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Re: Missing MAS in Maui


I am not sure how we can have a discussion
of channel bandwidth without using the term
"Hz", "MHz, or "GHz".  But I suppose when we
do use these terms we ought to agree on the 

If one talks about the rate of digital symbols 
in a serial link, I am accustomed to usage of
of the word "baud".  When describing the analog
bandwidth necessary to achieve the transmission
of the digital symbols I expect to see "Hz", "MHz",
or "GHz" used.  When we discuss channel bandwidth, 
it is absolutely necessary to consider rise and fall
times, else we might underestimate the required 
channel bandwidth.

If we use "baud" and "GHz" interchangeably, I think
we run the risk of confusing the issues.  


At 03:31 PM 12/21/99 -0700, you wrote:
>a lot of alliteration from assertive article authors...
>At 08:24 PM 20-12-99 -0600, Patrick Gilliland wrote:
>> >You're confusing signaling rate with bandwidth. The fastest signaling
at the
>> >transmitter for PAM5x4, that being an NRZ multi-level code, is 2.5 GHz.
In my
>> >MAS update presentation in Kauai, I specified the following Tx and Rx
>> >bandwidths, which correspond to your numbers:
>> >
>>I apologize for the confusion on my part; when one uses 
>>the "GHz" symbol I understand it to mean frequency.
>rich's comment is accurate -- a 5-Pam 5Gbaud line rate corresponds to a
>fundamental line frequency (here i ignore rise- and fall-time to
illustrate a point).
>the terms are not interchangeable yet people are not always careful with that
>distinct quality.

Only if we are sending sine waves is your statement true.
I do not believe the current HARI/MAS proposal advocates 
the use of sine waves at 2.5GHz over multimode fiber for
digital transmission.
It seems the real numbers for channel bandwidth for the
HARI/MAS proposal are really in excess of 5GHz +/- 10%,
depending on the cook.  This number could be reduced at 
the receiver port to 3.5-4.0 GHz again depending on the 
choice of filter type, etc.

>> >I agree. This is the well known 1300 vs. 850 nm tradeoff, but has
nothing to do
>> >with MAS.
>> From my point of view it has a lot to do with MAS.
>>MAS is the reason behind any SNR degradation at the 
>>transmitter.  With 1300nm, there is room below the 
>>laser safety limit to increase transmit power and
>>therefore offset the SNR degradation occasioned by
>you two seem to be talking past each other.
>for the same average power level no one can dispute that multi-level 
>modulation entails added power penalty for x-PAM, where 'x' is any
>integer larger than 2.  yes, one can easily argue that LW diodes offer 
>an advantage over SW diodes in power budget simply because an eye 
>safety limit for LW is much higher than for SW.  but this viewpoint is 
>narrow in that it does not allow for 'effective' SNR improvement through 
>use of schemes like forward error correction (FEC).
>now, right away someone will complain about higher cost of more
>complex silicon.  but then i argue that side emitters are already very
>expensive, and in the long run all of us really want to push cost out
>of optics and into silicon anyway.
>FEC details have not been described, but with it you can separate
>design issues for SNR and MAS.


I agree with most of what you have here.  FEC is one of
the best points this proposal has to offer.  It is true
we can achieve lower bit-error-rates with lower SNRs via
the use of FEC.  I agree with you also there is not a need
to fear the complexity of the Silicon (or whatever) IC because
of the benefits to be gained from the FEC provisions.  However,
FEC comes with the price tag of higher baud rates.

I do have an issue with your assumptions about the cost of 
edge emitting lasers.  Let's look for the least expensive 
laser in today's market.  By extension of your logic, the 850nm 
VCSEL should be the lowest cost.  But the 850nm VCSEL is not
the lowest cost 850nm laser available today.  It is the edge 
emitting 850nm laser which has a price less than 1/2 the price
of the 850nm VCSEL.  It is because the volume is much higher
in the edge emitting variety.


>>I would not lump multilevel in with WDM because WDM is a 
>>mainstream technology, widely deployed, and well character- 
>>ized.  Multilevel optical signalling has not been deployed
>>anywhere to my knowledge.
>but MAS can be applied whether or not there is also a cost-effective 
>solution in (D)WDM, thus affording the best of both worlds.


WDM does not scale in the same manner as MAS and is therefore
not a true channel in the classical sense.  WDM is really a
collection of channels (JBOC) which can no doubt be combined 
into one higher capacity channel.

MAS allows implementation as a single channel.  The potential  
difficulties with MAS are not theoretical, rather practical.
I do not see them as interchangeable unless we define a class
of transceiver which has a significant set of functions within 
to handle all the serialization/deserialization, deskewing, and
multi-level protocol multiplexing/demultiplexing which would
be necessary to make the interchanging of the two transparent 
to the host system.

I would argue for a much simpler optical transceiver definition.

Best Regards,

Pat Gilliland