On 2012-03-15, Chris Albertson <albertson.ch...@gmail.com> wrote: >> Basically you have a trade-off between frequency error and time offset. >> IIRC, polling quickly will reduce the offset, but you get greater frequency >> variations. > > > There is nothing magic about NTP. You have the exact same trade off > if you try and sync your wrist watch to the big town clock on the > tower. > > If you look at the tower every hour and set the wrist watch to match > you will have very small offset between the watch and the big clock. > But if you do this the _rate_ of your wrist watch will never be > spot-on except by luck. > > On the other hand if you sync your watch once then wait a week or a > month to see how much it has gained or lost you may see quite a bit of > "offset" but now you have good data on the relative rates of the two > clocks > > NTP did not invent this trade off, it existed 200 years ago.
Unfortunately it is not that simple. That rate changes by significan amounts. Thus the rate you get after a week may be very different than the rate you get after an hour. That, I submit, is the chief obstacle to having an accurate clock. And that change in rate does not fit with the "Allan variance" assumptions (the noise source is not of the type assumed) > > > > > Chris Albertson > Redondo Beach, California _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
