RE: bit ordering on XSBI vs SFI-4
The XSBI should not be labeled in terms of MSB and LSB. bit significance has
no meaning at that interface.
Originally Ethernet was defined to send most significant byte, least
significant bit first. That is still the way the length field is sent. On
the other fields, we now leave defining byte order significance up to the
protocols obove Ethernet. If you look at the payload of a 64B/66B data
block, the MSB is may the 8th data bit and the LSB may the 57th data bit.
There is no meaning to bit significance when one looks at quantities larger
than a byte because it depends on the byte significance.
When looking at the XSBI, bit significance becomes even more irrelevant
because byte boundaries may fall anywhere in the 16 bits. The terms LSB and
MSB have no relevance to the XSBI.
It seems that there are two ways of handling this and both result in
inconsistancy that will confuse some readers. 802.3 has always numbered bits
in primitives so that bit 0 was the first bit to be transmitted and the
highest numbered bit was the last bit transmitted. If we make
tx_data-group<0:15> and rx_data-group<0:15> work the opposite way, we will
be likely to confuse readers who are accostumed to the convention used in
the rest of the standard.
If SFI-4 numbers bits in the opposite order, then we have two choices.
Number tx_data-group and rx_data-group opposite of the way the other
primitives such as TXD and RXD.
Leave the tx_data-group and rx_data-group numbering as it is and add a note
that explains that tx_data-group<0> is the same as SFI's bit 15 and
tx_data-group<15> is the same as SFI's bit 0. Does SFI actually call their
signals tx_data-group and rx_data-group? The explanation will be clearer if
the names are not exactly the same.
It is less confusing to be consistant in bit numbering within the standard
and explain the difference between bit numbering of this standard vs other
related standards than to change our bit numbering order somewhere in the
From: Jscquake@xxxxxxx [mailto:Jscquake@xxxxxxx]
Sent: Monday, March 26, 2001 1:28 PM
To: Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx; stds-802-3-hssg@xxxxxxxx
Subject: Re: bit ordering on XSBI vs SFI-4
In a message dated 3/26/01 12:28:50 PM Pacific Standard Time,
Subj: RE: bit ordering on XSBI vs SFI-4
Date: 3/26/01 12:28:50 PM Pacific Standard Time
From: Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx (Jonathan Thatcher)
Before offering any opinion on this, I would like to know what impact there
would be on measurement and test equipment, if any.
Clearly, a piece of equipment expecting a serial PRBS pattern would need the
bits in a specific order. No?
Let me state again that any changes (if any) would only be in relabeling and
is purely a logical construct. There was/is neven intention to actually have
Ethernet packets sending bit stream data out on a serial link in a MSB first
manner. If Pat (clause 49) or anyone else does see this as becoming the case
then I would back off from this effort and just leave things as is. This
user (customers of modules makers) to be careful in knowing which bit is
sent out first on the link. Leaving things as is OR relabeling the XSBI to
never stop implementers. The serial PMA is a "dumb" device ...
Just a try here to make a simple suggestion ...
If I relabeled the XSBI to have MSB transmitted first then Pat coming out
from her clause could possible reword saying that the LSB should be mapped
the MSB of the XSBI in the case of a serial link.
Quake Technologies, Inc.
2880 Zanker Road, Suite 104
San Jose, CA 95134
Tel: 408-922-6888 x108