Wi-Fi warning

Wi-Fi versus a direct connection

By changing from a Wi-Fi connection to a LAN connection, both to a local GPS/PPS locked stratum-1 NTP server, I was able to dramatically improve performance on a Windows-7 PC running ntpd 4.2.7p398.

Here is the view seen by MRTG, with 5-minute samples of the offset reported by NTP:

and here is the same offset but plotted directly from the loopstats produced by NTP. The black line is an hour-by-hour RMS offset from the mean value:

The jitter shows an improvement from an averaged value of around 3 milliseconds, to a value just under 1 millisecond (0.977 ms) a limit set by Windows-7. [This limit is much less in Windows-8, resulting in potentially even better performance.]

Finally, you can see that the frequency is much more stable with the direct LAN connection:

Acknowledgements

My thanks to Martin Burnicki for triggering my investigation, after I asked a question on the comp.protocols.time.ntp Usenet group.