GPON: 4‑Port Subscriber Enclosure

Good day, dear readers.

Today I'd like to start a short blog series on subscriber enclosures (distribution boxes or splice closures). As I mentioned earlier, subscriber connections are made in enclosures where the incoming cable is terminated with SC/APC connectors and matching adapters. So far, we've purchased enclosures from several suppliers, but there were only two design variants. Below, I've shown them in photos — with their contents, opened up, and with the splice tray removed and placed next to the lid. The rotating splice tray locks into place with a latch. In the open position, it provides easy access to the optical adapters.

Variant No. 1.

Variant No. 2.

Both variants side by side.

As you can see from the photos, the main difference between these enclosures is the splice tray design. In the second variant, it's much more convenient to work with, and there's space to install an optical splitter — which is quite important and desirable for GPON applications. I'd also note the more convenient holders for heat‑shrink splice protectors and their greater number. In the first variant, the protector size doesn't match the holders very well, and the splitter had to be placed in the bottom right corner of the box, squeezed between the adapter holder and the wall.

Overall, "Variant No. 2" gives a much better impression of a well‑made, user‑friendly product: the lid screw doesn't fall out when you open it, the key feels more solid, securing the central strength member is easier (a single screw), the adapter ports are numbered, the lid latches more securely, and a protective tube for the fibers is included.

As for drawbacks, I'd note the following for both enclosures: they can only be mounted to flat surfaces with screws — which makes pole mounting awkward. The seal for the subscriber cables is a single piece — when you need to add a new subscriber, you have to pull it out, feed the cable with the connector through, and re‑insert the seal over all the cables. It would be much easier if each entry had its own individual seal (or you could just cut this one into four pieces). I hope the manufacturers take note of these issues and update the design.

All in all, "Variant No. 2" suits us perfectly and has proven itself well in our passive optical network. It's also suitable for any FTTH application.

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

Flat FTTH Drop Cable

Preparing a standard modular optical cable for splicing involves several seemingly simple steps:

• Stripping the outer sheath to the required length;
• Removing the aramid yarns;
• Flushing the optical modules with a special compound — D'Gel — to remove the water‑blocking gel;
• Wiping off the remaining D'Gel with purified gasoline;
• Opening the modules and cleaning the optical fibers — usually with gasoline as well.

Modular optical cable with a central strength member.

For an experienced splicer, these operations are routine and take about 5‑10 minutes per cable. A bit trickier is preparing a "loop" entry — feeding the cable into a splice closure without cutting the modules. This usually involves two longitudinal cuts along the sheath on the section to be opened, plus two transverse cuts at the cable anchor points. After that, the sheath comes off like peeling a banana. Cleaning the modules becomes more complicated too, since they're twisted together and it's hard to wipe each one individually to remove all the gel. You also have to be extra careful not to damage the modules — and therefore the fibers inside them. The "loop" entry allows you to significantly reduce the number of splices — unused fibers just pass straight through the closure without being cut. Plus, in my opinion, it improves reliability, since there's less chance of extra loss or fiber breakage at splice points.

It all sounds well thought out and reliable — in theory. In real life, though, working conditions aren't always ideal: rain, wind, freezing temperatures, cramped risers, dusty attics — you name it. And that's when you start wishing for a cable that could be prepped in seconds, without any cleaning solvents. Sometimes cable designers simplify the construction: they use water‑blocking tape instead of gel, a single central module instead of several, experiment with the amount of aramid yarn, the thickness of the central strength member, or the density of the polyethylene sheath. But none of that really hit the spot — I wanted something simpler :)

Apparently, I wasn't the only one thinking this way — and that's how subscriber drop cables for PON connections came about. These were flat, indoor cables, white in color, with the fiber(s) sandwiched between two strength members. Subscriber cables typically use ITU‑T G.657 fiber — bend‑insensitive, with low loss at tight radii. In practice, the cable turned out to be quite easy to work with, even without special tools — just split it in half along the grooves molded into the sheath, and you're ready to handle the fiber itself. Manufacturers didn't stop there and released an outdoor version — black in color. But as it turned out, using this cable outdoors is risky — its tensile strength is too low, even when the glass‑reinforced plastic rods are replaced with steel wire.

Flat FTTH drop cable — design and cross‑section.

At the CSTB exhibition, I finally saw exactly what I was looking for — Fujikura's flat outdoor cable with an additional strength member in the form of a steel wire about 1 mm in diameter. Despite its light weight, this allows spans of up to 50 meters — though that recommendation applies specifically to special anchor clamps for flat cables. For longer spans, the sheath can't take the strain and splits at the attachment point. If you really need a longer span, it's better to carefully separate the steel wire and tie it to a secure anchor point — that way you can go up to 80 meters.

Sheath damage at the attachment point.

Manufacturers, thinking about user convenience, went even further — they developed special connectors that mount directly onto the cable. No extra adapters or distribution boxes needed — you can make a patch cord and plug the cable straight into the equipment. I'm planning to write a separate post about those connectors soon.

SC/APC field‑installable connectors for drop cables.

To sum it up: in my view, flat drop cables — simple and affordable — are perfectly suited for their purpose: connecting endpoints, standalone buildings, shops, and private homes, using any technology, including GPON. But I'd also recommend not cutting corners — go for cable with "bend‑insensitive" G.657 fiber. When you're pulling it through tight channels and conduits, that significantly reduces the risk of micro‑bend losses — which means more reliable data transmission and greater peace of mind for you.

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

The Approach We Chose for Subscriber Connections

As part of our GPON subscriber deployment project, we specifically purchased an Ilsintech Swift F1 fusion splicer. The network rollout plan called for 100% coverage of the residential area, with all cable lines fully pre‑terminated for subscriber connections. To achieve this, we installed splitters inside the distribution enclosures, or terminated the fibers with SC/APC connectors and mounted optical outlets. The installation crew would only need to run a drop cable from the subscriber's premises to the pole where the optical box is mounted, terminate both ends, and connect it to the network and the ONT — or simply use a pre‑made patch cord.

However, if the subscriber enclosure doesn't allow a pre‑connectorized cable to be fed in, the drop usually has to be pulled through drilled holes, along beams in attics, tucked into baseboards, and so on. During all this handling, there's a real risk of damaging the connector — which would undo all the effort. Of course, you can use factory‑made cables with protective caps and built‑in pulling loops, but these solutions are still quite expensive.

There are also field‑installable connectors available, where the fiber is mechanically spliced inside the connector body. All you need is a stripper and a cleaver. In practice, though, achieving acceptable loss levels with these connectors is tricky — you need a microscope to inspect the cleave quality, and the loss can drift over time. I've even seen cases where the fiber broke completely, taking the link down. You also have to qualify the connection from both ends — install one connector, test it, send a signal down the line, install the second connector, and check the signal level again. If the loss is too high, you have to re‑terminate one of the connectors — and if that doesn't fix it, the other one too. To do this efficiently, you need two technicians on site, each with a full termination kit. And worst of all, you lose time troubleshooting and get that nagging feeling of doing unnecessary work — which is just unpleasant.

With Ilsintech's splice‑on connectors, we're expecting better quality splices because we can actually see the splice quality during the process and check the splicer's loss estimate. There's still a chance of a bad splice or mechanical damage when plugging the connector in, but the odds are much lower.

I'll write a separate post about the connectors and the splicer itself soon — because things turned out to be not quite as rosy as the reviews and sales reps made them sound. Every time, I'm reminded that there's no substitute for hands‑on experience — though sometimes it comes at a pretty steep price, and I don't just mean money.

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

Preparing an Optical Distribution Frame (ODF)

I'd like to share my experience with preparing a distribution frame (ODF) for installation. We try to buy pull‑out or swing‑frame ODFs, because if something goes wrong, it's much easier to replace a damaged adapter or resplice a pigtail.

The model shown in the photo isn't the best design — when you pull the frame out, it drags the cable along with it. It's much better when the cable stays fixed and only the module tray moves.

In that respect, NAG's swing‑frame ODFs are pretty good — but they have a drawback too: you can't recess the frame slightly into the enclosure, so there's a risk of damaging connectors when closing the cabinet door.

On the left side of the photo, you can see some mounting brackets taken from other wall‑mount ODFs and splice closures — these were salvaged from unused cable entry ports. You can end up with quite a collection of these over time :)

To fit these "new" brackets, we had to do a bit of modification — drill a few extra holes. They're marked with blue and red arrows in the photo.

And here's what we ended up with:

Cable ties are fine, but I prefer screws and nuts :)))

Next, we insert the adapters, label them, install the pigtails, route them, and trim them to fit the cassette. We often don't number the pigtails — instead, we use a visual fault locator during splicing to identify the right pigtail.

Here's a photo without the cassette for clarity:

And with the cassette in place:

We're planning to route the cable along the back wall of the frame from right to left, with the modules feeding directly into the cassette. I already visited the installation site beforehand to figure out the best way to bring the cable into the frame.

But really, repositioning the mounting brackets and pigtails shouldn't be a problem.

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