Kurt,
You raise an important point. There is no such thing as
accuracy in the
product, only a statistical estimation of the resolution,
precision and
other statistics. There is no specific accuracy statement
possible in
this thing. It's like saying, how pretty is your
girlfriend.
Dave
Kurt Roeckx wrote:
> On Sun, Feb 18, 2007 at 10:33:38PM +0000, David L.
Mills wrote:
>
>> todd,
>>
>> That's exactly what I had in mind, and that's why I
phrased the values
>> as I did in the spec. Note also jitter and wander
are formally defined
>> and so is delay, dispersion and distance.
>
>
> I've looking for which terms are defined in the draft,
and this is what
> I came up with:
> - resolution: Let p be the number of significant bits
in the second
> fraction. The clock resolution is defined 2^(-p), in
seconds.
> - precision: The clock precision is defined as the
running time to
> read the
> system clock, in seconds. Note that the precision
defined in this way can
> be larger or smaller than the resolution. The term rho,
representing the
> precision used in the protocol, is the larger of the
two.
> - offset: The offset theta represents the
maximum-likelihood time
> offset of
> the server clock relative to the system clock.
> - delay: The delay delta represents the roundtrip delay
between the client
> and server.
> - dispersion: The dispersion epsilon represents the
maximum error inherent
> in the measurement. It increases at a rate equal to the
maximum
> disciplined
> system clock frequency tolerance PHI, typically 15
PPM.
> - jitter: The jitter psi, defined as the
root-mean-square (RMS) average of
> the most recent time offset differences, represents the
nominal error in
> estimating theta.
> - wander: The RMS average of past frequency offset
differences
> represents the oscillator frequency stability or
frequency wander
> OMEGA
> - distance: The synchronization distance LAMBDA =
EPSILON + DELTA / 2
> represents the maximum error due all causes.
>
>
> So, what I couldn't find was one for accuracy. The
distance is
> probably closest to that.
>
> But I want to give an example, so I know I understand
things right. If
> my kernel returns the time in microseconds, but only
updates it say at
> 100 Hz. I think my resolution is 10 ms, but would set p
to 6, so I get
> .0156 seconds. If I can read the clock say 1000 times a
second, I think
> my precision would be 1 ms. But since I'd read the same
value 10 times,
> I'd have to set rho to the resolution.
>
>
> Kurt
>
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