On 17/03/14 13:50, Joe Gwinn wrote:
In article <[email protected]>, William Unruh
<[email protected]> 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)
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
anyway.
IRIG-B004 DCLS can provide really good performance if you let it.
To get *good* PTP performance, comparable to your IRIG-B, prepare to do
a lot of testing to find the right Ethernet switches, and then replace
them all. Redoing the IRIG properly start to look like cheap and
straight-forward.
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
systems.
Yes. My question is basically a query about the current state of the
art.
The state of the art is not yet standard and not yet off the shelf
products, if you want to call it PTP.
Cheers,
Magnus
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