Multiplexing is a way of sending multiple signals or streams of information over a communications link in the form of a single, complex signal.

Multiplexing is used for two main reasons. The technique makes it possible for network devices to communicate with each other without having to dedicate a connection for each pair.

Secondly, it allows for a more efficient use of resources, especially when equipment is scarce or expensive, or on occasions where installing a new communications link would cause disruption. For example, being able to share cables between services in busy towns and cities keeps costs down for public administrators and minimise the disruption to the public while allowing homes and businesses to function and flourish.

Multiplexing works differently on the technology being used. Analogue radio transmissions are usually multiplexed using frequency-division multiplexing. This involves dividing the bandwidth on a communications link into subchannels of different frequency width. Each frequency carries a different signal.

Optical networks transmit signals on light waves. In these networks, signals are multiplexed by sending data for different communications channels on light waves of different wavelengths. This is called wavelength division multiplexing.

Digital networks use time division multiplexing, transmitting multiple signals over the same channel in alternating time slots.

Finally, code division multiplexing is widely used in digital television and radio broadcasting as well as 3G cellular networks. Code division multiplexing relies on assigning each separate signal an identifying code to distinguish between the various channels operating on the same medium.