[ntp:questions] Query about NTP accuracy

Andy Yates andyy1234 at gmail.com
Fri May 22 23:50:08 UTC 2009


Unruh wrote:
> Andy Yates <andyy1234 at gmail.com> writes:
> 
>> Hal Murray wrote:
>>> In article <4a15e001$0$18238$da0feed9 at news.zen.co.uk>,
>>>  Andy Yates <andyy1234 at gmail.com> writes:
>>>> Does anybody have any figures that shows the effect on accuracy of an
>>>> NTP v3 client using a stratum 1 server rather than a stratum 2 or 3
>>>> server? It's all in a GE LAN based scenario, commercial stratum 1
>>>> servers connected to GPS and stratum 2 and 3 servers are typically
>>>> dedicated Linux boxes.
>>>> However I'm been pressed to supply an SLA for accuracy. My argument is
>>>> that although you can get your stratum one server to synchronize to
>>>> microseconds of UTP, as soon as the client uses NTP v3 over the LAN,
>>>> even a GE LAN, then the accuracy degrades and putting well designed well
>>>> specified stratum between the boxes is not going to decrease accuracy
>>>> sufficiently to warrant purchasing many stratum one appliances.
>>> What sort of accuracy are you interested in?  1 ms?  10 ms?  100 ms?
> 
>> Hi Hal
> 
>> Its up to us to specify what we think the SLA should be - the guide is
>> "as accurate as possible"!
> 
> That is of course completely idiotic. They do NOT want it as accurate as
> possible. That would cost them millions of dollars and is not in fact
> needed. What are they using the time for? what kind of machines are they
> ( Windows, linux, BSD, special home grown OS?)
> 
> 
>>> How stable is your temperature?  (Both the room and the CPU load.)
> 
> ntp is terrible if anything varies ( absurdly long settling times).
> 
> 
>> Temperature will be very stable, the DC is the very well specified and
>> scrupulously engineered - no cables blocking air flow etc. Generally
>> speaking the CPU is over specified.
> 
>>> What is the load on the LAN between the clients and servers?
>>> (Delay is not a problem.  Variation in delay is a problem.)
>> The NTP will be on a separate management LAN to the production traffic
>> so not subject to the variances that application load has on the network.
>>>
>>> I suggest you measure it.  Start with your current system.
>>>
>>> Setup a box as a ntpd system and tell it to use several target boxes
>>> as servers and turn on logging.  peerstats will tell you the difference
>>> between your local clock and the target system.
>>>
>> I'll look at the current NTP infrastructure however its completely
>> different to the new. The old has 3 geographically diverse GPS receivers
>> plus a GPS and radio source on the roof of the data centre and we use
>> network components to provide the intermediate strata between stratum
>> one and client - and have not really had many issues after almost 10
>> years of use. However, requirements are changing and we will probably be
>> using dedicated stratum 2/3 servers as required.
> 
> Does teh GPS have PPS ( puse per second) or are they just using the nmea
> time signal? Radio about as bad as GPS with just NMEA for timing. 
> 
> 
Hi Unruh

it uses IRIG-B but can use PPS amongst others - However I should have
said "as accurate as possible using a well designed NTP system". Our
stratum zero sources and stratum 1 sources will be v.accurate - its the
distribution via NTP that were trying to get our heads around.

Regards
Andy




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