[ntp:questions] SNTP with 1ms of precision?
marcelopimentacs at gmail.com
Wed Jun 16 19:56:39 UTC 2010
2010/6/16 Rob <nomail at example.com>
> Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> > The idea to have this is not overload the network asking time every
> > to keep my accurancy in 1ms in 100% of time. And the second reason is
> > how many ms your clock will be wrong in 59 sec without a frame to
> > my clock again? PCs clock for example is not good, that's why I need a
> > good crystal.
> Why are you causing your own problems by sticking to SNTP instead of
> using NTP which was designed to overcome the problems you fear?
Rob, my understading about the use of SNTP and NTP is: while SNTP provides
time synchronization within *one *network, NTP allows a global time
synchronization on the internet. SNTP provides the current time, the current
number of leap seconds and the warning flags marking the intriduction of a
leap second correction.
The NTP algorithm is much more complicated than the SNTP algorithm. NTP
normally uses multiple time servers to verify the time and then controls the
slew rate of the device. The algorithm determines if the values are accurate
using several methods including fudge factors and identifying time servers
that don't agree with the other time servers. It then speeds up or slows
down the device drift rate so device time is always correct and there won't
be any subsequent time jumps after the initial correction.
SNTP usually uses just one Ethernet Time Server to calculate the time and
then it "jumps" the system time to the calculated time. It can, however,
have back-up Ethernet Time Servers in case one is not avaiable(not two at
the same time).
In my case, I have* only one* network. My Time Server is not a machine, is a
meinberg GPS. In my point of view, if my source time were machines, maybe
NTP could be better to find a middle line between all these machines used as
time servers. But if I am using a very good and reliable GPS(Meinberg) with
a lot of satellites giving it the correct time, and it's pluged directly in
my switch, I think that in this case, NTP will not make any difference.
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