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Re: XAUI AC coupling


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

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,
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