The authors have investigated the maximum distance reach for wavelength division multiplexed (WDM) fiber optic transmission system (FOTS) where optical signals are transmitted with two different per channel bitrates (i.e., 10 and 40 Gbit/s) and for optical signal modulation three different modulation formats are used, such as NRZ-OOK, 2-POLSK and NRZ-DPSK. These chosen formats encode information by manipulating with the different optical signal freedom degrees: amplitude, phase and state of polarization. The system’s maximum distance reach was detected for the worst case scenario of the spectrally most efficient WDM system’s configuration: [1st, 4th and 7th channels: NRZ-OOK, R= 10 Gbit/s] – [2nd, 5th and 8th channels: 2-POLSK, R=40 Gbit/s] – [3rd, 6th and 9th channels: NRZ-DPSK, R=40 Gbit/s]. It was done using iteration loops that consisted from chromatic dispersion compensation module (DCM), optical signal inline amplifier and one span of standard single mode fiber (SSMF). One such loop emulates one sector of a fiber optic transmission line (FOTL). It is found that the maximum distance reach strongly depends on the investigated system’s average spectral efficiency and the length of used SSMF span.