Brandon Martin lists.nanog at monmotha.net
Thu Apr 23 07:36:23 UTC 2015

On 04/23/2015 12:01 AM, Mikael Abrahamsson wrote:
> If you use 80km SFP+ then they should be able to handle the CD
> (chromatic dispersion) of your 68km fiber stretch, and if you have a
> power problem, then you can solve that by adding EDFA mid-span.
> CD causes "noise" (OSNR) to the receiver, it doesn't cause your power
> levels to be low. So if you want to solve your power problem, add EDFA
> mid-span. If you want to be able to use 40km optics (they might be
> cheaper), add DCM as well if the manufacturer rates them as not being
> able to electronically compensate for dispersion more than 40km.

Should you find yourself on the edge (or unknowing) of the dispersion 
tolerance of the 80km modules you would like to use, 120km-tolerant 
modules are also somewhat readily available these days, including from 
fiberstore.  They don't have the power to shoot 120km without external 
(generally mid-span) amplification, but they will tolerate the 
accumulation of ~120km worth of chromatic dispersion.  Thus, you can do 
120km of fiber (typ.) with EDFAs in the span for power budget reasons 
but without an accompanying DCM at each hop.  Since the commodity DCMs 
cost almost as much as commodity mid-power EDFAs, these days, that could 
be a significant cost savings.

As always when buying whitebox/commodity networking goods, careful 
review of the specifications and testing in your proposed application is 
in order.
Brandon Martin

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