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RE: PMD discussion - parallel solutions

The viability of parallel solutions keeps coming up and I feel compelled to
respond (sorry - usually I resist the temptation).

I believe parallel is the lowest cost transceiver for 10G---it's hard to beat
one alignment for an array of VCSELs with one IC that sets the drive currents
for the entire array.   Contrast that to (presumably) WDM's multiple alignments
and multiple ICs or at least setpoints for each laser.  Or to the higher speed
ICs required for serial.  The 'problem' with parallel is that at some run length
it no longer is the lowest cost solution even if it is the lowest cost

Although parallel is a great application for 10G (and higher) I did not feel it
was a proper solution for 10GbE because of the distance objectives and market
timing.  First, 12-wide parallel transmitters and receivers already exist from
several vendors and by the time this standard is finished you would want to go
at a higher data rate with fewer fibers.  A parallel 4-wide looks very promising
but when I used the link model to analyze it at 3.125 it runs into the 3.6 dB
ISI limit just short of 300m.  (Although technically one could argue 100m is all
that is needed because you could 'conceivably' gang up installed
fiber---practical implementations would install new ribbon cable).  So it failed
to meet the distance objectives.  One could install a higher bandwidth fiber but
why not go serial then since that meets the same distance objective and by the
time the standard is finished will be a lower cost solution (providing IC cost
trends stay the same).

I look at parallel and serial playing a leapfrog game where there is a market
window for parallel that eventually closes when the higher data rate serial IC
costs come down.  Now throw WDM in the mix and ask yourself if packaging costs
are dominated by alignments (traditional?) or by components (specifically ICs).

More recently a 4-wide parallel scrambled approach has been proposed that would
meet the 300m distance objective.  So now the question is by the time the
standard is finished will it be a lower cost solution?


Mark Donhowe
W.L. Gore & Associates
750 Ott's Chapel Road
Newark, DE  19713
ph: (302) 368-2575
fax: (302) 737-2819