[time-nuts] Time in a cave

Oz-in-DFW lists at ozindfw.net
Thu May 14 07:15:20 EDT 2015


On 5/13/2015 4:44 PM, Tucek, Joseph 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. 
Sort of.  All I remember without combing through the notes were
subjective relative statements and no quantitative values.
>  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).  
There are four orders of magnitude between 1 millisecond and 100
nanoseconds - that's a heck of a stretch.  Is this what you really
mean?  What is the cluster doing that needs such good absolute time? 
did you mean 100 microseconds as a stretch (one order of magnitude.)
> UTC sync can comparatively be terrible; 10-1 ms is fine,
10 e -1 milliseconds as in 100 microseconds?  This is achievable but
will require some care.  Even 10 ms is 'pretty darn good' for all but a
very few industrial applications. Most datacenter applications I've done
only guarantee 100 ms absolute.  Internal distribution is much better of
course.
>  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.  
It doesn't until it gets really bad.  Like shattered bad. 
> I'm mostly looking at TCXO options. OCXO isn't out of the question, but rubidium doesn't seem to give $/value.
This begins to sound like you really don't know what you need and are
specing the best you can afford "to be sure." In my experience this is a
good way to get bitten, because you really are not sure.  Many
industrial applications require excellent relative accuracy within a
cluster. Time synchronizing chains of rotating machinery is surprisingly
demanding, but you almost don't care about absolute accuracy outside the
local clock domain. It's a rare application that /needs/ better than a
second absolute. Most of these are 'big science' projects or
infrastructure that covers a large geographical areas.  Time errors can
be catastrophic.  If you are working with one of the rare ones, you
really need to understand the real requirement and then design for that
with margin. If not, you should still be able to estimate the absolute
need and /then/ add margin to that. 
>> 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. 
PTP is both well understood and documented, but it sounds like it's not
for your industry and application. This tends to imply that it's not
time critical.
>  
>
> Currently, I think my two best options are: 1) CDMA enabled PTP appliance (set and forget), or 
No, for a few reasons:

    First, it's going away sooner than you think. Verizon says 2021, but
    they are doing everything they can to accelerate that.  I'll be
    surprised if it's still viable in 2018. Analog cellular was shut off
    in February of 2008, but was barely useable in metro areas several
    years before that. The operators shut off all but the absolute
    minimum capacity to save costs and provide incentive to move to
    newer technologies. Expect your cave to lose coverage much sooner
    than 2021.

    Second, while in-spec cellular is a good frequency reference, there
    is no requirement for absolute time that you have access to.  The
    time available over the air interface can be off by /minutes./ 
    Typically it's within seconds of UTC and many operators now do far
    better than that.  You are better off with WWVB or open Internet NTP
    in terms of predictable accuracy.

> 2) PTP appliance running as stratum 2 from good NTP.
Yes, or something you operate that is "network close" and has a GPS
reference.
>
> Thanks to everybody for the feedback.
>
> -joe

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