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

unruh unruh at wormhole.physics.ubc.ca
Tue Jan 5 18:02:16 UTC 2010


On 2010-01-05, B <bertil84 at gmail.com> wrote:
>> Dispersion is, the diff to stratum 0 (UTC).
> Root dispersion yes, but there is also a peer dispersion.
>
>> Can you explain what you mean by the phrase "expecting time"?
>
> 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
> expecting time. Becouse the distribution isn't known within these
> interval, I can't say that the time at stratum 3 will be within 2 ms
> relative UTC at 95% of the time.
>
> David L Mills wrote in an old thread:
> "Use ntpq and the rv billboard for the rootdelay, rootdispersion and
> jitter
> displays. Note the jitter display, which includes both peer jitter
> and
> selection jitter, is probably the best indicator of expected time
> quality. Read this as follows: the best estimate of the server time
> is
> the offset in the rv display, with jitter as the uncertainty about
> that
> value."
>
> Absolute error bounds is within this interval
> [OFFSET - DELTA/2 - DISPERSION, OFFSET+DELTA/2 + DISPERSION]
> DISPERSION, in this case PEER.DISPERSION is defined in RFC-1305, page
> 102 as the maximum error in OFFSET and the maximum error in ROUNDTRIP
> DELAY. PEER.DISPERION is the maximum error in the interval.
>
> If PEER.DISPERSION is bigger than JITTER.
> I will able to use OFFSET +/- PEER.DISPERSION as an"estimate" of the
> server time by analogy with what David L Mills wrote: "the best
> estimate of the server time is
> the offset in the rv display, with jitter as the uncertainty about
> that value."

No. The dispersion is a very very conservative ( read large) estimate of
the error in the time. Unless you are really really unlucky ( eg, each
outbound packet takes 1usec to get to the stratum 2 server, and 1 second
to get back) the dispersion is going to be very much larger than the
actual difference between your time and UTC. However, the only way you
can really know is to get a GPS with PPS, hook it up to your machine and
measure the difference in time between the Stratum 3 set clock and the
GPS time. Of course, if you have that gps, you might as well use it to
set the time and you then have a stratum 1 time. Assuming that you do
not have such a weird, consistantly assymetric link to your server, then
the jitter is probably a better estimate of the time difference from
UTC. 
In the science literature, this is called the difference between random
errors and systematic errors. The latter are always far far harder to
estimate than the former. 

>
> Hope you understand my question
> Best regards / B
>
>
>
>
>
>




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