When your voice is digitised, or a file is transferred from computer to computer, the result is a stream of 1s and 0s, which could be transmitted from point to point in any way. Possible transmission methods include optical (through free space or fibre), radio and microwave, and electrical.
Where a cable is to be used, the choice is between electrical transmission over copper, and optical transmission over fibre. Although optical transmission involves additional equipment to convert electrical signals to optical and back again, it has several advantages which lead to its increasing use in many applications.



Fibre's high bandwidth means the more data can be transmitted further, so use for long-haul telecommunications is almost standard these days (a pair of copper wires can transmit 2 telephone calls simultaneously - a single fibre over 80,000!).

Glass is cheaper than copper so for the same data carrying capability fibre cables cost far less than copper cables.

Fibre is unaffected by electro-magnetic interference, and so is ideal in electrically noisy environments.

No electrical current is transmitted down a fibre (glass is a good insulator!), so it is suitable in hazardous environments where electrical discharge would be a potential disaster.

No signal is radiated by a fibre (unless you bend it or break into it), making an all-fibre network secure from undetectable tampering.

As transmission technology continues to improve, higher and higher bandwidths are possible over the same fibre links without having to replace the cables.