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I'm doing some performance tuning and capacity planning for a low-latency application and have the following question:

What is the theoretical minimum round-trip time for a packet sent between a host in London and one in New York connected via optical fiber?

knorv
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    How many routers? How fast are the routers? Or are you asking about the speed of a signal through copper wire? – S.Lott Mar 16 '10 at 17:50
  • I think you might need some definition of "theoretical". I'm guessing you're probably not allowed to lay your own cable and put your application on computers hooked up directly to it. – Cascabel Mar 16 '10 at 17:51
  • @Jefromi: You can hope, but the question doesn't say, making it hard to answer. – S.Lott Mar 16 '10 at 17:52
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    @S.Lott: What I mean is, anything built in "modern" times uses fiber, not wire. – Cascabel Mar 16 '10 at 17:57
  • Yes, it's fiber :-) I've updated the question with that assumption. – knorv Mar 16 '10 at 17:59
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    Aside: 1 foot/ns is a decent approximation to the speed of light in vacuum. Both wire and fiber have effective indexes of refraction around 1.5, giving 8 inches/ns at the physical layer. Useful thing to know when you're wiring up plug-board computers. – dmckee --- ex-moderator kitten Mar 16 '10 at 18:19

2 Answers2

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I believe the index of refraction of fiber is around 1.5, and the internet reports it's around 5600 km from NY to London, so the theoretical minimum one-way is 5600 km / (c/1.5) =~ 28 ms. Round-trip is double that, 56 ms.

Up to you to do the real work of estimating latency through your routers and all.

P.S. The cables might not be straight :p

Edit: A bit of the wikipedia article on optical fiber pretty much contains all this information.

Cascabel
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    The theoretical minimum round trip time is actually twice that, so 56ms. For comparison with reality: I'm in Belgium and www.nyi.net has a ping of 89ms for me. Surprisingly low isn't it! – Wim Coenen Mar 16 '10 at 18:05
  • Oh, round-trip, my bad. Fixed! – Cascabel Mar 16 '10 at 18:08
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Just ask Hibernia, they currently are at 72ms and presently looking at 60ms by mid-2012.

http://www.a-teamgroup.com/article/andrews-blog-laying-cable-and-the-low-latency-gauntlet/

Steve-o
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