Fiber within specs but signal strength isn't high enough with LX/LH SFPs

baskervi · ‎10-04-2012

We have multiple runs of single mode fiber from 1 - 3 KM. They all terminate to an intermediate building approximately 150 meters from the building with the switches that are using the LX/LH SFPs on both ends. All patch cables are SM. There are no problems from the far ends to the building about 150 meters from the end building regarding interface errors, and everything seems to be great. We move the connection to the end building, and we start having a very high number of errors on the longer fiber links.

The folks that did the fiber installation did the testing and certification, and they said from end to end (even with the intermediate patch cable installed), our loss was at most 15 db. We were told it shouldn't exceed 20 db.

Is there any way to increase the laser power on the SFPs? Is there anything else we should look at? Thanks for your time.

Tom Randstrom · ‎10-05-2012

The 1000Base-LX/LH SFPs operate at the 1310nm wavelength. The fiber loss (budgetary) at 1310nm is 0.35dB /km. For your 2 to 6km fiber path (assuming a jumper cable in the middle) would have a light loss of 0.70dB to 2.3dB. The connector loss is 0.75 dB (typical is <0.30dB) per connected pair. If you had 4 connectors in a path, you would have an additional loss of 3dB; for a total path loss of 3.35dB to 4.5dB total loss.

Fiber loss = 0.7 to 2.3dB

Connector loss = 3dB

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Estimated path loss = 3.7 to 5.3dB.

If the network you are describing is a maximum of 6km long, the folks that performed the fiber installation need to find where the additional light loss is occurring. 15dB loss for this network is not operating properly. It is possible that you have a bad fiber splice, or more typical, the fiber is bent or pinched somewhere (splice case or other) such that it exceeds its bend radius, causing excessive splice loss.

If my assumptions are not correct, please provide a network diagram so we can understand and troubleshoot your issue.

Good luck!