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RE: XAUI AC coupling


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.


> -----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.
> -- 
> Best Regards,
> Rich
> ------------------------------------------------------- 
> Richard Taborek Sr.                 Phone: 408-845-6102       
> Chief Technology Officer             Cell: 408-832-3957
> nSerial Corporation                   Fax: 408-845-6114
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