[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

David J Taylor david-taylor at blueyonder.co.uk.invalid
Wed Mar 14 20:00:41 UTC 2012

> Hi David T,
> NOW .... you understand.
> I have 4 PC's connected to the LAN, plus my wife's work computer 3 days 
> / week, and on rare occasions my son's computer.  All are connected by 
> wifi.  All do pretty mundane things: web browser, email, sometimes 
> downloading patches, sometimes doing online backup.  My 4 are running 
> NTPD.  3 run windows most of the time and dual boot into linux.  My 4th 
> machine runs linux all the time.  When my wife is here, she does a 
> remote desktop type of thing into her work system.  Two of my PC's are 
> running Vista, one is running Windows 7.  Those three dual boot into 
> Ubuntu 11.04 and the always linux machine runs Ubuntu 11.04.  Almost all 
> the discussions I've had on this mailing list are for my Windows 7 
> machine.
> The path out of my house is:
> PC Wifi --> Wifi router --> wired router --> cable modem --> ISP --  
> internet
>> - is this PC connected over wireless or wired?
> Wifi G
>> - what else is going through the router?  Someone else downloading 
>> large files or using streaming audio or video?
> See above.  Normally, no huge data hogs.
>> - who else might be sharing your connection?
> It's cable.  Who knows?  I also get cable TV and telephone through the 
> same wire.
>> - what type of service do you have?  Presumably not dial-up!  But what 
>> speed?
> Comcast Cable
> Just tested it with speedtest.net
>      Ping to near city: 91 ms, Download: 29.63 Mbps, Upload: 5.3 Mbps
>> - having checked you speed, would you describe your connection as 
>> "stable"?
> See above for speed.  Generally, it's very stable.  However, for the 
> purposes we're discussing, I think it's latencies and delays that are 
> the problem.
> As I mentioned in my reply to David L, I'm not concerned over trying to 
> get stellar performance from internet servers.  I just want to get a 
> good GPS server system running and use the internet servers as a backup.
> Sincerely,
> Ron


Thanks for that clarification.  I think you should be getting /very much/ 
better performance from your Internet servers.  That ping is poor as well. 
Here I have 30 Mb/s down, just 1 Mb/s up, and NTP delays show as 18-34 ms 
(most in 22-30 ms).

To that end, if your cable modem has multiple ports, connect one of the 
PCs direct to the CM and run it as your local NTP server.  Wi-Fi doesn't 
help NTP.  Later, you can add GPS/PPS to that PC as well.  At the very 
least, connect your timekeeping PC direct to the wired router.  No Wi-Fi! 
For my main NTP server, I got a low-powered and fan-less Intel Atom 
system, and it runs FreeBSD.  It /only/ runs NTP, no interactive stuff at 

You might also consider getting rid of the two routers and just using the 
wireless one.  You might also see whether you can run NTP on your router - 
perhaps it's a model which can run the DD-WRT firmware.  I think some 
variants of DD-WRT can run NTP, but please check.  I have a WRT 54GL in my 
system where I run the DD-WRT firmware.


Online backup could well affect the performance of the connection for 
timekeeping, and your imminent Sure GPS/PPS will help enormously.

Just some thoughts which may help you along the way.


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