Re: PAM-5 at 1.25 Gbaud
Rich,
I hoped that you would come with a retraction.
My comments are placed within your email.
Jaime
Rich Taborek wrote on Jan 27, 2000:
> Jaime,
>
> My basic disagreement with your spreadsheet is that you seem to be simply
> summing optical intensity penalties, with little regard for how the penalties
> interact. In addition, some of these penalties are not quite accurate.
You would seem to have a special theory and simulation results
that you do not want to share with us. Please, be concrete.
> ....
>
> For example, for PAM5 loss, you note a penalty of 6 db optical. My understanding
> is that this penalty is an SNR penalty whose value is more correctly 4.1 db
> optical.
Now you are concrete. And you are wrong. The PAM-5 penalty
amounts to 6 dB optical signal power and 12 dB electrical signal
power.
You refer to the model presented by Del Hanson and David
Cunningham in York. In their presentation at York they do not
mention this issue at all. The reason was that this issue had
already been discussed and agreed upon in previous meetings.
See, for example, David Cunningham's presentation in Montreal,
July 99.
> The "swing" of a PAM5 signal at the receiver is of approximately the
> same amplitude from the lowest PAM5 level to the highest as it is for a PAM2
> (binary signaling) 0 -> 1 transition.
Here we agree.
> It is incorrect to simply subtract even
> 4.1 db from the link budget due to PAM5 loss, especially in light of coding gain
> which can significantly increase effective SNR.
Again, if you consider that something is incorrect,
please, describe concretely the right way of calculating
electrical SNRs and how to add coding gain.
I described in great detail the way that I calculate
the SNR for a PAM-5 system in the presence of
coding gain. See my presentation in Kauai, Nov 99.
Part II of that presentation, "BERs and RINs", deals
on how to include coding gain in the calculation of
the effective SNR. Part IV of that presentation,
"Coding Gain", presents simulation results that
verify the correctness of the theoretical models
developed in Part II.
>
>
> In York (September '99), Del Hanson proposed modifications to the GbE link model
> to extend the model to be used for 10 GbE PMD development. I also presented some
> issue modeling issues.
I went through your presentation in York. In this
presentation you do not present any concrete modeling
information. On the contrary. In your opening slide
you emphasize that the 1 GbE Link Model is NOT part
of any standard. And in your closing slide you state
that the development of a Universal Link Model for
10 GbE could turn into a "Holy Grail" effort.
This does not look to me as a support on your part
for any rational model.
> I believe that it's in the best interest of the HSSG to
> use a common, well understood and proven link model for 10 GbE PMD development.
I use the 1 GbE model for my simulations. I do not see
any significant discrepancies between Del Hanson's
presentation and my results. My link simulations results
seem pretty similar to Del Hanson's. For instance,
my link simulations give 2.5 dB (optical) ISI loss
for 62.5 um 500 MHz*km MMF and 3.5 dB (optical)
ISI loss for 50 um 400 MHz*km MMF at a fiber
length of 300 meters for on-off keying modulation.
This is consistent with Del Hanson's results of 3.6 dB
vertical eye closure penalty for the combined characteristics
of 62.5 um and 50 um MMF for 300 meters link length
(see Del Hanson's presentation in Kauai, Table 38-8).
>
> I suggest that you obtain a copy of Del's model and use or modify it to get a
> better apples to apples comparison of the various PMD proposals including
> yours.
The "installed MMF spreadsheet" is not intended to
be a replacement of all the details of the Gigabit Link
Model that are already well documented (see, for
example, David Cunningham's book or Del Hanson's
presentations). It is not either a replacement of the
complete description of the individual proposals.
The objective of the spreadsheet is to present briefly
on one page the main differences between the different
proposals targetted specifically towards the installed
multimode fiber.
**** ELECTRICAL SNR ****
The spreadsheet adds also an important item that
people did not have to deal with in 1 GbE: Signal to Noise
Ratios. In 1 GbE all the architectures (1000BASE-LX/SX,
50 and 62.5 um) use the same symbol rate in the fiber,
1.25 Gbaud. Hence, it made sense to concentrate only
on the signal power and receiver sensitivity (again
signal power) and ignore the noise power that was
similar in all the cases, since everyone had the same
receiver bandwidth.
The proposals that are targetted to supporting the
installed MMF in 10 GbE have a large variety of
symbol rates: 1.25, 1.5625, 2.5, 3.125, and 5 Gbaud.
Hence, we have to deal now with receivers that have
different bandwidths.
Therefore, a meaningful comparison between the
different proposals has to include also the noise
power of the receiver. Hence, the SNR becomes an
important figure of merit . Just saying how much
signal power we have at the receiver is not enough.
Jaime
Jaime E. Kardontchik
Micro Linear
San Jose, CA 95131
email :kardontchik.jaime@xxxxxxxxxxx