Just installed a 350' Fiber Optic cable for a client. Customers IT company came in to install the AT&T router and said the fiber optic cable is not working. IT company installed the router with a short patch panel cable at the junction block and said the router was working. We do not have sophisticated test equipment so we used a very bright flashlight and were able to see light in the fiber optic cable (Single Mode LC). Is it possible AT&T needs to boost the signal for this distance? Is there a cost effective way to test the cable without expensive test equipment? Cable was purchased pre-terminated and passed internal testing. We are new to the fiber optic side and install mostly ethernet cabling systems so please forgive our limited knowledge. We tried searching many forums but there is limited information that is more confusing than helpful.

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  • What optics/SFP's are you using at each end? – Chopper3 Jun 14 '18 at 6:42
  • Not sure the router / data box was provided by AT&T – user473961 Jun 14 '18 at 17:29
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    You have, by your own admission, limited knowledge of fibre optic cables and no test equipment. Why on earth are you installing fibre? A professional will know when to say to the customer "you need to get a specialist in for this part of the job"; this was one of those times. – MadHatter Jun 17 '18 at 7:25

350' is a negligible distance for SMF. Most long-wave PHYs do 10 km easily.

A very simple way to test the run is to hook up two 1000BASE-LX ports to it, check if they link and transmit data without errors. You can also put a loopback on one side (LC simplex cable) and the other side's port should link with itself (don't connect the switch to the production network, obviously).

Using 1000BASE-BX10 ports on each side, you can even check each fiber in the pair separately. Additionally, with a loopback on the far end you can connect two ports on one side through the run and the loopback.

A root cause for a failed fiber might be stressing the cable during installation or mismatching the fiber type - you can't mix single-mode and multi-mode fiber, even with a mode-conditioning patch the attenuation penalty is severe.

With pre-terminated fibre there's little to repair short of installing a new run.

  • Thank you for your response. While it is possible something happened during installation we were extra careful and used indoor / outdoor plenum which has a pulling strength of 100lbs. Based on our limited knowledge on fiber optic how would we connect two LX-ports and how would they communicate if the switch was not live? If we use a loop back where would we connect the 1000 base LX port in the router? Sorry for all the questions i tried to search for answers and did not find much that was helpful. Any suggestions for where we can learn more about fiber? – user473961 Jun 14 '18 at 16:39
  • 1000BASE-LX uses 1310 nm wavelength and expects single-mode fiber. You either need one router/switch with -LX on each side or use a simplex LC cable on one side and connect the other to an -LX (switch/router/NIC/converter) port. With the loopback as a basic link test, a switch or router with LLDP or CDP support should report its own info on the port. – Zac67 Jun 14 '18 at 17:02

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