In any communication system, the received signal is never is identical to the transmitted one due to some transmission impairments. The quality of analog signals will deteriorate due to transmission impairments as given above.
Attenuation:
The strength of a signal decrease with the increase in distance travelled over a medium. Attenuation means loss of energy. When any signal travels over a medium or channel, it loses some of its energy in the form of heat in the resistance of the medium. Attenuation decides the signal to noise ratio hence the quality of received signal. Attenuation is given in decibels as:
Attenuation (dB) = 10log10 (Pout/Pin)
Where, Pin= Power at the sending end
Pout= Power at the receiving end
Distortion (Harmonic):
Another meaning of distortion is change in shape of the signal. This type of distortion is observed for the composite signals made by different frequencies. If the medium is not perfect, then all the frequency components present at the input will not only be equally attenuated and will not be proportionally delayed.
Noise:
When the data travels over a transmission medium, noise gets added to it. Noise is a major limiting factor in communication system performance. Noise can be categorized into four types as follows: