[ntp:questions] Accuracy of NTP - Advice Needed

Chris Albertson albertson.chris at gmail.com
Fri Dec 23 18:58:52 UTC 2011


On Fri, Dec 23, 2011 at 2:09 AM, Terje Mathisen <"terje.mathisen at
tmsw.no"@ntp.org> wrote:

> The best (and probably only possible) solution that does give you
> single-digit us is to route a PPS signal to each and every server, then use
> the network for approximate (~100 us) timing, with the PPS doing the last
> two orders of magnitude.

I think the above is true.  Even then getting NTP to track the PPS at
the uS level is not "plug and play".  But there is no hop of doing uS
level over the gigabit either net.

Distributinng PPS is no simple task either.  I'm experiment here with
that.   Im using the the "extra" wire pairs in the install cat-5 wire
that s already in the walls.   Of course PPS can't go through a router
so you have to physically connect a twisted pair  from the source (In
my case this is a GPS) to every computer.   This is very hard to do in
a big campus, impossible really.  So PPS only works within a building
and only over about a "few" hundred meters of cable.

I'm using a simple RS232 driver chip from Max as the transmuter.
These are driven  by a simple "hex inverter" TTL chip.  The Max driver
converts the signal to -9 and +9 volts.   There is delay in this.
Some in the driver chips and some in the cable.  Cable is roughly
about nanosecond per foot.  You need to measure and remove this and
it's different for every cable run.

But most people with more than a few computers have wire closets and
plenty of unused cat-5 wire pairs.  This is a very cheap and good way
to distribute PPS in a building.  If you have two buildings buy a GPS
for each building.


Chris Albertson
Redondo Beach, California


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