Line Troubleshooting. Part 2

I don't know if it's actually true, but you constantly see posts online about the danger of an ONU getting stuck in PON networks. If the laser on an ONU stays on, it can bring down the entire branch it's connected to. Here's why. As I mentioned earlier, the OLT constantly sends commands telling each ONU when it's allowed to transmit upstream. It also sends periodic signals to check for new devices. Either way, every ONU needs to send data back to the OLT from time to time — and managing those transmission windows is the whole foundation of PON.

Now let's look at what happens in more detail. The OLT sends data and instructions to a specific ONU on 1490 nm, telling it, for example, that it expects upstream traffic and status info at a certain time. The ONU waits for that slot and starts transmitting on 1310 nm. But if a faulty ONU has its laser stuck on at the same wavelength, the OLT receives both the signal from the working ONU and noise from the stuck one at the same time. The OLT can't properly decode the data, so it retries. And it'll do this for every ONU on that branch — eventually none of them will be able to transmit successfully.

That said, these cases are pretty rare. I've only ever read about the risk — I've never actually seen anyone on a forum describe it actually happening. Plus, manufacturers build hardware safeguards into their equipment to prevent stuck lasers.

What's even worse is when someone plugs any 1310 nm transmitting device into a subscriber's fiber — like a media converter, a switch with an optical port, or an SFP module. Any subscriber with a basic understanding of fiber optics could do this. Whether they're trying to cause trouble or just curious about what happens doesn't really matter. The bottom line is that everyone else on that branch loses Internet access, and the operator gets all the complaints.

To be honest, I actually ran a test in the lab. I connected a 20 km media converter to a PON setup I'd built on a bench using CATV splitters. Total split ratio was 108 ways — three splitters in series: 6, 6, and 3 ways — with total loss around 21 dB. And the PON equipment kept working just fine. Maybe the media converter I used had a weak laser, but real-world PON losses are higher anyway. For reference, a typical 64‑way split using two 1×8 planar splitters gives about 20‑21 dB loss, plus another 3‑4 dB for connectors.

In general, the probability of an OLT receiver being blinded by external signals is pretty low — but it's still possible. That's why network designers usually include the ability to disconnect individual branches, all the way down to the subscriber level. The easiest way is to put splitters on connectors, so you can just unplug a branch. All you'd need is a pigtail spliced to the outgoing fiber and connected to the splitter tap through an optical outlet.

If you do run into interference and everything is spliced, your only option is to visit every subscriber and unplug their ONUs until you find the culprit — or call them and ask to power down their devices. But there's a good chance the subscriber won't be home. In that case, you have to grab your splicer and start breaking and re‑splicing fibers one by one. That's not exactly fun — splice closures are usually up on poles. If the connections are connectorized, though, you can isolate the source much faster by just unplugging branches. Start at the main closure to find the problem segment, then work your way down to the subscriber drops until you find the noisy connection.

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