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Re: XAUI AC coupling


I can best describe your response as "twisted".

XAUI is an optional interface for P802.3ae. I don't believe that I will
get any disagreement on this point. However, 802.3 voters voted
unanimously to include XAUI as a baseline proposal in P802.3ae. The
definition of XAUI in the standard has been allocated a clause,
specifically, clause 47.  

We (P802.3ae) need to develop XAUI specs. The ones in the baseline
proposal are a good start but are far from complete. If you were at the
New Orleans XAUI breakout session, and I believe you were for a short
time, you would have a good idea of how incomplete the XAUI specs really
are. One of the issues at hand is one of Tx and Rx coupling method.
Recent reflector discussion is already talking about the possible
specification of capacitor values for XAUI AC-coupling.

I'm taking my usual systems perspective and trying to have a serious
technical discussion on the issue of the requirement of a specific
coupling method for XAUI. I believe that this TECHNICAL discussion is
both appropriate and warranted. My point is that XAUI AC-coupling is NOT
required for interoperability. Therefore, it should not be a requirement
in the standard. Such a requirement, if any, would be listed in the PICS
of clause 47. I don't believe that asking for proof of an AC-coupling
requirement for XAUI is extreme at all. This proof is in line with the
PAR 5 Criteria and the KISS principle, some of the basic tenets by which
802.3 has historically made decisions and made its standards so

As further proof of the wishy-washy nature of the XAUI "requirement" to
AC-couple I will submit a presentation by Ali Ghiasi on this issue:
See slide 6 where the AC coupling caps are shown and accompanied by an
asterisk which reads:

* It is recommended were possible to use 0.75V for the common mode
to allow the user for possible direct connect in backplane applications.

Well, fact of the matter is that XAUI is ONLY a backplane (i.e.
chip-to-chip) application. There is no objective in P802.3ae to extend
XAUI with copper cables between equipment which is likely powered by
different supplies. Therefore, even the original Hari presentations
suggest that DC-coupling is appropriate for XAUI.

Lets please keep this discussion above board and on a technical level so
we have a chance of finishing this standard on time.

Best Regards,

Jonathan Thatcher wrote:
> Rich,
> While the discussion is worth having and your request for supporting
> information valid, I think that your conclusion that we must prove that it
> is required for interoperability a bit on the extreme side. There are lot's
> of things that we add to the standard, that have significant value, that are
> not required for interoperability. For example: XAUI.
> jonathan
> > -----Original Message-----
> > From: Rich Taborek [mailto:rtaborek@xxxxxxxxxxxxx]
> > Sent: Friday, September 22, 2000 11:57 AM
> > To: HSSG
> > Subject: Re: XAUI AC coupling
> >
> >
> >
> > All,
> >
> > I'd like to propose that clause 49, XAUI, remove any requirement for
> > coupling, AC or DC. The basis of this proposal is as follows:
> >
> > 1) XAUI is a chip-to-chip interconnect. As such, DC-coupling is
> > clearly appropriate and advantageous from an implementation
> > perspective, an example is when interfacing chips from like
> > logic families utilizing the  same power supplies. This is likely
> > to be the case in many implementations. Therefore, the standard
> > should not dictate AC-coupling when DC-coupling is adequate to
> > achieve interoperability.
> >
> > 2) I've reviewed all instances of the use "coupling" including fuzzy
> > variants in the 802.3 standard. There is no precedent for dictating a
> > specific coupling method for a chip-to-chip interconnect in the
> > standard.
> >
> > 3) Absolutely nothing will be taken away from the standard by
> > removing a requirement for AC-coupling. If AC-coupling is either
> > desired when not required or required for a specific implementation,
> > then the details for AC-coupling including the determination of
> > specific capacitor values,  the frequency spectrum of 8B/10B
> > transmission code, etc. are all well  documented and readily available.
> > 8B/10B transmission code is far and away the most commonly used and
> > well understood code in serial gigabit links including chip-to-chip
> > interconnects. From a signal coupling  perspective, except for
> > proportionally higher signaling frequency, there is no difference
> > between a single XAUI lane and a 1000BASE-X link. Note that for
> > 1000BASE-X, both AC and DC coupling is available from
> > transceiver module vendors.
> >
> > AC coupling was proposed as a requirement for the Hari interface which
> > was effectively renamed as XAUI. It has been carried into the baseline
> > proposals for P802.3ae. Now is the time to decide whether AC-coupling
> > is an interoperability REQUIREMENT. I challenge anyone to argue and 
> > prove that AC-coupling is required for XAUI interoperability. If such
> > proof is not forthcoming, clause 49 should be modified to remove any
> > requirement for AC-coupling.
Richard Taborek Sr.                 Phone: 408-845-6102       
Chief Technology Officer             Cell: 408-832-3957
nSerial Corporation                   Fax: 408-845-6114
2500-5 Augustine Dr.        mailto:rtaborek@xxxxxxxxxxx
Santa Clara, CA 95054