If you've made it this far, you now have the correct yellow fiber (Singlemode) running into a wall outlet in your home. The last step to have internet is to connect this outlet to your provider's Router (or ONT). To do this, you'll use a ready-made, flexible cable called an Optical Patch Cord.
But this is where many users make a fatal, costly mistake. They go to a store, see a cable that "fits", buy it, forcefully push it into the Router and... permanently damage the machine's socket.
In fiber optics, the shape and color of the plug is not a matter of aesthetics. It is a strict, international code. Let's decipher the initials (SC, LC, APC, UPC) to connect your equipment with the security of a professional.
The first difference concerns purely the physical size and shape of the plug (Connector). There are dozens of types, but in home networks (FTTH) two dominate:
Practical Tip: If your Router has an SC connector and the wall outlet has an LC connector, you can simply buy a Patch Cord that is SC on one end and LC on the other. The light will pass normally.
It's the big, square plug. It has a "Push-Pull" mechanism (you just push it in and you hear a "click"). It is the absolute standard for wall sockets (Rosettes) and for the entry of operators' equipment (ONT).
It is exactly half the size of the SC. It looks a lot like telephone/internet plugs (RJ45) in that it has a small, plastic clip (tongue) on the top that you have to press to remove. It is mainly used in professional Racks and Switches (in the so-called SFP modules), because it saves a lot of space.
Here lies the crucial detail. Inside the plastic plug, there is a small, white ceramic ferrule. In the center of this cylinder is the end of the fiberglass.
For light to pass from one plug to the other without loss, these two ceramic ends must touch each other. The way this end is cut and polished at the factory determines the color of the plug.
Always BLUE.
The edge of the glass is polished completely flat (actually it has a subtle, symmetrical curve, but the contact is perfectly vertical, at 0 degrees).
In legacy networks, in Data Centers internal connections and in fiber optic Ethernet equipment.
Always GREEN.
The edge of the glass is cut and polished to a strict 8 degree angle.
It is the ultimate, global standard for FTTH (Fiber to the Home) and cable television (CATV) networks.
When light reaches the junction of two plugs, a small percentage of it is inevitably reflected (bounces back towards its source). This is called Return Loss.
What if your provider's Router has a green (APC) connector, but you have a blue (UPC) patch cord in your drawer and you try to use it?
The Absolute Rule of Fiber Optics: GREEN ONLY GOES IN GREEN. BLUE ONLY COMES IN BLUE. (There is absolutely no exception to this rule).
When it's time to buy an optical cable for your home, look at the outlet on the wall and the socket on your Router. If both are green (as is the case with 99% of residential FTTH connections in Greece), you will ask for a "Singlemode, Simplex, SC/APC to SC/APC" cable. Remove the small protective caps, hear the characteristic "click" during insertion, and enjoy the speeds of light in complete safety.
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