[ntp:questions] NTP server with no network, no GPS signal

DaveB g8kbv at uko2.co.uk
Tue May 3 11:08:26 UTC 2011


In article <BANLkTi=jOa7WOdDbstB3Z4p+RP_ee7q5wA at mail.gmail.com>, 
albertson.chris at gmail.com says...
> 
> On Fri, Apr 29, 2011 at 4:10 AM, Cristian Seres <cristian-2 at contrasec.fi> wrote:
> > Hi!
> >
> > I am looking for a solution to install an NTP server in a very restricted
> > network which can not be connected to other networks even through a firewall
> > because of strict security policy. RS232 and usb are ok. Using GPS or DCF77
> > reference clocks would require a long antenna wire to get signal. Installing
> > the antenna cable would be expensive, but there is good LAN cabling between
> > a space with good GPS signal and the room with high security.
> 
> This is easy.  Install the GPS but use a very short antenna cable.
> Then the output of a GPS is in two parts (1) A serial data stream and
> (2) a pulse per second.   Both of these can go over the LAN cable.
> 
> You never want a long antenna cable because of the losses in a long
> run.  It is best to keep that cable short and send the data from the
> PS back over a digital cable

Using some RS232<>RS422/485 converters, then you can use as long as 
needed (within reason) UTP cable.   (Especially for the PPS signal.)

Ready made converter assemblies are available, as are the chips to make 
your own.  The extra delay is minimal, and will be stable in any case, 
as such converters are more or less analogue devices internally.

Use either PoE or local power for the GPS device itself (and serial 
adapters.)  Do not rely on feeding plain 12/5 or 3V DC up a "spare 
pair" in the UTP cable, the resistive losses can make that 
problematical.  PoE handles it much better if a local (to the GPS) power 
source can't be provided.

At the "secure" end, you will need more serial converters to get back to 
RS232, (& PoE injector if used) and run the NTP daemon on the secure PC, 
or use a separate timekeeping PC, on the "secure" LAN.   Depending on 
your detailed security needs.

Just my take on it, but I know nothing, even having done just this in 
the past for my own needs, so don't believe anything I say...

I built my serial adapters from parts I had, and found suitable PoE kit 
on a well known web based auction site.  12V in 12V out (with up to 0.7A 
available at the remote end, more than enough!) good regulation, with 
48V on the wire.   Lynksys kit I think it was.

Remember though, that anything like this in a commercial environment, 
could violate any local electrical safety rules, as it is not 
galvanicaly isolated, unlike normal LAN cable endpoints.  Plus, even if 
it was, a lightning hit sort of negates any of that...

Regards.

DaveB.




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