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

Richard B. Gilbert rgilbert88 at comcast.net
Thu Feb 12 22:21:40 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.
> 

A CDMA cell phone base station broadcasts a "reference signal" that 
supplies the time.  You cam buy a receiver for this signal which will 
supply you with time derived from GPS.

Google for CDMA time server.

<snip>




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