On Fri, Dec 5, 2014 at 3:35 PM, Charles Swiger <cswi...@mac.com> wrote:
> Well, we do have time enthusiasts around who like to achieve the best > precision they can, regardless of whether there is a specific business > justification or not. :-) > Sure but that doesn't help someone that just wants a simple (-minded) configuration to keep a few clocks in sync. > > If all your external clocks fail I suspect the typical user can depend > > on the disciplined virtual clock for days. > > For real hardware, sure-- once the intrinsic frequency drift has been > setup, > you can free-run for days into weeks without drifting too far. Cell phone > towers (especially CDMA) are a decent example of such fault-tolerant > systems. > c While perhaps strictly correct I was talking about the common crystal in a typical computer not the Cesium, Rubidum or OCXO in a cell site and there's really no basis for comparison. > What confusion? Certainly it's a decent paper to read.... > I misquoted. This "Without measuring the local clock against some other clock or oscillator" suggests comparing a clock to a better frequency reference but BIPM creates a virtual clock (with better Allen deviation) and everyone steers toward that. Perhaps you meant something else. Yes, you need to compare timestamps using purpose-built systems like a > TCXO, Cesium, or Rubidium clock > That wasn't my point. You need a purpose built NTP server to expose its virtual clock for comparison to an external frequency reference. Of course you need a "purpose" built reference but only in the sense that you'd use "real" counter rather than one in a voltmeter. > Even back in 2002 with very inexpensive commodity hardware, FreeBSD was > able to > achieve accuracy measured to ~260 nanoseconds: > Hmmmm. So phk uses a $1,500 rubidium standard as a system oscillator and you call it inexpensive and commodity. He also ran a particular install of BSD and a non-standard NTP. All of those are what I was referring to when I said purpose built system to measure the variance of an NTP disciplined virtual oscillator. By the way high resolution low-latency counters in computers have become commodity items. The software to use them -- not so much. It might be nice to conduct a similar experiment with Chrony but it's all pointless. As Bill suggests you want to measure typical performance in typical environments. That's the bit I said was interesting and I don't think the published numbers make it clear what's better. Frankly I suspect even that is pointless. If someone asked me how to do NTP on the cheap I'd say buy or build some number of Laureline-like PLLs in a box with a NTP packet emulator or some NTP servers off Ebay and run SNTP/OpenNTP on the clients. If you have a larger budget then buy Meinberg, Microsemi et. al. new. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
