[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Fri Mar 23 10:49:19 UTC 2012


unruh wrote:
> 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 
best approach:

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 
offset.

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
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"



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