Would a GPS hooked to the serial port of a Linux box do the same?

Dirk

On Mon, Jun 22, 1998 at 10:43:48PM -0700, Tony Li wrote:

...

Folks who run NTP in their nets might wanna check out:

http://www.coetanian.com/tss/tss100.htm

This is a reasonably affordable, NTP-ready, GPS based stratum 1 chimer, complete with 10BaseT interface. It's also getting close to being plug-n-play.

I've been beta testing one for a couple of weeks and it seems to chime reasonably well.

Tony

p.s. I have no finanical interest in this product or this company whatsoever. I'm just a time geek...

Any clock hooked up to a NTP server will make the server a stratum 1 server. It makes no statements about accuracy, just NTP hopwise closeness to truth. To NTP stratum, there is no difference between the thing Tony mentioned and a modem calling ACTS. Your accuracy of time will be much lower in the later cases, with what you proposed usually having errors in the single digit milliseconds. (rant #2) Lots of people say 'Why should I care about accurate time?' I have used NTP synchronized clock with both tcpdump and Cisco logging to chase problems. Sometimes I'm after big things, but other times I'm trying to figure out whether two things that are happening are the same or different. Being able to have time accurate to the few packet range is a real in separating these out. It's really hard to get there when you're talking about OC48s, but even fast ethernets have packets lasting in the 10s of microseconds. The nastiest problem I ever traced this way was a ethernet card on a bsd unix box that would occasionally drop interrupts when the system was under load. Being able to have two machines looking on the wires while the problem machine was acting as a router was the only way I found it. The events rarely lasted more than a couple milliseconds, so 10+ms timing just wouldn't have been good enough. Someone might have been able to guess the problem other ways, but with this I had tools to see the problem. So usually you don't need it, and if you don't have it you won't even believe you need it, but I have used it and always want it available. Not many (maybe none) customers are going to care below the 10s on milliseconds, but as a network engineers it could save you. (end or rant) jerry

we synchronize from the NTP server attached to Stonehenge... the first silicon based clock. not very accurate, and useless when its cloudy, but great for predicting the next ice-age. sorry, I couldn't resist (to explain, for those historically challenged, I found this on altavista... http://www.dcs.ex.ac.uk/studyRes/my-town/cs93dlh/stone.htm ) Paul ---- :r~/humour/signature :wq

It is noteworthy that certain manufacturers (Rockwell and possibly Trimble come immediately mind) do not synchronize the PPS pulse to the top of the second as standardized by USNO (and indirectly BIH) but rather just give you one pulse per second starting at an arbitrary (but accurately expressed in the NMEA sentence!) point in the second.

Welcome to the nasty world of using boxes not designed for time source as a time source. The Trimble accutime units (which are designed for time synch) do the right thing, they are the only ones I've used. I have gotten a couple companies to stop advertising that they can be a time source if they don't do it right. Not all GPS receivers are time sources! It may be imperfect, but GPSWorld magazine does an annual receiver survey, and they list applications each receiver was designed for. Using anything that doesn't say it is designed for timing is risky. Once you get any new stratum 1 online, always find a nearby trusted stratum 1 and peer, just to do a couple week sanity check before offering it live. an expect script on the ntpq or xntpdc output in a cron job should catch it if it goes weird. jerry

From: Shawn David Solomon <sdsolomo@iupui.edu> I'm glad we've wandered upon the topic of stratum 1 clocks. I've been looking for a clock that has the capability to provide clocking to Cisco ATM switches (BPX's) and also provide a clock source for our unix hosts. Do they make such a beast? is it a good idea to use the clock for both purposes? Actually, you're looking for two different things here: 1) a frequency standard that is traceable to a cesium or rubidium frequency source, with an output of 1 MHz, 10 MHz, 100 MHz, or whatever the PLL in the Cisco wants to see to discipline the line clocking, and 2) a time standard (synchronized to USNO) with a 1PPS output that is synchronized to the top of the second. My question would be, what is the goal here? All of the integrated plug-n-play NTP servers represent a tradeoff of money for time, but that may well represent a wise choice if you don't have someone on your staff who has built a stratum 1 NTP server before. If there is some reason to use the same clock for both? If not, there is a pretty good chance that you can set up two separate units, a GPS-based unit for the computers and a Cs or Rb standard for the ATM switch for less money than you would pay for a brand new integrated unit. This is particularly true if you're handy with surplus shops -- I have seen Rb standards going for as low as $800 (working, with plenty of life left in the tube), and if you have plenty of time and access to an already-calibrated frequency standard, even the calibration is well within the capabilities of any reasonably advanced technical generalist. If you want to talk more about this, let's take it offline before someone sends me email telling me that NTP is not an operational issue (bah!). :) ---rob PS: links o' the day (bouquet of options from ready-made to roll-your-own): http://www.truetime.com/DOCS/request.html http://www.tmo.hp.com/tmo/Summaries/English/#Frequency_Time_Standards_and_Sy... http://www.trimble.com/oem/om_timng.htm http://www.mot.com/AECS/PNSB/products/pnsbprod.html http://www.tapr.org/tapr/html/tac2.html

Some of you may be interested in this website about submicrosecond UTC on a FreeBSD machine http://phk.freebsd.dk/rover.html -- Michael Dillon - Internet & ISP Consulting Memra Communications Inc. - E-mail: michael@memra.com Check the website for my Internet World articles - http://www.memra.com

I looked at Stratum 1 clocks at SuperCom (I'm a time geek too, but it's hard to get sub-microsecond accuracy from a stem-wind chronometer...) TrueTime has some damn nice clocks that you can use to time standard telco and video stuff, but which also can have NTP capable Ethernet interfaces. GPS based, but you can add cesium oscillators, if you go for that sort of thing.... As I recall, prices range form about $5k to about $20k, depending. No affiliation at all with these guys, I just happened to have their catalog sitting on my desk. Stan