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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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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, 2011 at 19:35
  • And fiber cables aren't laid in straight lines neither.
    – Jader Dias
    May 28, 2011 at 19:37
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    @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, 2011 at 21:19
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    @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, 2011 at 21:21
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    @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 May 29, 2011 at 18:00
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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.)

source: http://www.networkperformancedaily.com/2008/06/latency_and_jitter_1.html

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The general conception of the speed of light in vacuum is 300,000 km/s, give or take. That is 300 km/ms or around 1,000 km in 3.3 ms (milliseconds). The minimum network latency for a 1000 km connection using optic fibers may be between 10 and 30 milliseconds, according to the answer of @kyle kanos to the question "How fast does light travel through a fibre optic cable?", in the Physics Stack Exchange.

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