Subject: [ippm] Calibration error calculation for rtt metric

From: Fabien Michaut <[email-masked]>
Date: Mon, 07 Apr 2003 17:08:07 +0200

I have a question concerning the calculation of the calibration error in 
RFC 2681 related to roundtrip delay metric.

The proposed calibration scheme is the following :

e = calibration error such that the reported value is in the range (true 
value -e) to (true value +e) at least 95 percent of the time.
and e = systematic error + random error

Here the error can be bounded by determining all the individual 
uncertainties : 2*Rsource + Hinitial + Hfinal + Hrefl.

Rsource is the clock-related uncertainties,
Hinitial and Hfinal are the host-related uncertainties,
Hrefl is the "Dst producing a response" uncertainties.

The proposed calibration consists in :
1. Using a known, isolated LAN in a lab,
2. Sending small test packets

In this case, we can approximate the roundtrip time to be zero. Then the 
measured delay contains only systematic and random error to a 95% confidence.
One way to compute the systematic error and the random error is to repeat 
the experiment many times. The systematic error would then be the median.
The 95% confidence interval would be the range from the 2.5th percentile to 
the 97.5th percentile of these deviations from the true value.  The 
calibration error "e" could then be taken to be the largest absolute value 
of these two numbers, plus the clock-related uncertainty.

So the question is : Why do we have to add the clock-related uncertainty. 
Isn't it already included in the measured delay?

Thanks,

Fabien

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