Voltage Levels and Logic
There is no 1 on a wire. There is a voltage, and there is a receiver that has decided in advance which range of voltages it will call one and which it will call zero. Digital logic is an agreement layered on top of an analogue quantity, and the reason firmware normally gets to ignore that is that the agreement usually holds — the hardware on both ends was designed to the same convention, so the bits you read are the bits that were sent.