This paper revealed the maximum transmission distance for wavelength division multiplexed (WDM) fiber optic transmission system (FOTS) over which the optical signals can be transmitted and then successfully detected on the other fiber end with appropriate error probability below the maximum permissible threshold. The studied mixed WDM system complies to the following configuration scheme: {[1^st (f_c=193.0250 THz), 4^th (f_c=192.8250 THz) and 7^th (f_c=193.2375 THz) channels: NRZ-OOK, R=10 Gbit/s] - [2^nd (f_c=193.1000 THz), 5^th (f_c=192.8875 THz) and 8^th (f_c=193.3000 THz) channels: 2-POLSK, R=40 Gbit/s] - [3^rd (f_c=193.1750 THz), 6^th (f_c=192.9625 THz) and 9^th (f_c=193.3750 THz) channels: NRZ - DPSK, R=40 Gbit/s]}. As it can be seen optical signals are transmitted with two different per channel bitrates - 10 and 40 Gbit/s and non-return to zero encoded on-off keying (NRZ-OOK) modulation format is used for the manipulation with optical signal's amplitude. Whereas NRZ encoded differential phase shift keying (NRZ-DPSK) is used for the manipulating with signal's phase and, finally, orthogonal binary polarization shift keying (2-POLSK) is used for the manipulation with signal's state of polarization. The maximum transmission distance was detected using the number of iteration loops which simulate the sector of FOTS line.