Signal received may differ from signal transmitted causing:
- analog – degradation of signal quality
- digital – bit errors
The most significant impairments are
- attenuation and attenuation distortion
- delay distortion
Attenuation–a decrease in magnitude of current, voltage, or power of a signal in transmission between points.
- If signal is too weak, it cannot be detected or errors may be introduced.
- Attenuation tends to be an increasing function of frequency as well as distance.
Delay Distortion–distortion of a signal occurring when the propagation delay for the transmission medium is not constant over the frequency range of the signal.
- Can cause intersymbol interference, i.e., the energy of one signal interval carriers over into the next–the result for digital transmission is a possible bit error.
- Can be compensated for by using equalization circuits (or line conditioning).
- Thermal noise–caused by thermal agitation of electrons in a conductor (No = k Temp is the noise power density–the amount of noise in 1 Hz).
- Intermodulation noise–due to the nonlinear combination of signals of different frequencies.
- Crosstalk–phenomenon in which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel.
- Impulse noise–a high-amplitude, short- duration noise pulse.