Paul wrote:
On Mon, Mar 17, 2014 at 8:50 AM, Joe Gwinn <[email protected]> wrote:
Yes. My question is basically a query about the current state of the
art.
Some NTP offsets (output may look funny if formatted) clock1 looking at
clock2 and clock3 (a Raspberry Pi). This suggests it can be as good as
your IRIG system.
Gig ethernet to Gig ethernet. ~22 days:
N Min Max Median Avg Stddev
244059 -0.098741335 0.019727433 9.586e-06 4.814598e-06
0.00038621792
Gig to Fast ~10 days
N Min Max Median Avg Stddev
112254 -0.000516264 0.000453913 1.127e-06 6.8736914e-06
4.2248166e-05
But without additional measurements you still don't know for sure if
this is the true time offset, or if there is an additional systematic
time offset (e.g. to an asymmetric network connection) which can't be
measured.
For example:
I have an NTP node running on my home network connected via an
asymmetric DSL line, getting the time from the internet.
If I monitor that NTP node from our company network which is connected
to the i'net via a symmetric line then my home NTP server seems to have
a low offset, below 1 ms.
However, if I let my home NTP node synchronize to a GPS receiver then
suddenly the the offset observed from our company network increases to a
few milliseconds.
The reason is that the way the asymmetry affects the polling my how
server does towards the internet, and the way the asymmetry affects the
polling of my home server from our company are in the same range, but
with reversed sign, so the effect is compensated and from our company's
point of view my home server is very accurate even though it isn't.
If my home server syncs to GPS then the accuracy isn't compensated
anymore and if I'm monitoring my my home server from our company there
now seems to be a time offset, where in fact there isn't one anymore.
Martin
--
Martin Burnicki
Meinberg Funkuhren
Bad Pyrmont
Germany
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