Re: Analog Interface for 10Gb/s Ethernet
jaime, i have a couple of comments on this:
At 11:09 AM 14-04-2000 -0700, Jaime Kardontchik wrote:
>[...]
>Mike Wincn, wrote on 01 Mar 2000:
>
> "...finally, there are probably many ways one can
> build a fast ADC but the more difficult challenge -
> for 10 Gbd x-PAM - is in building a fast linear DAC."
>
>The main reason of this difficulty is not just building
>the many-bit DAC but the combination DAC + laser_driver.
the quoted text appears to be accurate (though i would need to dig up
that article to be certain). however, the message is taken out of context
and i feel i must clarify some detail so that no one will be misled.
there are certainly some challenges associated with building fast, linear,
monolithic DACs, but in that article my concern was for direct modulation --
no intermediate stage between DAC and diode. use of an intermediate
gain stage can solve some of those problems, though it will likely introduce
others.
you've quoted that text more than once, as if to draw some kind of
conclusion. but for clarity sake, i didn't conclude anything. i didn't
imply anything could or could not be done. i did not imply one thing
should or should not be done. i simply said one task is more difficult
than another. i left things open for further discussion.
>[...]
>2) Present production-worthy VCSELs lase in the 850 nm window.
>At 850 nm the bandwidth of the installed MMF is only 160 MHz*km.
>At 5 Gbaud the eye closes completely at about 50 meters
>of fiber.
just as an aside, past experience with equalizers for copper-based
data comm suggests that simple time domain observation of
"a completely closed eye" is not sufficient to conclude that data
cannot be recovered and low BER not preserved. it can -indicate- a
problem, you may argue that is a 'necessary' condition, but it is not
sufficient. whether such equalization used there can also be used in
10GbE, or not, is an open question. if a given equalization scheme
-relies- on "a completely open eye" then i argue it is a poor choice.
>[...]
>Given the challenges of your proposal, wouldn't be reasonable
>to say that the technology needed for this proposal "is not here"
>and the right way to proceed is to be sincere and brave and
>recognize this fact and withdraw your proposal from further
>consideration by the 802.3ae ?
jaime, given the variety of 10GbE links that are currently on the table,
given that the group must narrow the selection to a small few, given that
those few should include the kind of thing an end-user would want to
use, wouldn't it be reasonable to ask you to offer some kind of summary
for any support you have for your proposed solution?
a clear statement could help narrow the selection and help move things
forward.