Any signal will slowly be lost over some distance. Not just in networking, but in many other situations as well. This is called Attenuation.
Think of sound for example. As you move away from the source of the sound, it will become quieter. This is because some of the energy of that sound is lost as it’s travelling over a distance.
This loss of energy, or power, is called attenuation. This applies to both wired and wireless networking.
A signal’s power is measured in decibels. Attenuation is the amount of power that a signal will lose per kilometre.
For example, OM5 multi-mode cable may lose 3dB of power per kilometre. However, single-mode fibre may only lose about 0.5dB per kilometre.
As a result of attenuation, cables and WiFi can only be used at a limited distance. Eventually, the signal will lose so much power that it is no longer usable. Broken or defective cables may even result in greater attenuation.
If a signal attenuates too far, a booster or repeater can be installed, to increase the power of the signal.