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Fibre Optic Basics
Jones88
2 Posts

I'm trying to get my head around fibre optics compared to copper. In my head copper is easy, if i've got 20 signals in the field going to a PLC, I may select a 20 pair cable (for example). However if i've got 100 signals in the field, and i want to use fibre then how do i know how many OM3 cables (for example) i need to use/what's the limitation on OM3 cable. Can someone point me in the direction of any literature that explains? 

1 Replies
mapj1
4444 Posts

It rather depends on the bandwidth of those signals coming in over the copper pairs. I agree for a short length you could have 20 pairs, and the same applies to fibre, but in both cases that is really wasteful, and normally  even over wire, you'd multiplex the signals in some way, perhaps in an analog manner with modulation on  a sub-carrier tone per signal, or in a digital way, either time division or digitisation and then as streams. 

So are these pairs carrying gigabit ethernet, or perhaps video or audio, or maybe really slowly changing voltages representing signals like temperatures  of a factory process?

Once you have a figure for the bandwidth needed for the combined signal, that can be worked back into a number of fibres.   Again, the carrying capacity  is length dependant, and you can shine more than one wavelength (think colour, but infrared) down a fibre and separate them with filters at the other end.

As a rough start, OM3 will take data  at a rate of perhaps  10gigabits per second for about quarter of a km and 40gigabits about 100m, if you want a lot more capacity or distance then single mode fibre OSxx  is the way to go. more on that here

 First realise that the ‘simple’ copper is not so simple, only when you know what the signals are, then translate to fibre.

Mike

 

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