[time-nuts] Low-long-term-drift clock for board level integration?
woody at pch.net
Tue Feb 21 01:35:57 UTC 2012
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On Feb 20, 2012, at 4:56 PM, Brooke Clarke wrote:
> It was my understanding that the receiving station knows the path taken, is that the case? Or are you saying even when it's the same path the time delay has large variations?
The sending station can map the path, using traceroute or its equivalent, and send that information to the receiving station. The path changes unpredictably, as circuits and routers go up and down, and the routing topology shifts. Path changes generally look like a shift in both baseline and jitter, which tips you off to re-map the path from the sending side.
And yes, even when the path is consistent, its delay generally varies a lot, depending upon the depth of the intervening routers' and switches' transmit queues and forwarding-engine loads, temperature, etc. What makes it possible at all is that the cost of a measurement is low, so you can keep measuring and measuring, and establish a minimum floor, which is what you'll get when, coincidentally, you happen to hit all empty transmit queues.
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