Tip of the Week – 12 Feb 2013
Tip of the Week:
Below is a difficult debug scenario that you may encounter on short fiber links.
Your boss ask you to setup a short GigE fiber optic link between your CO and a customer location over an existing 1km (0.6mile) single mode fiber cable span. You agree and tell your boss its simple and you will have the link ready in 30 minutes. You grab two 1310nm GigE SFPs and run out to the end points and install the SFPs in the appropriate equipment connecting the fiber jumpers to the 1km fiber cable’s fiber distribution panel (FDP). The equipment link light next to the SFP turns green and you think the job is done. Just to be sure you dig out the bit error rate (BER) tester to confirm the link integrity. You loop back the far customer end and connect the bit error rate tester at the CO end. To your surprise the tester is beeping erratically counting up errors, 1000 in the first minute, 5000 in the next minute, 40000 after a few more minutes. It shows a bit error rate of 10E-4. Totally unacceptable. Whatâ€™s wrong?
So you run back to the equipment room and grab an optical power meter and source. First you check the receive optical power at 1310nm, its -6dBm. Checking the SFP specs you see that this level is well within the SFP’s operating range of 0dBm to -20dBm. Then you decide to measure the total fiber loss with your power meter and source at 1310nm. It measure at 6dB. You remember that when the fiber cable was installed the OTDR trace showed a total fiber loss over the 1 km span of only 0.5 dB. It must be the fiber jumpers you think, so you replace all the jumpers and try the BER test again. No luck, just as many errors as before. Ok then it must be bad SFPs, so you replace one then the other. Again, no luck still tons of bit errors coming in. Is there something wrong with the bit error rate tester? You check it and loop it back on it self. This works fine, so it is ok.
Four hours have gone by now and the link is still not ready. You are getting upset, your colleges are wondering why you are running back forth and your boss is wondering if you know what you are doing. You are ready to call the equipment manufacturer to complain about @&#% faulty equipment they sold you. But before you do that, you decide to try one more test since the 6dB loss is quite puzzling for this short of link.
You run back to the equipment room again and dust off the OTDR, and grab the manual too since its been a while since you used it. You run a trace at one end and it shows you a nice straight line with no intermediate connectors or splices and a fiber span loss of 0.6 dB. This is strange since the power meter showed you a span loss of 6dB. You run to the other end of the fiber link and try the trace again. This time the OTDR shows a fiber span loss of 0.7 dB. Hmmm, you read through the manual some more and check the optical return loss (ORL) reading. It shows 8dB. This is way to low, and should be closer to 30dB. There must be a connector issue at the OTDR end. Unfortunately the OTDR trace cannot see that connector because of the OTDRâ€™s dead zone of 50 meters. You need a dead zone fiber.
You run back again to the equipment room and dig out the dead zone fiber and then run the trace again. Yup, the OTDR shows that the connectors at the fiber distribution panel have very high reflectance. You try to clean it but it does not help. They need to be replaced. You check the other end and same problem. Connectors at both fiber span ends have to be replaced. You call a fiber splicer and he replaces all connectors the next day and leaves you with a $600 bill. You again run an OTDR trace and now the ORL is 28 dB, much better!
You now check the fiber span loss and its 1dB, much better too. The bit error rate tester now records zero errors, nice and clean. One day has gone by, you finally tell your boss the link is ready and you hand him the $600 bill.
What happened? The poor quality fiber connectors were reflecting the signal back into the fiber that ultimately interfered with the original signal. The low loss in the short fiber link did not attenuate these reflections much when they were bouncing back and forth in the fiber resulting in an interfering signal being received with the original signal, similar to an echo. The SFP minimum receiver sensitive is specâ€™ed at -20dBm, but often the SFP will receive signal powers much lower than that.
Moral of the story, be sure to perform an OTDR trace with a dead zone fiber and check ORL on new OSP fiber cable connector installs.
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To learn more about this topic and other fiber optic network topics, refer to my following books (publisher McGraw-Hill):
“Planning Fiber Optic Networks“, ISBN 0-07-1499199 available from Amazon.com .
â€œFiber Optic Installerâ€™s Field Manualâ€, ISBN 0-07-135604-5 available from Amazon.com.
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