[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




More information about the questions mailing list