[ntp:questions] local refclock and orphan mode...
stenn at ntp.org
Wed Nov 23 22:54:56 UTC 2011
> Harlan wrote:
> > If such networks have what they think is a valid use case for
> > simultaneous use of both local refclocks and orphan mode, it would be
> > Good to hear what that case is.
> I may not be understanding the question correctly, but one
> similar-sounding real-world application comes to mind:
> There is a planned radar system where a GPS receiver distributes GPS
> System Time via IRIG-B. There are a number of Intel x86-64 servers
> running RHEL (with MRG) supporting realtime applications software. It
> is necessary that this applications code be synchronized to GPS System
> Time, so applications can stay in synch with the radar hardware command
> One way to achieve this is to provide an IRIG receiver card, the Linux
> I/O driver needed to access the IRIG card, and a compatible reference
> clock driver compiled into the NTPv4 daemon. (Symmetricom offers such a
> triplet; there may be others as well.) This allows application code to
> use ordinary kernel-provided timers to trigger software to "make the
> donuts" exactly when needed.
> An alternative approach, successfully used on prior radars, is to have
> the IRIG card (or custom equivalent hardware) generate hardware timing
> interrupts, which interrupts are turned into UNIX signals sent to the
> application code.
If there are multiple machines with these refclocks then just "mesh"
them together, and in this case I don't see why either orphan mode or
the local refclock is needed.
If I'm missing something, I can see that in the "old days" a local
refclock would have been good to keep the machines sync'd together, and
now orphan mode would do the same thing.
I do not see that in this case that *both* the local refclock driver
*and* orphan mode would be useful.
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