North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: cheap GPS
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 <[email protected]>, 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 [email protected]
|