[ntp:questions] Query about NTP accuracy

Andy Yates andyy1234 at gmail.com
Sat May 23 00:03:50 UTC 2009


Unruh wrote:
> Andy Yates <andyy1234 at gmail.com> writes:
> 
>> Unruh wrote:
>>> 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.
>>>> The reasons is that I would rather scale by adding strata - its a very
>>>> big data center with thousands of clients and has several "zones" that
>>>> are isolated. However some opinion is suggesting we run IRIG-B between
>>>> the GPS receiver and a bunch of stratum 1 servers and clients access
>>>> these directly. Much more expensive and any increase in accuracy from a
>>>> client experience may be negligible.
>>>> 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.
>>> I think you need some specifications. What accuracy do you really need?
>>> Without that you are simply wasting your, and everyones time and money.
>>>  Direct GPS connection can give you
>>> 1 usec. Lan connection to those can give you 20usec average. Then each
>>> stratum will loose about 20-50 usec (my experience). Do you need better than msec
> 
>> Hi Unruh
> 
>> Totally agree about specification - trouble is the customers include
>> third parties who don't issue specifications so we've been tasked to
>> make it v.accurate, industry leading etc. however I'm not going to
>> encourage blowing cash on diminishing returns. Thanks for the experience
>> figures, this is exactly what I need.
> 
> 
> Note also that it can depend on the network harware. When we ran 100Mb/s
> switches, the delay time was astonishingly regular and repeatable. Now
> that we have switched to Gb  switches it is not. It much worse. There
> seems to be a one way "quantization" It used to be that the round trip
> time from the machines to the ntp stratum 1 source was consistantly
> 140-160usec. Now it varies, with one level still at the 140usec, but
> other round trip times of 280 or even longer (on the same machine).
>  And they seem to be one
> way delays ( the worst kind). Ie, you really need to select the switches
> between your stratum 1 sources and your machines carefully to get the
> best behaviour. 
> 
> 
Interesting - I'll look for that during testing as we have a choice of
several switches from 2  different vendors.




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