# What's the minimum network latency for a 1000 km connection using optic fibers?

I know most of the network latency for short distances is due to router processing times. But for longer distances the speed of light also counts. And it's different from the speed the light in the vacuum. What is it?

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related question serverfault.com/questions/61719/… – Jader Dias May 29 '11 at 17:43

## 2 Answers

A typical index of refraction for optical fiber is 1.62, therefore the speed of light in a fiber is approximately 3e8 m/s / 1.62 = 1.85e8 m/s. Therefore it would take at least 1000000 m /1.85e8 m/s = 0.0054 s to travel that distance. Note that this value doesn't cover the extra distance traveled by the light from bouncing side to side.

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I expect more than this theoretical 5.4 ns/m, isn't there any source that says what actual latencies are? – Jader Dias May 28 '11 at 19:35
And fiber cables aren't laid in straight lines neither. – Jader Dias May 28 '11 at 19:37
@Jader, lol, first you ask for "minimum network latency" and then you complain they aren't realistic enough? You must be confused. – akappa May 28 '11 at 21:19
@akappa I'm not confused. I'm asking the the "minimum network latency", not the "minimum latency through an ideal-but-yet-not-manufactured-medium" – Jader Dias May 28 '11 at 21:21
@Jader Dias is right on that last point - the refractive index is a property of bulk material and can't take reflections into account. The actual delay factor due to these reflections depends on the angle of light rays entering the fiber. The maximum angle is determined by the NA of the fiber. Note that for thin (single-mode, less than 10 lambda diameter) fibers it doesn't even make sense to think about light rays bouncing around in the fiber... see en.wikipedia.org/wiki/Optical_fiber#Single-mode_fiber – Jonas Heidelberg May 29 '11 at 18:00

Distance Delay is simply the minimum amount of time that it takes the electrical signals that represent bits to travel down the physical wire. Optical cable sends bits at about ~5.5 µs/km, copper cable sends it at ~5.606 µs/km, and satellite sends bits at ~3.3 µs/km. (There are a few additional microseconds of delay from amplifying repeaters in optical cable, but compared to distance, the delay is negligible.)

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