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.
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.
From: Geoff Thompson [mailto:gthompso@xxxxxxxxxx]
Sent: Friday, August 22, 2008
10:15 AM
To: Swanson, Steven E
Cc:
STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3BA] 802.3ba XR
ad hoc next step concern
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.