); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Thinking outside the square.... What you need is a low frequency transmitter exactly in the middle of the ring...
Neville Michie On 05/08/2007, at 5:01 AM, Jack Hudler wrote: > I was thinking of timing protocol only without any Ethernet on the > fiber > (Trying to think outside the box, which gets in more trouble than you > know!). I'm not sure why Ethernet was mentioned, maybe they wanted > to piggy > back on the existing net. That scenario sounds like IT bureaucratic > nightmare that I'd want to stay away from. > > If Pablo is going to design their own hardware, perhaps something like > 1550nm transmission on a high grade single mode fiber that should a > nice > loss budget for the splices on 27km. Then design/adapt a simple timing > protocol that would support their desires. > > Anyway it sounds like a fun project! > > Jack > -----Original Message----- > From: Magnus Danielson [mailto:[EMAIL PROTECTED] > Sent: Saturday, August 04, 2007 6:16 AM > To: [EMAIL PROTECTED] > Cc: time-nuts@febo.com > Subject: Re: [time-nuts] Timing on Ethernet > > From: "Jack Hudler" <[EMAIL PROTECTED]> > Subject: RE: [time-nuts] Timing on Ethernet > Date: Fri, 3 Aug 2007 20:57:12 -0500 > Message-ID: <[EMAIL PROTECTED]> > >> Thanks Magnus, I guess I was dating myself :). > > Jack, no worries! :-) > >> Pablo, is this going to be used for timing throughout the entire >> CERN site >> or just instrumentation on 27km LHC? >> If it's just the LHC then what about using an open fiber/copper >> ring to >> distribute timing signals from both ends. >> Then you should be able to monitor propagation delay and >> compensate at > each >> distribution node. > > He would need to have active nodes on a continous basis regardless. > The > distance you get from 10Base-T or 100Base-T is not sufficient. Each > node > would > not only need to regenerate signals put also redo the two-way- > transfer. For > a > 27 km ring you probably would like to consider fiber as a real > option unless > the spacing between points is dense enought. Multi-mode fiber would > maybe do > the trick (up to about 2 km), but single-mode would do it easilly. > GE SFP > modules is fairly cheap and should work well. If you don't stress the > distansce > too much you can't fail. Only thing to care about is not to feed it > a too > hot > signal on the receiver side when you have a small distance, but > that is only > a concern for short stretches with long-shot modules. > >> I would think timing at less than 1ns would be easily obtainable >> using a >> consistent material for transmission. > > The problem for these distances is mostly in the end terminal design. > Getting > a sufficienly high resolution on the TICs, canceling biases, avoiding > ambiguities. The underlying math is however trivial. > > What I would do is to steal some of the capacity (not bandwidth!) > and have a > repeating ping going down the line with all the necessary info and > measure > the > carrier on the preamble end of that package. In order to create the > slot for > it > a "false" collision is created in the maxmimum package time > justbefore the > time the test-packet is sent. Then the packet can be sent on an > even clock > multiple of the base clock (40 MHz or whatever). Just after the > test package > is sent normal traffic is allowed again. The Ethernet bitclock is > locked up > to > the base clock (locking a 25 MHz crystal to a 40 MHz is trivial in > this > game). > Locking of the bitclock removes the temperature-induced relative > wandering. > A > clear lock also removes the beating between the clocks. Depending > on how the > TIC interpolation is intended to be done this may or may not be a good > thing. > One very dirty method is simply to lock them slightly off so they > have a > continous beat pattern. Considering however that we normally have a > +/- 100 > ppm > control range on a +/- 50 ppm clock we can only use a maximum of > +/- 50 ppm > for > our deliberate adjustment of the mark. It is however sufficient to > keep the > integration times fairly short and to some degree act like an analog > interpolator. The interpolator design does not have to be advanced > to get > some > useable depths of resolution. > > When one builds a network, one must recall that the resolution in the > pseudo- > range measures and thus the roundtrip will translate to the one-hop > resolution > and that the multi-hop systematic error growns linearly with it. The > measurement noise will cause the traditional square-root-of-hop > increase. > If you going to do a 5 hop network with 1 ns bounds, then each node > can't > contribute with more than 200 ps of systematic error. > > Cheers, > Magnus > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.