Even with the long and variable ping time, time sync can work. The reason it can work is that time is not re-sync'd in one "ping" but time sync is an on-going process that occurs over a period as long as hours.
Think about adjusting the rate of a clock by hand. The computer (NTP) can (and does) use the same method. You sync it up as best you can then come back in 24 hours and see it your clock has gained or lost time then fix the rate and continue this process forever, eventually some balance is reached as you learn to make fine adjustments. So having a few tens of milliseconds of error in the communications channel is not so bad if we can let the clocks runs for a long time and also we can average MANY measurements, NTP uses quite a few tricks and works about as well as it can given the nature of the communications channels it is given work with. PTP has one more trick it can use that really solves the problem., PTP depends on special hardware that time stamps the messages so it knows the ping-like delays you see. But PTP's problem is that it requires special PTP compatible hardware. There are people in academia who have sync close to on order 100 nanoseconds over WiFi using PTP. I think this is a do it your self thing right now. It would not be really hard to build a WiFi router form Linux or BSD then add PTPd on each end. There might be a commercial solution, I don't know. But in any case don't expect your system clock to bounce around like the ping times do. NTP is good enough that it does an average. I run NTP in some WiFi connected autonomous robots and it works well enough to keep internal log files sync'd between robots. But I only need a few tens of milliseconds for that. On Fri, Jan 13, 2017 at 11:35 AM, Chris Caudle <[email protected]> wrote: > On Fri, January 13, 2017 11:40 am, Bob Camp wrote: > > The ping response is anywhere from 2 ms out to 400 ms. Most of > > the time it's in the 3 to 9 ms range. Simply taking that > > down to < 1 us would be a really big deal. > > I doubt that the response time will get that low, rather the time sync > will be moved lower in the hardware stack so that the variation stays > below 1us so it can be compensated as a systematic offset. Basically a > Wi-Fi version of the hardware time stamping that a lot of NIC's do now for > PTP support. Just a guess at this point. > > -- > Chris Caudle > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > 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 -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
