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Re: Where does XAUI fit

Dear Walter,

>  Walter Thirion <wthirion@xxxxxxxxxxxx> writes:
> I'm slightly confused by your comment:
> > Rich Walker wrote:
> > All PMD's could take their input from pure XGMII and convert back to
> > XGMII at their output.  It makes no sense to burden all the PMDs with
> > complex rules for converting between the idiosyncrasies of 
> > other coding
> > schemes. 
> I'm not sure how this fits with the PMD.

Not surprising.  I admit to using the terms wrongly.  Perhaps it would
be better if I said PCS rather than PMD.  I'm relatively new to the
precise but arcane nomenclature of 802.3.  One nice side effect of this
discussion is to help me to tighten up my use of the language. 

> I do agree that XGMII should be the specified interface between the
> MAC/RS and the PCS.  Further, if we treat XAUI as an XGMII extender,
> then XAUI should take in XGMII and spit it back out the other side
> (maybe 20" of copper later...). 


> On the other hand, if XAUI is to be used between the PCS and the PMA,
> then its inputs and output can be whatever the group thinks is the
> best signalling interface between those layers, but by definition
> (since it would be below the PCS), in this instance XAUI would be
> transporting whatever coding the PCS used.  But even this could be
> done with an envelope as opposed to a decode/recode. 

Every layer, in addition to any layer-specific function, must at least
transmit logical Ethernet signalling information. 

XGMII is perhaps the most succinct specification of a pure-logical Ethernet 
signalling layer.  (please pardon my non-standard language here...)

What I'm trying to say is that any idiosyncratic PCS coding of a particular
PMD should never need to be supported by the PCS of any other PMD-style.

Any /A/ columns or scrambled idles used by XAUI should stop at the XAUI
boundary.  The 64b/66b serial PCS/PMD has no business accepting or
transmitting these codes. 

This is easy to see if you are willing to view XAUI as a copper PCS/PMD. 
It should, like all the other physical transmission systems be defined
w.r.t.  the lowest common denominator of logical Ethernet signalling:
eg: XGMII. 

In this way, all PCS/PMD combinations act as simple logical XGMII
extenders, whether 20 inches over copper, or 40 km over fiber, or at
several variants in cascade. 

kind regards,
Rick Walker