[ntp:questions] Is dispersion > jitter in all situations

B bertil84 at gmail.com
Wed Jan 6 10:23:33 UTC 2010

On 5 Jan, 22:20, David Woolley <da... at ex.djwhome.demon.invalid> wrote:
> B wrote:
> > I want to know the accuracy on a certain NTP-server at stratum 3. It
> > is easy to calulate the absolute error bounds that wont be exceeded
> > with this equation
> > OFFSET +/- [DELTA/2 + DISPERSION]. This will in my case be OFFSET +/-
> > 4 seconds, but I need to know more precise, ie an indicator of
> In that case there is something wrong in your configuration! ntpd will
> start ignoring servers if their root distance is rather less than this
> and it would be an unusual system where the leaf nodes go so long
> between polls that they can accumulate the additional dispersion needed
> to reach 4 seconds.

Oh, sorry for my typo.
In my case the absolute error bounds is -0,72 +/- 3,9 ms, not seconds!
There is almost a dedicated network with optical fiber and no(very
little) asymmetris. There is a huge network which got three GPS-clocks
as stratum 0, reference clocks for the primary servers. Below you can
see the selected servers, all in [milliseconds].

Stratum 2
ref clock     st  when  poll reach  delay  offset    disp
.GPS.             1    55    64  377     0.7    0.02     0.1

Stratum 3
ref clock     st  when  poll reach  delay  offset    disp
xx.xx.x       2   154   256  377     5.7   -0.74     0.6

For my logging application and master thesis report I need to know a
more narrow interval, and I know it is more accurate than the absolute
error bounds [-4.62, 3.18]. So a smaller interval of expected time is
Unruh you are probably right, but aren't you talking about
synchronization distance(sometimes called root distance), DELAY/2 +
DISPERSION, as a conservative estimate of the error.

I thinking if it is ok to use OFFSET with DISPERSION as the
uncertainty about that value. If only DISPERSION is a conservative
value for that too, at least I got a narrower interval of [-0.72 -
0.7, -0.72 + 0.7]=[-1.42, -0,02] milliseconds.

I really appriciate your help!  // B

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