Re: Recommendation on NTP appliances/devices

[Julien Goodwin <nanog () studio442 com au>]

On 04/04/14 21:48, Saku Ytti wrote:
> On (2014-04-04 20:37 +1100), Julien Goodwin wrote:
>
> > > Meinberg[0] pegs rubidium at ±8ms per year, if you need NTP to do say single
> > > direction backbone SLA measurement you want to have microsecond precision.
> >
> > Those two statements don't go together.
>
> Point I was making is that free-running rubidium is not accurate enough for
> QoS measurements of IP core.

Free running oscillators are fine as long as you know what the actual
specs are (and have the unit measured to that).

> > Also outside the HFTers most of us don't care about a few milliseconds
> > (sure an extra 50ms can be a pain, but is trivial to measure).
>
> Jitter in backbone is low tens of microseconds, if you want to measure how
> that changes over time, free-running rubidium is not going to cut it.

What you'd actually measure is a side affect of buffer depth at any point.

Show my anything short of a classic SONET transmission system (or
perhaps sync-E) where you actually have something with jitter that low.

So what, that sends IP packets, are you using to *measure* it. I can
imagine using an FPGA hard clocked to a reference oscillator (and even a
TCXO is good enough) doing it, but I'm not aware of any actual
implementation of this. Again, short of the HFT world I just can't
imagine how this could actually matter.