Re: [HSSG] Higher speed trade offs
Hi Hugh and others
I have been following this mailing list with interest and wanted to
comment on Hugh's statement in his email.
"It strikes me that if the sources and destinations of many carriers are
co-located and correlated then coding can eliminate inter signal
interference."
I am trying to understand what the advantage is of merging 10 10G
channels into one 100G channel versus keeping the 10G channels separate.
It seems to me all the buffering and SAR requirements required are only
of value if we do take advantage of all the dimensions. Obviously this
is something we've done in 1000BASE-T and 10GBASE-T by running some kind
of FEC over all four dimensions.
However we've already had a discussion on how FEC adds latency and that
may not be acceptable in short-haul applications. Also, decoding a 10
dimensional code would not be trivial, though the potential coding gain
would be large, allowing dense packing of wavelengths. Also, if there is
significant correlation across the dimensions/wavelengths we can take
advantage of that using maximum-likelihood detection techniques. Again
the complexity and latency become issues. However the maximum likelihood
approach is interesting in that it can be utilized without compensating
for any bulk skew mis-match between the dimensions/wavelengths.
I look forward to seeing how this work develops.
Cheers
Stephen
------------------------------------------------------------------------
Dr. Stephen Bates PhD PEng SMIEEE
High Capacity Digital Communications Laboratory
Department of Electrical and Computer Engineering Phone: +1 780 492 2691
The University of Alberta Fax: +1 780 492 1811
Edmonton
Canada, T6G 2V4 stephen.bates@ece.ualberta.ca
www.ece.ualberta.ca/~sbates
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Hugh Barrass wrote:
> Andrew and others,
>
> It often amuses me that technical principles from one field of invention
> seem to leak into other fields. The mechanism that you suggest strikes
> me as very similar to Discrete Multi-Tone modulation, used in DSL. There
> are some considerable advantages of compact multi carrier systems over
> higher baud rate single carrier systems. I guess it's only a matter of
> time before someone comes in (or back) with optical multi-level
> signaling to make the matrix complete :-)
>
> Not being an optical expert allows me the freedom to look at this from
> the outside and to suggest some ideas that may (or may not) be
> completely hopeless. Has anyone considered the use of FEC codes designed
> to correct errors caused by ultra-fine WDM spacing? It strikes me that
> if the sources and destinations of many carriers are co-located and
> correlated then coding can eliminate inter signal interference.
>
> Isn't communications theory fun? :-)
>
> Hugh.
>
> Ellis, Andrew wrote:
>>
>> Dear all,
>>
>>
>>
>> I have recently joined this reflector, and I would like to make one
>> observation which may be of benefit to the group. However, please
>> forgive me if I speak out of turn, am confused about some of the
>> acronyms or cover material already agreed.
>>
>>
>>
>> There appears to be some considerable debate relating to the trade-off
>> between reach, data rate, buffer requirements and the capabilities of
>> electronics with manufacturability at a reasonable cost. This has lead
>> to the M lanes at N Gbit/s per lane discussion, with M varying between
>> 1 and 10 and N taking values between 100 and 10 Gbit/s respectively.
>> Higher numbers of lanes appears good for electronics cost and reach,
>> whilst a higher serial data rate appears good for minimizing buffer
>> requirements and maintains a narrow occupancy of the optical spectrum
>> (to allow operation in a WDM environment, in turn allowing multiple
>> 100 Gbps Ethernet links to operate over the same fibre).
>>
>>
>>
>> I would like to propose the HSSG to consider the use of a new
>> modulation format, currently known as “Coherent WDM”, in order to
>> simultaneously meet all of these requirements. In Coherent WDM, we use
>> a single laser source, minimizing inventory. This source is either a
>> mode locked source, producing multiple carriers, or a standard cw
>> source followed by at least one sine wave driven modulator (10 GHz in
>> this example) in order to generate an optical carrier for each lane.
>> These carriers are then modulated using an array of modulators (one
>> for each lane, and each driven at 10 Gbit/s in this example), with,
>> for example, a PIC similar to the one proposed by Drew Perkins. This
>> produces a single 100 Gbit/s (in this example) signal, occupying a
>> small spectral width of very close to 110 GHz which is transmitted as
>> a single entity over a link (either point-to-point or a WDM network).
>> The compact spectrum and careful design of the PIC and drive circuits
>> combine to give negligible skew between the lanes, minimizing buffer
>> requirements. You thus obtain the key features of the high serial data
>> rates. It has been demonstrated that the reach of a Coherent WDM
>> system is dominated by effects proportional to the data rate of each
>> lane rather than the total data rate, and 10 Gbit/s electronics may be
>> used. You thus also obtain the key features of a high lane count, low
>> serial data rate link.
>>
>>
>>
>> I would be very happy to provide further details of Coherent WDM
>> should anybody reading this contribution feel that it is appropriate.
>>
>>
>>
>> Thank you for your attention
>>
>>
>>
>>
>>
>>
>>
>> Andrew Ellis
>>
>>
>>
>>
>>
>> Senior Research Fellow
>>
>> Photonic Systems Group
>>
>> Tyndall National Institute and Department of Physics
>>
>> University College Cork
>>
>> Ireland
>>
>>
>>
>> Phone: +353 21 490 4858
>>
>> Fax: +353 21 490 4880
>>
>> e-mail: andrew.ellis@tyndall.ie <mailto:andrew.ellis@tyndall.ie>
>>
>> web site: www.tyndall.ie/research/photonics-systems-group/index.htm
>>
>>
>>