Range using single-mode SFPs across multi-mode fiber - was Re: NANOG Digest, Vol 47, Issue 56

Justin M. Streiner streiner at cluebyfour.org
Wed Dec 14 20:54:47 UTC 2011

On Wed, 14 Dec 2011, Keegan Holley wrote:

>>> inappropriate. We are attempting to use Juniper single-mode SFPs (LX
>>> variety) across multi-mode fiber. Standard listed distance is always
>>> for SFPs using the appropriate type of fiber. Does anyone out there
>>> know how much distance we are likely to get? Thanks for your help in
>>> advance.
> Single mode just has a smaller core size for the smaller "beam" emitted by
> laser vs. LED.  it works although I've never done it outside of a lab (MM
> is cheaper). As for the distance it theory that should come down to the
> optics and your transmit power.  Hopefully this is just a cable connecting
> the router to a long line.  I've never heard of a 10K MM fiber run since SX
> optics can't shoot that far.  You should be able to get through the 500m or
> so that MM optics are rated for, but YMMV (errors, light levels, bounces,
> etc etc)

In a nutshell, don't do it if at all possible.  This issue gets significantly
worse at 10G.  If there's any way to get SMF in place for this link, do it.

In practice, you will likely get something less than the rated distance, 
particularly if the MM fiber in question is an older type, such as OM1. 
If you're using OM1, mode-conditioning jumpers at both ends are pretty 
much a must.

The problems with shooting an LX/LH beam over MMF are threefold:
1. Attenuation on some flavors of MMF can be significantly higher than an 
equivalent run of SMF.
2. Modal dispersion on MMF will scatter and distort the LX beam, likely 
resulting in link errors because the receiver can't recover the data 
3. Shooting a 9 micron beam into a 50 (or worse, 62.5) micron core, and 
getting enough of the beam to reach the 9 micron target at the other end 
to result in a recoverable signal is problematic.


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