Multimode FO cable and long service loops

[gkoncic86]

Hi everyone,

I would like to hear the opinion of at least 3-4 people who did something similar in the praxis…

So let´s say that I have 200 – 300 meter long Multimode fiber optic cable and service loops of approximately 15 meters on every end.

Should that be a problem for speed of 1 Gbps, considering multimode dispersion, different speeds of propagation for every wavelength and because of so much loops and reflection angles…. ??

As I have researching it should not be a problem with single mode fiber, but what is with multimode I read somewhere  that it can happen that the signals on receiver end receives different wavelength at a different time and if this difference is big enough that it can make signal unusable because than it would mix with edge of the next signal ??

Anyone has experience on field, testing or only theory ?

[chrihussey]

Compared to single, multimode fiber has a much larger core diameter and is more susceptible to chromatic dispersion, whereas multiple reflections of the original wavelength can arrive at the receiver at different times.

However, as long as one stays within the distance operating specifications of the multimode fiber this should not be an issue. One thing to consider is the type of multimode fiber as there are variations. With 62.5 micron fiber you could get from 722 to 902 ft with Gig. With 50 micron multimode, depending on the OM rating, you can get from 1640  to 1804 ft with Gig. In addition, one could use SM optics on MM fiber and with the aid of mode conditioning patch cords you could also get up to 1804 ft. 

The distances you reference, 200 - 300m,  are within the specs for using MM fiber at Gig speeds. I've never had an issue at these distances and in some cases have pushed the envelope a bit further. The biggest benefit with the MM fiber is that the optics are considerably less expensive than the SM optics.

Hope this is helpful.

[gkoncic86]

HI, tanks for the answer!

The only thing that I am here worried about is that if I make so much service loops (let´s say 30 meters on a cable length of for example 300 meters), that with that I could increase chromatic dispersion, or anything like that which would make such influence on the signal that it is not working anymore.

So as I heard from a college of mine, with multimode cable you should NOT have service loops, or if you have them, then they should be very short and with the biggest possible bending radius.

What is your opinion ?

[chrihussey]

I've never had an issue with fiber that was caused by a service loop. Naturally one should not exceed the bending radius, but other than that I don't think it should be an issue. I can't speak to your colleague's experiences, but in my 20+ years it's never been a problem.

[gkoncic86]

Cool, thanx!

And what do you think, what is the longest service loops that you had - with multimode cable, was it more like 2-3 meters, or more like 20-30 meters ?

[chrihussey · Accepted solution]

For the most part, service loops are on average 10 meters. More than that and you're just wasting space in your IT closet. I talked with one of my cable installers and it is not uncommon to have 20 - 30 meter service loops with outside aerial fiber on poles.