[time-nuts] worst case jitter of PTP (IEEE1588) derived clock?

Javier Serrano javier.serrano.pareja at gmail.com
Thu Mar 3 09:38:31 UTC 2011


On Thu, Mar 3, 2011 at 3:54 AM, Chris Caudle <chris at chriscaudle.org> wrote:

> Has anyone found reference which derives the worst case clock jumps to
> expect when using PTP (IEEE1588-2008) and how to derive a phase  noise
> spectrum from that?
>

As Magnus says, this is highly dependent on implementation. For "standard"
implementations, in which a non-controllable oscillator is used in the PTP
clocks, the digital servo has a sampling rate which varies with network
latencies (which themselves vary with network load), so it is virtually
impossible to give a precise answer to your question.


> I'm looking at a protocol which uses 1588-2008 to create a common clock
> between networked devices, and those devices are supposed to derive other
> clocks (e.g. sampling clocks) synchronized to the 1588 clock.


We have exactly that requirement at CERN, and the White Rabbit project (
http://www.ohwr.org/projects/white-rabbit/wiki) is our answer to it. The
main idea is very simple: PTP does not tell you anything about the nature of
the oscillator you should use in a PTP device. Instead of using a
non-controllable oscillator, we derive our clock signal from the incoming
Gigabit Ethernet data stream. The rx_clock from the PHY is always good, even
in the absence of Ethernet packet transfer. By doing that, we have a system
which is easier to analyze (constant sampling rate) and we require much less
PTP traffic since PTP now only has to deal with very slow thermal effects
(like fiber length changes). To our knowledge, and as per the ISPCS 2010
plug fest, White Rabbit is currently the most precise PTP implementation. We
are preparing a document with a phase noise PSD plot for reference and
recovered clocks using a 5-km fiber link. Typical clock signal jitters are
below 100 ps and UTC-transfer accuracies are below 1 ns. For a quick
overview of WR you can see
http://svn.ohwr.org/white-rabbit/trunk/documentation/presentations/WR_Tomek_BE_TechCommittee/javier_ni_talk.pdf

Cheers,

Javier


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