[time-nuts] worst case jitter of PTP (IEEE1588) derived clock?
lists at rtty.us
Thu Mar 3 16:50:29 UTC 2011
In a real network you may have multiple switches and a lot of variable
traffic. Your noise is coming from many sources. You make some high level
choices in terms of sample rate. Without pretty exact information, you are
going to have a hard time coming up with a useful answer.
Data with box A plugged directly and only into box B is much of what's out
there. It only tells a small part of the story, and often it's a bit hard to
work out exactly what is being shown.
The practical answer is - this is one of the last things you will set. You
get a working network up and then see what you are getting. Beat it up with
real traffic and see what it does. Loop through adjust and observe.
The alternate solution - let the local sources generate the phase noise
inside your mask. Make the PLL very narrow (<< 1Hz) so it's outside the
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Caudle
Sent: Wednesday, March 02, 2011 9:55 PM
To: time-nuts at febo.com
Subject: [time-nuts] worst case jitter of PTP (IEEE1588) derived clock?
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?
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. If you are
locking a sampling clock to that, you will need to know the worst case
phase noise so you can design the PLL bandwidth, etc. So far haven't
found the right reference which derives that, just some very gross maximum
offset type of numbers, nothing which shows whether you might go +/-
maximum offset in one cycle, or whether it drifts slowly to that level of
I don't recall seeing much about PTP (Precision Time Protocol) on the
list, compared to NTP. Are many people using it yet?
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