As we defined an information channel, we may define a communication channel
as the set of components in the universe that implement the functionality
needed for the communication process to take place. Because communication requires
two or more information processes, a communication channel always consists
of two or more information channels in series. By using the word needed,
we refer only to those components that directly and obviously contribute to the
operation of the channel, ignoring the fact that a butterfly moving on the other
side of the world does affect the performance of a communication system on the
reader’s side of the world.
Using this model of communication, we may measure both the amount of
information that is present at different points in the model. For example, we may
measure the amount of information entering the system as well as capture and
describe the information at the output of the first and second processes and at
intermediate points in a communication hierarchy