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At the last meeting we discussed briefly
what is the “legacy” channels we should attempt to cover. Obviously it’s not a 10GBASE-KR channel –
not enough lanes. As has been suggested by others, I agree
that we should look at 40GBASE-KR4 channels as the ‘legacy’ we should attempt
to support… especially for the bladeserver market. Kinda funny, because KR4 is
not common in the market yet, but will be soon… so it’s a “future legacy.” As Rich points out PCB technology moves at
a snail’s pace compared to silicon. I started designing boards in 1988 and the
technology – especially for volume PCs & servers - is largely unchanged. In
fact, your Smart Phone uses much more advanced PCB tech than an IA server. Look
at the size of a Server motherboard – if we force a technology that doubles (or
more) the square-inch cost of the PCB, that is not a good fit for this market. Relating
this to the objectives terminology below, if “improved FR4” means Megtron-6,
that is impractical. We have another external force is this
market: regulatory requirements for RoHS (restriction of hazardous substances).
Over the last few years the PC/Server market made the transition to
lead-free PCBs in support of RoHS. Our next exciting challenge is Halogen
Free. That includes getting rid of the bromine & chlorine flame retardant
common in FR4 today. This makes a fundamental change in the epoxy dielectric
properties. (Actually the transition to lead-free was easier electrically –
it was primarily a change in surface finish away from HASL; didn’t affect the dielectric.)
There is active industry effort underway to improve the electrical performance
of HF PCBs… but the first stop along that path is just getting them up to “common”
FR4 performance. Finding a cost-effective, manufacturable, Halogen Free PCB
solution with the electrical performance of Megtron-6 (or -4, or Nelco-13SI)
may be a long way off… but more investigation is needed here. For more
background on HF, see: http://thor.inemi.org/webdownload/projects/ELSC/HFR-Free_PVC-Free_Timeline.pdf Tying this back to a length objective for
backplane channels is difficult, but I think we had the same issue in 802.3ap. We
assumed “improved” FR4 to achieve 1m. But in practice I think most implementers
chose to stay with lower cost PCB material & implement shorter channels. In
802.3 we are used to having an objective for length – but that’s an awkward
fit. Dave From: How about dielectric material that is
projected to be used in X percent of the market in 201#? That plays to
broad market potential. Silicon technology is expected to still have
exponential growth. Changing “backplane/line card” board technology on
the other hand is like turning a battle ship. Given that, we
should at least evaluate how much of legacy product present in 201# we can
cover given the disparity of the silicon vs. board technology pace.
Simply put, coverage could be one of the metric to compare proposals.
Unfortunately it’s still crystal ball. … Rich From: Havermann, Gert
[mailto:Gert.Havermann@xxxxxxxxxxx] All, besides the grammar I would like to
have some kind of reference to the "enhanced FR-4
Material". There are so many different anhanced materials available (low
loss, ultra low loss, low dk glass...) all having a different influence on the
length factor we are trying to fix (not to mention the cost differenve between
materials). Regards Gert Havermann Absender ist HARTING Electronics GmbH &
Co. KG; Sitz der Gesellschaft: Espelkamp; Registergericht: Bad Oeynhausen;
Register-Nr.: HRA 5596; persönlich haftende Gesellschafterin: HARTING
Electronics Management GmbH; Sitz der Komplementär-GmbH: Espelkamp;
Registergericht der Komplementär-GmbH: Bad Oeynhausen; Register-Nr. der Komplementär-GmbH:
HRB 8808; Geschäftsführer: Edgar-Peter Duening, Torsten Ratzmann, Dr. Alexander
Rost Von: John D'Ambrosia
[mailto:jdambrosia@xxxxxxxxxxxxxxx] All, Not sure this message got forwarded to
the reflector, so forwarding. Regards, John From: Kolesar, Paul [mailto:PKOLESAR@xxxxxxxxxxxx] John and group, The objectives are on the right track, but
there are a few grammatical and clarity issues. The first is an issue with the wording at
the end of each. The phrase “up to at least” is illogical and confusing
because it combines words that specify maxima (i.e. up to) with words that
specify minima (i.e. at least). While this phraseology may be following
some precedent of former objectives, it only serves to cloud the real intent.
In the past these objectives have always been interpreted as the
requirements for minimum reach. Therefore I propose that they each simply
state it as such by replacing “lengths up to at least” with “lengths of at
least”. Further, I do not know what is intended by
the phrase “for links consistent with lengths”. Why use the word
“consistent”? Is there a perception of some increased flexibility or some
other advantage? Please explain. If the advantage is ambiguity, I
would prefer stating objectives more crisply. Unless your rationale for this word choice
is compelling, I propose combining these two issues into the following new
phrase: “for link lengths of at least”. Lastly, it would be simpler and clearer to
place all the adjectives describing “traces” before the noun rather than some
before and some after. Rearranging these yields ”over improved FR-4
copper traces”. With all three of these changes, the two
draft objectives become: ·
Define a 4-lane 100
Gb/s PHY for operation over improved FR-4 copper traces for links lengths of at
least “X” m. ·
Define a 4-lane 100
Gb/s PHY for operation over copper twin-axial cables for links lengths of at
least “Y” m. Of course all these grammatical
improvements do not address the main issues which are the values of X and
Y. But that is what study groups are for. Regards, Paul Kolesar From: John D'Ambrosia [mailto:jdambrosia@xxxxxxxxxxxxxxx] All, I wanted to try and foster some discussion
on the reflector regarding objectives for the project to help all focus their
planning of presentations for March. So what do we appear to have consensus on
so far? a) We are in a study group looking at
100GbE over backplane and copper twin-ax b) Legacy support indicates broad market
potential would be aided by 4 lane solutions What appears to need further consensus
building? Well the big ones would seem to be reach for both backplane and
cu cabling objectives. So if we can combine where we appear to
have consensus with what we need to resolve, the following two statements could
be used as strawmans for objectives for the group to work towards (leaving the
reach #’s as variables for now): ·
Define a 4-lane 100
Gb/s PHY for operation over copper traces on improved FR-4 for links consistent
with lengths up to at least “X” m. ·
Define a 4-lane 100
Gb/s PHY for operation over copper twin-axial cables for links consistent with
lengths up to at least “Y” m. Feedback? John |