I see a common misconception here. Most of the concepts about NTP can be explained using a common wristwatch.
One is that more frequent checking to a standard keeps the time closer to the standard. That would be true only if you set your watch to match the standard each time. NTP does not do that. It plays with the "fast and slow" lever and adjusts the rate. To set a rate the longer you wait between checks the better. It takes time to see it you are running fast or slow. Frequent jumping the clock to match a standard while ignoring the rate adjustment produces a sawtooth shape error function. Making small adjustments to the rate is better and is what NTP does. NTP can't really know it's own absolute error from "true" UTC time. It if did it would be simple enough to make it zero. All it can do is report statistics about how in the recent past it differed along with an "error bar" on that. To really know how NTP is doing you need a second local clock that is about at least an order of magnitude better. Practically speaking for most of us that means a timing grade GPS receiver. But how do you know you have that set up right and you have accounted for the cable and internal delays. You get a second GPS, hopefully a different type. The good news is that these GPSes are dirt cheap. Under $20 if you hunt around. But it is absolutely true that unless you have access to "true UTC" and also have a way to double/tripple check then you can never know if your NTP server is accurate. You will need a way to measure phase differences in the PPS signals too. None of this stuff is very expensive and there seem to be endless supply from China. But it takes time to build up your ability to measure. So if you want an NTP server that works at the uS level, first assemble a timing lab that can measure nS. New modern timing grade receivers cost under $100 and are good for single digit nanoseconds. Finally, you are ready to start. Setting up an NTP server that does better then 1uS requires a very stable internal clock in side the PC. Most PCs are unsuitable and their internal clock frequency will track the building's air conditioning/heating cycles. If these cycles are faster than NTP's control loop there is not much the software can do. People have resorted to hardware mods like temperature control and upgrading the crystal clock on the computer main board. I found an Atom powered main board that does not need a heat sync, very stable. The bottom line is that getting NTP to sub uS is entering the realm of "rocket science". You can do it but not by tweaking software parameters and reading NTPs own status print outs. That method is fine for mS but not for uS BTW there're some guys in a lab at work (not me) who are measuring "femtoseconds". I'm impressed. -- Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
