Multiplexing was first used in the 1870s, but what is it and how is it used? What are the various types of multiplexing? What about keyboard matrices and Charlieplexing?
Multiplexing, or MUXing, is a way of reducing the number of expensive wires, cables or RF transmitters by combining several signals over one medium.
It was originally used in telegraphy in the 1870s to allow more than one person to communicate at a time, and was developed by George Squier in 1910 for the telephone carrier network.
The opposite of MUXing is DEMUXing, which is a way of separating all the combined signals at the other end.
These days it is used in TV and radio stations, telephone networks, satellite communications, you are even using multiplexing to read this article and watch this video as you are sharing the Internet with billions of other users.
In electronics there are three common methods of employing multiplexing; Time, Frequency and Space.
- TDM – Time Division Multiplexing.
- FDM – Frequency Division Multiplexing.
- SDM – Space Division Multiplexing.
Time Division Multiplexing
In TDM, signals are combined in the time space by allocating a short, fixed timeslot where each source signal transmits a few bits or bytes.TDM is used primarily in digital communications, such as the old GSM network, but can also be seen in the old WAV audio format, which uses TDM to interleave it’s stereo channels much like a zipper.
Frequency Division Multiplexing
With FDM, signals are combined within the frequency space. Every TV and radio channel on the planet uses FDM as it shares the atmosphere when transmitting signals.
FDM can also be used in cable television and can be referred to as WDM, Wavelength Division Multiplexing, when talking about fibre optic communications.
Space Division Multiplexing
With SDM, signals are combined within the space space…
Good examples of this is your landline telephone with it’s multi-pair wires, a humble I2C bus, an Ethernet switch with it’s many ports, mesh networks such as ZigBee, or your wireless Access Point, which uses phased array antennas to transmit several signals at the same time.
There’s also several other sub methods:
Polarization Division Multiplexing
PDM, which uses light polarization in fiber optics or orthogonal polarization in wireless to transmit two signals simultaneously. It is used primarily in fibre optics, microwave links and satellite communications.
Code Division Multiplexing
CDM, is something you have been using for many years and is seen in the humble mobile phone. Often also referred to as CDMA, it employs frequency hopping or DSSS to achieve multiplexing.
Orbital Angular Momentum Multiplexing
Finally, OAM, is a very recent technology that takes PDM further by using the orthagonal quantum states of light to multiplex signals. I won’t go into this here as it’s still only in research.