or scheduling information help the receiver identify each message. Packet networks often combine these techniques across different layers of the system.

Optical fiber can also use different wavelength bands to carry multiple channels. The underlying electromagnetic waves superpose in the same physical fiber, while optical components separate their bands at appropriate points. In every case, engineering must keep channels distinguishable despite noise, imperfect filters, timing errors and physical limits. Allocation on paper becomes useful only when the real transmitter and receiver can maintain that separation.

The key distinction is between sharing a medium and confusing the messages. Multiplexing deliberately supplies an organizing rule that the receiver knows. Frequency separation allows simultaneous use of different spectral regions; time separation allows successive use of the same region. Both turn one costly physical connection into a shared resource, while preserving enough structure to reconstruct its separate streams.