Jan Hoevers wrote: > Dennis Hilberg, Jr. wrote on 27-10-2007 20:40: >> Right, but your system is only *accurate* to it's synchronization distance, >> which is the root dispersion + 1/2 the root delay. >> >> Your server, as viewed from my stratum one, has rootdelay=29.432 ms and >> rootdispersion=46.082 ms. That's a synch distance of 60.798 ms, even though >> your reported offset in the same output is offset=-0.939 ms. > > I'll have to do some reading on that first. > > I've tested my system from two different locations over the internet > (and compared it with gps). It is certainly not 60 ms off.
No no, it's not 60 ms off, it's -0.939 ms off. > I guess that's the maximum possible error, with these servers and > network paths. But again, I have to read a bit more about this first. Yep, synch distance (root dispersion + 1/2 root delay) = maximum error, with respect to the root of the ntp chain. > I didn't claim I can guarantee accuracy better than 1 ms, but skipping > the nanokernel and bringing timing granularity back to 1 ms would > degrade accuracy. David Mills explains it a bit better in a post on the comp.protocols.time.ntp newsgroup on 8/30: David L. Mills wrote: > Richard, > > 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. > > Dave > > 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. Sorry, I should have worded that differently. -- Dennis Hilberg, Jr. [EMAIL PROTECTED] NTP Server Information: http://saturn.dennishilberg.com/ntp.php _______________________________________________ timekeepers mailing list [email protected] https://fortytwo.ch/mailman/cgi-bin/listinfo/timekeepers
