[ntp:questions] Isolated Network Drift Problem

Unruh unruh-spam at physics.ubc.ca
Wed Nov 26 00:43:15 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>Cal Webster wrote:
>> On Tue, 2008-11-25 at 09:43 -0500, Richard B. Gilbert wrote:
>> [...]
>>>> What's the best way to determine which of our NTP servers provides the
>>>> best local clock?
><snip>
>>> Consider that the Garmin GPS18LVC has a pulse per second output and 
>>> costs less than $100 US.  If you can site an antenna with a good view of 
>>> the sky, you can have a stratum 1 server of your very own and have the 
>>> time accurate to within a millisecond or less.  Note that while the GPS 
>>> is accurate to 50ns or better the process of getting the time into your 
>>> computer may introduce several hundred microseconds of uncertainty.
>> 
>> Interesting... We'd have to run a long cable though, the computer lab
>> has no windows and the roof is the most logical place to put an antenna.
>> I don't think USB will handle that much line loss. It's a single story
>> section of the building so we're probably talking about 100 meters or so
>> to get a view of the sky. I'll need a version that can handle the remote
>> distance. I'll request a purchase but it could be months before we see
>> it. 
>> 
>> After a bit of googling I found an excellent write-up on how to use one
>> of these for an NTP server [http://time.qnan.org/ "Using a Garmin GPS 18
>> LVC as NTP stratum-0 on Linux 2.6"]
>> 

With your 100m setup you really want a buffer amp on the line. At 100m, the
one way trip is .3ms, with reflections every .6ms which might make the
system a bit weird. To get rid of the reflections you need a 100ohm
termination if you use cat5e cable to lengthen the 5m wire on the receiver.



>Your NTP server need not live in a computer room; it can be anywhere 
>that you have a LAN connection!  A PC that has been retired from desktop 
>service can be recycled as an NTP server.  A "486/33" has more than 
>enough computing power to be an NTP server.  You'll probably have to 
>settle for a Pentium because I think the museums have cornered the 
>market in 486s! ;-)

>Some flavor of Linux would be a good choice of O/S.




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