cheap GPS

Jerry Scharf scharf at
Fri Aug 20 18:11:57 UTC 1999

On Fri, 20 Aug 1999, Jeremy Porter wrote:

> Most telcos do use use GPS for timing, however they also use
> Cesium standards for backup also.  Don't forget CDMA systems use
> GPS/Cesium for clocking also, and CDMA won't work at all without
> accurate clocking.  There are only about 2 companies in the world
> making precision time/frequencey references for high speed telecommunications
> networks.  The NTP term "stratum" is derived from the old
> primary refence clocks used by the Bell network to provide
> timing.
> The little birdies tell me that some of this equipment failed its
> week rollover test the first time, but patches were quickly made.
> Hopefully all the telco's using the equipment read the engineering
> field notices....
> In message <19990820125326.F24455 at>, Jared Mauch writes:
> >
> >	I'm interested in knowing if there are any telcos that
> >are using a GPS for their ckt timing, and this will cause that timing to
> >break, and those of us that take "clock source line" from M13's, etc..
> >will have problems with our channelized ckts (dial, ct3, etc..?)
> >
> >	Anyone here privy to that type of information, and can
> >you comment?
> >
> >	- jared
> >

There are two types of timing that are used by telco networks, relative
timing for signal recovery and absolute timing for timestamps, billing and
the like. The one most people care about is the signal recovery timing.
There has been a major move from the classic Bell time distribution system
to independent GPS based timing sources, it's a whole load cheaper and
as/more reliable. The idea behind these systems are that you have a stable
oscillator (rhubidium or cesium (usually two of them in an oven) and
something that figures out how far off of the desired frequency the
oscillator is. The GPS is used as the "discipline" source since it's
accuracy doesn't drop over time like the oscillator. The same basics are
true for both land based and mobile base station clocks (as well as TV
networks ...). Depending on the specifics of the failure mode for the GPS
side, the clock could either fail completely, or loose the discipline and
slowly drift off exact time. 

The reality is that no one selling these systems (there are about a half
a dozen now) hasn't done full testing against a simulator and gotten
patches out to all their customers in case of problems. Also, since the
problem has been well known in the GPS community for many years, these
kinds of problems shouldn't be a surprise this week. Probably Y2K has
helped the customers take this more seriously. I'm not too worried about
my circuits still working after the rollover.

I think absolute time for billing is a whole different can of worms, and
not subject to good generalizations. (If someone couldn't bill me for a
while, I'd just have to live with it.)


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