[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second
"terje.mathisen at tmsw.no" at ntp.org
Fri Mar 23 10:49:19 UTC 2012
> No I would not. That is not what ntpd does. It really does throw away 7
> of the samples and never uses them. The whole question is what is the
> best statistic to use. I do not believe that the "shortest roundtrip
> time" is that best statistic. If you could convince me it is, I would be
> more than happy to have ntp use it.
In _some_ scenarios, keeping only the minimum rttsample is indeed the
I have been working for a couple of years on the new cell phone network
in Norway (we've replaced everything, including every single base station!).
Even if GSM and related cell phone standards does not require the same
absolute timing precision as some of those used in the US, there is
still a requirement for a _very_ stable frequency base in order to do
transparent handover from one base station to the next, and the relative
offset determines the maximum doppler that can be handled.
In order to be considered OK, we can't accept more than 50 ppb frequency
Handling this with up to 50 ms sawtooth variation (with periods up to
several hours) in the one-way latency means that the vendor require
sampling periods of up to 10+ hours, with multiple packets/second and
then keeping a single packet at the end.
Of course, the main requirement is to start with a _very_ stable time
base, in this case double-oven OCXOs with daily drift rates in the
fractional ppb range!
> IF the roundtrip times were to vary by factors of 2 from one instance to
> the next, I might be persuaded that it was the best statistic. But it
> does not in almost all cases where ntpd is used. It varies by a few
> percent ( with maybe an occasional blip with larger delays.) I have huge
> reams of data to support my statement.
So do I, and I would have agreed with you a month ago, but I have gotten
actual measurement data from the base station vendor showing really
_huge_ packet-to-packet jitter values.
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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