[ntp:questions] how to have offset < 1ms

nemo_outis abc at xyz.com
Thu Apr 15 08:09:56 UTC 2010

unruh <unruh at wormhole.physics.ubc.ca> wrote in
news:slrnhsd78b.gnt.unruh at wormhole.physics.ubc.ca: 

> On 2010-04-15, nemo_outis <abc at xyz.com> wrote:
>> John Hasler <jhasler at newsguy.com> wrote in 
>> news:874ojdmx12.fsf at thumper.dhh.gt.org:
>>> Richard B. Gilbert writes:
>>>> One possible solution is using radio controlled clocks.  I have a
>>>> wristwatch that uses a radio signal to correct itself.  I also have
>>>> a wall clock that does the same.  Both work very well.
>>> They won't work very well inside a steel ship or far out to sea. 
>>> They use VLF (60kHz/77kHz) signals that only propagate a thousand
>>> miles or so and not at all through steel.
>>> In any case it is clear that the ACEB box can provide the
>>> performance he needs if he can only communicate with it.  He has a
>>> networking problem, not a timekeeping problem.
>> Spot on!
>> Regards,
>> PS  Which means we can now dispense with much of the speculation
>> regarding GPS and its putative implementation, howsoever configured.
> Not clear what you thought was spot on. Radio control is not GPS. It
> is true that the equipment ( at a cost of maybe a 1000 times that of
> GPS) can keep time, but so far the evidence is that it is useless as
> it cannot actually report that time at all accurately. Whether it is
> network, or the actual ntp card in the box we do not know ( the pings
> indicate it is not a network problem) Again, putting a laptop on the
> same network as the box, and doing a tcpdump on the ntp packets could
> tell you if the delay is in the network or in the box. (If t3-t2 is
> 29ms, it is in the box. If t3-t2 is 1usec, it is in the network)

IMHO (although I'm not all that H :-) what was spot on was the 
observation that the OP had a networking problem, not a timekeeping 

If the ACEB box is performing to spec the OP has no need for GPS or radio 
as a timekeeping source (although those might well improve his 
timekeeping, I provisionally accept his lax criterion that the single-
source ACEB box is "good enough" for his purposes).  Accordingly, his 
core problem is distributing that ACEB signal (or one derived from it) - 
a network problem.

Now, as you discuss above, resolving that network problem may require 
some detective work to discern whether the network problem originates in 
the ACEB box's NIC, the switch, or elsewhere.  Those investigations and 
their outcome will depend on the available tools and other resources, not 
least of which is the OP's insight (or lack thereof).   If the trouble is 
found, workarounds may be possible (e.g., replace the NIC, bypass the 
switch with a direct connection, etc.).


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