51 lines
2.7 KiB
HTML
51 lines
2.7 KiB
HTML
<!-- $NetBSD: measure.html,v 1.1 1998/12/30 20:20:35 mcr Exp $ -->
|
|
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML Strict//EN">
|
|
<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="ftp://ftp.udel.edu/pub/people/mills/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><address>David L. Mills (mills@udel.edu)</address></body></html>
|