You have upgraded your home to FTTH. In the first weeks the internet "flew". All of a sudden, you start experiencing dropouts, speed drops dramatically, or you see a scary red light (often labeled LOS - Loss of Signal) flashing on your Router or ONT. The internet went down.
In the old copper networks (ADSL/VDSL), the drop in speed was usually due to moisture in the road wells, bad weather or electromagnetic interference. Optical fiber, as glass, is completely immune to them.
When an optical fiber has a problem, the causes are almost always purely mechanical or optical. As engineers, troubleshooting follows specific steps. Let's look at the usual suspects and how you can fix basic problems without waiting for the technician.
If we had to bet on the cause of an optical network failure, 80% of the time it's a dirty connector.
As we have seen, light passes through a core only 9 micrometers (µm) in diameter. A speck of dust, some pollen from the open window, or a fingerprint on the tip of the green plug (SC/APC) is enough to completely block the Laser or reflect it back.
This is the most classic mistake made when arranging the living room. In an attempt to hide the yellow patch cord behind the TV cabinet, you fold it sharply or tie it tightly with plastic ties (Zip-ties).
To keep the light inside the core, the fiber relies on the phenomenon of "Total Internal Reflection". If you bend the cable beyond a certain limit, the angle changes. The light, instead of being reflected, "pierces" the walls of the mantle and is lost in the room. This light leakage is called Macrobending Loss.
Check the entire path of the cable from the wall outlet to the Router. If the cable makes an "angle", straighten it. The golden rule is: Each curve of the fiber should be at least the size of a coffee saucer (radius 3-5 cm). Use only Velcro straps to gather the cables, never hard ties.
The fiber doesn't just stop working if it's cut in two. Often, the damage is invisible from the outside.
The yellow wire was found under the sofa leg, or someone stepped on it with a heel. The outer yellow plastic appears intact, but the glass inside has shattered, creating an impenetrable barrier to the Laser.
Technicians use a tool called a Visual Fault Locator (VFL). It looks like a lens that emits a powerful, visible red Laser. They connect the VFL to one end of the cable. If the glass is broken internally somewhere along the way, red light leaks through the crack and the plastic cable "glows" brightly at that point like a lightsaber!
Unfortunately, if the glass is shattered, it cannot be fixed. The only solution is to replace the Patch Cord with a new one.
Sometimes, the glass is in perfect condition, but the electronic component that receives the light is malfunctioning.
If you talk to your carrier's tech support, they will remotely check your line and report "Loss in Decibels (dB)". The light signal is measured on a negative scale (dBm).
Optical fiber is perhaps the most reliable means of data transmission ever devised by man. Eliminates interference and annihilates distances. However, it requires respect for the nature of its material: glass.
By keeping the plugs (the green APC terminals) spotlessly clean, avoiding sharp bends and protecting the cable from mechanical stress, you ensure that your FTTH investment will work seamlessly for decades.
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