In order to look at possible stumbling blocks
ahead of us at Sponsor Ballot time, I sent a note to Geoff Thompson and to
Bob Grow with the following query:
Do either of you see any problem with 802.17 using a "standard 802.3
PHY" and supporting frame sizes less than 64 Bytes", especially if the
data rates of interest are 1 and 10 Gbits/s? Would you expect anyone
in 802.3 to be upset with 802.17 requiring the PHY to support the small
frame sizes? (I realize that you can't know for sure how everyone
will respond to this question. I am just looking for your gut
reaction here. Thanks.)
As I look ahead at the Sponsor level ballot, I want to steer around
stumbling blocks and want to get an early peek at whether there is any
reasonable standards conformance based reason to want to avoid requiring
802.17 PHYs from supporting these small frame sizes. I expect that
we will have a fair number of 802.3 voters in our sponsor ballot pool.
As it turns out, both Bob and Geoff
have strong concerns with what we are doing.
Geoff's has kindly given me permission
to post his reply which was:
"A
"standard" 802.3 PHY can not be used unaltered if it has Auto-Negotiation.
(i.e.
you can't just buy Ethernet chips and use them in 802.17 applications)
We
haven't even looked at the ramifications of having radically different
path lengths between transmit and receive
You
lose a great deal of your resiliency if you use 1000BASE-T links in
an RPR ring.
Basically, I have grave reservations about using Ethernet chips in an
application where you are throwing out a significant number of the basic
design assumptions that were in hand when the PHYs were designed. These
assumptions are not necessarily documented in any other fashion than that
it was an Ethernet chip being designed. That, in and of itself, sets a
large number of conditions. I don't think you get to use an Ethernet chip
without examining each and every departure from Ethernet assumptions on an
engineering basis at the detail level.
Do I know off the top of my head what ALL of the considerations are?
No way! I haven't ever had the occasion to look at the appropriate level
of depth that is required to develop product. I'm probably not even the
right guy to do it as I am not a PHY designer at this point in my life.
Can I think of a few off the top of my head? Yup, see above."
Bob Grow's reservations were even stronger. There are two
conclusions we should draw from Geoff's remarks.
(1) We should be studying very carefully, not only the requirements
that we get from reading the specifications for the PHYs that we pick up,
but the assumptions that went into the development of those PHYs
(2) We need to do comprehensive analysis to justify any change to the
assumptions used in developing those PHYs.
The second conclusion implies that we need to do a great deal of work
before assuming that the PHYs can be used for small frame size (less than
64 Bytes). We may also want to ask the question as to whether
the use of short frames is worth the risk of breaking Ethernet PHYs,
especially is we are unable to do a comprehensive review of the
Ethernet PHY requirements. We should also look at the implications
of large frame size, even though large frame sizes are used in proprietary
products today.
Note that problems uncovered during Sponsor Ballot can completely
derail the standard and the schedule. We don't want to be making any
significant changes to the draft at sponsor ballot time. Certainly
changing the length of control frames would be highly significant.
We have work to do. Let's decide how to best address it early enough
to stay within our present schedule.
Best regards,
Robert D. Love
President, Resilient Packet
Ring Alliance
President, LAN Connect Consultants
7105 Leveret
Circle Raleigh, NC 27615
Phone: 919
848-6773 Mobile: 919
810-7816
email: rdlove@xxxxxxxx
Fax: 208
978-1187