On Wed, Jun 22, 2016 at 08:16:20AM -0400, Vanderpool, Clyde wrote: > I guess a little bit of both. From looking things up on google or > different forums all I could find was kind of circular (i.e. 'frequency > adjustment is the computer adjusting it's frequency') I figured it had > something to do with the clock pulse of the individual machines but I don't > want to guess. I have to present some information and I want to make sure > I sound reasonably intelligent. Any info would be appreciated.
In general, there are two important variables when synchronizing two clocks. The "offset" is the instantaneous time difference between the two clocks. If you perfectly synchronize two wrist watches by pushing their pins in at exactly the same, then at that moment, the offset will be zero. After that, the two watches will slowly drift away from each other. This happens because the quartz crystals oscillate at slightly different rates. Another word for the rate is "frequency". The difference between the rates of two clocks can be represented as a fraction or percentage. Normally the difference is small, and so you will see "parts per million" (PPM) and "parts per billion" (ppb) used instead of percentage. Regarding the ptp4l program, it is not enough for it to simply correct the time offset, it also must change the rate of the local clock to match that of the master clock. The program uses the PTP to estimate the time and frequency offsets from the master and correct them by applying a frequency adjustment to the local clock. The amount of frequency adjustment is shown in the log in units of ppb. There are tons of papers to read about this topic. This page and the site might interest you: http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm Or try this classic David Mills paper: https://www.eecis.udel.edu/~mills/database/papers/time.pdf Here are all of the Mills papers: https://www.eecis.udel.edu/~mills/papers.html These two were helpful to me: @TechReport{mills1993precis, author = {David Mills}, title = {Precision synchronization of computer network clocks}, institution = {University of Delaware Electrical Engineering Department}, year = 1993, month = {November}} @TechReport{mills1992model, author = {David Mills}, title = {Modelling and analysis of computer network clocks}, institution = {University of Delaware Electrical Engineering Department}, year = 1992, month = {May}} HTH, Richard ------------------------------------------------------------------------------ Attend Shape: An AT&T Tech Expo July 15-16. Meet us at AT&T Park in San Francisco, CA to explore cutting-edge tech and listen to tech luminaries present their vision of the future. This family event has something for everyone, including kids. Get more information and register today. http://sdm.link/attshape _______________________________________________ Linuxptp-users mailing list Linuxptp-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linuxptp-users
