Steve
I would be against introducing a 2nd PMD with just 150 m reach unless
the group is willing to adjust the reach of
40/100GBase-SR4/SR10 base propose. It is just too small an increment
when we already have seen proposal for
250 m on OM3
http://www.ieee802.org/3/ba/public/jul08/dudek_02_0708.pdf .
Unless the 2nd PMD has some significant benefit with 2-3x reach
increase otherwise it would better left-off as an
engineered solution, MSA, or use SFP+.
Duplex MMF has been the most successful optical interface, but I am not
sure the same would be true with
Ribbon MMF when duplex MMF PMDs become available! I do understand that
duplex MMF likely
will not be part of current 40G/100G PAR.
Thanks,
Ali
Swanson, Steven E wrote:
Geoff,
This is certainly one way to
look at it. While I agree that it has been the tradition of 802.3 that
we offer more to the market than just "overwhelming odds," I also think
that "market coverage" is equally important and I believe that the odds
are significantly lower than 1 out of 12 tries that a 150m link will
fail even with the current baseline specs (and I think we can easily
get a 150m worst case design). While
we have worked hard in 802.3 to work on "worst case design" we have
often "compromised" that position. 10GBASE-LRM is the most recent
example of a standard that is not worst case design - in fact, we
define two launches just to mitigate failures but I digress.
The reasons that I think the odds of a 150m link
failing are small are as follows:
I think the 1/12 failures would only occur if
all links were 150m. If you believe the Flatman survey, 90% of links
are less than 100m so 90% of the links will work all of the time with a
standard that is designed to support 100m. Even if the other 10% are
all 150m, the failure rate is less than 1% overall. I think the number
less than 100m is smaller than 90% but even if it was 80%, the failure
rate would be less than 2% overall. And this also assumes that
EVERYTHING is worst case. If one always designs for worst case, we
incur unnecessary costs and place our customers in a difficult
position.
What should we tell our customers who have link
lengths longer than 100m and want (or require) a standardized solution?
Should we tell them to buy a 10km single-mode solution?
I think we can get to a 150m solution and we
should continue to try. I am always amazed at the controversy that
seems to follow multimode fiber, particularly since it has proven over
and over again to be the most successful optical solution for Ethernet.
We need to get it right and should not give up because it is a hard
decision.
Best regards,
Steve
Steve-
In answer to your question
Because it has been the tradition of 802.3 (and I strongly
believe a foundation of the success of 802.3 in general) that we offer
more to the market than just "overwhelming odds" (quantified in the
earlier message as 92% or only 11 times out of 12 tries). What we have
worked to in 802.3 is significantly closer to "worst case design" than
that. We have argued over the years about what that has meant but we
have certainly never dipped that low.
What I believe that John has argued for (and not unreasonably) is the
following.
We provide assured operation at 100 meters
If you want to go to 150 meters, the odds are very strong that you can
succeed as long as you are will to lower your expectations from plug
and chug to trying your way through a half dozen parts at each end in
order to get a set that works.
His thesis, as I understand it, is:
1) It is not worth the extra investment (time and money both) to get
a different standard with the extra reach.
2) Even if we do #1, the market won't pay anything for it. They will
just go through the select and try route in order to save the extra
money and separate inventory hassle.
Given the odds that it will work over 90% of the time, I would agree.
Am I willing to reduce our customers' overall chance of success to 92%?
No !!
Sincerely,
Geoff Thompson
At 08:55 AM 8/21/2008 , Swanson, Steven E wrote:
John,
Thanks for all your
work on this; I have to study it more and would like to see the actual
presentation but I would offer the following comment:
If the following
statement is true, why do we have an objective of 100m rather than 150m?
"Do nothing to the
standard and when 150 m of OM3 or 250 m of OM4 is desired just plug in
the fiber. The odds are overwhelming that it will work."
Thanks,
Steve
From: PETRILLA,JOHN [mailto:john.petrilla@xxxxxxxxxxxxx]
Sent: Wednesday, August 20, 2008 11:23 PM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: [802.3BA] 802.3ba XR ad hoc next step concern
Colleagues
I m concerned that the proposal of creating a new objective is leading
us into a train wreck. This is due to my belief that it s very
unlikely that 75% of the project members will find this acceptable.
This will be very frustrating for various reasons, one of which, almost
all the modules expected to be developed will easily support the
desired extended link reaches, will be discussed below.
I don t want to wait until our next phone conference to share this in
the hope that we can make use of that time to prepare a proposal for
the September interim. I ll try to capture my thoughts in text in
order to save some time and avoid distributing a presentation file to
such a large distribution. I may have a presentation by the phone
conference.
Optical modules are expected to either have a XLAUI/CLAUI interface or
a PMD service interface, PPI. Both are considered.
A previous presentation, petrilla_xr_02_0708, http://ieee802.org/3/ba/public/AdHoc/MMF-Reach/petrilla_xr_02_0708.pdf
has shown that modules with XLAUI/CLAUI interfaces will support 150 m
of OM3 and 250 m of OM4. These modules will be selected by equipment
implementers primarily because of the commonality of their form factor
with other variants, especially LR, and/or because of the flexibility
the XLAUI/CLAUI interface offers the PCB designer. Here the extended
fiber reach comes for no additional cost or effort. This is also true
in PPI modules where FEC is available in the host.
Everyone is welcome to express their forecast of the timing and
adoption of XLAUI/CLAUI MMF modules vs baseline MMF modules.
To evaluate the base line proposal for its extended reach capability, a
set of Monte Carlo, MC, analyses were run. The first MC evaluates just
a Tx distribution against an aggregate Tx metric. This is to estimate
the percentage removed by the aggregate Tx test. The second MC
evaluates the same Tx distribution in combination with an Rx
distribution and 150 m of worst case OM3. The third MC repeats the
second but replaces the 150 m of OM3 with 250 m of worst case OM4.
Worst case fiber plant characteristics were used in all link
simulations.
The Tx distribution characteristics follow. All distributions are
Gaussian.
Min OMA, mean = -2.50 dBm, std dev = 0.50 dBm (Baseline value = -3.0
dBm)
Tx tr tf, mean = 33.0 ps, std dev = 2.0 ps (Example value = 35 ps)
RIN(oma), mean = -132.0 dB/Hz, std dev = 2.0 dB (Baseline value =
-128 to -132 dB/Hz, Example value = -130 dB/Hz)
Tx Contributed DJ, mean = 11.0 ps, std dev = 2.0 ps (Example value =
13.0 ps)
Spectral Width, mean = 0.45 nm, std dev = 0.05 nm (Baseline value =
0.65 nm).
Baseline values are from Pepeljugoski_01_0508 and where no baseline
value is available Example values from petrilla_02_0508 are used.
All of the above, except spectral width, can be included in an
aggregate Tx test permitting less restrictive individual parameter
distributions than if each parameter is tested individually. In this
example distributions are chosen such that only the mean and one std
dev of the distribution satisfy the target value in the link budget
spreadsheet. If the individual parameter is tested directly to this
value the yield loss would be approximately 16%.
The Rx distribution characteristics follow. Again, all distributions
are Gaussian.
Unstressed sensitivity, mean = -12.0 dBm, std dev = 0.75 dB (Baseline
value = -11.3 dBm)
Rx Contributed DJ, mean = 11.0 ps, std dev = 2.0 ps (Baseline value =
13.0 ps)
Rx bandwidth, mean = 10000 MHz, std dev = 850 MHz (Baseline value =
7500 MHz).
For the Tx MC, only 2% of the combinations would fail the aggregate Tx
test.
For the 150 m OM3 MC, only 2% of the combinations would have negative
link margin and fail to support the 150 m reach. This is less than the
percentage of modules that would have been rejected by the Tx aggregate
test and a stressed Rx sensitivity test and very few would actually be
seen in the field.
For the 250 m OM4 MC, only 8% of the combinations would have negative
link margin. Here approximately half of these would be due to
transmitters and receivers that should have been caught at their
respective tests.
The above analysis is for a single lane. In the case of multiple lane
modules, the module yield loss will increase depending on how tightly
the lanes are correlated. Where module yield loss is high, module
vendors will adjust the individual parameter distributions such that
more than one std dev separates the mean from the spread sheet target
value. This will reduce the proportion of modules failing the extended
link criteria. Also, any correlation between lanes results in a module
distribution of units that are shipped having fewer marginal lanes than
where the lanes are independent.
So while there s a finite probability that a PPI interface module doesn
t support the desired extended reaches, the odds are overwhelming that
it does.
Then with all of one form factor and more than 92% of the other form
factor supporting the desired extended reach, the question becomes,
what s a rational and acceptable means to take advantage of what is
already available? A new objective would enable this but, as stated
above getting a new objective for this is at best questionable.
Further, it s expected that one would test to see that modules meet the
criteria for the new objective, set up part numbers, create inventory,
etc. and that adds cost. Finally, users, installers, etc. are
intelligent and will soon find this out and will no longer accept any
cost premium for modules that were developed to support extended reach
- they will just use a standard module. There s little incentive to
invest in an extended reach module development.
I ll make a modest proposal: Do nothing just hook up the link. Do
nothing to the standard and when 150 m of OM3 or 250 m of OM4 is
desired just plug in the fiber. The odds are overwhelming that it will
work. If something is really needed in the standard, then generate a
white paper and/or an informative annex describing the statistical
solution.
Background/Additional thoughts:
Even with all the survey results provided to this project, it s not
easy to grasp what to expect for a distribution of optical fiber
lengths within a data center and what is gained by extending the reach
of the MMF baseline beyond 100 m. Here s another attempt.
In flatman_01_0108, page 11, there s a projection for 2012. There for
40G, the expected adoption percentage of links in Client-to-Access
(C-A) applications of 40G is 30%, for Access-to-Distribution (A-D)
links, it is 30%, and for Distribution-to-Core (D-C)links it is 20%.
While Flatman does not explicitly provide a relative breakout of link
quantities between the segments, C-A, A-D & D-C, perhaps one can
use his sample sizes as an estimate. This yields for C-A 250000, for
A-D 16000 and for D-C 3000. Combining with the above adoption
percentages yields an expected link ratio of C-A:A-D:D-C = 750:48:6.
Perhaps Alan Flatman can comment on how outrageous this appears.
This has D-C, responsible for 1% of all 40G links, looking like a
niche. Arguments over covering the last 10% or 20% or 50% of D-C
reaches does not seem like time well spent. Even A-D combined with
D-C, AD+DC, provides only 7% of the total.
Similarly for 100G: the 2012 projected percentage adoption for
C-A:A-D:D-C is 10:40:60 and link ratio is 250:64:18. Here D-C is
responsible for 5% of the links and combined with A-D generates 25% of
the links. Now the last 20% of AD+DC represents 5% of the market.
Since the computer architecture trend leads to the expectation of
shorter link lengths and there are multiple other solutions that can
support longer lengths, activating FEC, active cross-connects, telecom
centric users prefer SM anyway, point-to-point connections, etc., there
is no apparent valid business case supporting resource allocation for
development of an extended reach solution.
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