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Re: Analog Interface for 10Gb/s Ethernet


I included my responses in the body of your e-mail below:

At 11:09 AM 4/14/00 -0700, Jaime Kardontchik wrote:
>Hello Nariman Yousefi,
>In your presentation in Albuquerque, March 2000:
>   "Analog Interface for 10-Gb/s Ethernet"
>   by: Pieter Vorenkamp, Kevin Chan, Myles Wakayama
>   and Nariman Yousefi
>in slides # 11 and 12 you describe the specifications
>of the analog transmitter. Essentially, you use an
>8-bit 5 Gsample/s CMOS current DAC to drive a VCSEL.
>Several questions arise:
>1) You will need VCSELs with SFDR (Spurious Free
>Dynamic Range) of about:
>   SFDR = 20*log(2^8) + (2/3)*10*log(2.5e+9)
>        = 111 dB*Hz^(2/3)
>This SFDR spec for a VCSEL is quite a challenging spec.
>See, for example, the recent paper:
>   "Dynamic Range of Vertical-Cavity Surface-Emitting
>    Lasers in Multimode Links"
>   by: H. L. T. Lee, R. V. Dalal, R. J. Ram
>       and K. D. Choquette
>   IEEE Photonics Technology Letters, vol 11, pp 1473-75,
>   November 1999
>Is there a laser manufacturer that could provide production-
>worthy 5 Gbaud VCSELs with the above specs at a reasonable
>cost and within the time frame of the present Task Force ?

- Our system simulations assumed 1310nm wavelength to achieve more than
300m on existing MMF. Please look at the conclusion slide of our
presentation labled 10Gb/s PMD using Trellis Coded Modulation. The main
purpose of the Analog Interface presentation was to show the structure of
the implementation and show that parallelization can be used to achieve
high speed operation in CMOS.  The laser could be either FP or DFB and it
does not change that conclusion. It would have been more consistent to show
a DFB laser in the trasnmitter slides.

>                ********
>The design of an 8-bit current DAC at 5 GHz in CMOS is
>quite a difficult task. I will quote in this respect the opinion
>of one of the designers of the T-waves' team of Transcendata,
>who was faced with the same problems:
>Mike Wincn, wrote on 01 Mar 2000:
>   "...finally, there are probably many ways one can
>   build a fast ADC but the more difficult challenge -
>   for 10 Gbd x-PAM - is in building a fast linear DAC."
>The main reason of this difficulty is not just building
>the many-bit DAC but the combination DAC + laser_driver.
>The laser is a quite high-current consuming device. Using
>a VCSEL provides a significant advantage in this respect,
>since VCSELs have much lower threshold current than other
>types of lasers, easing the design of the CDAC+driver
>However, this leads to my next question to you:
>2) Present production-worthy VCSELs lase in the 850 nm window.
>At 850 nm the bandwidth of the installed MMF is only 160 MHz*km.
>At 5 Gbaud the eye closes completely at about 50 meters
>of fiber.
>In another presentation by your colleagues,
>   "10Gb/s PMD using PAM-5 Trellis Coded Modulation"
>   by: Oscar Agazzi, Nambi Seshadri and Gottfried Ungerboeck
>   Albuquerque, March 2000
>on slide # 7, they sustain that - even after adding all the
>complex Tomlinson-Harashima pre-equalizers in the transmitter
>and forward equalizers in the receiver - the maximum reach
>at 850 nm will only be 160 meters.
>Hence, your proposal that you submitted to the 802.3ae Task
>Force is really targetting a maximum of 160 meters on installed
>MMF, and not 300 meters (or even 500 meters, as was sustained in
>the Dallas meeting and corresponding spreadsheet).
>Hence, my second question to you is:
>Could you clarify to the 802.3ae Task Force what is the
>real maximum link length target on installed MMF of the
>proposal you submitted ?
>I would say it is only 160 meters on installed MMF. Maximum.
>And after all the complex pre- and post-equalizers and precoder
>initialization schemes.
>                        ***********
>Given the challenges of your proposal, wouldn't be reasonable
>to say that the technology needed for this proposal "is not here"
>and the right way to proceed is to be sincere and brave and
>recognize this fact and withdraw your proposal from further
>consideration by the 802.3ae ?

We ultimately have to design a product that will be successful in the
industry. Many successful products and standards in the past required
innovation and solving challenging problems. If we did not think that PAM-5
serial is practical, we would not have presented it.  

>It would not be the first time that an attractive proposal,
>with many good merits, has to be withdrawn because the
>technology is just "not here". See, for example, the fate of
>the "serial at 12.5 Gbaud" proposal.
>A clear withdrawal would clarify the field and would allow
>other proposals on the table to gather momentum and support
>from ASIC vendors, laser vendors, etc.
>Jaime E. Kardontchik

Nariman Yousefi
Director Networking Engineering

PH  (949) 585 5450
FAX (949) 453 1848
e-mail : Yousefi@xxxxxxxxxxxx