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Re: [10GBASE-CX4] Working Paper Available on IEEE site





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