MAS vs. WWDM vs. Serial TDM

[Rich Taborek <rtaborek@xxxxxxxxxxxxxxxx>]

I find the discussion of points of view on the various 10 GbE PHY
proposals to be very interesting and long haul space requirements to be
very informative. However, I'm a bit concerned about making choices so
early in an IEEE effort since the Study Group hasn't even met yet. I
encourage all reflector participants to continue to help educate the
masses rather jumping to conclusions like: 

>First, let me say that I agree that long-term (say 6-10 years out) a
>low-cost 10-Gb/s serial solution may be the simplest and lowest cost
>solution.  That having been said, I think that with today's technology
>(and for several years out) WWDM will be the lowest cost and most useful
>technology for 10-GbE LAN applications.
>
>Brian Lemoff

- and -

>What if HSSG defined a common fiber module interface for both WDM to
>serve the LAN applications and Serial to serve the WAN applications?
>
>Shawn Rogers

Now that the weekend is here I have a bit of time to get my 2 cents in
about the advantages of Multilevel Analog Signaling (MAS). I'll use
Brian Lemoff's note of May 7, 1999 as a forum for my comments as he
addressed both the serial TDM and WWDM proposals in his note: 

>First, let me say that I agree that long-term (say 6-10 years out) a
>low-cost 10-Gb/s serial solution may be the simplest and lowest cost
>solution.  That having been said, I think that with today's technology
>(and for several years out) WWDM will be the lowest cost and most useful
>technology for 10-GbE LAN applications.

MAS requires only one of the 4 laser and photodetector sets required by
WWDM, requires only a simple single channel stream like all standard
Ethernet variants, requires no waveguides, and a single-chip CMOS
implementation looks feasible. Since the laser/photodetector is likely
the highest cost component of both MAS and WWDM, I'd say that the
potential cost of MAS vs. WWDM is likely to be significantly lower. I
remember a survey run by Ahmad Nouri, Compaq, during GbE development
(http://grouper.ieee.org/groups/802/3/z/public/presentations/mar1996/ANgigsur.txt)
where implementation cost was the #1 Customer rated Ethernet technology
selection criteria.

>Fiber:   A 4 x 2.5-Gb/s WWDM module in the 1300nm band should still
>support useful distances of up to 300m on the installed base of 62.5
>micron core fiber.  The SpectraLAN approach, like 1000LX, will
>simultaneously support multimode and single mode applications (up to
>10-km) with a single transceiver.  All 10-Gb/s serial approaches that
>have been proposed (excluding multilevel logic) will require new fiber
>to be installed in premises applications.

Agreed. The Transcendata MAS scheme employs "T-Waves" and is the only
scheme proposed which is capable of compensating for dispersion. All
others schemes must assign a link penalty to dispersion in the form of
an Inter-Symbol Interference (ISI), Mode Partition Noise (MPN) or other
link penalty. The ability to transport multiple bits per Baud (ala Fast
Ethernet and GbE over UTP-5) and compensation are the methods which
allow the use of longer multimode and single-mode links than all other
proposed schemes.

Dispersion compensation allows MAS technology to be deployed in "bad"
fiber environments (e.g. DMD, etc.) as well as for long-haul links
including the "gopher bait" fiber mention by Bill St. Arnaud. MAS
technology should be compatible with most EDFA's (these amplifiers are
linear). MAS technology should also be compatible with WDM systems (i.e.
as a "feed").

>Laser Cost:  At 2.5-Gb/s, low-cost uncooled, unisolated DFB lasers can
>be used with no side-mode suppression requirement (double moded lasers
>are okay) up to 10km.  These lasers are readily available today in die
>form at costs not that much higher than the FP lasers used in 1000LX. 
>Linewidth, RIN, and Jitter requirements at 2.5-Gb/s are MUCH easier to
>realize with high yield and low-cost electrical packaging than they are
>at 10-Gb/s (not to mention 12.5 Gbaud). Optical isolation will probably
>be required to achieve the necessary noise and linewidth requirements
>for a 10-km, 10-Gb/s serial link (Lucent presented an unisolated FP
>solution for 1km.  The data they showed for a 10km uncooled DFB link
>required isolation).  Given this, I believe that the 4 lasers required
>for WWDM will be many times lower cost than the single laser required
>for serial.

...and the 1 low-cost uncooled, unisolated DFB laser which may also be
used for MAS will be 4 times lower cost than the 4 lasers required for
WWDM. These lasers easily meet the linearity requirements for MAS.

>Optical Packaging Cost:  The 1000LX standard has forced transceiver
>vendors to develop low-cost automated alignment and precision die attach
>systems for aligning edge-emitting lasers to single-mode fiber. In our
>WWDM solution, we are leveraging such a system to robotically assemble
>and align our 4 lasers and MUX in a fast, low-cost process.  On the Rx
>side, only multimode alignment tolerances are required to align the
>demux to the detector array and glue it into place. The mux and demux
>optics themselves are low-cost parts (many times lower cost than a
>micro-optical isolator). The mux is a simple, unpolished, unpigtailed,
>silica waveguide chip (several hundred devices on a standard 4" wafer). 
>The demux is an injection-molded plastic optical part, requiring minimal
>assembly.  This may sound complicated, but it is not expensive.  As we
>get further into the standards discussions, we'll provide more details
>that should help convince the skeptics that this is a realistic and
>low-cost solution.

A MAS optical package consists of a connector shell (e.g. SC, SFF,
etc.), one laser, one photodiode/preamp. This is as simple as it gets.

>Electronics:  WWDM at 2.5-Gb/s per channel works with existing low-cost
>Si electronics.  10-Gb/s serial Tx and Rx IC's will require processes at
>least 4 times faster. Add to this the tighter jitter and noise
>requirements, the poorer performance of dielectric circuit boards, the
>higher laser current requirements (required to push relaxation
>oscillation frequencies 4 times further out), and you have a difficult
>electrical problem to solve.  The cost associated with the electronics
>and electrical packaging is likely to be much higher than that for 4ch
>WWDM for several years.

MAS electronics are different than mainstream On-Off Keying (OOK)
optical link electronics, but the function of each block is fairly
similar. The most difficult part of MAS is the high speed D/A logic on
the transmit side. However, our current direction of exploring a
single-chip CMOS implementation should make the D/A complexity much
easier to swallow. MAS T-Wave signals force zero-crossings to greatly
simplify receiver clock recovery circuitry. Given the 4 channel WWDM
approach, channel skew, and receiver power penalty, I'd say that MAS and
WWDM electronics are likely to be comparable in scope.

>Scalability:  Bryan made a good point that a 10-Gb/s serial solution
>adopted now could be combined with WWDM later to provide even higher
>capacity (e.g. 40 Gb/s).  Why not adopt the WWDM (4 x 2.5 Gb/s) solution
>now, when 10-Gb/s lasers and electronics are still very expensive, and
>then in a few years, increase the channel rate to 10-Gb/s.  Either
>solution for 10-GbE is scalable to 40-Gb/s when it is combined with the
>other.

I kind of like the MAS over WWDM using 10-Gbps lasers where 4 MAS
channels run at 40 Gbps over 4 WWDM colors and the 10-Gbps laser drive a
single fiber at 160 Gbps. This should take care of the 100 Gigabit
Ethernet market :-) 

>Eye-safety:  The proposed power budget for SpectraLAN meets the Class I
>eye-safety requirement by a comfortable margin.  At 1550nm it would be
>even better, but increased fiber dispersion and the lack of
>well-characterized fiber in the LAN make this a more difficult option. 
>It should be noted that 4 lasers means 6-dB less eye-safe power
>available per laser, but at 4 times the speed, for a given IC process, a
>typical receiver will be less sensitive by at least 6 dB, negating the
>eye-safety advantage inherent in the serial approach.

MAS signal are average optical power balanced and similar in nature to a
"slower" OOK signal carrying the same amount of data. 

I'll add EMI to the list of MAS advantages. MAS signaling is narrowband,
resulting in emissions at 10 Gbps similar to those found in GbE PHYs. I
anticipate that all other schemes will have substantial difficulty
meeting emissions requirements. This is not a PHY characteristic to be
taken lightly.

>"Inherent Simplicity":  A serial approach is "inherently simple".  The
>question which we must answer over the coming year is which approach
>makes the most practical sense from a performance and cost perspective,
>given the technologies that are available today.

Hear, hear.

-- 

Best Regards,
Rich

------------------------------------------------------------- 
Richard Taborek Sr.    Tel: 650 210 8800 x101 or 408 370 9233       
Principal Architect         Fax: 650 940 1898 or 408 374 3645
Transcendata, Inc.           Email: rtaborek@xxxxxxxxxxxxxxxx 
1029 Corporation Way              http://www.transcendata.com 
Palo Alto, CA 94303-4305    Alt email: rtaborek@xxxxxxxxxxxxx