Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

RE: bit ordering on XSBI vs SFI-4



Instead of changing the order of transmission for 10G Ethernet (and affecting

three clauses), isn’t it preferable to place a note in the XSBI portion that

states that while the bit numbering assigned to the XSBI may differ from

that of implementations, the first bit on the fiber must always be the one

labeled in the standard as XSBI bit 0?




- Tom


P.S. I think a problem here is that there is a notion of “MSB” and “LSB”

with respect to the OIF SFI-4 interface (the OIF spec says that bit 15 is

the “MSB”). I believe this is silly, and it’s also what causes all these

assumptions about requiring inversion of connections on the board and

so on. I don’t know why a SERDES device should associate arithmetic

significance with its inputs and outputs. Perhaps we should get OIF to




-----Original Message-----
From: Jscquake@xxxxxxx [mailto:Jscquake@xxxxxxx]
Sent: Friday, March 23, 2001 3:49 PM
To: stds-802-3-hssg@xxxxxxxx
Cc: pat_thaler@xxxxxxxxxxx; Tom Alexander
Subject: bit ordering on XSBI vs SFI-4


Hello All,

As the clause editor for clause 51, I have received feedback that the present
bit ordering for the XSBI interface, i.e. LSB sent first,  will cause
confusion to people that have used the SFI-4/SONET modules.

Background on this topic ... an OC192 serial stream has the MSB bit first
while Ethernet networks have the LSB bit sent first. Early 802.3ae
discussions, we decided to leave the LSB bit order as is so that the PCS
(clause 49)  to XSBI will have no inversions. This leaves the WIS (clause 50)
to do the inversion. This is
the present status of draft 802.3ae rev 3.0.

The concern is that system people will be confused by the fact that XSBI
modules and SFI-4 modules have "reversed" ordering AND if they are not
careful  the PCB layout will be reversed.

To reduce this risk, a change that CAN be made to make the XSBI bit ordering
consistent w/ SFI-4 and also send the MSB bit out first. This would result in
three changes

1) clause 51 - reverse the order to MSB bit out first
2) clause 50 - remove section on reversing the bits from the PCS
3) clause 49 - "modify/add" section to send out the data with MSB bit first

My request to the reflector is to give some guidance here on the direction for
a change or none change.

Comments please. Thanks in advance.

Justin Chang
Quake Technologies, Inc.
2880 Zanker Road, Suite 104
San Jose, CA 95134
Tel: 408-922-6888 x108
Fax: 408-922-6827
email: justin@xxxxxxxxxxxxx