The bandwidth of a channel is the range of frequencies that it can transmit with reasonable fidelity.
For example, if a channel can transmit with reasonable fidelity a signal whose frequency components vary from 0 Hz (dc) up to a maximum of 5000 Hz (5 kHz), the channel bandwidth B is 5 kHz. Likewise, each signal also has a bandwidth that measures the maximum range of its frequency components.
The faster a signal changes, the higher its maximum frequency is, and the larger its bandwidth is.
A signal can be successfully sent over a channel if the channel bandwidth exceeds the signal bandwidth.
To understand the role of B, consider the possibility of increasing the speed of information transmission by compressing the signal in time. Compressing a signal in time by a factor of 2 allows it to be transmitted in half the time, and the transmission speed (rate) doubles.
Now, to transmit this compressed signal without distortion, the channel bandwidth must also be doubled. Thus, the rate of information transmission that a channel can successfully carry is directly proportional to B.
More generally, if a channel of bandwidth B can transmit N pulses per second, then to transmit KN pulses per second by means of the same technology, we need a channel of bandwidth KB.
So, the number of pulses per second that can be transmitted over a channel is directly proportional to its bandwidth B.
The Electromagnetic Spectrum
The range of electromagnetic signals encompassing all frequencies is referred to as the electromagnetic spectrum.
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