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

unruh unruh at wormhole.physics.ubc.ca
Thu Jun 17 09:58:11 UTC 2010


On 2010-06-16, Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> 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?

ntp relies on the outgoing and ingoing times are the same. They could be one
year, but if they are both exactly the same ( both exactly one year)
ntp can deliver a time accuracy of 1nsec. 
Yes, I typically get time accuracies of tens of usec on a local network. 
Even over adsl frin home to work  I get 1 msec easily ( and yes I can detect the accuracy by
having a gps PPS receiver at both endw to test the accuracy to 1us)

level)
>
> 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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