Noise is any undesired signal in a communication circuit. Another definition calls noise unwanted disturbances superimposed on a useful signal, which tends to obscure its information content. There are many varieties of noise; however, the four most important to the telecommunication/data communication technologist are thermal noise, intermodulation noise, crosstalk and impulse noise.
Noise refers to any external and unwanted information that interferes with a transmission signal. Noise can diminish transmission strength and disturb overall communication efficiency. In communications, noise can be created by radio waves, power lines, lightning and bad connections.
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Thermal noise
Thermal noise occurs in all transmission media and communication equipment, including passive devices. Every equipment element and the transmission medium itself contribute thermal noise to a communication system if the temperature of that element or medium is above absolute zero.
Intermodulation (IM) noise
Intermediation (IM) Noise is the result of the presence of intermodulation products. If two signals of frequencies F1 and F2 are passed through a nonlinear device or medium, the result will contain IM products that are spurious frequency energy components. IM products may be produced from harmonics of the desired signals in question.
Crosstalk
A disturbance caused by electromagnetic interference, along with a circuit or a cable pair. A telecommunication signal disrupts a signal in an adjacent circuit and can cause the signals to become confused and cross over each other.