Parts of the signal may be delayed and arrive at different times at the receiver. An easy way to observe this is with an AM radio, since AM has properties that make it possible to hear many of the effects of transmission impairments. An AM signal will fade as the receiver gets farther away from the transmission tower. This occurs with FM also. In fact, the attenuation is greater for higher frequencies.
Delay distortion is most interesting, especially in fiber optics.
An example of this is the rapid oscillation of an AM signal which is very far away. We will discuss this further in Chapter 3. Noise is also an interesting phenomenon, and one which we can quantify. We can also use this to take a closer look at the maximum possible transmission rate in the presence of noise. Let's take each type in turn.