NIST NTP servers
andreas at naund.org
Tue May 10 22:46:05 UTC 2016
> Boss: That sounds expensive. How much are we talking?
> IT guy: $300
Over the past year we made engineering samples to deploy to datacenters.
The goal was to use GPS and PPS to discipline ntpd appliances and serve
as stratum 1 to other NTP distribution servers without the $5k price tag
of commercial NTP 1RU gear. We also deliberately not pursued the path of
running antenna coax from the roof to a receiver, as that is not an
option in all the datacenters where we need to deploy.
These appliances were built for lesss than $150 as
(a) Raspberry-Pi with GPS receiver board
(b) Garmin 18(x) LVC with DB-9 to an "older whitebox server"
In my experience, most locations inside datacenters where you have good
power and network connectivity have not "good enough" GPS signal reception
due to servers emitting lots of RF noise in the range 1-2 GHz on the
L-band. A brand new suite in the datacenter had OK GPS quality, but
once we added 20+ racks with 40 servers each, GPS reception was pretty
much gone. This hardware was an active antenna with less than 6 feet of
cabling routed to the top of the network cabling rack. Most smartphones
can run an app to show you the GPS signal on the phone, just walk around
your datacenter and compare the signal.
The only workable solution was to move the GPS clock to a location
where it had good GPS signal but neither redundant network nor conditioned
power (aka. "on my desk near a south facing window"). It also works pretty
well "in my garage".
In places where GPS reception is good, you can achieve 10E-06 seconds
accuracy over time even with cheap hardware. If you chose to run the DB-9
NMEA0183 and GPS as "serial port passthrough" to a VM on a Hypervisor
you can still get better than 10E-03 seconds accuracy.
Andreas Ott (Time-Nut) K6OTT +1.408.431.8727 andreas at naund.org
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