NIST NTP servers

Eric Kuhnke eric.kuhnke at
Wed May 11 20:15:48 UTC 2016

Cellular carriers also use GPS timing for many reasons that are not readily
apparent at the layer 3 router/IP/BGP network level. One big need is RF
related, back-to-back sector antenna frequency re-use with GPS synced
timing on the remote radio heads, such as an ABAB configuration on a tower
or rooftop site.

The same with some much less costly near consumer grade WISP radio
platforms and PTP radio systems nowadays.

In such a configuration the GPS timing signal from the local GPS receiver
(small cone shaped or puck antennas at the site) is actually the primary,
and whatever NTP-based GPS signal the network node might have access to is

On Wed, May 11, 2016 at 12:10 PM, Mel Beckman <mel at> wrote:

> No, many cell carriers run their own completely independent timing
> networks. I support some head-ends where they have rubidium clocks and a
> T1-delivered time source. They do reference GPS, and many cell sites have
> GPS as a backup clock (you can see their conical antennas on the very top
> of the tower). But most cellular providers are very protective of their
> time sources. They’re also typically clocking SONET networks too, which
> requires BITS.
>  -mel
> JAshworth said:
> > CDMA and GSM are false diversity: both network types nodes *get their
> time*
> > from GPS, so far as I know.
> > On May 11, 2016, at 10:54 AM, Valdis.Kletnieks at wrote:
> >
> > On Wed, 11 May 2016 15:36:34 -0000, "Jay R. Ashworth" said:
> >
> >> CDMA and GSM are false diversity: both network types nodes *get their
> time*
> >> from GPS, so far as I know.
> >
> > I'll make the fairly reasonable assumption that most readers of this
> list have
> > networks that span multiple buildings.
> >
> > If somebody is managing to figure out that you have a GPS in Building
> 37, and a
> > GPS-based CDMA up on the corner of Building 3, and the *other* 4 clocks
> at
> > other locations and getting close enough to all of them at the same time
> to
> > conduct a successful spoofing attack, all just to move your time source a
> > few seconds off....
> >
> > ...  then the fact that GPS is spoofable is probably *NOT* your biggest
> > security problem.
> >

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