Re: [802.3ae] XAUI Rj TR comment
Hi
Certainly this topic has created lots of traffic. Instead of
trying to explain how CDR works or how to add random jitter,
lets analyze precedence and application.
First:
Fiber Channel initially specified DJ and RJ separately.
Second:
Initiated by FC optical group in 1996 FC start specifying
RJ=TJ-Max(RJ), to facilitate optical transceiver compliance
and margins. Effectively by this change the max RJ now
could reach TJ limit with TJ=0.
Third:
XAUI is an FR4 backplane interconnect primarily limited by DJ
and not RJ. The FC historical change of RJ=TJ- Max(DJ)
probably puts more limitation on a cooper interconnect than
first thought.
We need to think how to optimize XAUI interconnect for today
and tomorrow. Limiting max RJ in a copper interconnect has
no penalty as you would not be able to transmit at full RJ
for more than 0", but provides additional level of flexibility
and robustness.
Thanks,
Ali
Mike Li wrote:
>
> Folks:
>
> I am not a regular email contributor for IEEE 802.3, but I read emails from
> reflector
> on a regular basis. Here are some of my observations after reading though
> all the
> XAUI jitter related discussions on RJ and DJ.
>
> 1.) We have to agree the definition and term on DJ, RJ, and TJ.
> FC MJS document is probably one of the well thought and adopted publications
> for serial/parallel link communication. Unless we all agree the definition
> and term
> for jitter, any subsequent discussion will lead us nowhere.
>
> 2.) Assuming that we adopt the MJS definitions and terms, i.e.,
>
> Jitter: Any deviation from ideal and is a statistical process
> DJ: bounded
> RJ: Gaussian, unbounded to beyond 1e-12
> TJ: (UI-opening) at BER function of 1e-12
>
> The rest parameters and pdfs related to jitter can be UNIQUELY
> determined by mathematics
> and there should be zero error and ambiguity. We have done numerous math
> simulations
> to prove that is the case.
>
> 3.) These is no basic physics and mathematics to support the "double delta"
> DJ pdf.
>
> The DJ root cause can come from many different noise sources, such as
> reflection,
> ringing, bandwidth limiting medium/device, periodical modulation, crosstalk,
> etc. We all
> know that their pdfs are different and are not "double delta" at all.
>
> 4.) DJ pdf and DJ value can be uniquely determined given the TJ and RJ pdfs
>
> We have shown that DJ pdf and peak-to-peak value can be determined via
> "deconvolution". There is no need to assume "double delta" even from
> a simplifying point view.
>
> 5.) We have presented our study on the consequence of using "double delta"
> in estimating
> DJ value, and we have shown that the error can be as large as 30%, in
> comparison with
> the actual DJ value in applications.
>
> The document was uploaded to T11 back to 1999. The reference number is :
>
> T11/99-432v0, and the title is : "What is the correct definition for DJ"
>
> 6.) We plan to do a systematical presentation on DJ, RJ, TJ pdfs and their
> interrelationship
> in November's Plenary meeting.
>
> We feel that we have the obligations to share some of our experiences in
> the
> process of defining and understanding jitter. It is important to maintain
> the
> integrity and accuracy of IEEE 802.3 document, and to make sure all that the
> methods
> developed by the committee are mathematically and physically sound.
>
> Thanks !
>
> Mike Li
>
> Wavecrest
>
> > -----Original Message-----
> > From: THALER,PAT (A-Roseville,ex1) [mailto:pat_thaler@xxxxxxxxxxx]
> > Sent: Wednesday, August 01, 2001 11:43 AM
> > To: Lindsay, Tom; Howard A. Baumer; HSSG_reflector (E-mail)
> > Subject: RE: [802.3ae] XAUI Rj TR comment
> >
> >
> >
> > Tom,
> >
> > Jitter always seems to be a difficult subject to sort out and
> > your remark
> > below caused me to do some checking on RJ vs. DJ.
> >
> > I've looked all through the 802.3 standard and our draft.
> > There doesn't seem
> > to be any definition of RJ or DJ. Processes can certainly be
> > random without
> > being random or Gaussian. Deterministic means if a set of
> > conditions is set
> > up we know what will result. The roll of a die is random
> > though the result
> > is bounded.
> >
> > If we are using dictionary words with a different or more
> > restricted meaning
> > such as random = Gaussian (or truncated Gaussian where the truncation
> > happens past 1E-12) then we should define our terms. Since we specify
> > deterministic jitter and total jitter, we should at least
> > have a reasonably
> > rigorous definition of "deterministic jitter."
> >
> > I also notice that in some places jitter is divided into RJ
> > and DJ, but in
> > other places in 47 it is RJ, DJ and sinusoidal. 52.9.10.4 (and the
> > equivalent subclause of 53) divide jitter into random,
> > deterministic and
> > bounded.
> >
> > Crosstalk is deterministic in that given a fixed adjacent
> > signal and a fixed
> > coupling function one can determine the crosstalk. However,
> > the crosstalk at
> > a receiver is often the result of multiple disturbers
> > coupling in each with
> > its own function and the signals aren't correlated to the
> > received signal.
> > Therefore, the sum of the crosstalk looks like a truncated
> > Gaussian. Even if
> > the definition of RJ is Gaussian up to at least 1E-12, it
> > isn't clear to me
> > that crosstalk would fall outside that definition. I don't
> > recall seeing any
> > studies on the distribution of crosstalk for XAUI or for our optical
> > receivers.
> >
> > I would expect crosstalk to be part of RJ rather than DJ.
> >
> > Regards,
> > Pat
> >
> > -----Original Message-----
> > From: Lindsay, Tom [mailto:tlindsay@xxxxxxxxxxxxxxxxxxxx]
> >
> > See below, Tom
> >
> > -----Original Message-----
> > From: Howard A. Baumer [mailto:hbaumer@xxxxxxxxxxxx]
> > Sent: Tuesday, July 31, 2001 11:36 AM
> > To: Lindsay, Tom; HSSG_reflector (E-mail)
> > Subject: Re: [802.3ae] XAUI Rj TR comment
> >
> > >>>>snip<<<<<
> > - We're still confused on how you would ever get 0.55UI of RJ. If
> > crosstalk adds so much jitter,
> > **TL - crosstalk is expected to be bounded, and therefore more
> > effectively deterministic (the definition of RJ is
> > unbounded/Gaussian to
> > least below 1E-12, and DJ is all other stuff).
> >
--
Ali Ghiasi
Broadcom Corp.
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