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.