[179623] in North American Network Operators' Group
RE: DWDM and EDFA and DCM
daemon@ATHENA.MIT.EDU (Jameson, Daniel)
Thu Apr 23 12:34:15 2015
X-Original-To: nanog@nanog.org
From: "Jameson, Daniel" <Daniel.Jameson@tdstelecom.com>
To: Brandon Martin <lists.nanog@monmotha.net>, "nanog@nanog.org"
<nanog@nanog.org>
Date: Thu, 23 Apr 2015 16:34:05 +0000
In-Reply-To: <5538A0F7.9030906@monmotha.net>
Errors-To: nanog-bounces@nanog.org
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 tol=
erance of 73.5ps/nm but you don't want your dispersion number to ever go n=
egative. If it's a single-span only rule of thumb is use the next size s=
maller than the measured fiber distance maintaining at least 10km on the b=
ottom 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 th=
e ring pass a channel through, or is only optically amplified, you need t=
o calculate DC along the entire path, ensuring that the DC number never go=
es negative.=20
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-channe=
l DCM, it gets really messy as the system grows. And remember always clean=
-scope-clean!
-----Original Message-----
From: NANOG [mailto:nanog-bounces@nanog.org] On Behalf Of Brandon Martin
Sent: Thursday, April 23, 2015 2:36 AM
To: nanog@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=20
> (chromatic dispersion) of your 68km fiber stretch, and if you have a=20
> 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=20
> levels to be low. So if you want to solve your power problem, add EDFA=20
> mid-span. If you want to be able to use 40km optics (they might be=20
> cheaper), add DCM as well if the manufacturer rates them as not being=20
> able to electronically compensate for dispersion more than 40km.
>
Should you find yourself on the edge (or unknowing) of the dispersion toler=
ance of the 80km modules you would like to use, 120km-tolerant modules are =
also somewhat readily available these days, including from fiberstore. The=
y 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 E=
DFAs, these days, that could be a significant cost savings.
As always when buying whitebox/commodity networking goods, careful review o=
f the specifications and testing in your proposed application is in order.
--
Brandon Martin
