[ntp:questions] NTP on embedded Linux with GPRS connection

unruh unruh at invalid.ca
Thu Nov 24 13:39:42 UTC 2011

On 2011-11-24, David Lord <snews at lordynet.org> wrote:
> mass85 at tlen.pl wrote:
>>> I've used gprs with usb dongle and on either Ubuntu or NetBSD.
>>> The main problem was the gprs being very slow to respond until
>>> it had adjusted to any significant level of traffic. I'm not
>>> sure if Ubuntu uses ntpdate at startup or if it's redundant
>>> with NetBSD but either way it wasn't possible to get a reliable
>>> timesetting at startup until I used 'burst'. It might be that
>>> iburst would have been sufficient by then but I had setup a pool
>>> server so using that option was ok for me and I had also set a
>>> long default poll interval.
>> Ntpdate works pretty fast, I can get time set in 1 or 2 seconds. The
>> problem is that connection setup can last long or even fail.
>>> At startup rtt was sometimes around
>>> 10 seconds and when connection was fully established the lowest
>>> rtt was above 150ms. I would not rely on any timings being
>>> useful for adjustment of hwclock.
> In my case with rtt of 10s the difference between the paths
> could result in the hw clock being wrongly set.
> You seem to have a different problem.
>> Why not? My NTP can synchronise in 15minutes and then keep control
>> over system clock. I assume that when it synchronises time and adjusts
>> system clock, it knows what it's doing and it is doing it quite
>> accurately. RTC can be at least 1s off every 24h, so it is quite much.
>> Is there any other ready to use way than "hwclock --adjust" to
>> calibrate RTC? Could NTP do it?
> That is what the drift file is for after you have an initial
> calibration. You should not need to continuously recalibrate.

Which drift file? Not ntpd's. That is the drift of the system clock, not
the rtc. 

> David
>> Marcin Adamski

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