Whatever the quantities monitored at the data source (whether electrical or physical), the cost to transmit each quantity through a separate channel would be prohibitive. Think of the equipment and cables or frequency spectrum required to monitor and transmit several hundred or thousands of measurands! One way to conserve resources is to share time or frequency spectrum with techniques such as Time-Division Multiplexing (TDM) and Frequency Multiplexing (FM), respectively.

Today, the most popular form of telemetry multiplexing (originally called commutation, as in an electric motor’s commutator) is TDM. Here, each channel is serially sampled for an instant by the multiplexer (see figure below).

When all channels have been sampled, the sequence restarts at the first channel. Thus, samples from a particular channel are interleaved in time between samples from all of the other channels. An example of a simple interleaved data stream is shown below as the output of the commutator:

Since no measurand is monitored continuously, sampling must be accomplished fast enough so that the value of each measurand does not change significantly during intervals. In practice, each parameter is measured at a rate of three to five times the highest frequency of interest. There's a definite science to selecting efficient sampling rates and measurand positions!