Mbps and MHz are different

Even though the transmiting computer sends out square wave pulses, the medium see them as a group of sinusoidal waves.There lies the difference between Mbps and MHz . Medium is worried about mhz while the users, like you and me, are worried about mbps For example CAT6e ethernet cable is having the quality of handling 500 mhz. Because of this, it is able to transmit 10Gbps. Where as CAT3 cable is having the quality of 16mhz. Hence it is able to transmit only 10mbps.
In case of cat3, if you send 100mbps, the shape of the pulse will not be recognized by the received properly. Because, the higher frequecies are be attenuated almost completely when the signal reaches the 100 meter distance.