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Transmission Impairments

Analog signal consist of varying a voltage with time to represent an information steam. If the transmission media were perfectly, the receiver could receive exactly the same signal that the transmitter sent. But communication lines are usually not perfect, so the receive signal is not the same as the transmitted signal. For digital data this difference can lead to errors. Transmission lines suffers from three major problems

1. Attenuation
2. Delay distoration
3. Noise

Attenuation:

It is the loss of energy as the signal propagates outward. The amount of energy depends on the frequency. If the attenuation is too much, the receiver may not be able to detect the signal at all, or the signal may fall below the noise level. For reliable communication, the attenuation and delay over the range of frequencies of transmission should be constant.

Destortion:

The second transmission impairment is delay distortion. Communication line have distributed inductance and capacitance which distort the amplitude of signals and also delay the signals at different frequencies by different amounts. It is caused by the fact that different Fourier components travel at different speed.

For digital data, fast components from one bit may catch up and over take slow component from bit ahead, mixing the two bits and increasing the probability of incorrect reception.

Noise:

Noise is a third impairment. It can be define as unwanted energy from sources other than the transmitter. Thermal noise is caused by the random motion of the electrons in a wire and is unavoidable.

Cross talk:

Similarly cross talk is a noise that is caused by the inductive coupling between two wires that are closed to each other. Sometime when talking on the telephone, you can hear another conversation in the background. That is cross talk

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