[OT] Splitting PPS?

ASSI Stromeko at nexgo.de
Thu Nov 14 16:21:48 UTC 2019

Richard Laager via devel writes:
> I'm working with a small, Canadian Internet exchange that offers an NTP
> service. They originally had one CDMA and one NTP master clock NTP
> server feeding (via NTP) 3x Ubuntu + NTP Classic servers facing the
> world. This is in a data center. They have one GPS antenna mounted on
> the roof.
> CDMA service has finally be killed off in their area, so that's dead.
> Their GPS master clock died. So at this point, they're just Internet
> stratum 2 / 3.
> The tentative plan is to upgrade/reinstall Ubuntu, switch to NTPsec, and
> deploy NTS. That's all easy.
> It'd be nice if we could also replace the dead GPS master clock with a
> directly connected GPS receiver, so we get PPS into the server (e.g. via
> a serial port). That would make them stratum 1. I recently did this on
> my own with a ublox 6 evaluation kit that I picked up for cheap on eBay.
> For this project, we could buy an equivalent current generation EVK-M8T
> for $250 from e.g. Digi-Key.

If the price range you indicate is not a blocker and they want something
that just works out of the box, get a LeoNTP server and be done with
it.  Get another two if they can afford it and maybe add at least one
other GPS antenna (there must have been a feed for the CDMA I'd think?)
and then disperse any number of stratum-2 NTP serveres around the data
centre that the clients can synchronize with.

You could certainly roll your own NTP servers, maybe even more cheaply,
but you'd spend quite some work to get it all set up in a way you can
rely on in the face of various failures and you'd essentially still have
no holdover capability.

> Is there an easy way to split an RS232 level PPS (and optionally the
> GPS's TX for gpsd) into three systems?

Redundancy at the receiver level would be more useful, so a high-gain
GPS antenna and then a splitter (could still be passive if you only
split three or four ways depending on the gain of the antenna).  You
should also have a cold spare antenna that can get installed immediately
if the active on fails.

Splitting PPS gets iffy when the servers aren't very close together and
I'd opt for either converting to and from differential signaling over
ethernet cable or to/from optical fiber (which allows re-use of existing
infrastructure in the datacenter).  I#d use point-to-point wiring either
way in order to not take down all servers at the same time.

> What is the most cost-effective way to create three stratum 1 servers?

While NTP reports stratum-1 when you are synced to GPS, you should have
at least four hour holdover capability if you want to serve customers.
Splitting the GPS or PPS doesn't give that to you, and in fact any
number of servers you create off such a split are _not_ redundant in the
sense that NTP would use them (NTP by itself won't know, you have to
take care of using them correctly).

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