[ntp:questions] What is the "best" synchronization possible over the network?

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Thu Feb 12 21:42:27 UTC 2009


In article <49920E17.6050007 at tla.org>,
 ntp at tla.org (John Ioannidis) writes:
>The problem setup: two locations, both within the United States, neither 
>has roof access so no GPS reception is possible.  How do you synchronize 
>them with better than 50-microsecond accuracy?  Straight NTP over the 
>Internet doesn't do the trick.  They don't need to actually be 
>synchronized to "real" time, they only need to be synchronized to each 
>other.  Assume reasonably unlimited resources (running a private fiber 
>plant across the continent *is* unreasonable).
>
>I've looked into slaving something off a voice-carrier time base, but 
>that's not accurate enough.  Maybe something over raw SONET would do the 
>trick?

SONET gets you frequency, not time.  The tick marks (frames) are not
synchronized across various SONET links.  Even the frequency isn't
locked if the two ends come from different phone companies.

I think you have several basic approaches.

If cell phones work where you need the time, that may be an alternate
to GPS.  I don't know what flavor of cell service they use.



One approach is to get good clocks at both ends and let them coast.
Atomic clocks are $50K to $100K ballpark.  They have long term
accuracy of 1E12.  So you get to coast for roughly 50*1E12 microseconds
or 50E6 seconds.  That's 58 days, so it will be a lot of work to keep
them synchronized.  I haven't worried about factors of two.  You might
get better than the spec accuracy if your air conditioning is good...


Another approach would be to build an extension cord to a place
where you can get an antenna on the roof, setup a GPS unit there,
and send the PPS signal to where you need it.  This may require more
research/testing than you want to do.

Is roof access really impossible?  How much would it cost?

Do you have any windows?  Have you tried a GPS receiver there?
With a good timing receiver, you only need 1 satellite.  (You need
several for setup, only one to keep going.)

-- 
These are my opinions, not necessarily my employer's.  I hate spam.




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