[ntp:questions] Polling interval in FreeBSD vs. Windows

David Woolley david at ex.djwhome.demon.invalid
Thu Jan 20 07:38:46 UTC 2011


Miroslav Lichvar wrote:
> On Tue, Jan 18, 2011 at 09:48:08PM +0000, David Woolley wrote:
>> That was my point.  Unruh's main issue is that, on modern LANs, the
>> dominant low frequency error is in the local clock, rather than the
>> measurements.
> 
> That's the theory behind NTP.
> 
I should probably have said mid frequency.  The specific problem with 
NTP that he complains about is that it sets it loop time constant too 
long (or more precisely fails to adapt it down fast enough) such that 
mid-frequency clock errors are removed from the error measurement by the 
loop filter and the time drifts badly in response to them.  This is the 
behaviour you would want in respect of mid-frequency network delay 
variations, e.g. the sort of problem I was describing where the delay 
jumps up for a couple of ours around 1pm when everyone is browsing 
during their lunch break.

(As the temperature variations (and network delay variations) often have 
squarish shapes, they actually have a broad spectrum.)




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