On 2011-04-18, Chris Albertson <albertson.ch...@gmail.com> wrote: > On Mon, Apr 18, 2011 at 3:49 PM, unruh <un...@wormhole.physics.ubc.ca> wrote: > >>> Thanks for all the great answers. ?Now for a harder question, how does the >>> accuracy of the local clock source >>> affect the accuracy of ntpd. >> >> What local clock source? > > That is a good question: "Which clock?" One clock might be the > reference clock for a stratum 1 NTP server. This is typically a GPS > receiver and these vary a LOT. Some can supply a pulse per second > with 1 sigma error of only a hand full of nanoseconds and other GPSes > have hundred of microseconds of error. That is a factor of over > 10,000 difference in accuracy.
That sounds like the difference between a PPS gps receiver or one whith simple NMEA output. The latter should NOT be considered a stratum 0 receiver. > > The other clock is the crystal inside the computer that NTP tries to > "discipline" by using the above reference clock. I think what matters > most is "stability" rather than "accuracy". In other words it is > fine if this oscillator runs at the wrong speed but not good if the > speed is not constant with respect to time. The best oscillators are > housed in temperature controlled ovens but these are not common inside > PCs. What is worse is that modern cpus in order to conserver energy keep changing their rate, which makes them really difficult to use as a good rate. > > > > > > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
