Can I Use a Multimode Fiber Patch Cable with...

Posted on 2008-06-19
Last Modified: 2012-05-05
I have two sites that are connected via singlemode fiber (about 2000 feet run) and on each end the fiber terminates into a fiber distribution panel.  Can I use a multimode patch cable between the panel and the gbic?
Question by:nugentgl
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Accepted Solution

Roachy1979 earned 125 total points
ID: 21823352
The problem with this is the difference in size of cores between the single and multimode fibre.

Singlemode fibre has a core of between 8 and 12 microns, while multimode has a core of either 50 or 62.5 microns

The problem is that as light passes down the thinner fibre it should be "caught" by the thicker fibre, which possibly might work.  However, for light travelling in the other direction, a lot of the signal will be directed into the cladding of the fibre, resulting in loss of light.....

In short, it probably won't work due to the amount of light loss, although for the cost of a fibre patch lead you might get lucky......
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Expert Comment

ID: 21823451
In addition to this the transmission wavelengths are different for single and multimode - , so thinking on you would probably need a single/multimode media converter.....


Author Comment

ID: 21823671
What is strange is that the existing fiber patch cable is 62.5/125 (multimode, right) on both ends and has been connected like this for 4 years.  The reason I am changing is that I am moving the traffic on both ends to new switches with new gbics (before they were CISCO gbics in a 2948g).

What is the worst that will happen?  Will it fry a gbic?

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Expert Comment

ID: 21823939
So the sending and receiving devices are both multimode :)It's just the fibre in the middle thats single mode.... in that case ignore my comments about the transmission wavelengths...

No risk of frying the gbics....the worst that will happen is light loss on the jump between muti -> single mode as light gets fired into the cladding rather than the core....  If the amount of light still travelling down the core is adequate, the device will still function.....if not, it won't.....worst case scenario, you try, fail and then have to get media conversion...

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Expert Comment

ID: 21826997
I'd be willing to bet you'll end up with bandwidth limitations.
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Expert Comment

ID: 21845403
I'd agree that this isn't the most efficient (or recommended way of doing things) but with budgetary constraints it might work.... although not effectively.....

Author Comment

ID: 21845471
So it works.  Basically everything is single mode (fiber run, fiber distribution panel, and gbic's) except for the patch cable from the fiber distribution panel and gbic's.  I installed Qcheck to test the bandwidth between two servers that have gig connections connected directly to the switch that has the gbic's installed and the bandwidth is about 94Mbps.  This seems low but it also might be the free qcheck tester since the test file size only maxes out at 1meg.

Any suggestions for another free bandwidth tester?
Is 94Mbps normal for a gig connection?

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Expert Comment

ID: 21845857
I'd be willing to bet that the losses occur as a result of the jump from single/multimode and back can test the losses using an OTDR (although these are expensive and it would be a case of hiring the unit).  The only way to avoid this would be to take the plunge and get some single mode gbics (which are very expensive) and single mode patch leads.  The other alternative is to get a set of single/multimode media converters.....again, pricey though...

I've not used the Qtest tool but iperf is quite reliable.....


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