"The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point."
Our main purpose is to transfer digital information – which is a sequence of bits 0’s and 1’s – from one system to another through a communication channel. Let us return for a moment to the concept behind simple digital logic where logic 0 can be assigned to one voltage level while logic 1 to another. Provided the static discipline is followed, all our system electronics has to do is to be able to differentiate between these two levels.
Of course, digital circuits also have real voltages and real currents which are never exact. The infinite voltage levels are just broken into two discrete regions. Hence, they remain close to that state when the real world noise is introduced, which makes the digital circuits very robust and noise tolerant. This breaking down of real numbers on a continuous scale into discrete regions is the basis of the whole digital revolution.
This particular philosophy is very close to human nature too. Think about it: we like to classify everyday stuff into clearly defined boxes. For example, day and night, black and white, good and bad. The reality, however, is continuous in the sense that the brightness of light varies throughout the 24 hours, there are multiple shades of grey, and people are much more complex than being simply good and bad.
Digital communication follows the same principle but allows for multiple discrete regions. Let us explore why and how. Since many textbooks on digital communications concentrate on mathematical derivations of the concepts, we explain these concepts from more of an intuitive viewpoint without rigorous proofs. Those texts are highly recommended for readers seeking mathematical derivations.