Currently, almost everyone uses information transmission services. To meet all the needs, the power and data transmission rates in modern optical communication systems have reached enormous values. At high transmitted powers, undesirable nonlinear effects occur in the optical fiber, which reduce the efficiency of the entire optical system. Nonlinear effects in optical fibers impose various limitations on the communication channel, and understanding such effects is practically necessary. Four-wave mixing is the dominant nonlinear effect present in optical networks with wavelength-division multiplexing. The general provisions on the mechanism of manifestation of four-wave mixing in an optical fiber are substantiated. The article presents a variant of the classification of nonlinear effects. The types of four-wave mixing are considered. The paper shows the dependence of the FWM effect on the number of signals in the path and the phase ratio between the interacting signals. The authors propose ways to increase the reliability of optical signal transmission in fiber-optic transmission systems.