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Bob,
You
have bought forth some intriguing remarks and rationale
responses to a very specific question dealing with less
than 64 byte frames
on
802.3 PHYs. Thanks for the delving into this. BTW, I do believe careful
(safe)
PHY
use is always a wise thing:)
Geoff/Bob, (or anyone who know more in this area) could
you elaborate
on
what exactly are Ethernet PHY's limitations specifically related to
frame size?
Apparently, reading the 802.3 specifications it does
not jump across
that
any limitation is implied.
Are
there upper limits on ordered_set code densities?
Even
non-standard Jumbo frames use standard Ethernet PHYs and that seems to
work
(at
least at the PHY level) implying no lower limit on ordered_set code
densities.
I also
don't understand the following and its relationship to
802.17:
1. The
relevance of 1000BaseT (i.e. use of 4 pairs of twisted wires between adjacent
stations) in RPR
2. Why
a standard PHY that has auto-negotiation cannot be used.
3.
Finally there is mention that 802.3 PHYs have not been characterized when the
transmit
and
the receive path delays are different.
What
in RPR gives rise to the delay difference on spans connecting two adjacent RPR
stations
that
has implications that off beyond Ethernet?
raj
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
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