[ntp:questions] Re: Varying Offset Values
Richard B. Gilbert
rgilbert88 at comcast.net
Sun Aug 20 16:26:28 UTC 2006
David Russell wrote:
> I have been observing the values that a stratum 2 server has for its clock
> offset from multiple stratum 1 GPS based time servers using ntpq -p.
> Two of the three time servers show an offset agreement in the 50-200
> microsecond range. But the offset for third, which is at the same site as
> one of the first two, has been off as much as 10 milliseconds from the other
> two but now is down to 800 microseconds.
> The first two timer servers are installed at different sites and the third
> is installed on an adjacent subnet to one of the servers in the first group.
> I am showing a consistent offset for two stratum 1 servers at different
> sites but a significant difference between two stratum 1 servers that have a
> nearly identical path to/from the stratum 2 server.
> It seems to me that the data is telling me that the third stratum 1 server
> is drifting all over the place and should be taken out of service and fixed.
> My question is: if I had three stratum 1 servers installed side by side that
> were perfect time keepers, shouldn't my stratum 2's clock offset from the
> each of stratum 1 servers be nearly the same?
I wouldn't worry about it! A stratum of 1 does not say anything about
the accuracy, actual or potential! It only says that the server is one
hop away from a reference clock of some sort. If the reference clock
happens to be an HF or VLF receiver getting a signal from WWV or WWVB,
it could be off by considerably more than ten milliseconds.
I have a WWV receiver that I have used as a reference clock. It's a
PST/Traconex Model 1030 that selects the best signal from the 2.5, 5.0,
10.0 , 15.0 or 20 MHz broadcasts. Depending on the current radio
propagation conditions it could deviate from my GPS receiver by anywhere
from a few microseconds to a few HUNDRED milliseconds. (When it's good,
it's really good and when it's bad it's really bad!) Just to make
things REALLY fun, WWVH in Hawaii broadcasts on the same frequencies as
WWV and I sometimes get a better signal from Hawaii (I'm in New Jersey)
than I do from Colorado!
Ntpd should be able to figure out who has the right time if you've
configured it properly! That's why it's recommended that you configure
four or more servers; ntpd can then just say "poor old xxxx has had a
little too much to drink; we'll just ignore him till he sobers up!"
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