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

Mark Foster blakjak at blakjak.net
Wed Dec 14 15:06:06 CST 2011


On 15/12/11 09:54, Justin M. Streiner wrote:
> 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.

I sense confusion in the above.

- LX drivers on MM fibre can work with Mode-Conditioning patch leads and
can give you significant distance wins, particularly if you're using
legacy OM1 Fibre. 
- SX drivers on SM fibre is not something i've ever seen done, I can't
imagine why you'd do it - even if SX drivers are cheaper.

>
> 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.

If you're not pushing your distance too far it'll probably be fine, to
be honest.
Back in the day when I was working on large legacy campus fibre runs,
220 metres was the max distance we considered OK for SX drivers and OM1
fibre (for gig ethernet).  Mode conditioning leads would push this out
to say, 900m trustworthy.  If your distance is >900m I would suggest a
fibre upgrade is on the cards.

Again, the above all assumes mode-conditioning in use.  If you're not
mode-conditioning your effective range is going to be very short - to
the point of unusability - and I'd be concerned about the affects of
'overdriving' fibre that is not set up for the use of low powered lasers
and was instead optimised for LEDs, which obviously put out a lot less
power.

Mark.



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