Comment Re:How do they do it? (Score 1) 145
You can, that's the good part about it, many times signals reflect when they hit an improperly terminated connection (impendance mismatch).
This holds for both optical and electrical signals. Given the propagation time and speed, length is trivially calculated. There's special hardware to do this for you.
See also: http://en.wikipedia.org/wiki/Impedance_mismatch