[ntp:questions] IEEE 1588 (PTP) at the nanosecond level?
joegwinn at comcast.net
Mon Mar 17 12:50:08 UTC 2014
In article <lg61s4$ong$3 at dont-email.me>, William Unruh
<unruh at invalid.ca> wrote:
> On 2014-03-16, Joe Gwinn <joegwinn at comcast.net> 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)
IEEE 1588-2008 does say one nanosecond, in section 1.1 Scope.
I interpret it as aspirational - one generally makes a hardware
standard somewhat bigger and better than current practice, so the
standard won't be too soon outgrown. IEEE standards time out in five
years, unless revised or reaffirmed.
> > 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.
I forgot to mention a key point. We also have IRIG hardware, which
does provide microsecond level sync errors. The hope is to eliminate
the IRIG hardware by using the ethernet network that we must have
> One of the key problems is getting the packets onto the network (delays
> withing the ethernet card) special hardware on the cards which
> timestamps the sending and receiveing of packets on both ends could do
> better. But it also depends on the routers and switches between the two
Yes. My question is basically a query about the current state of the
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