[ntp:questions] SNTP with 1ms of precision?

Marcelo Pimenta marcelopimentacs at gmail.com
Wed Jun 16 15:50:47 UTC 2010


2010/6/15 unruh <unruh at wormhole.physics.ubc.ca>

> On 2010-06-14, Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> > Hi everybody!!
> >
> > My question is about Time Accuracy of NTP/SNTP protocol. I want to know
> if
> > is possible to have precision of 1ms(could be < 1 ms?) using SNTP in a
> > network with 50 hosts(is the same with 100 hosts?) using only swithes, no
> > routing. All these hosts are syncronized with a Meinberg GPS. The traffic
> is
> > low(~25%). Is it possible to have the same time accuracy of IRIG-B for
> > example?
>
> I think you mean accuracy of 1ms. Yes. it can. On a local network,
> 100usec is easily possible ( and depending on the routers, it can be
> better than that).
>

Yes, you are right, accuracy of 1ms. On a local network 100usec?? Even if we
use only switches(no routers), how is that possible if I have 4 types of
Latency increasing about 80us? Algorithms to compensate the delay? Even in
SNTP?

Anyway, the question is about 1 ms. To get 100usec I'll use PTP.



> >
> > The internal clock in my devices(hosts) starts with the crystal
> frequence. A
> > typical offset or frequence tolerance of the crystal is less then
> > 1ms/minute. The resolution is about 0.9ms and the devices acts as unicast
> > clients, asking GPS every minute.
>
> Why?  The whole purpose of ntp is to discipline your clock so that it
> keeps time much better than that.
>

The idea to have this is not overload the network asking time every second
to keep my accurancy in 1ms in 100% of time. And the second reason is about
how many ms your clock will be wrong in 59 sec without a frame to discipline
my clock again? PCs clock for example is not good, that's why I need a very
good crystal.


> Not sure what you mean by "the resolution is about .9ms" What is your
> device?
>

I mean that my device can show me variations of 0.9ms, is the smallest
possible increase of time the clock model allows. I work with protection
IED(intelligent electronic device), is a term used in the electric power
industry to describe microprocessor-based controllers of power system
equipment.

If something happens in 458ms, I will get exacly 458ms. Some equipment have
 so I can trust in variation that I can capture.

>
> > Is it enough? The Time Accurancy could be better if I could ask GPS twice
> a
> > minute? A broadcast implementation with GPS sending time every minute
> could
> > help?
>
> It depends on what you want the time for.
>

I need accurancy at least of 1ms in 100% of time.

>
> >
> > I know that these kind of issue have many variables but the main question
> > is: Is possible to have precision of 1ms(could be <1ms) with SNTP?
>
> Yes, depending on the competence of the writer of the SNTP software.
>

So, the problem is not in the standard but into implementation of SNTP to
not loose time to correct the clock and something like that?


> >
> > Thank you very much
>
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