In this paper, the authors proposed a procedure, which determines where the interruption of the quality of transmission starts. Based on the algorithm, simulations of transmission for specific values are executed. The analysis of the results showed how the BER and Q-factor change depending on the length of the fiber, i.e. on the number of amplifiers, how the number of multiplexed channels and the flow rate per channel affects on a transmitted signals. Analysis of DWDM systems is designed for systems with flow rates of 2.75 Gb/s and 12 Gb/s per channel in the software package OptiSystem 7.0.