On Wed, May 23, 2012 at 2:59 PM, Bob Camp <li...@rtty.us> wrote: > Hi > > Ok, to be 1000: 1, you would take the 0.2 to 0.5 ms that you see on the LAN > and take it up to 200 to 500ms. That's *way* worse than anything I have ever > seen for a serial server over a LAN.
Yes, 200 ms would be insanely poor. NTP with a direct connected GPS can run the micro second level. The 100x to 1000x worse I'm talking about puts NTP at the 0.5 to 1.0 ms level. With PPS connected directly to the DCD line on a Linux based server the hardware counter is captured with a typical error of 1 micro second. Over the LAN or USB we see this error at about 1 or 2 milliseconds So what I'm saying is by using the LAN you give up micro seconds for milliseconds or about a factor of 1000. You are right. It is never anything at all like 200 ms. We get better than 200 ms error over the Internet from servers 1,000 miles away. In general you should expect a 1 uSec level error from a direct connect to from a GPS's PPS to the DCD line of a serial port and a mSec level error from USB or LAN and tens of ms error from an Internet connection. Where NTP really shines is extracting 10ms level timing from an unreliable connection that has much longer then 10ms delay. It really is not so impressive that it can get u-sec level performance from a directly connected Trimble Thunderbolt. The bottle neck is the PC hardware. You really never can do better then 1 uS with a generic PC. > > Bob > > On May 23, 2012, at 5:15 PM, Chris Albertson wrote: > >> On Wed, May 23, 2012 at 2:02 PM, Bob Camp <li...@rtty.us> wrote: >>> Hi >>> >>> What ever degradation the serial stream sees on the LAN, the resulting NTP >>> output will see once it's on the same LAN. It's unlikely you will see more >>> than a 2:1 net degradation no matter what is going on. The flywheel in the >>> NTP algorithm will likely help you in this case to actually improve things a >>> bit. >> >> Have you actually tried this and measured? 2:1 is very optimistic. >> Typically it is 1000:1 or worse >> >> But you are right that it may not matter. For most uses if the >> computer's clock is correct at the 0.1 second level they are happy. >> but this is a "time nut" mailing list and some of us like to get NTP >> to run at the uSecond level. Useless as that might be. >> >> >> >> Chris Albertson >> Redondo Beach, California >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
