Re: [802.3BA] 802.3ba XR ad hoc next step concern
To help with
understanding what the industry WRT best practices...
There is an EIA standard
for data cable wiring and labeling. Try EIA-606.
There is also ITIL-v1,
ITIL-v2, and now ITIL-v3
BR,
Gus
Scott-
While your scenario is quite plausible, even likely. It
is nothing we as 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
user keep it from happening?
The answer
is:
1) Good
operational
practices
2)
Good record keeping
If it were my network, there would have be tags at each
end of the fiber 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
professionally 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
much".
Geoff
At 08:07 AM 8/22/2008 , Scott Kipp wrote:
All,
I disagree with some premises 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 little scenario. Joe the installer needs to go 130 meters and
begins plugging in the ends of the fiber to different modules and a
combination 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 still
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
responsible.
The reason we have standards (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
difficult task of choosing only one way to extend the link or can we talk
about several 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.
TGIF,
Scott
From: Geoff Thompson [mailto:gthompso@xxxxxxxxxx]
Sent: Friday,
August 22, 2008 9:15 AM
To:
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.