DWDM and EDFA and DCM
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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