Line Troubleshooting. Part 3

I'd say this section is still mostly theoretical, because I want to think out loud a bit about using an OTDR to find high-loss points in fiber lines. We do have an OTDR at the company, and I've even used it successfully — but only on regular point‑to‑point links, like from a node to a building or between two nodes. In a PON setup, though, you've got optical splitters dividing the signal. Since our company also offers cable TV services alongside Internet access, the trace on the OTDR screen looks somewhat familiar. In cable TV networks, we also use optical splitters — and in fact, PON is designed to carry the TV signal on the same fiber to subscribers, using the 1550 nm wavelength band. That's the classic wavelength for CATV, just like 1310 nm. But you can't use 1310 nm in PON — that's the upstream wavelength from subscribers to the OLT. Which immediately creates a problem: you can't take an OTDR measurement from the headend on 1310 nm, even if only one subscriber is connected. And what if there's more than one? Plus, any attempt to measure would knock all subscribers on that PON branch offline.

I should probably explain a bit about how an OTDR works. It's an incredibly useful tool for anyone working with cables. In a nutshell: the OTDR fires a laser pulse into the fiber — it knows exactly how long, how powerful, and when it sent that pulse — and then starts measuring the reflected signal. The signal reflects back from the fiber material itself (that's called backscatter) and from any imperfections along the way. By analyzing how long it takes for the reflection to return and how strong it is, the OTDR builds a visual map of the fiber. You can see excessive loss as a downward step on the trace. Loss usually shows up at bad splices or where the fiber's bent too tightly. Reflections from imperfections, on the other hand, show up as spikes. These are typically connectorized joints (where the signal crosses from one medium to another) or cracks in the fiber. Since the OTDR knows the round‑trip time and the speed of light in the fiber, it can calculate the distance to each event. So you get a complete profile of loss and distance from the measurement point. If you know the cable route and span lengths, you can usually pinpoint exactly where the problem is.

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