[If GPS dies] "Internet activity would slow to a crawl, because many backbone operators rely on precise GPS time stamps to route data. " http://www.wired.com/wired/archive/11.05/start.html?pg=6 Sounds like bullshit to me, data clocks should be able to run without being fully synched externally, constantly. Maybe very occasional minor glitches at boundaries of clock domains. Any SONET Gurus wanna comment? [The article is full of hysterics like this. What it *doesn't* say is that the rockets have been used to launch OsamaSat, HusseinSat, etc., none of which exist of course, no such hardware here] --- Number theory makes my brain hurt.
On Sun, Apr 20, 2003 at 09:35:43PM -0700, Major Variola (ret) wrote:
[If GPS dies] "Internet activity would slow to a crawl, because many backbone operators rely on precise GPS time stamps to route data. " http://www.wired.com/wired/archive/11.05/start.html?pg=6
Sounds like bullshit to me, data clocks should be able to run without being fully synched externally, constantly. Maybe very occasional minor glitches at boundaries of clock domains.
Much of the current telcom plant from CDMA cellphones to many high capacity fiber muxes, switches and rings depend on GPS timing and operate with a decentralized timing hierarchy with each major node having its own primary reference locked to GPS time. And CDMA cellsites in particular cannot perform proper handoffs without GPS accuracy timing. Even such technology as low tech as police radio trunking systems and pagers now depend on GPS for accurately synchronizing multiple ("simulcast") transmitters so they don't interfere with each other. It is, however, true that most of these facilities use accurate rubidium or sometimes even cesium clocks as backup for GPS outages and even CDMA cellsites often have high precision ovenized oscillator frequency standards that will meet the timing drift specs for up to a couple of days with no valid GPS signal. And some network facilities can fall back to locking their timing to other facilities (eg the data coming in from a fiber) rather than GPS - it really depends on how much care and thought went into the system design and whether any of the backup timing modes are tested regularly. But yes, GPS time and frequency is VERY widely used, and has revolutionized the timing and synchronizing architectures of many kinds of telcom networks and systems. It ain't 1975 when all timing and frequency for the whole CONUS radiated out of a master oscillator in an underground bunker in Hillsboro Mo, and essentially everything was slaved to it by layers of PLLs and local disciplined oscillators. And having a GPS broadcast time base has made life much easier for system designers - time of day accurate to around 25 ns is now available for less than $1K anywhere you want it with very high reliability as is frequency accurate to parts in 10^13. Back in the old days these were unheard of numbers... with maybe 10 ms timing accuracy being considered good. And yes, all of this is to some degree or another - sometimes quite severely - vulnerable to a long term GPS outage of hours or days or longer. Virtually all of these GPS timing systems are designed to stand short GPS outages, as they can happen due to sky coverage, satellite failures, or routine maintenance. And yes, malicious jamming is a real threat. What else is new ? -- Dave Emery N1PRE, die@die.com DIE Consulting, Weston, Mass 02493 PGP fingerprint 1024D/8074C7AB 094B E58B 4F74 00C2 D8A6 B987 FB7D F8BA 8074 C7AB
Variola quoted and commented:
[If GPS dies] "Internet activity would slow to a crawl, because many backbone operators rely on precise GPS time stamps to route data. " http://www.wired.com/wired/archive/11.05/start.html?pg=6 Sounds like bullshit to me, data clocks should be able to run without being fully synched externally, constantly.
There are at least three interesting GPS failure modes - Whole system fails at once - Whole system becomes less accurate - Local areas get jammed The article's talking about a failure mode where the satellites gradually wear out over a couple of years and nobody replaces them. From a telco perspective, this is annoying, because GPS is cheaper and better than what we used to do before we adopted it about 5 years ago, and we've got a few hundred billion dollars less ready cash then we did then, and the people who really knew synchronization well have mostly retired or moved to dotcoms, but we could always hire enough of them back to do the job for a lot less money than it would take to redesign GPS, on the off-chance that we haven't replaced enough of the old phone infrastructure with VOIP for it to simply Not Matter by then. It's not like this is Air Traffic Control. [Summary: If you blew GPS out of the sky without warning, major telco synchronization would degrade a bit but not much, voice calls would be more likely to get noise, but not much, internet connections might have a fractional percent more TCP retransmits, and a random number of things that didn't have other timing sources would break until people fixed them. Cell phone systems are the big telecom uncertainty, and of course Air Traffic Control, nuclear missiles, and similar apps.] As Tyler said, SONET synchronization is a complex topic, and telco synchronization is more complex than that. There are two basically separate problems - timestamping / NTP Network Time Protocol, used by lots of Internet stuff, because ISPs sometimes need to know what time it is. - synchronization used in synchronous telco transmission media, which doesn't care about time of day, just about phase. ISPs care about this because they don't like lots of dropped bits, but it takes a lot of dropped bits to really bother TCP. Everybody uses GPS for everything these days, because they _can_, but high-end GPS equipment is about 5 orders of magnitude more accurate than the most demanding NTP things ISPs use it for, and cheap GPS equipment usually gets you millisecond precision, which is about 3 orders of magnitude more than most things care about, and most of them don't care very much. WWV radio clocks were Really Just Fine, and ISPs who needed better clocking could build decent NTP systems. Routers don't care much about phase, because each T1 or fiber interface usually gets timing from the transmission line independently. Voice transmission equipment cares more about phase, because it uses Time Division Multiplexing to switch the voice bits, so a whole voice switch runs on the same clock source - it it's talking to somebody else whose clock is different and drifts by more than the line buffers can fix, it drops a frame of bits or adds a frame of 0s, which will cause brief noise on voice calls or trash one or two data packets. Telcos care about this because they not only care about voice quality, they have SLAs with customers for the number of errored seconds per day/month/etc. that cost money that cost money if they don't meet them, but it takes a lot to get down to cell-phone-in-traffic quality. Telecom sync equipment is categorized in different strata, and normally a box will have a clock that accepts timing from outside and has a Stratum-N-quality holdover clock inside that's guided by the feed. There are lots of different ways to measure the accuracies of these things. A Stratum 1 clock is whatever master timing source you're using, and a Stratum 2 clock is supposed to be able to hold its own for about 5 days before the first slip and not do more than a slip every couple hours after that. A Stratum 3 clock can slip about 10/hour the first day and 132/hour free-running. SONET needs somewhere between Stratum 3 and Stratum 4 to stay connected. The best timing feeds between buildings run on copper T1s, not fiber. GPS is Stratum 1 quality, as long as you've got a good holdover clock, typically Stratum 2 quality for telco offices. A decade ago, a main AT&T, MCI, or Sprint office would have a feed from a Stratum 1 source, and a Stratum 2 clock of its own, and would feed a bunch of smaller offices which had their own clocks. MCI and Sprint tended to use Loran for some of their timing sources; AT&T used really expensive clocks in main offices, and had more offices. An important concept for AT&T is the Building Integrated Timing Source - there's a master clock for each building that feeds all the hardware. Most of the other professional telcos do something similar; I'm not sure if all the upstart newcomers do. Since then, most AT&T offices have their own GPS clock, except a few percent with Bad Radio Magic or building issues which get fed by other nearby offices. I'm not sure how local telcos get their timing - presumably a similar combination of GPS, local clocks, and feeds. They have more small offices which are closer together, but also the clocks keep getting cheaper. Cellular systems are the main telco case I don't understand; some of the newer systems care much more about timing, but the application that needs the most precision is 911 location, because that needs to triangulate between multiple offices as opposed to just handing off calls at the optimum time to avoid drops. It's possible that something could go wrong here, but I'd guess that cell towers are also more likely to be fed by copper T1s, which would give them excellent sync if they need it. (GPS is also useful for finding the precise location of a tower, which phone-locator ICBM-delivery apps care about, but you only need to do that once; the things don't move except in earthquakes.) at least with some of the newer systems, but I'd be surprised.
participants (3)
-
Bill Stewart -
Dave Emery -
Major Variola (ret)