RE: bit ordering on XSBI vs SFI-4
I agree with Justin's solution and analysis.
Some further arguments:
It is clear that SFI-4 and XSBI were intended to be harmonized from the
inception, but SFI-4 could not be referenced because it is an implementation
agreement, not a standard, so it was copied, and then diverged in the
bit-ordering. Even SFI-4 was the result of common practice in the industry,
before OIF adopted it. XSBI is a interface that is new to Ethernet, so prior
Ethernet bit-ordering convention should not be an issue.
I propose that the bit-ordering should match between XSBI and SFI-4, as this
interface represents a simple, common transceiver module interface that can
be used to support multiple standards. I think this was the original
intention of specifying this optional physical instantiation. Bit
relabelling to cause reordering on XSBI represents only a renaming of the
bits, and this operation is not without precedent, as this is done right now
in the WIS (clause 50).
Lastly, I think it is useful to point out the utility of getting this
harmonized, when all we're really talking about is the names of the bits:
Any implementer using a 16 bit LVDS interface with forward clocking for a
transceiver module will be presented with the same bit order, regardless of
whether such a module is a XSBI or SFI-4 interface.
I recommend the following changes, very similar to those suggested by
Justin, but perhaps slightly simplified (and will submit comments for D3.0).
1) Move the "relabelling" operation from Clause 50 to Clause 49 (this
relabels this bits "on their way out" and "on their way in" respectively)
2) Change the bit order in Clause 51
This seems to me to still be a EDITORIAL change as relabelling (renaming)
bits should not a technical change at the interface. The alternative is to
explain to implementers why the bit labels are different between SFI-4 and
XSBI, when they were intended to be the same, and guide them laboriously
through Ethernet and SONET bit-order conventions towards a successful
PS: This suggested change will remove any possible Bit Ordering Anxiety
(BOA) and possibly reduce the design review time. I, myself have been the
victim of BOA, which is a terrible affliction, so I would not wish this on
any implementer. BOA must be stopped.
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