Line Troubleshooting. Part 1

When designing a PON network, you're constantly balancing optical budget against maintainability. As they say, you can never have too much budget — so ideally, the entire path from OLT to subscriber would have no connectors at all, just splices. But that's not really practical. Imagine having to strip the incoming cable, glue a connector onto the fiber, and plug it straight into the OLT's SFP port. I'd love to have equipment that could re‑coat the fiber with buffer material in the field. On top of that, epoxy‑termination is a pain — you need special tools and gear for polishing, curing, and inspecting the polish quality.

So most operators use pre‑terminated pigtails instead — these are factory‑made fiber tails with a connector already glued, cleaved, polished, and tested for loss. Then you just splice the pigtail to the cable fiber inside splice closures or distribution frames, which protect the splices from damage and the elements. Cross‑connections to outside lines are done through optical outlets. So right off the bat, we've already got one connectorized joint in our link — actually three: two at the OLT port (one on each side of the adapter) and one at the ONU input. These are unavoidable. The number of additional connectors depends on the designer.

In our network, as I mentioned before, we add one more connector at the splice closure where the subscriber drop cable connects, plus two connectors on the top‑level splitter — one at the input and one at the tap. That gives us a total of six connectorized joints in the link. Typical loss per connector, according to manufacturers, is about 0.5 dB, which adds up to 3 dB total for all six. For splices, industry standards allow up to 0.1 dB loss, but in practice that's a very conservative number. Fujikura, for example, measured their splicers' average losses in the 0.01‑0.03 dB range — which, as you'll agree, is way lower than the budget you'd normally reserve.

Connectorized joints are a potential source of trouble, and the more you have, the higher the chance of something going wrong. Plus, connectors at the top of the network affect service for many subscribers at once. In my experience, I've run into excessive loss on adapters quite often, and replacing the outlet usually fixes the problem.

This article is a translation of the original Russian-language post.My journey of learning GPON