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Re: equalization

Vipal and All:

You did a great initiation in equalization issue.  There are more complex 
issues than originally thought.  Those are very natural development to find 
more problems, while we are discussing.  Do we have time, or we do not have 
time is quite an arbitrary decision depending what is the criteria.  In 
general, market will not disappear, just because equalization issues will 
take more time to come up as enhancement later.

The basic effect of optimum launch is to remove modal BW from OFL to optimize 
BW, but nothing to do with curing DMD.  Therefore, we should not count on 
optimum launch to reduce DMD.  However, optimum launch proposed by FOTP-203 
happens to concentrate over 75% of flux within 15 um radius, and it can help 
to minimize DMD effect.  Nevertheless, it was not designed for the irregular 
frequency response caused by DMD; therefore, it may not help equalization 

The reasons:

Equalization is required at the corner frequency and the high frequency 
attenuation region, but not the flat low frequency region.  The accurate 
prediction of the corner frequencies (DMD has multiple fc), and the shapes of 
attenuation slopes (DMD can have multiple slopes) are required to design 
effective equalizers.  The optimum launch by FOTP-203 has nothing to do with 
all these requirements.  We have to spend time to characterize the frequency 
responses of the installed MM fibers, which is our task, but not FO2.2.1.  It 
simply will need time to do it.  It can be done, if we have years of time to 
do it as an enhancement.

Even if we have time to do equalization as enhancement, we still need 5-PMDs, 
but not 2-PMDs, or 3-PMDs.  There are time-to-market issue, and variety of 
technology options to stimulate competition issues.  The serial approach 
needs many new technologies, which CWDM/WWDM do not need.  The monotonic 
serial approach will be lacking challenge from the readily available 
CWDM/WWDM approach; as a result, users will suffer, which will affect the 10 
GbE market growth.   

One thing we do not know yet is what is the price we are going to pay for the 
complex equalization, if we get one?  The average users will weigh cost, 
availability, timel-to-reach market issues.  We are dealing the whole 
technology and marketing issues.  To over simplify the PMD issue as a pure 
mathematicaly-met issue is too easy.


Ed Chang
NetWorth Technologies, Inc.        



 Upon reflection, I think it is possible that signal distortion for
 multimode fibers will be less troublesome than that created by
 wireless multipath. For one thing, by restricting the excitation of
 modes to a controlled subset, we may not see the occurrence of
 multipath effect. The pulse may not split; it may just spread
 according to the classical bandlimiting effect. In that case, we can
 use spectral compensation to overcome ISI. Second, even if you see
 the multipath effect, it may vary in time so slowly that we can
 treat it as time invariant, unlike wireless multipath.
 Of course, I agree that before we raise our hopes, we need to
 scrutinize this idea.
 If more than 25% of us think that there isn't enough time to reach a
 judgment about this equalization idea because we need to move on
 quickly, I will drop this idea. Before that, I would like us to give
 the DSP folks a chance to talk to us. This is a potential solution
 that can meet Objectives 1 and 2, and keep the total number of PMDs
 to just 2 or 3.