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

Ryan Malayter malayter at gmail.com
Tue Jun 15 13:47:03 UTC 2010


On Mon, Jun 14, 2010 at 3:03 PM, 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?
>
> 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.
>
> 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?
>
> 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?
>
> Thank you very much

Yes. No. Maybe.

A lot of factors come into play, such as the characteristics of the
switches, the characteristics of the network cards, the
characteristics of the load on the network and servers, etc. There are
no guarantees with the NTP protocol.

I will report that I *typically* see offsets of <0.5 ms on a network
of ~20 newer Dell and HP servers connected to a pair of Cisco 3750s
via Gigabit Ethernet. The Stratum-1 source is CDMA (essentially GPS).
But during nightly backups or during periods of intense load, I see
excursions out to 5 ms or more from the perspective of a monitoring
system on the same network. This behavior more than meets the needs of
my applications, but might not meet the needs of yours.

--
RPM




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