[Cerowrt-devel] 10GigE nics and SFP+ modules?

[Chuck Anderson]

> thank you all for sharing your knowledge!
> 
> In my life I've punched holes in coax, run arcnet, climbed mountains
> and tall buildings to run wifi P2P links and cut and crimped more
> twisted pair than I care to remember, but my exposure to fiber's
> methods has been sadly limited, til now.
> 
> I still don't quite have a grip on how and when to use various forms
> of cwdm or what wavelengths make the most sense, when...

"Normal" optics (sometimes called "grey") run at the standard
wavelength defined for whatever standard they are made for, for
example 1000BASE-SX runs at 850nm over multimode fiber, and
1000BASE-LX runs at 1310nm over singlemode fiber.  Similarly,
10GBASE-SR is 850nm and 10GBASE-LR is 1310nm.  These are the commonly
available wavelengths and would be what you would normally use unless
you have a specific reason to do otherwise.

You use CWDM or DWDM "colored" optics when you have a scarcity of
fiber strands and you need/want to run multiple signals or links
across them.  For example, for metro fiber or long haul fiber, you get
charged by how many strands you lease and how long the run is.  With
cheap passive CWDM or DWDM multiplexers, you can run multiple 1gig or
10gig Ethernet links across just 2 strands of metro fiber, or even a
single strand with "bidirectional" optical modules which use a
different wavelength for each direction.  As another example, if you
want to run Fibre Channel alongside Ethernet over the same strands you
can also do that with WDM.

Longer haul links may require more sophisticated active WDM gear and
amplifiers and can get very expensive.  Historically, these active WDM
systems also had (and may still have) "transponders" to convert from
the standard "grey" wavelengths 850nm or 1310nm installed in the
routers/switches to the WDM wavelengths used on the WDM links.  It is
becoming more common though to forego the expense of the transponders
and instead install the WDM optics directly into the routers &
switches via removeable SFP (1gig - 4gig) or SFP+ (8gig, 10gig)
modules.

The C in CWDM stands for Coarse, and the spectrum is divided into 4,
8, or less commonly 16 wavelengths, from 1270nm to 1610nm.  Here is a
good pictorial:

http://a2net.eu/images/dwdm.gif

The D in DWDM means Dense, and there are 40, 80, or more wavelengths
much more tightly packed together, usually wholly within the 1550nm
band.

More here:

http://a2net.eu/en/solutions-fiberoptics/