[ntp:questions] WiFi & NTP.

David L. Mills mills at udel.edu
Fri Aug 31 05:58:47 UTC 2007


For better or worse, the NTPv4 spec and the reference implmentation 
define precision as the time to read the system clock currently a few 
hundred nanoseconds. The resolution on the other hand is the smallest 
increment that can be measured by the system clock, usually one 
nanosecond with Unix timespec structures. Your interptetation of the 
root dispersion is not pecisely correct. What is correct is that the 
root synchronization distance, equal to the root dispersion plus half 
the root delay, is th maximum errot bound.


Richard B. Gilbert wrote:
> Dennis Hilberg Jr wrote:
>> Jason Rabel wrote:
>>> Those numbers seem much more reasonable.
>>> The root dispersion is the maximum error between the local clock and the
>>> root of the NTP chain. Your time on each system is accurate to within 
>>> the
>>> range given above.
>> My problem is that I misunderstood you when you said 'precision'.  I 
>> was simply referring to offsets grabbed from ntpq.  Perhaps Guy was too.
> The NTP world uses the word "precision" to designate the value of the 
> low order bit kept by the system clock; e.g. the smallest possible 
> difference between the two consecutive time stamps.  It's generally 
> specified as a negative power of two.
> My Sun Solaris (SPARC) boxes have a precision of -21 but an accuracy of 
> plus/minus 5 milliseconds when synchronized from the internet.

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