Re: [10GBASE-CX4] Working Paper Available on IEEE site
Kamal,
In Roseville the group unanamouslly went with a Tx driver template for
the Annex 48B.2 low frequency pattern. Go to
'http://www.ieee802.org/3/10GBCX4/public/dec02/Directions_Roseville.pdf";
and take a look. Clause 47 XAUI is meant to opporate without any
equalization anywhere. The specified compliance channel in clause 47
works without any Tx pre-emphasis or Rx equalization. The whole point
of XAUI was to define a driver and receiver that would work without any
equalization over 20" of FR4 backplane. That channel is more like a 3m
IB cable or even better. Further more in a backplane, a chasis, where
the connections can vary beyond the XAUI compliance channel each and
every connection is a known quantity and can be setup in that system as
such. As soon as you make the connections via a cable you no longer
know what the connection is. If you look at all other 802.3 cable
connection standards that require equalization to work the Tx and the
cable is always speciified in detail. When you do this you take all
unknowns out of the system. We can discuss this again in Vancouver, but
in Roseville the vast majority of the group agreed with this approach.
I do agree that the current Rx and Tx specifications are not consistant
and need to be made that way. All the numbers that were put in as
directed by the results from Roseville were put in to draw out
discusions, presentations and the supporting work that is required. So
please bring any and all supporting data and information you have so we
can get these numbers all modified to be consistant and produce a great
spec.
Howard
Chuck Harrison wrote:
>
> Hi, Kamal,
>
> I support the general model of TX emphasis, and designing the
> spec so that adaptive RX eq is *not* required. (It can be a value-add
> for any vendor who thinks it's worth the cost.) So I think we're
> basically in agreement about how implementation should shake out.
>
> I still don't think the XAUI experience is a good test of the
> wild & wooly interop world we face in CX4 because so often the
> backplane and line cards are co-engineered. Architecturally,
> fixed TX and vendor-specific RX is more robust in the long run,
> when the channel is going to be quite variable.
>
> Given that we both want a system based on TX pre-emphasis,
> a well-specified channel, and passive RX eq, the only difference
> is whether the signal is standardized at the TX end or the RX
> end of the link. (I don't think we can standardize both places
> because the channel varies.) With the question rephrased that
> way, what do you see as the relative benefits of TX vs RX signal
> standardization?
>
> Side note: If we standardize at one end, we can always include
> an *informative* section about behavior at the other end. And
> the compliance part will probably specify testing at certain
> limit conditions, which is almost like normative text.
>
> Cheers,
> Chuck
>
> kdalmia@marvell.com wrote:
> >
> > Chuck,
> >
> > XAUI was primarily targeted at backplanes. Cable environment is not much
> > different from the backplane environment - backplane trace lengths can and
> > do vary a lot. Most intuitive approach is to solve the length variation
> > problem by "adaptive RX equalization". However, it has been well proven in
> > backplane environment that adaptive RX equ is not the most optimum
> > technique from a practical implementation point of view - specifically,
> > power and performance. Instead, it is better to "over de-emphasize" - it
> > works well for short and long links (cable or traces).
> >
> > Regards
> > Kamal
> >
> > ===============================
> > KAMAL DALMIA
> > Technical Marketing Manager
> > Marvell Semiconductor
> > 700 First Av.
> > Sunnyvale, CA 94089
> > Phone: (408) 222 8979
> > ===============================
> >
> >
> > Chuck Harrison
> > <cfharr@erols.com> To: kdalmia@marvell.com
> > Sent by: cc: stds-802-3-10GBCX4@ieee.org
> > owner-stds-802-3-10gbcx4@majordo Subject: Re: [10GBASE-CX4] Working Paper Available on IEEE site
> > mo.ieee.org
> >
> >
> > 01/02/2003 10:15 AM
> >
> >
> >
> > Kamal,
> >
> > kdalmia@marvell.com wrote:
> > >
> > [...]
> > > Instead, I believe, the RX and the Channel should be specified (with TX
> > > implied). Current XAUI specs have RX sensitivity and jitter tolerance
> > > numbers. This implies that the burden is on the TX to deliver a "clean"
> > eye
> > > at the RX.
> >
> > The big difference between CX4 and XAUI is that we now need to
> > tolerate a dramatically larger range of channel-loss behaviors.
> > An architecture which uses fixed TX preemphasis and a single-
> > parameter cable model gives RX designers a well-defined problem
> > to solve. It allows (but does not require) RX eq based on
> > estimated cable length. If you invert this -- fixed RX behavior
> > and vendors trying to put their smarts into transmit EQ -- you
> > can never get as good a system result because the TX side has
> > no way of knowing about the channel characteristic. And if
> > both TX and RX sides (not to mention the cable vendors!) are
> > blindly trying to be "smart" then interoperability is at
> > serious risk.
> >
> > In the variable-channel CX4 environment, I would *strongly*
> > lean towards Howard's proposal -- fixed TX step response, and
> > a narrow tolerance on the channel's deviation from the nominal
> > cable model.
> >
> > Cheers,
> > Chuck Harrison
> > Far Field Associates, LLC