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diff --git a/contrib/ntp/html/measure.htm b/contrib/ntp/html/measure.htm new file mode 100644 index 0000000..a06261d --- /dev/null +++ b/contrib/ntp/html/measure.htm @@ -0,0 +1,50 @@ +<html><head><title> +Time and Time Interval Measurement with Application to Computer and +Network Performance Evaluation +</title></head><body><h3> +Time and Time Interval Measurement with Application to Computer and +Network Performance Evaluation +</h3><hr> + +<p>The technical memorandum: <cite>Time and Time Interval Measurement +with Application to Computer and Network Performance Evaluation</cite><a +href="http://www.eecis.udel.edu/~mills/database/memos/memo96a.ps"> +(PostScript) </a> describes a number of techniques for conducting +experiments typical of computer network and transmission systems +engineering. + +<p>In most experiments in which time is involved, it is necessary to +develop estimates of time, frequency and measurement errors from a +series of time measurements between the clocks of a number of computers +and ancillary devices interconnected by some kind of computer network. +However, time is not a physical quantity, such as mass, nor can it be +measured relative to an absolute frame of reference, such as velocity. +The only way to measure time in our universe is to compare the reading +of one clock, which runs according to its own timescale, with another +clock, which runs according to a given timescale, at some given instant +or epoch. The errors arise from the precision of time comparisons and +the accuracy of frequency estimates between the timescales involved. + +<p>The usual data collected during a performance run of some experiment +might include time offsets, time delays, frequency offsets and various +error statistics. While time offsets between two clocks can be measured +directly, frequency offsets can be estimated only from two or more time +offsets made over some time interval in the experiment. In practice, a +sequence of time comparisons can be performed over the lifetime of the +experiment and the instantaneous frequency estimated either in real time +with a recurrence relation, or retrospectively with a polynomial fit to +the data. + +<p>Estimating time and frequency errors in real time has been studied by +a distinct subspecies of physicists who have made a career of the +technology involved. Various means including autoregressive models, +Kalman filters and simple weighted-average algorithms are used +extensively by national standards laboratories to model cesium-clock +ensembles. These techniques have been adapted to computer network and +transmission engineering problems as well. This memorandum explores +issues in performing experiments of this type and summarizes various +techniques found useful in practice. + +<hr><a href=index.htm><img align=left src=pic/home.gif></a><address><a +href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a> +</address></a></body></html> |
