Jameson, Daniel Daniel.Jameson at tdstelecom.com
Thu Apr 23 16:34:05 UTC 2015

Rule of thumb is you need Dispersion compensation for any single  span over 60Km.  10G/STM64 has a CD Tolerance of 1176 ps/nm, 40G/STM256 has a CD tolerance of 73.5ps/nm  but you don't want your dispersion number to ever go negative.  If  it's a single-span only  rule of thumb is use the next size smaller than the measured fiber distance  maintaining at least 10km on the bottom end,  65km would use a 40km dcm  70km  would use a 60km dcm.  As long as each site does OEO you can do Dispersion hop-by-hop,  if any node on the ring pass a channel through,  or is only optically amplified,  you need to calculate DC along the entire path,  ensuring that the DC number never goes negative. 

DC should be inserted between the egress of the combining mux and the post-amp to maximize your launch power.  Try to stay away from channel-by-channel DCM, it gets really messy as the system grows.  And remember always clean-scope-clean!

-----Original Message-----
From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of Brandon Martin
Sent: Thursday, April 23, 2015 2:36 AM
To: nanog at nanog.org
Subject: Re: DWDM and EDFA and DCM

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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