[ntp:questions] What is the "best" synchronization possible over the network?
John Ioannidis
ntp at tla.org
Fri Feb 13 04:27:22 UTC 2009
Hal Murray wrote:
> 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.
Gene Miller suggested these devices:
http://www.endruntechnologies.com/network-time-server.htm
They are very reasonably priced: under $4K for the full-featured time
server, plus $3K for the Rb oscillator. So long as neither VZ nor S go
out of business, we're in good shape!
>
>
>
> 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...
>
Actually, 50E6 seconds is 578 days. It's not out of the question to
have a third clock always sync-ed to GPS, fed off a 48V battery, and
drive it to the data centers twice a year to slave the local oscillators.
>
> 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?
It involves problems at layer 9 :(
>
> 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.)
>
No windows, I use Solaris :) There is, however, cellphone reception in
the datacenters, so the EndRunTechnologies devices should work.
Thanks again
/ji
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