Recommendation on NTP appliances/devices

Fri Apr 4 09:37:54 UTC 2014

On 04/04/14 17:29, Saku Ytti wrote:
> On (2014-04-03 21:25 -0700), Will Orton wrote:
> 
>> There are commercially available NTP servers with GPS + Rb oscillators... for NTP 
>> use you could basically let it sync up a couple days, disconnect the GPS and let 
>> it freerun. You'd still be within a millisecond of GPS even after a couple years 
>> most likely. Reconnect it to GPS for a couple days every 1-2 years to resync it. 
>> More fun and cheaper to build your own I'd bet, if you had the time.
> 
> 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.

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).

It's one thing to be a time-nut (I'm certainly one), but recognise that
straight NTP, well deployed, even syncing from the pool is sufficient
for nearly all use cases not needing sub-millisecond precision.

> I think most GPS chipsets today do Glonass also, maybe it's partly because
> Russia has import sanctions for those who don't do, or maybe simply because it
> gives better user experience as sync is found earlier.
> 
> But is there NTP product which you can configure to GPS-only mode or
> Glonass-only mode, which I think might be closer to Rob's idea of redundancy.
> As if you use both, 'poisoned' source would break all of your kit.
> But that is probably not easy to solve with two sources, if half is GPS and
> half is Glonass, and Glonass starts sending bad timing information, what do
> your NTP clients do, average to the middle?
> 
> [0] http://www.meinbergglobal.com/english/specs/gpsopt.htm
> 