[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

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

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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