Attenuation in Data Communication

Suppose two persons are talking over a wired connection. The sound, receiver hears is not loud enough to understand what the caller is saying. However, the caller is saying with enough loudness. So, what makes the volume go low at the receiver. The reason is attenuation, and it is caused due to the imperfection of the transmission medium.

Fig 1: Attenuation 

In this case, wire is the transmission medium. Consider a voltage V is applied across a wire of resistance R, and current I flows through it for time t. Some of the electrical energy is lost to overcome the wire's resistance. The energy lost is dissipated as heat which is calculated as H = Rt*I^2 Similarly, when the signal travels through the transmission medium, it experiences some resistance. The signal loses some of its energy to overcome the resistance. This loss of energy of a signal is called attenuation. It decreases the amplitude of the signal obtained at the receiver, which reduces the volume of the sound the receiver hears. The signal's amplitude is increased with devices called amplifiers to compensate for the losses due to attenuation. It makes the sound at the receiver loud enough.

So, along with a wire or transmission medium, the signal loses its strength as it moves from point A to point B and gains strength from point B to point C. The loss or gain in signal's strength is measured in dB and calculated as 10log10(P2/P1), where P1 and P2 are the powers of the signal at two points. If the dB value is negative, then the signal is attenuated. If the dB value is positive, then the signal is amplified.

Let us take an example. A signal travels through the transmission medium, and its power is reduced to one-half, then one can calculate the dB value as dB = 10 log10 [(P1/2)/P1] = -3 dB It means the signal is attenuated by 3 dB.

We can also add the dB numbers. Therefore, one can calculate the dB value between two points by adding the dB values at several points. To understand this, let us take the transmission medium where the signal is first attenuated by 3 dB, then amplified by 7 dB. Finally, it is attenuated by 3 dB. One can find the resultant dB value for the signal between points A and D by adding the dB values between each set of intermediate points. It comes out to be 1 dB. It means the signal has gained power as it moves from point A to point D.