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Re: [HSSG] Higher speed trade offs



Rich Taborek made several presentations proposing an optical PAM scheme during the HSSG led by Jonathan Thatcher that became the 802.3ae project.




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From: Hugh Barrass [mailto:hbarrass@xxxxxxxxx]
Sent: Thursday, September 14, 2006 9:44 AM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [HSSG] Higher speed trade offs


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? :-)


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



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