[time-nuts] Low-long-term-drift clock for board level integration?

[Bill Woodcock]

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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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