[time-nuts] can of worms: time-of-day in a community radio station

[Achim Gratz]

Fiorenzo Cattaneo writes:
> I have been quite puzzled about the asymmetric nature of my home Cable
> Modem connection to the Internet in regard with the offset discrepancy
> I observe. The "last mile" asymmetric nature of Cable Modem (Comcast
> in my case) is not very high compared the delta I see between my
> stratum-1 servers at home and the nearest public stratum-1 NTP server.
> The upload/download discrepancy would be much less than 100
> microseconds - as you point out in your calculation the worst is 33
> microseconds for an upload speed of 12 Mbps, which is a drop in the
> bucket compared to other potential sources of jitter, switch queueing
> delays or asymmetric routing. The latter is not an issue in my case.

The asymmetry from different bandwifth up-/downstream is only a small
part of the effective up-/downstream difference.  Dan Drown has measured
that:

https://blog.dan.drown.org/tag/asymmetric-latency/

I would think the jitter distributions are skewed as well, so some of
that gets finally interpreted as an offset.  If you can trust the clocks
on either end of the connection to be accurate, then you can set the
offset to zero and solve for the asymmetry instead of the usual
assumption of zero asymmetry to solve for the offset.

Plotting the offset against the RTT like Dan does is instructive, but
you won't always get such nice graphs.  I've attached a plot of how one
of my stratum-1 has seen the three PTB servers through my DSL connection
during the last three months.

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So while the connection is stable you can sort of remove the extra
offset that comes from a changing RTT, but at any one time you might
jump to one other set of correlation lines and get that "correction"
wrong.


Regards,
Achim.
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