I am noticing a thread (one among several) being discussed
in this multi-threaded discussion that I consider disturbing given
our history of link standardization.
Normally we define a link by developing a transceiver specification
for a defined data rate running over a specified media (or a referenced
media specification) at a maximum link distance under worst case
Now we all know the that a slightly longer link distance will
work, sometimes significantly longer! But we DO NOT discuss it! Why?
Because we are NOT prepared to guarantee it worst case, and we normally
limit ourselves to the world of worst case analyses.
Geoff Thompson wrote:
While your scenario is quite plausible, even likely. It is nothing we
a standards group can prevent. It's gonna happen. Now in a practical
sense (and OUTSIDE of the standards arena) how does a vendor or end
keep it from happening?
The answer is:
If it were my network, there would have be tags at each end of the
AND a notation in the link record database (Remember, BICSI has best
practices for how to handle all of this stuff. It's a bad idea not to
follow them, especially in this sort of case.) that noted that this was
an "out of spec" link and had a pointer to a procedure for the
appropriate "exceptional handling" required. Do I think this is
a good idea for twisted-pair links handled by casual users? No. Do I
think that a data center manager would think it is reasonable and
something that can be handled within an appropriately and
managed cabling system? Yes.
Given that the above is the case. It WILL eat into the market for any
different product for a longer reach, especially a marginally longer
reach. The other question then becomes how much cost handicap this
customer will be willing to endure across his entire transceiver
population to duck this minor problem. I believe that the consensus is
that for distances over 100 meters the answer is "not
At 08:07 AM 8/22/2008 , Scott Kipp wrote:
I disagree with some
that have been put forward and feel that we need to proceed on the
present course to defining how to extend the reach beyond 100
meters. The time to throw in the towel has not come.
Let's run through a
scenario. Joe the installer needs to go 130 meters and begins
plugging in the ends of the fiber to different modules and a
finally works and the link comes up. The link works fine for a few
months or even years and then the module degrades a little (but is
within the 100 meter specification) and the link begins to have bit
errors. Joe the installer is long gone (or should hide after the
link fails) and nobody knows what happened. Both modules still pass
any tests based on the standard, but the link is failing and no one is
The reason we have
(like Geoff says) is because we want reliable links to work all of the
time over the lifetime of the parts. When a link fails, we can
determine the cause and have responsibility. I agree that the links
are defined conservatively and will work longer most of the time, but I
disagree with John's premise that there is no value in the XR solution
that goes significantly farther. Responsible installers will rely
on standards based parts to go the rated distance. We have several
ways we can make OM3 links work for longer than 100 meters.
Do we have the
of choosing only one way to extend the link or can we talk about
ways to do this in an informative annex? The hard part is choosing
one solution because we have so many camps with different
solutions. The usual problem is that we have no clear winner - no
Usain Bolt - in the XR race beyond 100 meters.
From: Geoff Thompson
Sent: Friday, August 22, 2008 9:15 AM
Subject: Re: [802.3BA] 802.3ba XR ad hoc next step concern
In answer to your question
Because it has been the tradition of 802.3 (and I strongly
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
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
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
money and separate inventory hassle.
Given the odds that it will work over 90% of the time, I would
Am I willing to reduce our customers' overall chance of success to
At 08:55 AM 8/21/2008 , Swanson, Steven E wrote:
Thanks for all your
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
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
Sent: Wednesday, August 20, 2008 11:23 PM
Subject: [802.3BA] 802.3ba XR ad hoc next step concern
I m concerned that the proposal of creating a new objective is leading
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
extended link reaches, will be discussed below.
I don t want to wait until our next phone conference to share this in
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
Optical modules are expected to either have a XLAUI/CLAUI interface or
PMD service interface, PPI. Both are considered.
A previous presentation, petrilla_xr_02_0708,
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
The Tx distribution characteristics follow. All distributions are
Min OMA, mean = -2.50 dBm, std dev = 0.50 dBm (Baseline value = -3.0
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 =
Spectral Width, mean = 0.45 nm, std dev = 0.05 nm (Baseline value =
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
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 =
Rx bandwidth, mean = 10000 MHz, std dev = 850 MHz (Baseline value =
For the Tx MC, only 2% of the combinations would fail the aggregate Tx
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
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
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
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.