On Sat, Feb 18, 2012 at 11:55, <[email protected]> wrote: > Hello, > > the situation before I had a DCF77 clock: I synchronized my > NTP-Server against a few Stratum-0 Servers (ptbtime2.ptb.de, > ntp1.sda.t-online.de, ntp1.fau.de). > At the statistic site from the pool, I saw that my offset (in good > times) oscillates about 0 with +/- 10ms. That was ok for me.
Were the offsets equally balanced between positive and negative? > I bought a DCF77 clock and connected it to my NTP-Server: > > server 127.127.8.0 mode 16 prefer > fudge 127.127.8.0 time1 0.251 > > Now I have a problem: > On a second independent computer I ran NTP and let it > syncronize against my NTP-Server with the DCF77 clock > and against ptbtime2.ptb.de. I changed the "time1" to a > value, so that my NTP-Server and ptbtime2.ptb.de a > almost the same (low difference of the offset). Please tell us the time1 value that brought your clock into apparent alignment with this national time reference. > But the statistics of the pool now show me, that my > NTP-Server has an offset of 10ms +/- 2ms. If I change the > "time1" so that my NTP server show a good value on the > pool statistic site, I get a huge difference (>10ms) to > ptbtime2.ptb.de. Please tell us the time1 value that minimized your offset with the pool monitoring. > How can I calibrate my NTP-Server correctly? NTP relies on network paths being symmetric -- the same latency from A to B as from B to A. If the path is not symmetric, the result is an offset error. Often the access link is inherently asymmetric such as ADSL. In contrast, your DCF77 receiver suffers offset error due to propagation delay between the transmitter and receiver, as well as delay getting the signal into your computer. You cannot be sure your calibration is correct using only network sources, due to the asymmetry likely present. You may be able to come close by checking against different remote sources as you have tried with PTB and the pool, but both share the access link from your premises which may be the major source of error. Cheers, Dave Hart _______________________________________________ pool mailing list [email protected] http://lists.ntp.org/listinfo/pool
