Re: XAUI AC coupling
Larry,
The standard needs to differentiate between a specific XAUI application,
such as an XGP Transceiver Module interface and XAUI in general. I
believe that your comments are applicable to a 10 Gigabit Pluggable
Transceiver Module.
Best Regards,
Rich
--
> Larry Miller wrote:
>
> From a user:
>
> This looks like a replay of the Transceiver/SERDES mess on 100 Mb/s
> SFF and 1G 1 x 9 systems.
>
> What happened there was that nobody agreed on the input common-mode DC
> level of either direction. Users were lectured to about "classic" PECL
> coupling networks by companies whose own wares did not follow it.
> Bipolar chips wanted a different DC input level from CMOS chips, which
> was again different from GaAs designs.
>
> Worse, in many cases the transceiver inputs relied on the Thevenin
> coupling network to set the (correct) DC bias point.
>
> The net result was that many transceiver/SERDES combinations did not
> operate together at all. In the end, vendors ended up having to build
> two versions of everything-- AC coupled and DC coupled. And there were
> still incompatible combinations.
>
> We ended up insisting on AC coupled designs for everything just to get
> interchangeability of parts between vendors, time, temperature, phase
> of the moon, etc. And we also ended up insisting that the transceivers
> have the caps IN the units, where they belong from an EMI standpoint.
>
> I suspect that our company will again insist on capability for AC
> coupling. That way each vendor gets to control the DC characteristics
> on "his" side of the coupling capacitor. The corollary here is that
> each vendor is responsible for providing an input circuit bias means
> that sets the DC level to the "sweet spot" for that chip.
>
> Larry Miller
> Nortel Networks
>
> -----Original Message-----
> From: Rich Taborek [SMTP:rtaborek@xxxxxxxxxxxxx]
> Sent: Saturday, September 23, 2000 12:43 PM
> To: HSSG
> Subject: Re: XAUI AC coupling
>
> Vipul,
>
> 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,
> Rich
>
> --
>
> 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
Chief Technology Officer Cell: 408-832-3957
nSerial Corporation Fax: 408-845-6114
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