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RE: XAUI AC coupling


I agree with Jonathan that the caps fall in the same category as XAUI.  That
is: optional, but not required.  Specing XAUI as mandatory is costly and
unnecessary, and would require proof of need.  Same should go for the caps.

On the other hand, I agree with Jonathan that the caps fall in the same
category as XAUI.  Leaving the "unnecessary but often used" out of the spec
invites incompatible implementations, which is why we specified the optional
XAUI.  Is it the same situation for the caps?  

If failing to specify the caps could cause people to come up with
incompatible implementations, then we should specify them and make them
optional.  This was the motivation behind XAUI.  But if it's a trivial issue
then let's not waste time on it.  This is the motivation behind not
specifying too much, such as whether data pins should be arranged in
clockwise increasing order or not.  Perhaps in the future, we will have to
do that too because swapping the order cannot be done reasonably. 

My guess is that the caps are trivial, because worst case they're easy to
change at any point during design or manufacturing (unless you're
integrating them!).  As Rich points out, we're not going between boxes (no
surprises that vendor X's cap choice doesn't work with vendor Y after

So, I'd recommend leaving them out unless there is feeling that they need to
be specified to ensure compatibility between AC coupled implementations.
Then, I'd specify them as optional unless it could be proven that AC
coupling was required.  

-Simon Sabato
-Intel Corporation 

-----Original Message-----
From: Jonathan Thatcher [mailto:Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx]
Sent: Friday, September 22, 2000 2:11 PM
To: 'rtaborek@xxxxxxxxxxxxx'; HSSG
Subject: 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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