David L. Mills wrote: > Richard, > > There were several different architecture computers considered in the > 1995 and 1998 studies, incluing SPARC, Alpha, Intel and several lab > instruments. All oscillators conformed to a simple model: white phase > noise (slope -1) below the intercept, random-walk frequency noise (slope > +0.5) above the intercept. This is equivalent to your model. > > Additional data are in the nanokernel documentation. The only > differences are in the (x, y) intercept. You don't need das Buch to > justify this model; there is evidence all over the place. Clocks of all > kinds from cold rocks to Cesium oscillators all show very similar > chacteristics, whether modelled in the time domain or frequency domain. > > It's easy to make your own Allan characteristic. Just let the computer > clock free-run for a couple of weeks and record the offset relative to a > known and stable standard, preferable at the smallest poll interval you > can. The PPS from a GPS receiver is an ideal source, but you have to > jerry-rig a means to capture each transition. > > Compute the RMS frequency differences, decimate and repeat. Don't take > the following seriously, I lifted it without considering context, but > that's the general idea. Be very careful about missing data, etc., as > that creates spectral lines that mess up the plot. > > p = w; r = diff(x); q = y; i = 1; d = 1; > while (length(q) >= 10) > u = diff(p) / d; > x2(i) = sqrt(mean(u .* u) / 2); > u = diff(r) / d; > x1(i) = sqrt(mean(u .* u) / 2); > u = diff(q); > y1(i) = sqrt(mean(u .* u) / 2); > p = p(1:2:length(p)); > r = r(1:2:length(r)); > q = q(1:2:length(q)); > m1(i) = d; i = i + 1; d = d * 2; > end > loglog(m1, x2 * 1e6, m1, x1 * 1e6, m1, y1 * 1e6, m1, (x1 + y1) * 1e6) > axis([1 1e5 1e-4 100]); > xlabel('Time Interval (s)'); > ylabel('Allan Deviation (PPM)'); > print -dtiff allan > > Dave > > Richard B. Gilbert wrote: > >> Unruh wrote: >> >>> "David L. Mills" <[EMAIL PROTECTED]> writes: >>> >>> >>>> David, >>> >>> >>> >>> >>>> 1. I have explained in very gory detail in many places how the time >>>> constant is chosen for the best accuracy using typical computer >>>> oscillators and network paths. See the briefings on the NTP project >>>> page and especially the discussion about the Allan intercept. If you >>>> want the >>> >>> >>> >>> >>> The Allan intercept is predicated on a very specific model of the >>> noise in >>> a clock ( as I recall basically random gaussian noise at high >>> frequencies, >>> and 1/f noise at low). It is not at all clear that real computers comply >>> with that. >>> >>> >>>> best accuracy over the long term, you had better respect that. Proof >>>> positive is in my 1995 SIGCOMM paper, later IEEE Transactions on >>>> Networking paper and das Buch. I abvsolutely relish scientific >>>> critique, but see the briefings and read the papers first. >>> >>> >>> >>> >>>> 2. To reduce the convergence time, reduce the time constant, but >>>> only at the expense of long term accuracy. An extended treatise on >>>> that is in das Buch, especially Chaptera 4, 6 and 12. I would be >>>> delighted to hear critique of the material, but read the chapters >>>> first. >>> >>> >>> >>> >>> While you may know what in the world Das Buch is (Hitlers Mein >>> Kampf?) I do >>> not. Nor do I know where to get it. >> >> >> >> Computer Network Time Synchronization: The Network Time Protocol by >> David L. Mills (Hardcover - Mar 24, 2006) >> >> Available from Amazon.com. You may be able to find a copy at a >> University Book store. Be prepared for "Sticker Shock". It ain't >> cheap! Publishing in small quantities is EXPENSIVE!!! It's different >> when you can amortize your setup costs over 50,000 copies! >> >> "Das Buch" is unlikely to become a best seller! >>
David, Why are you telling me this? My contribution to this thread consisted of the above exposition of the publication data and availability of "Das Buch". _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions