TRANSMISSION IMPAIRMENT

Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.

Topics discussed in this section:

• Attenuation
• Distortion
• Noise

Causes of impairment

Attenuation

• Attenuation means a loss of energy.
• When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm.
• To compensate for this loss, amplifiers are used to amplify the signal.
• To show that a signal has lost or gained strength, engineers use the unit of the decibel.
• The decibel (dB) measures the relative strengths of two signals or one signal at two different points.
• The decibel is negative if a signal is attenuated and positive if a signal is amplified.

Distortion

• Distortion means that the signal changes its form or shape.
• Distortion can occur in a composite signal made of different frequencies.
• Each signal component has its own propagation speed through a medium and, therefore, its own delay in arriving at the final destination. Differences in delay may create a difference in phase.
• The shape of the composite signal is therefore not the same.

Noise

• Noise is another cause of impairment.
• Several types of noise, such as thermal noise, induced noise, cross-talk, and impulse noise, may corrupt the signal.