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On Feb 20, 2012, at 11:07 AM, Jim Lux wrote:
> One question about the 100 microsecond spec.. is that a worst case at any
> instant, or is it an average spec over a day (so a consistent diurnal
> variation would cancel out)
Well, it's not a hard limit, it's a target at which our system would be
performing at a level we'd be happy with. Obviously some measurements will be
less accurate, others more so, and we get what we get.
One of the primary reasons for doing link-by-link unidirectional delay
measurement is to estimate the length or routing of cables. Each microsecond
of inaccuracy is 650 feet of inaccuracy in the estimate. 650 feet isn't of any
interest. But twelve miles (100 microseconds of inaccuracy) is starting to get
up into the range where you might not be able to distinguish one city from
another, for instance. One millisecond is 123 miles, and we might well be into
another country or something at that point.
Measuring queue depths and estimating degrees of resource contention or
processing inside routers is a more complex topic, but similar in that it tends
to still be interesting in the 100 microsecond range, but less so at another
order of magnitude less precision.
-Bill
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