[ntp:questions] NTP no internet connection

Unruh unruh-spam at physics.ubc.ca
Sat Feb 9 21:25:50 UTC 2008


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

>flyersix wrote:
>> I have a network that I would like to cordinate the time in without
>> exposing it to the internet for NTP servers.
>> 
>> Could I just setup NTP on one of my internal servers and set the
>> clients to go to it for time syncs?  I know the time won't match the
>> internet time but my thought is if I only need to change the time on
>> the one server and then let the clients all go to it to update their
>> time.
>> 
>> Is this possible?

>Possible?  Yes.  Useful?  Maybe!

>The problem with this is that your clock is not being disciplined.  This 
>means that, in addition to not necessarily having the correct time, it 
>may not be ticking at exactly 1 second per second.  Even worse, if the 
>temperature is not controlled, the clock frequency and, therefore, the 
>tick rate, can change from hour to hour.

So what? The other clocks will simply follow that one, whatever its time.
If its clock drift rate is 300ppm there might be trouble, but in general
that is not what happens.


>Now, imagine other machines trying to synchronize with this server.  I 
>think of it as one drunk driver trying to follow another.  Nobody is 
>going to crash into anything but the various machines trying to 
>synchronize to the server will probably be all over the map.

What does "all over the map" mean. they will be withing a few tens of
microseconds of that server. 


>If  you need or want tight synchronization, get an inexpensive GPS 
>timing receiver.  A Garmin GPS18LVC can be had for less than $100 US.
>You will need a soldering iron, a five volt power supply and a suitable 
>connector (probably DB9 or DB25) to plug into a serial port.

Yes, that will give him an excellent time control But it does not seem he
wants it. 
So why should he spend $100 and a day of his time installing it. 


>If you can site the antenna where it will have a good view of the sky, 
>you should be able to synchronize your server to it with an accuracy 
>that might be as good as 50 to 100 nanoseconds.  Thus synchronized, the 

Well, no, it will be within about 1usec, not nanosec. 

>server should be stable as a table and should synnchronize your other 
>machines quite closely, say within 50 or 100 microseconds.  A lot will 
>depend on the latencies within your LAN.  Lightly loaded gigabit 
>ethernet should do very well indeed if the network is small.  Network 
>switches can introduce unwanted random latencies.  100MB Ethernet also 
>works quite well.  I don't recall ever having used NTP over 10MB 
>Ethernet but it's been years since I've seen one of those!

It works fine.





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