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

B bertil84 at gmail.com
Tue Jan 5 15:37:52 UTC 2010


> 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."

Hope you understand my question
Best regards / B









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