On Mon, Feb 20, 2012 at 12:15 AM, mike cook <[email protected]> wrote: > Le 20/02/2012 07:18, Bill Woodcock a écrit : >> >> Murphy says we won't. Bell curve, again. A very few will have good >> symmetric paths to Stratum-1 servers, most will have mediocre asymmetric >> paths, and some will have nothing usable at all. >> >>> Are you targeting homes, offices, or machine rooms? >> >> The vast majority will be office buildings. A few datacenters. Probably >> only a handful of homes. >> > With the exception of the mobile units, the units' position can be > determined with google maps or local survey data prior to installation so if > you are not relying on the GPS receiver for time sync, why not forget it, > set the data prior to shipment and invest the saved cost on a better > oscillator.
I've been thinking that all along. The GPS receiver is not going to be useful at all for timing. Even for the first setup, even if GPS was able to get you "perfect" time as soon as you connect top NTP that perfect time will be wiped out with NTP's idea of the time. So it's pointless. Now that I've read you are building hundreds of these then I'd say there is ABSOLUTLY ZERO chance of ntp working at the uSec level. Yes it might work now and then but NOT all 100+ of your systems. For that to happen would would require a statistical miracle right up there with buying a dozen winning lottery tickets in one day. It might happen. But getting 100+ NTP installations to work that well is near impossible. You WILL need GPS and even then getting to 100% is hard. Some sites simply will not have a good view of the sky or if in a high rise building running antenna lead down 25 floors through conduit would cost to much. The ONLY way to get 100% success rate with hundreds of installations is to have multiple options. "coockie cutter" or turn-key just can't work. MOST places will not have a good enough network connection for uSec level times. Some data centers will. Small offices and homes will not. Most places will have a way to set up GPS. Then in the remaining places you can use cell phone based radio clocks. They cost more then GPS and have 100 times worse performance but they might be the only option in most cases. You said the person installing this will be rather clueless. The "fix" for that is good customer tech support. You can walk them through the decision process about what kind of clock is best for their site then help them set it up and test it. Testing is key. Have a few fall backs if the tests fail. Bottom line is that open loop timing at the level of 10 uSec per year costs well into 7 digit figures (Cesium clocks) so you need some connection to the outside world. So must choose a connection type. The available options are 1) NTP. Attractive because it is free but in most cases it will not meet your accuracy requirements, only in places with "good" networks like co-location facilities and some large data centers or if you are lucky and the customer already base GPS connected NTP servers on his local network (many places do have this. It is common with larger companies) 2) GPS. Works perfectly, not expensive but it does require an antenna that can see much of the sky. You may need a long antenna lead cable and running long wires in a commercial building can be expensive. 3) CDMA reference clocks. These can work at the uSecond level any place there is decent CDMA cell phone coverage. The clock and its antenna can be located indoors. Here is an example of one: http://www.endruntechnologies.com/pdf/UnisonCDMA.pdf 4) There may be other ways to connect to the outside, phone modems, WWB broadcasts. Loran is still running in some parts of the world. Your customer may already have a precision timing system of some kind and you can use that. (We use IRIG in our lab) But you do this case by case. If this were me. I'd have GPS as the default option but then tel the user he does not need it if a GPS connected NTP server is already installed at his facility, then he could just use NTP. Then use CDMA as a fall back if those fail. Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
