[ntp:questions] Re: NTP precision
Tom Smith
smith at cag.zko.hp.com
Fri Sep 23 18:12:55 UTC 2005
Brad Knowles wrote:
> At 4:03 PM +0000 2005-09-23, Tom Smith wrote:
>
>> However, I'm not sure of the proper way to put these together.
>
>
> We've tried to put some useful definitions at
> <http://ntp.isc.org/bin/view/Support/NTPRelatedDefinitions>. This may
> be of some help to you.
>
> If you have any further questions on these definitions, or you have
> any suggestions for clarification, you can either make those yourself
> via the standard TWiki procedure, or you can mention them here and we
> will try to make the appropriate updates at some point in the
> hopefully-not-too-distant future.
>
Thanks Brad.
Jitter
Short-term variations in Frequency with components greater than 10 Hz.
The estimated time error of the system clock measured as an exponential
average of RMS time differences. [This needs some work. :-)]
Dispersion
Represents the maximum error of the local clock relative to the
reference clock. [reported only for individual peers using "rl [ID]"]
Root Dispersion
This is a number indicating the maximum error relative to the primary
reference source at the root of the synchronization subnet, in seconds.
Only positive values greater than zero are possible. [It's in milliseconds
in ntpq]
Reading between the lines, and adjusting for the actual units reported
by "ntpq -c rl 0" (milliseconds), it seems to say:
Estimated error of the current local system time[?] relative to [what?]:
"true" time[?] = systemtime[?] +- jitter
Maximum error of the current local system time[?] relative to the source
at the root of the currently selected chain of sources:
root server time = systemtime[?] +- rootdispersion
Maximum error of the current local system time relative to
"true" time:
[???] "true" time = systemtime +- rootdispersion [+- some other error?]
It's still not crystal clear to me, so I don't really know how
to improve the wording.
-Tom
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