[ntp:questions] NTP constant delay ?

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Dec 27 12:30:28 UTC 2012


On 27/12/2012 10:11, Ran Shalit wrote:
[]
> Hi,
>
> SNPT performance with talks about accuracy of 100msec are shown in the
> following white paper:
> http://www.cisco.com/en/US/tech/tk869/tk769/technologies_white_paper09186a0080117070.shtml
> In out configuration we are talking about 2 machines, one using Linux
> SNTP client, the other I do not know, but assume it is RT Operating
> system. both machines are connected through ethernet.
> It seems that if we use SNTP (as is required right now), we can't
> assure more accuracy then 100msec (according to that document), which
> might be probematic with our video sync requirements which are talking
> about less then 40msec.
> I appreciate any comments or ides,
> Regards,
> Ran

It seems that you will be limited by what the RTOS offers.  Your Linux 
system will usually run full NTP and be able to offer time over SNTP to 
the RTOS system.  Check whether the RTOS can run NTP, and if so, use it. 
  If not, run SNTP and try to keep the updates as frequent as possible, 
perhaps every 60 seconds. I am suggesting here that the RTOS be synced 
to the Linux system directly over the LAN.  I suspect that the CISCO 
document is referring to less direct connections when it talks of 100 ms.

If you are syncing both PCs to a third source, then that's different.

If I understand it correctly, the danger with SNTP is that it will step 
the clock rather than alter the clock's rate, and this might result in 
lost video frames.
-- 
Cheers,
David
Web: http://www.satsignal.eu



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