[ntp:questions] Re: Does NTP model clock skew?

Affan affanahmed at gmail.com
Sun Feb 13 20:05:54 UTC 2005


David Woolley wrote:

> It's origins are in control engineering, not in statistics.  Look
> in the literature for proportional integral controllers.  These (or
> the more general proportional integral differential form) are
commodity
> items in industrial process control.
>
I will try and look this up .. thanks for the pointers


> >                               made through a two way exchange
between
> > the NTP server (another question.. are the server and client
supposed
>
> The innformation flow is generally one way (peer mode complicates
this, but
> even then the primary information flow is in one direction for
extended
> periods - indefinitely if things are all working and stable).  I
suspect
> you are thinking that NTP generates a consensus time; it doesn't, it
> distributes time from a single source, UTC time.
I undestand that, but I wasnt talking about the information flow, I was
talking about the two way packet exchange through which the propagation
delay and offset are calculated... I must have not been clear enough.


>
> > to be on the same network i.e. one hop away?)- and it changes
something
> > in oscillator PLL. But it doesnt do anything like what e.g. RBS
does -
> > meaining that it does not form a line which can be used to say..
yes
> > this is a value of skew that i have (based on some line drawn to
best
> > fit the datapoints).
>
> The processing in NTP has its history in analogue hardware designs
which
> have very limited memory (however these techniques are also
intrinsically
> fast to compute as well).  There are people (e.g. Nick McClaren of
> Cambridge University, who has a statistics background) who argue that
it
> would be better to use statistical methods that look at a number of
recent
> data point each time and try to make the best estimate of the skew
and
> offset based on those (taking into account the effects of the
corrections
> already applied.  My statistics isn't all that strong, but I do have
> a gut feeling that there may be some value in that.  However, part of
> the error term is the oscillator itself, and there is a definite
limit
> of how far back you can usefully go (characterised by the Allen
Intercept
> figure).
thank you.
Affan




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