> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On
> Behalf Of unruh
> Sent: Sunday, December 25, 2011 4:46 PM
> To: [email protected]
> Subject: Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency
> steps?
>
> On 2011-12-25, Charles Elliott < <mailto:[email protected]>
[email protected]> wrote:
> >> You might look at the peerstats file and also look at the
> "roundtrip"
> >> time. I might be that occasionally one of the paths from wireless to
> >> computer gets shorter ( clearer signal?) and ntpd will then take
> that
> >> as
> >> a good value, and an earlier value, and try to correct for that
> offset-
> >> -
> >> which it does by stepping the frequency.
> >
> >
> > This comment raises an interesting issue. There is a large,
> significant,
> > and negative
> > correlation between "Delay" and "Offset." The larger the delay, the
> more
> > toward
> > minus infinity the offset tends. Recall that in the regression
> equation
>
> That says that the noise is occuring in one of the paths, rather than
> the other.
>
> > Y = BX + A, B is the correlation between the variables X and Y. So
> if the
> > correlation is significant, this implies that there is a relation
> between
> > them.
> > I can't think of a physical relation between delay and offset, so if
> NTP
> > finds
> > a relation, there has to be something wrong.
>
> ntp assumes that the outgoing and incoming trips are the same time. If
> not, you get an offset. Thus if the return trip takes 10 min and the
> outgoing 1ms, ntp is going to say that the remote clock is five minutes
> further behind than it is. On a wireless I could easily imagine that
> the
> trip times are not the same, and that one way of the other is
> preferentially got a higher jitter than the other.
That's true. But that the offset becomes more negative as the delay
increases implies that the response trip always takes longer than the
request trip. Isn't that somewhat unlikely? Why would not the request and
response times be equal on average? That is, on average why don't the path
asymmetries cancel each other out?
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