[time-nuts] Time in a cave

Bob Camp kb8tq at n1k.org
Wed May 13 22:43:20 UTC 2015


Hi

Any CDMA device is a *really bad* idea. It’s perfectly legal to run CDMA several seconds out of sync with UTC. There are a number of networks that
do exactly this. 

PTP is fine as long as *all* your network infrastructure (switches / hubs / routers) is PTP enabled. If any of your links are not fully PTP stamping devices
you will get into time issues as your network load varies. 

Bob

> On May 13, 2015, at 5:44 PM, Tucek, Joseph <joseph.tucek at hp.com> wrote:
> 
> In response to Oz-in-DFW
> 
>> Given your description here, I'm guessing a millisecond or ten 
>> will do that as long as local cluster relative accuracy is maintained.
> 
> Spot on; I hope I'd made it clear earlier, but perhaps I've been communicating poorly.  My goals w.r.t. to sync to UTC and w.r.t. holdover are very loose.
> 
> I do need time sync intra-cluster to be tight (sub millisecond, 100 nano as a stretch goal).  UTC sync can comparatively be terrible; 10-1 ms is fine, and I can live with "bad NTP, 100 ms" if I must.  From specs, */really/* good quartz is my limit and /good/ quartz is acceptable, so long as it doesn't mess with the intra-node PTP tightness.  I'm mostly looking at TCXO options. OCXO isn't out of the question, but rubidium doesn't seem to give $/value.
> 
>> Yes, the master will have a fairly low phase noise local oscillator as
>> it's internal reference. Everything will synch to that.  If all you are
>> doing is syncing the local cluster you don't even care about time
>> outside. This is true for most industrial applications that are just
>> syncing machinery.
> 
> Thanks for the info.  PTP isn't as well understood/documented as NTP, so I've not been as certain about my decisions. Of course, that is fair for a relatively new standard.  
> 
> Currently, I think my two best options are: 1) CDMA enabled PTP appliance (set and forget), or 2) PTP appliance running as stratum 2 from good NTP.
> 
> Thanks to everybody for the feedback.
> 
> -joe
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