2000-03-29 16:38:44 +04:00
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<html><head><title>
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Time and Time Interval Measurement with Application to Computer and
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Network Performance Evaluation
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</title></head><body><h3>
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Time and Time Interval Measurement with Application to Computer and
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Network Performance Evaluation
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</h3><hr>
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<p>The technical memorandum: <cite>Time and Time Interval Measurement
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with Application to Computer and Network Performance Evaluation</cite><a
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href="http://www.eecis.udel.edu/~mills/database/memos/memo96a.ps">
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(PostScript) </a> describes a number of techniques for conducting
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experiments typical of computer network and transmission systems
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engineering.
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<p>In most experiments in which time is involved, it is necessary to
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develop estimates of time, frequency and measurement errors from a
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series of time measurements between the clocks of a number of computers
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and ancillary devices interconnected by some kind of computer network.
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However, time is not a physical quantity, such as mass, nor can it be
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measured relative to an absolute frame of reference, such as velocity.
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The only way to measure time in our universe is to compare the reading
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of one clock, which runs according to its own timescale, with another
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clock, which runs according to a given timescale, at some given instant
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or epoch. The errors arise from the precision of time comparisons and
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the accuracy of frequency estimates between the timescales involved.
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<p>The usual data collected during a performance run of some experiment
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might include time offsets, time delays, frequency offsets and various
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error statistics. While time offsets between two clocks can be measured
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directly, frequency offsets can be estimated only from two or more time
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offsets made over some time interval in the experiment. In practice, a
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sequence of time comparisons can be performed over the lifetime of the
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experiment and the instantaneous frequency estimated either in real time
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with a recurrence relation, or retrospectively with a polynomial fit to
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the data.
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<p>Estimating time and frequency errors in real time has been studied by
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a distinct subspecies of physicists who have made a career of the
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technology involved. Various means including autoregressive models,
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Kalman filters and simple weighted-average algorithms are used
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extensively by national standards laboratories to model cesium-clock
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ensembles. These techniques have been adapted to computer network and
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transmission engineering problems as well. This memorandum explores
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issues in performing experiments of this type and summarizes various
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techniques found useful in practice.
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2000-04-22 20:46:49 +04:00
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<hr><a href=index.htm>Home</a><address><a
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2000-03-29 16:38:44 +04:00
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href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a>
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</address></a></body></html>
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