Multiplexing Definition

The multiplexing is a technique which comprises passing two or more information through a single transmission medium. It enables to share the same resource between several users. There are two techniques of multiplexing:

  • Time
  • Frequency.

Principle

Using a multiplexer it combines a signal.

Time multiplexing

In the case of time multiplexing, the multiplexer functions as a switch. Each signal switches in turn at high frequency, a synchronization of frequency and ensures phase on both sides for each signal to restore where and as it must.

In a digital acquisition, it is used in particular to be able to use an analog/digital converter with several simultaneous inputs and outputs.

The use of multiplexing is also by modern computer transmissions (USB, IEEE 1394, SSA, Serial ATA …) and in the transmission of digital television channels (digital broadcasting package, digital terrestrial television).

Frequency multiplexing

Optical multiplexing no longer distributes signals over time but in frequency space. Although more abstract in principle, it was he who was first invented (see the phone and Samuel Morse).

It is to pass much information simultaneously by varying the length of a wave of the light emitted. This is also spatial [multiplexing].

More simply, we send several colors simultaneously on a single optical strand. This has allowed in particular to increase the transmission capacity of the current optical fibers without very significant extra cost.

Uses

Time-division [multiplexing] began to be used in aircraft so that each passenger could control his own lamp without connecting each switch of each passenger to each lamp switch.

  • It is this multiplexing that often introduces a small delay between the moment when it presses the switch and the one where the lamp comes on (it would have been instantaneous if the switch had been on the bulb instead of being in the arm of the wheelchair, but would have forced the passenger to lift his arm, see Ergonomics).
  • The use of Time-domain electronic [multiplexing] is also in the automotive industry. It consists of passing a signal multiplexed by a wire, and the power supply by another cable.
  • A demultiplexer at the end is responsible for re-dividing the signal into as many independent signals. In particular, the use of [multiplexing] is to control all rear lights by only these two wires.
  • One of the drawbacks of this technique and that the loss of one of the two leads to the failure of the set of taillights.
  • For mobile telephony, it uses three types of multiplexing: frequency [multiplexing] (FDMA), time-division multiplexing (TDMA), and code division [multiplexing] (CDMA). Frequency [multiplexing], the use of a spatial alias, was already in analog telephony.
  • The GSM standard uses frequency and time [multiplexing]. Finally, UMTS will resort to code [multiplexing].

The use of multiplexing is also in the entertainment industry. Mainly in lighting, where data transmission via a DMX cable uses it.