I am comparing my cable TV wire to my network wire. In a TV cable wire, different frequencies are used by different TV channels. And since a certain channel use a fixed frequency, I think the only left way to represent different signal is with the carrier wave's amplitude.

But what about the network wire? For all the network cards with the same type, do they also use different frequencies to send signals just like TV cable? I vaguely remember that they use frequency adjustment to represent signals. So the frequency should not be a fixed one. So how did all the network cards that sharing the same medium differentiate their own signal from others?