Eric S. Raymond writes: > Achim, you and Gary *both* get to write glossaries covering terms like > precision, accuracy, drift, and related stuff. Give it your best > shot(s). If, after a reasonable period of time, I have a glossary > only from one of you, tha person wins and the glossary gets blessed > and added to the official documentation.
That is a bit of a can of worms as you have already seen, both in the original thread and the answers here. Just the two terms you mention are used in different ways for different things and so far we haven't even determined whether we can or want to unify on one set of definitions throughout all of NTP or maybe keep domain-specific meanings where appropriate. Another line along which these terms split is whether they are applied to continous or discrete quantities, whether you are talking about a single number or a statistical moment and whether the thing you are talking about is a stochastic variable, a physical quantity or something you do a calculation with. For NTP, I'd think the only physical continous quantity of interest is time, but it only ever gets processed as a quantized numerical value. NTP als cannot directly measure the time, instead it approximates it by various means, in particular in the form of time differences. That approximation requires both statistical inference and numerical calculations. So there are at least three domains that need a glossary: representation of absolute and delta time (both conceptual and in implementation), the statistical inference and the implementation of these operations as numerical algorithms. Many of the terms used for ntpd are actually referring to internal algorithmic variables of the control loop rather than an estimate of some measureable quantity. The frequency offset for instance does describe the deviation of the system clock from the ideal frequency only when both the derivative of the frequency offset and the time offset are both zero. In all other cases it's a mixture of the (not measured) offset of the clock frequency and an additional offset introduced by the FLL/PLL that tries to keep the time offset as close to zero as possible. Similarly the time offset isn't the actual offset, but a measurement corrupted by multiple noise sources. Regards, Achim. -- +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+ DIY Stuff: http://Synth.Stromeko.net/DIY.html _______________________________________________ devel mailing list [email protected] http://lists.ntpsec.org/mailman/listinfo/devel
