Tech Article

Types of noise and how they affect the network

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.

Thermal noise occurs in all transmission media and communication equipment, including passive devices. It arises from random electron motion and is characterized by a uniform distribution of energy over the frequency spectrum with a Gaussian distribution of levels. 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. Whenever molecules heat above absolute zero, thermal noise will be present. The more heat generated or applied, the greater the level of thermal noise.

Intermodulation (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. These components may be inside or outside the frequency band of interest for a particular device. IM products may be produced from harmonics of the desired signals in question, either

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as products between the harmonics or between a harmonic of one of the signals and the other basic signal or between both signals themselves. The products result when two (or more) signals beat together or "mix."

Crosstalk refers to unwanted coupling between signal paths. There are essentially three causes of crosstalk: (1) electrical coupling between transmission media, such as between wire pairs on a voice-frequency (VF) cable, (2) poor control of frequency response (i.e., defective filters or poor filter design) and (3) nonlinear performance in analog (FDM) multiplex systems. Excessive level may exacerbate crosstalk. Analog transmission is distorted by crosstalk and it will deteriorate the BER performance of a digital path.

Impulse noise is a noncontinuous series of irregular pulses or noise "spikes" of short duration, broad spectral density and of relatively high amplitude. In the language of the trade, these spikes are often called "hits." Impulse noise degrades telephony only marginally, if at all. However, it may seriously corrupt error performance of a data circuit.

A transmission engineer in telecommunications is often called a noise fighter.

About the author: Roger Freeman has worked in telecommunications since 1946 when he joined the Navy and became an aviation radioman. Later, Roger served as a radio officer in the merchant marine for nearly 10 years. He then held several positions with ITT assigned to their Spanish Standard Electrica subsidiary. He also served the International Telecommunication Union as Regional Planning Expert for Northern Latin America based in Quito, Ecuador. Roger is bilingual. His last employee position was principal engineer with the Raytheon Company, Marlboro, MA where he took early retirement in 1991 to establish Roger Freeman Associates, Independent Consultants in Telecommunications. He has been giving seminars in telecommunication disciplines at the University of Wisconsin, Madison for nearly 20 years. Roger has been writing books on various telecommunication subjects for John Wiley & Sons since 1973. There are seven titles which he keeps current including the two-volume work, Reference Manual for Telecommunication Engineers, now in 3rd edition. He holds two degrees from NYU. His Web site is and his e-mail address is

This was first published in March 2005

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