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Communication started with optical communication – through fire and beacons. People used different coloured smoke to indicate different information.
The first formal way of communication was done in 1792. It was a French gentleman byname Claude Chappe, he set up a link across entire France with semaphore signals.
As early as the 1950s, communication system with electrical/electromagnetic communication was very well established, but the limitation was bandwidth.
All naturally occurring signals are analogue in nature.
The Nyquist Criteria states that the sampling frequency must be twice the largest frequency content of the analogue signal.

Carrier frequency indicates the frequency of the carrier signal, whereas, the bandwidth is decided by the message that is to be transmitted.

The frequency spectrum of a sinusoid with frequency fC is represented as two impulses (Dirac δ-function) at frequencies ±fC.
Signal bandwidth is a consequence of the spectral width of the modulating signal, whereas linewidth is the property of the carrier signal.
Amplitude shift keying: If the amplitude of the electric field is modulated depending on the bit, the scheme is known as ASK or OOK.

The measure of the magnitude of called spectral efficiency.
The transmitter may be followed by a coupler, a switch, a router, a multiplexer, depending on the
kind of the network. A wavelength multiplexer may be required to multiplex the two signals.