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Re[2]: 1310nm vs. 1550nm -> Eye Safety + Attenuation

     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