There are basically two types of signals that can be sent over a transmission line: analog or digital. Analog signals vary continuously in either amplitude or frequency. An example of an analog signal is a dimmer-controlled light: as the control is rotated, the intensity of the light slowly increases or decreases. The dimmer switch is actually varying the voltage amplitude of the output analog signal. The other type of signal transmission is digital. A digital signal has only two states: on or off. Digital transmissions are analogous to a light switch. Think of the eye as a receiver. As the switch which is flipped on and off, the eye sees the light flicker on and off. In the same way a digital signal is transmitted to a receiver conveying this same on-off information.