SC Fibre problem??

There are a couple of things to check first off that don't require test equipment. One being to make certain you are dealing with the same type of fiber. Be certain that the patch cords and the fiber are both multi or single mode. Then be certain what the switch requires. They have to be the same.

Second, be certain you can get light through the fiber links. Try lighting a fiber strand from the closet at one end with a flashlight (actually, there are adapters made for this) and see if you can see the flash of light at the other end. DO NOT TRY THIS WITH THE EQUIPMENT PLUGGED INTO THE FIBER! If the equipment uses a laser to light the fiber, you can permanently damage your eyes.

If you can see light, then you need to call in someone with fiber test equipment like a light meter and/or an OTDR to test the fiber.

It is possible that the patch cords you are using on the LC links are doing a double roll. You have to connect the transmit fiber at one end to the receive at the other. Sometimes the fiber is rolled at the interconnect cabinet, sometimes with the patch cord. If your fiber is being rolled twice, then your equipment is trying to transmit both directions on the same strand and the other end can't "see" what the other is sending.

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Justin Time
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I am currently trying to upgrade a site from old hubs to new switches, there is interconnecting sc and st fibre between the different buildings. The backbone switch I installed was a 3750 with 4 LC SFP's. The buildings that had ST interpatching worked fine on the new switch but the buildings that had SC fibre interpatching wouldn't work at all.

The following is an example that worked, from the backbone 3750 it starts: LC - ST, then to the local 2950sx switch the fibre is ST - MTRJ ...worked fine...

The following is an example that didn't work, from backbone 3750 it starts: LC - SC, then to the local 2950 sx switch the fibre is SC - MT RJ..wouldn't work..

Is there something I'm not doing? both st sites were ok but the sc sites wouldn't physically come up..

I really do apprecitate any help on this.

Thanks in advance Paul


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Hi Paul,

I have a feeling that the problem may be caused simply by cross-overing the patch cords twice instead of once as you are supposed to be doing. Here is the thing: since the STs cannot be combined into a duplex cord assembly, their position is not fixed and you have a flexibility to flip the connectors if they don't work in a particular position. Since both SC and LC connectors are clipped together in a duplex cord, you may lack this flexibility. Unless you take the clip off, obviously. Some manufacturers' cord construction does not allow to separate the connectors, but most do. If you cannot separate the SC end, try the LC. It does not matter which end gets switched.

In a situation like this a visual fault locator is an indispensable tool. If you do troubleshoot connections often, you may want to get yourself one.

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