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

[Fiorenzo Cattaneo]

Achim, thank you for your graph and for the link to Dan Drown blog and
your suggestions. I am going to collect data and replicate his
research. I doubt I'll discover anything new, but it's interesting
nevertheless to look into these time series data.


-- Fio Cattaneo

Universal AC, can Entropy be reversed? -- "THERE IS AS YET
INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."

On Mon, Oct 21, 2019 at 12:06 PM Achim Gratz <Stromeko at nexgo.de> wrote:
>
> 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.
>
>
> 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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