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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 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. |