What is Latency ?

Over the last months i had several discussions with Customer and Colleagues on the topic of “Latency”. So i try to make some statements here around this topic and try to clarify some of the confusion.

1. Fibre Optics (Wikipedia)
Latency is largely a function of the speed of light, which is 299,792,458 meters/second in vacuum. This would equate to a latency of 3.33 microseconds for every kilometer of path length. Because the index of refraction of most fibre optic cables is about 1.5, light travels about 1.5 times as fast in a vacuum as it does in the cable. This works out to about 4.9 microseconds of latency for every kilometer. In shorter metro networks, the latency performance rises a bit more due to building risers and cross-connects and can bring the latency as high as 5 microseconds per kilometer. It follows that to calculate latency of a connection, one has to know the distance traveled by the fiber, which is rarely a straight line, since it has to traverse geographic contours and obstacles, such as roads and railway tracks, as well as other rights-of-way. Due to imperfections in the fiber, light degrades as it is transmitted through it. For distances of greater than 100 kilometers, either amplifiers or regenerators need to be deployed.

So we can assume the following:
Metro distances (0-30km) ->Latency per km = 5 microseconds (0.005 milliseconds)
Larger distances (30-300km) ->Latency per km = 4.9 mircoseconds (0.0049 milliseconds)

So if you use a 20km Fibre Optics connection the Latency from this would be (20km x 5 microseconds) x 2 (roundtrip) = 200 microseconds (0.2 milliseconds)

So if you use a 100km Fibre Optics connection the Latency from this would be (100km x 4.9 microseconds) x 2 (roundtrip) = 490 microseconds (0.49 milliseconds)

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