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WWDM vs. 10Gb/s serial

     I will try to respond to some of Bryan Gregory's remarks regarding 
     CWDM vs. 10-Gb serial. By the way, I will refer to it as WWDM 
     (SpectraLAN is HP's implementation of WWDM), since CWDM is apparently 
     used to refer to 400-GHz spaced telecom systems, and this has caused 
     some confusion among some people on this reflector.
     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.
     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.
     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.
     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.
     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.
     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.
     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.
     "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.  
     I hope I have at least provided a few reasons why 4x2.5-Gb/s WWDM might be 
     better than a 10 Gb/s serial approach, at least in the near-term.  There is 
     still a lot to be learned, a lot to be demonstrated, and an awful lot of 
     discussion to be had before one solution is chosen over another.  
     -Brian Lemoff
______________________________ Reply Separator _________________________________
Subject: Re[2]: 1310nm vs. 1550nm -> Eye Safety + Attenuation
Author:  Non-HP-bgregory (bgregory@xxxxxxxxx) at HP-PaloAlto,mimegw2
Date:    5/6/99 9:32 AM

     In response to Bill's email... regarding the EDFA issue, I'd imagine 
     that this would only be used in a small number of cases with a serial 
     10GbE approach.  I don't think it needs to be a core concern of the 
     group, but in some dark fiber trunking applications it can be useful.  
     I am most concerned about wavelengths vs. eye safety, and wavelengths 
     vs. fiber attenuation.  This could end up being a real killer.  Four 
     lasers @ 850nm or 1310nm put out quite a bit of light in an eye 
     sensitive range.  As I remember, four lasers at 1550nm offer a lot 
     more margin.  A single source at 1550nm could be very strong and still 
     meet the eye safe requirements.  This increase in power combined with 
     lower fiber attenuation would reduce some of the link distance 
     problems that we're bound to run into.  
     Also, long term I can't see how [4 lasers and an optical mux] + [4 
     photodiodes and an optical de-mux] would be better than a single 
     source and photodiode.  There is a lot of difficult packaging involved 
     in the CWDM approach.  I think the CWDM solution offers a quicker path 
     to market because most of that technology is available today. But long 
     term a single 10 Gb source (uncooled DFB without isolator) has a lot 
     of advantages.  It is intrinsically much simpler.  I think the board 
     layout and chip-sets will eventually support this as well.  If the 
     standard wanted to be able to scale beyond 10 gigs, even the serial 
     10Gb solution could allow further CWDM scaling.
     Bryan Gregory

______________________________ Reply Separator 
Subject: RE: 1310nm vs. 1550nm window for 10GbE
Author:  "Bill St. Arnaud" <> at INTERNET
Date:    5/6/99 10:38 AM

Hmmm.  I just assumed that 802.3 HSSG would be looking at 1550 solutions as 
well as 1310 and 850

I agree with you on longer haul links it makes a lot more sense to operate 
at 1550

I am not a big fan of EFFA pumping.  It significantly raises the overall 
system cost. It only makes sense in very dense wave long haul systems 
typically deployed by carriers.

CWDM with 10xGbE transcivers should be significantly cheaper.  That is 
another reason why I think there will be a big market for 10xGbE with all 
those transceivers every 30-80km on a CWDM system. However there is a 
tradeoff.  There is greater probablity of laser failure with many 
transceivers and the need for many spares.  I figure somewhere between 4-8 
wavelengths on a CWDM and transceivers is the breakpoint where it is 
probably more economical to go to DWDM with EDFA. Also EDFA is protocol and 
bit rate transparent.
An EDFA will ..(edited).....  But EDFA window is very small, so wavelength 
spacing is very tight requiring expensive filters and very stable, 
temperature compensated lasers at each repeater site.  Also laser power has 
to be carefully maintained within 1 db otherwise you will get gain tilt in 
EDFAs. A loss of a signal laser can throw the whole system off, that is why 
you need SONET protection swicthing. But companies are developing feedback 
techniques to adjust power on remaining lasers to solve this problem.
A single 10xGbE transceiver will .(edited)....??? Probably less.  So 6 
10xGbE transceivers will equal one EDFA.  No problems with gain tilt.  If 
you lose one laser you only lose that channel, not the whole system.  
Protection switching not as critical, etc
Bill St Arnaud
Director Network Projects