North American Network Operators Group

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Re: cheap GPS

  • From: Michael Heller
  • Date: Fri Aug 20 15:37:05 1999

At my last job we had a Telecom Solutions setup.  It consisted of a
redundant receiver that spoke to the satelites and thus the cesium clocks
and a redundant set of rhubidium clocks.  If I recall correctly the
rhubidium clocks were supplying the T1 interfaces with a clock source and
they would adjust themselves off of the reference cesium clocks.  According
to the manufacturer, the whole thing could remain autonomous for a month if
the cesium reference was unavailable. Very nice equipment.

At 11:11 AM 8/20/99 -0700, Jerry Scharf wrote:
>
>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@puck.nether.net>, 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.)
>
>jerry
>
>
>
>
Michael Heller
Sr. Systems Engineer
Earthweb, Inc.
212.448.4175
mikeh@earthweb.com