To make an immediate voltage change in zero time requires an infinite amount of energy. The square wave is built from many harmonics or sine waves at higher frequencies. Each product in the square wave sum represents another harmonic at an integer multiple of the original frequency. The bandwidths of transmission media are limited If you have unlimited bandwidth, you can transmit all of the harmonics for a square wave and the wave will appear square. Therefore the wave is distorted .
All transmission media have limited bandwidth.
Each form of energy has different properties and requirements for transmission Transmission media may attenuate and distort the signal more or less. The transmitted energy is carried through some sort of medium . Transmitter encodes data as energy and transmits energy through medium. Different media have different transmission characteristics in terms of cost and bandwidth, etc.