In a message dated 7/19/00 2:00:24 AM, vipul.bhatt@xxxxxxxxxxx wrote:
>As we think about 10G on installed MMF, there is one issue we
>haven't discussed - equalization. Perhaps thinking about it will
>throw more light and provide another perspective.
>In theory at least, equalization looks very promising....
>In reality, there are a couple of challenges, applicable to both
>850 nm and 1310 nm cases...
>1. DMD: Can equalization overcome DMD? Some have suggested
>that DMD can be modeled as a multipath effect, something that
>the folks in wireless industry know how to deal with. When viewed
>in terms of a transversal filter, the multipath problem boils down
>to having enough taps and setting their coefficients.
>I am asking if this idea is worth discussing.
It is interesting to see what the folks in the 802.11 (WLAN)
have achieved after several years of full-time work. First,
they (or, more correctly, their predecessors) invested a great
amount of years characterizing the multipath environment
(indoor offices, buildings, factories). Then they (again their
predecessors) developed statistical models that can reduce
the amount of data into a small set of manageable equations
(Rice and Rayleigh distributions) that have been found
experimentally to represent fairly well the wireless multipath
environment. Based on this rich amount of previous work,
the 802.11a and 802.11b Task Forces dedicated about two
additional years to develop Standards for the physical layer.
I do not see where we have this prior characterization work
in the area of installed multimode fiber. It could take years to
accumulate the needed data and achieve a similar degree of
confidence in the presumable multipath-type models for MMF
that would fit these data.
Is there anything else we could learn from the 802.11 guys ?
The 802.11a Task Force adopted the OFDM approach to reach
(on paper) a respectable maximum data rate of 54 Mbps.
In OFDM the data is sent on 48 parallel channels riding on
different frequencies (sub-carriers). In this way the symbol rate
on each subcarrier is smaller and a longer symbol period can
be used. Using longer symbol periods minimizes the effects of
multipath delay spreads. And no equalization of the multipath is
needed (or it is a trivial one-tap equalizer after the FFT (Fast
This sounds familiar. Something essentially similar is being
proposed in 10 GbE: we call it 4-WDM. We divide the data into
4 parallel streams riding on four different wavelengths, so the
symbol period in each of them is large enough that we can live
with the spread in rise and fall times in the multimode fiber.
The other Task Force, 802.11b took a different path: brute force
equalization. They barely reached (on paper) an 11 Mbps
maximum data rate. After they finished their standard they realized
that they would not be able to compete with the 54 Mbps 802.11a
Standard in the market. So they have now a new Study Group to
enhance the speed of the 801.11b. Now, guess what ?
In the new study group there are a lot of voices that are proposing
to use OFDM (read CWDM) instead of equalization. Although there
are additional reasons why equalization is problematic in high-speed
WLAN (the need for short preambles), the general trend today in very
high-speed wireless LANs is to avoid the onerous fight (equalization)
against multipath and instead reduce the symbol rate using parallel
channels (OFDM) so that the multipath effects become
a manageable problem.
To summarize: might be that equalization is not the right way
to go. And even if it were - it is not a task that can be solved
in weeks to the satisfaction of the members of the 10 GbE
Task Force. It will not be the factor that would change significantly
the results of the previous votes regarding the MMF proposals.
On the contrary, I would expect a backfire and dissapointment
after so many rosy expectations being raised on the Reflector.
I would suggest to prepare the Drafts for the MMF proposals
based on link lengths achievable without equalization. If a
company wants to pursue the equalization path - it could do
it and offer it as a potential differentiation advantage of its
future 10 GbE products.
Jaime E. Kardontchik
San Jose, CA 95131