Standard set of terms for precision, accuracy, related concepts.
Gary E. Miller
gem at rellim.com
Mon Apr 24 19:40:21 UTC 2017
Yo Kurt!
On Mon, 24 Apr 2017 19:30:03 +0200
Kurt Roeckx <kurt at roeckx.be> wrote:
> On Mon, Apr 24, 2017 at 10:07:47AM -0700, Gary E. Miller wrote:
> > frequency offset:
> > The difference between the ntpd calculated frequency and the
> > local system clock frequency (usually in parts per million, ppm)
> > jitter, dispersion: The short term change in a value
>
> For me jitter is about chances in the interval. Some examples which
> I think are relevant are:
> - For a PPS signal, the signal generated by the source is not
> exactly 1 second each time, but on average it's 1 second.
> - For the same PPS signal, there is a variable delay between
> the source generating the edge and the PC having seen it
> and measured the time.
> - A packet send over a network doesn't always have the same delay.
So put that in a nice two sentence explanatory paragraph. Something
that fits at the bottom of an ntpviz page, suitable for newbies.
> dispersion for me is the standard deviation
Like yes, but standard deviation is a very precise math formula.
σ = standard deviation
xi = each value of dataset
x = the arithmetic mean of the data (This symbol will be indicated
as mean from now)
N = the total number of data points
∑ (xi - mean)^2 = The sum of (xi - mean)^2 for all datapoints
Leading to:
σ = √[ ∑(x-mean)^2 / N ]
Very few sqrt() in ntpd, and none near dispersion. Comceptually they
are similar, measures of the population variance, but σ = standard deviation
is a very specific thing not used in ntpd.
σ is used in ntpviz.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com Tel:+1 541 382 8588
Veritas liberabit vos. -- Quid est veritas?
"If you can’t measure it, you can’t improve it." - Lord Kelvin
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 488 bytes
Desc: OpenPGP digital signature
URL: <https://lists.ntpsec.org/pipermail/devel/attachments/20170424/ea3f4e0a/attachment.bin>
More information about the devel
mailing list