Richard B. Gilbert schrieb: > Jeff W. Boote wrote: > >> Heiko Gerstung wrote: >> >>> Richard B. Gilbert schrieb: >>> I was referring to sub-microsecond accuracy over the network, it >>> seems that the OP has no chance to use a hardware ref clock. >>> >>>> My only knowledge of PTP is based on some earlier messages here but >>>> I believe that it must have the same problems as NTP over the internet. >>> >>> >>> >>> PTP uses hardware timestamping at the MII/PHY level, this will not >>> help when you use the Internet, but maybe the OP has his own nice WAN >>> with low-jitter connections available. You can get in the lower >>> nanoseconds with PTP over Ethernet, but only in very small networks >>> or by using PTP-aware infrastructure components like switches with >>> integrated hardware timestamping.. >> >> >> I do have access to a very nice WAN with extremely low-jitter >> connections. I can likely remove nearly all buffered devices from the >> path - and get nearly the equivalent of a really long cross-over cable. >> >>>> <commercial for $1000+ product snipped> >>> >>> >>> >>> The mentioned LANTIME/NDT is basically an oscillator that is >>> disciplined by NTP. And yes, its too expensive for using it at home :-) >> >> >> Right - eventually it all comes down to price. How much accuracy can >> you afford... But, this at least sounds interesting. >> >> Thanks for the pointers. >> >> jeff > > Jeff, > > I think the killer keywords here are "disciplined by NTP". (BTW, I > thought it was GPS disciplined) If it is, in fact, disciplined by NTP, We have several different NTP time servers, synchronized to GPS, NTP, IRIG B and so on. The NDT is the one that is using another NTP server as its primary reference.
> it is no better than your NTP time source, be it reference clock, or > internet server. If it is, as I thought, disciplined by GPS, then it's > just a high grade crystal oscillator if you cannot apply the GPS signal > to it. I question whether it can maintain "sub microsecond accuracy" > without the GPS discipline. Sub-Microsecond accuracy is not possible with it, our measurements in a separated LAN show an accuracy of +/- 30 microseconds. > It's a very nice toy and, if I should hit the lottery for a couple of > million, I'll probably buy one. I would not call it a toy, there are actually people doing serious things with it, but we are getting off-topic :-) > If you really need that sub microsecond accuracy and must live with the > constraint of "no radio reception" your best bet is a Cesium clock. It's > expensive, but once calibrated by NIST it should drift no more than 400 > nanoseconds per year. If that 400 nanoseconds is significant, you'll > want to have it calibrated annually. Best regards, Heiko _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
