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Re: XAUI AC coupling


I'd like to set the record straight: Neither I nor anyone else involve
in this thread has ever recommended mandatory DC-coupling. The P802.3ae
approved XAUI baseline, developed initially by the Hari group which
included experts from Ethernet, Fibre Channel and InfiniBand, ended up
requiring AC-coupling. I have proposed removing this requirement and
allowing either AC or DC-coupling. My reading of your first paragraph
suggests that we are in violent agreement. If this is indeed the case,
then the only remaining decision is whether AC and DC-coupling of XAUI
is described in the standard or not. I'm happy going either way on this.
I favor leaving the details to the implementer.

In your second paragraph you seen to waver from violent agreement and
note a very specific application of XAUI as a system ASIC to transceiver
module link. I maintain that this specific application is well outside
the scope of the standard and only representative of one of a myriad of
applications for XAUI. For example, a simple early 10GE XAUI application
is to couple the XAUI output directly to a laser diver and
post-amplifier set of a WWDM module. The XAUI interface is short, the
laser driver to XAUI interface is likely to be custom, and DC-coupling
is appropriate. As I have pointed out in prior notes, a prevalent XAUI
application will be as a fixed chip-to-chip interconnect not involving
optical modules at all. It is a straightforward implementation detail to
select either AC or DC-coupling in the latter scenario. The standard
should not dictate sub-optimal implementations.

Vipul, I can't seem to place you in either the "Mandatory AC-coupling"
or "Allowable AC or DC-coupling" categories. From your last note it
seems like you're abstaining. I'd also like to get other's perspective
on this issue.  

Best Regards,

Vipul Bhatt wrote:
> Rich,
> Let's see if we can refine the question, in the hope of making progress.
> We do have some common ground: If a PHY module is going to be purchased
> by a switch manufacturer, it will likely end up as a pluggable or
> solderable module, at the XAUI interface. To manage the multiple
> buyer-supplier scenarios, it is best to use AC coupling. If, however, a
> PHY module is going to be integrated by the switch manufacturer on a
> single board, then XAUI becomes the switch manufacturer's internal design
> responsibility, and they should have the freedom to choose DC or AC
> coupling. Just as it would be wrong to burden a pluggable module with
> mandated DC coupling, it would be wrong to burden an integrated single
> board design with mandated AC coupling.
> Beyond this common ground, where we go from here becomes an interesting
> choice. One approach would be to leave the coupling issue to the
> implementers. Another approach would be to say, the popular purpose of
> XAUI is to allow easy separation of switch and PHY module
> responsibilities, and to allow the implementation of pluggable or
> solderable PHY modules. To help achieve that purpose in a bullet-proof
> fashion, we should mandate AC coupling. This is the "majority gets its
> way" approach. At the moment, I favor the second approach. To make
> further progress, it will help to know what others think.
> Regards,
> Vipul
> vipul.bhatt@xxxxxxxxxxx
> (408)542-4113
> ===============
> > -----Original Message-----
> > From: owner-stds-802-3-hssg@xxxxxxxx
> > [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Rich Taborek
> > Sent: Saturday, September 23, 2000 12:55 AM
> > Cc: HSSG
> > Subject: Re: XAUI AC coupling
> >
> >
> >
> > Vipul,
> >
> > OK, I asked for it... Now I'm forced to respond.
> >
> > A PHY module to ASIC connection, with the ASIC being in-line and
> > eventually connected to a switch fabric, is very likely to be a XAUI
> > implementation. If the module is pluggable, then it is very likely that
> > the XAUI link would be AC-coupled to insure maximum interoperability.
> > However, if the module is fixed, there are non negligible cost,
> > reliability and performance advantages to employing DC-coupling instead
> > if applicable. There is no risk. My point all along, is that
> > coupling is an implementation detail and should not be a standard
> > mandate.
> >
> > Your PECL example reflects information an implementer should be able to
> > glean from a manufacturers data sheet. Many XAUI devices will be fixed.
> > The implementer simply reads the relevant data sheets for both XAUI
> > devices and determines whether or not AC-coupling is required. If
> > AC-coupling is not required, the implementer may choose to DC-couple.
> > The determination of signal coupling requirements is standard practice
> > for chip-to-chip interconnects.
> >
> > Your third paragraph seems to describe a scenario where an implementer
> > makes a bad decision to employ DC-coupling where the device specs for
> > the two XAUI devices employed in the link dictated AC-coupling. I was
> > unaware that the purpose of the standard was to force suboptimal
> > implementations in case an implementer misinterprets device
> > data sheets.
> >
> > I believe that most implementers would be incensed by such imposing
> > regulations. I certainly hope that the same implementer doesn't rely on
> > the standard for all other aspects of XAUI link implementation, such as
> > power supply decoupling, trace layout, connector choice, via design,
> > etc. to insure that their XAUI links work reliably.
> >
> > I don't understand the relevance of LVDS to this discussion, please
> > explain.
> >
> > I agree that if either an implementer is uncertain about DC bias or DC
> > bias itself is uncertain, that AC-coupling should be used. However, you
> > seem to be describing a scenario where too much uncertainty exists. It
> > is highly likely that the operation of the XAUI link will be uncertain
> > in this case.
> >
> > To conclude, your assumed XAUI configuration is system ASIC to
> > transceiver module which exemplifies only one possible XAUI application
> > and one in which DC-coupling is applicable and preferred in many
> > instances. In addition your desire is to impose suboptimal
> > implementations on all XAUI links in case an implementer
> > happens to make a mistake. I have to respectfully disagree that either
> > argument dictates that XAUI AC-coupling is technically required.
> >
> > Best Regards,
> > Rich
Richard Taborek Sr.                 Phone: 408-845-6102       
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