Vectoring is the culprit
A copper line has a limited capacity to transmit data. This is partly because a copper line is sensitive to noise. To optimize capacity, your modem uses vectoring technology. You can compare this with the noise canceling of your headphones. Your modem listens for the noise on the line, and then sends an opposite signal to eliminate the noise
To do this properly, all modems in your street must work in harmony. One bad apple can cause trouble to the whole neighborhood. To prevent this, Proximus only allows certified devices on their network.
What causes interference on a DSL line?
Copper lines are subject to interference from radio signals and electromagnetic signals. For example, these devices can cause noise through radio signals: Wi-Fi transmitters, baby monitors, cordless telephones and broadcasting equipment. Electromagnetic disturbances can be caused by light dimmers, speakers, microwave ovens and screens, among other things.
The copper lines themselves also interfere with each other. They sit in large numbers close together over long distances and can thus exchange signals. This is called crosstalk, and in the past you could take that quite literally: you could sometimes hear someone else's conversation on your landline.
Why are you free to choose your own fiber router?
With fiber, Proximus always installs a fixed modem (also called Optical Network Termination or ONT). The router that you attach to it, cannot cause network disruptions. Therefore, no certification is required, and you can choose your router yourself.