RE: XAUI update from Interim meeting
Mike,
I find it difficult to understand how a receiver can work with a floating
mask with the center of the transition allowed to occur over a 0.6 UI
window. Consider the following pathological transmitter case:
999 of 1000 transitions out of the transmitter have jitter that falls
withing a .1 UI window
1 out of 1000 transitions falls .5 UI after the .1 UI window.
This transmitter's output would meet a 0.6 UI floating window spec.
A receiver PLL locking to that signal is going to locate the bit cell edge
somewhere within the .1 UI window. The receiver will sample the level half a
UI from where it believes the bit cell center is. When the transition occurs
.5 UI after the window it will occur after the sampling point and an error
will occur.
Now this case may be a bit extreme, but I have seen signals in a copper
system where most of the transitions were relatively tightly grouped and
there were a few outlying transitions to one side so it isn't all that
unusual.
The receive PLL will locate the average bit cell edge. Jitter affects the
receiver to the extent that edges are skewed from the average bit cell edge.
The receiver's ability to withstand jitter doesn't depend on the center of
the distribution of edges. It depends on the distance of edges from the
mean.
Pat Thaler
-----Original Message-----
From: Mike Jenkins [mailto:jenkins@lsil.com]
Sent: Friday, June 08, 2001 6:29 PM
To: Kesling, Dawson W
Cc: Serial PMD reflector (E-mail)
Subject: Re: XAUI update from Interim meeting
Dawson, et al,
Thanks much for your summary on the XAUI spec changes. I have
a comment/question concerning the summary item stating: "The
templates were centered in the data eye and not allowed to float."
Unless I misunderstand, this change causes an inconsistency in
the specification. In Fibre Channel, at least, eye masks (or
templates) have been defined such that the left-most vertex is
one half the total jitter (i.e., TJ/2) from the left edge of the
bit period (i.e., at 0 UI). The right-most vertex is symmetrically
TJ/2 from the right edge of the bit period (i.e., 1 UI), as shown
in my crude character graphics below (if you have fixed-width font).
_______ ___________
\ / \
\ / \
\ :<- TJ/2 ->:<- TJ/2 ->: / \
\: : :/ \
\ : / \
/ : \ /
/ : \ /
/ 0 UI \ /
/ (bit \ /
________/ boundary) \___________/
If the further constraint is added that 0 UI must be the mean of
the jitter (i.e., zero crossing) distribution, then that FORCES a
mask failure if the distribution is skewed, as it generally is in
copper transmission media.
The practice in Fibre Channel (for copper media) has been to freely
adjust the X position of the waveform. This permits a waveform with
jitter approaching the spec limit of TJ to fit legally on the template.
The proposal to constrain the mean of the distribution to be at 0 UI has
been voted down several times in the Fibre Channel jitter working group.
If I am missing something, I would appreciate being clued in.
Regards,
Mike Jenkins
"Kesling, Dawson W" wrote:
>
> Attached is a summary of XAUI progress and issues for those of you who
could
> not come to the Interim meeting last week. All changes and refinements
will
> be included in draft D3.1 due to be posted to the 802.3ae web site by
6/11.
>
> If you have questions that aren't answered in the attached summary and
can't
> wait until the draft is posted, please send them to the reflector audience
> in general so that other attendees can help me answer them. I probably
can't
> respond quickly to ALL the questions that might otherwise be sent to me
> personally!
>
> Thanks to all who participated so helpfully in St. Louis last week.
Special
> thanks to Anthony Sanders of Infineon and Tom Lindsay of Stratos Lightwave
> for their extra effort on XAUI jitter leading up to (and after hours
during)
> the Interim.
>
> -Dawson Kesling, Intel
> Editor, Clause 47
>
> <<St_Louis_summary.ZIP>>
>
> ------------------------------------------------------------------------
> Name: St_Louis_summary.ZIP
> St_Louis_summary.ZIP Type: application/zip
> Encoding: Base64
--
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