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I have a doubt about MMF and SMF cables. If MMF can handle more than one signal light at the same time and both MMF and SMF support full-duplex, why isn't MMF faster than SMF cable?

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As the name implies a multi mode fibre can carry multiple modes of light. This is not the same thing as multiple signals.

The multiple modes of the same signal only leads to modal dispersion which stretches out the signal in the time domain, due to the different propagation speed of the modes, making it more difficult to detect at the receiver end. This leads to a lower speed both in theory and practice than a single mode fibre which doesn't suffer from modal dispersion.

Both MMF and SMF fibre can carry multiple signals through the use of wavelength division multiplexing (WDM), although this is much more common with SMF as its characteristics allow it to be used over longer distances where it generally becomes more economical to use WDM equipment.

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  • Mode has little effect on speed. It does limit distance; higher speeds are more susceptible to dispersion.
    – Ricky
    Oct 27 '15 at 18:13
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    @RickyBeam, of course dispersion affects speed. Seen another way; For a given baud rate, dispersion will limit your distance. For a given distance, dispersion will limit your baud rate.
    – kll
    Oct 28 '15 at 7:23
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    It's worth noting that either can also carry multiple wavelengths at the same times, and thus, multiple signals (though this is more common on single-mode because single-mode is more commonly used at distances where it pays to use multiple-wavelength equipment rather than just add another fiber.)
    – Ecnerwal
    Oct 30 '15 at 20:54
  • @Ecnerwal updated my answer to that affect. Good point :)
    – kll
    Nov 1 '15 at 2:59

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