I'm in charge of the new network installation in the new office of my company, and I have troubles wiring two network rooms (one for servers and incoming internet and the other one for wiring closets) via an internal optical fiber installation.

In both network rooms, I have a panel with Duplex Multimode optical fiber with SC connectors, linking the two rooms with 2 optical fibers. I chose to use Netgear Prosafe Gigabit switches to link the two network rooms, (https://www.netgear.com/business/products/switches/smart/GS752TP.aspx#tab-techspecs), 1000Base-SX Duplex fiber SFP modules to connect them with fiber (https://www.startech.com/Networking-IO/sfp/modules/1000base-sx-sfp-module~SFP1000SXST), and LC to SC optical fiber to wire them to my panels (https://www.amazon.com/Mcl-Samar-Jarreti-Optique-multimode/dp/B004Y0YTFY/ref=sr_1_1?keywords=FJOM3%2FSCLC-1M&qid=1560791159&s=gateway&sr=8-1).

So, both my switches have the SFP module, outputting Duplex LC connectors to Duplex SC on my panels via the optical fibers I bought.

My switches are working correctly, I can discuss from one another with RJ45 copper connections between them, and the SFP ports are ok, as I can stack my switches with SFP copper modules and cables and still send pings betweend one computer on each switch. It's really my design of the fiber connection that must be off.

Have I mistaken something about optical fiber installations? Is some of my hardware inadequate or incompatible? In the meantime, I'm contacting the company who wired the internal optical fiber, to make sure they are OK on their side.

Here is a simple diagram to visually check my installation. Note that there are other optical fiber SC connectors in both my panel because I have other wiring closets rooms, but with the same kind of installation (I am still trying to figure out how to make a single one work) enter image description here

I'm a beginner in network engineering and still don't know if this is the right place to ask this kind of question but I'm in a bit of a dead-end and have little time left so I'm a bit desperate!

Thanks so much if you can help me :)


2 Answers 2


In addition to Ron's excellent answer:

1000BASE-SX runs on OM3 for up to 550 m officially - hope you're not exceeding that (too far).

Also with the majority of switches, the SFP modules must be brand compatible with the switch - not necessarily sold by the original vendor but explicitly compatible with the original part. Check out the documentation. Managed switches usually log the event in case they reject a module. [edit] The manual doesn't mention whether branded SFPs are required - you might need to contact support. [/edit]

In case the duplex matching as suggested by Ron doesn't work out you might want to check each fiber and patch cable separately.

Shining a bright flashlight or a laser pointer into one end lights up the other end (works for moderately short runs). That allows you to check if they got the pairing right and if the cross-overs are there. If you can't work in a team you can get SC-SC simplex cables to use as remote loopbacks. With most 850 nm transmitters, some portion of the light is even visible to the naked eye or to an electronic camera.

Similarly, you can use an SC-SC female-to-female adapter to check your patch cables.

Once you've verified correct operation of each single part the whole will work.

  • Hi Zac67, thanks for your thorough answer. You and Ron were right about crossing one end of the duplex wiring, but the biggest issue was that the optic panel was badly labeled and thus I ended up connecting some SC connectors that were related to a completely different end of my topology. As you pointed out, I've had to check each fiber to discover this issue. Thank you for your help!
    – ManuHeel
    Jun 20, 2019 at 7:16

The first thing to do when troubleshooting is to check the physical layer. That will include things like making sure that you have installed to polarity correctly. For example, the fiber at position A on one patch panel needs to be at position B on the patch panel on the other end. Your copper connections will probably automatically adjust the polarity when using a straight-through cable, but fiber connectors do not do that, so you must make sure that the transmit on one end gets to the receive on the other end.

This is directly addressed by the ANSI/TIA/EIA 568-B Commercial Building Telecommunications Cabling Standard:

Connecting Hardware and Polarity

Optical fiber shall be installed with odd numbered fibers having Position A at one end and Position B at the other. Even numbered fibers will have position A and B reversed from the odd numbered fibers. When using the 568SC connector or other duplex connectors, the above polarity must be maintained.

When using copper cabling, it must be straight through from termination to termination, but optical cabling must be crossed from one termination to another.

The reason fiber cabling is crossed is simple: when connecting fiber from one device to another, the fiber strands need to be crossed so that Tx on one end goes to Rx on the other end. For this reason, fiber patch cables are crossed, but when you have horizontal cabling, you have two fiber patch cords which cancel each other out. When the horizontal cabling is crossed, you end up with three crossed cables, making the whole a crossed cable, leading to success.

You also need to make sure that the fiber standard is correct for the optical transceivers in use. If you try to use single-mode fiber with multi-mode transceivers, or vice versa, then you will have problems. Also, the grade of the fiber must match the distance for the standard.

  • Hi Ron, thanks for your thorough answer. You and Zac67 were right about crossing one end of the duplex wiring, but the biggest issue was that the optic panel was badly labeled and thus I ended up connecting some SC connectors that were related to a completely different end of my topology. Thank you for your help!
    – ManuHeel
    Jun 20, 2019 at 7:14

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