On 2014-03-17, Martin Burnicki <[email protected]> wrote: > William Unruh wrote: >> On 2014-03-16, Joe Gwinn <[email protected]> wrote: >>> I keep seeing claims that Precision Time Protocol (IEEE 1588-2008) can >>> achieve sub-microsecond to nanosecond-level synchronization over >>> ethernet (with the right hardware to be sure). >>> >>> I've been reading IEEE 1588-2008, and they do talk of one nanosecond, >>> but that's the standard, and aspirational paper is not practical >>> hardware running in a realistic system. >> >> 1ns is silly. However 10s of ns are possible. It is achieved by Radio >> Astronomy networks with special hardware (but usually post facto) > > Why should 1 ns be silly?
> > If you have a counter chain clocked by 20 MHz then the timestamps > captured when PTP packets are going out or are coming in have a > resolution of 50 ns. I am not saying that a computer or a piece of hardware cannot have a 1 ns resoltuion. I am saying that imagining that I can sychronize two such clocks over the network or via gps is. > > If your hardware can be clocked a 1 GHz then the resolution could be > increase to 1 ns. > > Of course I know high resolution is not the only thing you need for high > accuracy, but it's a precondition. > > You'd need hardware (FPGA?) which can be clocked at 1 GHz, and even in > the hardware signal processing you'd need to account for a number of > signal propagation delays which you can eventually ignore at lower clock > rates. > > So of course the effort becomes much higher if you want more accuracy, > but this is always the case, even if you compare NTP to the "time" > protocol, or PTP to NTP. > >>> I've seen some papers reporting tens to hundreds of nanoseconds average >>> sync error, but for datasets that might have 100 points, and even then >>> there are many outliers. >>> >>> I'm getting PTP questions on this from hopeful system designers. These >>> systems already run NTP, and achieve millisecond level sync errors. >> >> Uh, perhaps show them to achievement of microsecond level sync errors? >> That is already a factor of 1000 better than they achieve. >> >> One of the key problems is getting the packets onto the network (delays >> withing the ethernet card) special hardware on teh cards which >> timestamps the sending and receiveing of packets on both ends could do >> better.a But it also depends on the routers and switches between the two >> systems. > > Of course all involved network nodes needed to be able to timestamp at > this high resolution, otherwise the overall accuracy would be degraded. > > And, it would probably be easier to achieve this accuracy with an > embedded device with dedicated hardware than with a a standard PC and a > NIC connected via the PCI bus. > > If there were a 1 GHz oscillator on the NIC used for timestamping then > you still have to provide a way to relate the timestamps from the NIC to > your local system time. If the only way to do this is via the (PCI?) bus > then the accuracy could suffer from bus latency, arbitration, etc. > > On a dedicated hardware the same oscillator/high resolution counter > chain could be used for system timekeeping, and to timestamp network > packets, which makes things much easier. > > > Martin _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
