RE: XAUI AC coupling
Rich,
If mine is twisted, yours is a full double helix :-) Let me demonstrate.
If the cap is not required and can be proven so by not proving otherwise, we
must have specified a set of logic levels that guarantees that there is
little or no common mode offset between chips. Why not, then, specify a cap
and ask to have someone prove that the common mode offset specification is
required? Can't prove it? Then we should reject a 0.75 common mode voltage.
Sound absurd? I agree.
Please note, I am not taking a position on the choice. I don't feel
qualified to do so. There are many reading this that are much more qualified
that I (we?).
The point is that you win an argument by first establishing the assumptions.
I, for one, have not YET accepted yours.
So, a few questions for those "in the know:"
1. What is the impact of this decision on parts that operate at different
voltages and use different technologies (e.g. a "MAC Chip and a WDM
Transceiver")? Yes, I reject the assumption that these could and should use
the same technology.
2. What is the impact of this decision on low jitter design?
3. What is the impact of this decision on EMI generation at the board level?
4. What is the impact of this decision on signal integrity?
5. What is the impact of the ground differentials common between high power
boards in a system?
6. What else am I forgetting to ask?
jonathan
> -----Original Message-----
> From: Rich Taborek [mailto:rtaborek@xxxxxxxxxxxxx]
> Sent: Friday, September 22, 2000 3:49 PM
> To: HSSG
> Subject: Re: XAUI AC coupling
>
>
>
> Jonathan,
>
> 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
> successful.
>
> 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:
> http://grouper.ieee.org/groups/802/3/10G_study/public/nov99/gh
> iasi_1_1199.pdf.
> 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
> voltage
> 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,
> Rich
>
> --
>
> 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.
>
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