[ntp:questions] Re: ntp & adsl

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Tue Jan 31 08:08:11 UTC 2006

Folkert van Heusden wrote:
> Hi,
> I've heard that NTP acts funny when it tries to sync over an ADSL
> line.
> That seems to be the case indeed:
> And there it is: 20ms offset. From what I've heard this is caused by
> ADSL being asymetric.
> Now I was wondering, can't this be fixed by telling the NTP daemon how
> much asynchronous the line is? I mean: it can be easily measured what
> the down- and upload bandwidth is.


Have you measured and removed the delay for the DCF77 device?  There 
receiver bandwidth will cause a delay, as will the propagation delay of 
the radio signal.

Compared to GPS, at this location servers on a 10Mb/s down / 384Kb/s up 
Internet Cable Modem connection show an offset of 2 - 3ms, and I would 
expect ADSL to be similar.  (Although the throttling technique in a CM is 
different to ADSL).

The asymmetry issue is an interesting one.  Whilst I don't see anything 
being added to NTP to cope with the problem, could one (in a test 
environment) achieve anything by measuring the delay from both ends?  This 
would give you eight timing measurements instead of four.


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