Steve,
Last year during the 100GE SMF PMD 10km
reach discussion you proposed a framework for how to deal with multiple SMF
reach applications that is an excellent model for how to move forward with
multiple MMF reach applications.
Specifically what we learned during the
extensive study of SMF reaches is that 10GE 10km SMF PMDs are used in a
variety of ways. One set of applications (we estimated at
approximately 50% of applications) needed reaches from 4km to 10km.
The 802.3ae 10GBASE-LR specification is directly applicable to those
applications. There was another set of applications (primarily in the data
center and approximately 50% of applications) that needed between 300m
and 4km reach, however requiring a much higher number of connectors then
the 4 assumed in the 802.3ae standard.
802.3 HSSG decided that the best path
forward was to adopt a single 100GE 10km SMF PMD objective, with a 4 connector
assumption, which would serve all existing applications that are served today by
10GBASE-LR (10km.)
At the time you made an excellent
suggestion that we should include an informative appendix in the 100GE 10km SMF
PMD specification to give guidelines to end users as to how the number of
connectors can be traded off against reach. We agreed that there was a
benefit to your suggestion, despite recognizing that such an
informative appendix would not be a guarantee of the link always working with
the various reduced reach and higher connector number combinations. The reason
is that as the connector number increases the loss adds up statistically.
Further, the corner case of many connectors, with each connector
having worst case connector offset causing reflections, breaks any
reasonable link budget. Such a case is very unlikely but makes it very
difficult to write an 802.3 standard, (unless sophisticated statistical
models are used.)
So the beauty of an informative appendix is
that it gives more information to the end user from the experts writing the
standard then they would otherwise be available for how to trade off
connector number versus reach. Without such an appendix end
users have no guidelines for how to do this (as is the case for
10GBASE-LR.) At the same time, we avoid the daunting task of writing a
separate 802.3 SMF PMD standard for large connecter count applications that
guarantees that the link budget can be closed in all cases.
A similar approach to MMF will serve us
well. We should write a standard for 100m OM3 which is guaranteed to work for
the majority of applications (100m or less.) At the same time, we include an
informative appendix to give guidance to the minority of remaining applications
on how to use the 100m OM3 PMDs for greater reaches, for example by using better
fiber such as OM4. This will be much better then not providing any guidance, and
it will also avoid what will otherwise be a very challenging and contentious
task of writing separate standards that guarantee longer
reaches.
Chris
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.