[ntp:questions] IEEE 1588 (PTP) at the nanosecond level?

Magnus Danielson magnus at rubidium.dyndns.org
Mon Mar 17 22:35:43 UTC 2014

On 17/03/14 13:50, Joe Gwinn wrote:
> 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
> 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 

>> 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.


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