[ntp:questions] Re: offset values from ntpq and ntptrace

Tim Tanguy.ropitault at gmail.com
Mon Apr 3 13:32:20 UTC 2006

Thanks all of you for your answer, it has helped me to understand some
things... For David Wooley, here is my typical configuration:
- My company is building embedded switch router card wich is supposed
to implementent NTP protocol for synchronising its LAN... actually, my
boss want that I make a document for potentiel customer that will use
NTP for explaining them how does It work and when can they use NTP...
so It's a very open question because for example, my card may be not
connected to the Internet and may not have any reference clock (like
radio clock for example). So I have isolated 4 particular
1 - The card is connected to the Internet and we use a public server
for synchronizing the card.
2 - The card isnt connected and use its own clock as reference clock (I
know that it's not recommanded but...)
3 - The card isnt connected to the Internet and it use a reference
clock like GPS receiver.
4 - The card is sometimes connected to the Internet but mainly uses Its
own clock to synchronise itself.

Here are my early conclusions:
For configuration 1: We can use this configuration if we want to
synchronize our LAN not very accurately... approximately 10ms accuracy
to UTC time can be expected if everything is OK...
For configuration 2: We can use this configuration only for maintening
our LAN synchronized but without any accuracy to UTC time expected. We
can only say that our LAN will have clock that differed from each other
from 100us.
For configuration 3: I havent said a lot of thing because its depend on
the reference clock that we choose but it can guarentee an accuracy of
few us.
For configuration 4: It's a bad configuration because when the card are
synchronizing itself on its reference clock, it drifts from UTC time
and when you connect the card to Internet, bad things can happen like a
higher difference of 128ms between the card and public's server time so
it's a bad configuration.
Are my conclusions right???
Anyone can say more about that?
My boss is also asking myself especially precision for configuration 2,
wich is the typical configuration where our card will be used. He knows
that we wont have any accuracy compared to UTC time, but the only
things he wants is that our LAN is synchronised thanks to the card. So
he wants to know what are the parameters that are important for
allowing that. I answer:
- drift difference between card's clock and client clock should not be
higher that 500PPM
- card's clock should be thermal stability
- card OS shouldn't be windows NT but rather Unix moder system or Linux
wich have greater resolution...
are you alright?
One last thing, I heard a lot about PPS, and I think that there is no
use for PPS unless to have its own reference clock? Am I right? Thank
you very much, and sorry for my english but technical conversation are
sometimes difficult for a non-english guy...

More information about the questions mailing list