[ntp:questions] Loop Frequency and Offset

unruh unruh at wormhole.physics.ubc.ca
Tue Sep 27 00:29:01 UTC 2011


On 2011-09-26, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
> Hi Richard!
>
> On 26 September 2011 22:39, Richard B. Gilbert <rgilbert88 at comcast.net>wrote:
>>
>>  What is a typical offset of the loop without using special oscillators? Is
>>> less than 1 us achievable?
>>>
>>
>> I don't believe that accuracy of 1 microsecond , or less, is obtainable
>> without without installing a GPS Timing Receiver or an atomic clock of some
>> sort.
>>
>
> I have a Motorola Oncore UT+ that I'll be testing soon. I've just averaged
> the position of the antenna during 72 hours.
>
> Besides correcting the CPU clock frequency what can I do to improve the
> performance? The machines I have at the moment are showing worse than 1 us
> performance perhaps because I am not using GPS timing receivers.

Any(?) gps receiver which has PPS output does better than 1us.  Any
network does much worse than 1 us ( more like 10s of us to hundreds).

>
> Your chances of obtaining 1 microsecond accuracy using clocks on the
>> internet is just about nil!  OTOH very few people actually need
>> microsecond accuracy for anything other than "bragging rights"!
>>
>
> I know... :-) My cable connection is really bad for NTP... I can't get
> better than 3 us. That is why I have 2 stratum 1 GPS receivers onsite.

You will NOT bet 3us from any network connection. 
Are you sure you are not confusing microseconds (us) with milliseconds
(ms)?

And even with a high accuracy timing receiver you will not get better
than ab out 1us because of hardware interrupt latencies and variability,
etc (and thermal wandering of your computer clock) unless you have some
very specialised hardware. 





>
> Would you really care if you missed the first 75 microseconds of your
>> favorite TV program?  I didn't think so!
>>
>
> This is mainly a personal project and if it goes as intended it will be the
> first public stratum 1 available in my country. It is sad but even our
> National Observatory doesn't care much about time as a public service...
> Take a look at one of their stratum 2 public servers:
>
> tick# ntpq -p ntp02.oal.ul.pt
>      remote           refid      st t when poll reach   delay   offset
>  jitter
>==============================================================================
> +ntp01.oal.ul.pt .GPS.            1 u  235  512  377    5.139   -0.158
> 0.566
>  ntp03.oal.ul.pt .INIT.          16 u    - 1024    0    0.000    0.000
> 0.000
>  ntp04.oal.ul.pt 194.117.9.138    2 u  444  512  377    0.376   -0.518
> 0.040
> +ntp05.oal.ul.pt .GPS.            1 u  255  512  377    0.318    0.163
> 0.064
> *ntp06.oal.ul.pt .IRIG.           1 u  483  512  377    0.303    0.169
> 0.072
>
> 160 us offsets?? Now, take a look at my VoIP server from which all the desk
> phones in my company synchronize time:
>
> tick# ntpq -pn asterisk
>      remote           refid      st t when poll reach   delay   offset
>  jitter
>==============================================================================
> +10.0.2.10       .GPS.            1 u    7   16  377    0.191    0.001
> 0.004
> *10.0.2.9        .GPS.            1 u   16   16  377    0.187   -0.002
> 0.005
>
>:-) For me this is a good stratum 2 server :-)
>
>
>> The telephone companies, wired or wireless, rely on extremely precise
>> timing.  Radio astronomers really care about nanoseconds.  I'm sure that a
>> few others care about the nanoseconds.  Those who do need the nanoseconds
>> generally have the specialized equipment and know how required.
>
>
> And I care about microseconds just as a proof of concept, as a personal
> challenge. I've built two stratum 1 servers to be used in my company and
> saved thousands of dollars. Obviously Symmetricoms and Meinbergs have bells
> and whistles that ease administration but in the end they just provide time
> service. My two boxes do the same for a fraction of the cost.
>
> I was just wondering if there's anywhere else I should look at in order to
> improve performance.
>
> Cheers,
> Miguel




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