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Re: Range using single-mode SFPs across multi-mode fiber - was Re: NANOG Digest, Vol 47, Issue 56
From: "Justin M. Streiner" <streiner () cluebyfour org>
Date: Wed, 14 Dec 2011 15:54:47 -0500 (EST)

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 correctly. 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.

jms


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