The authors have evaluated the maximum permissible transmission distance for the purposed configuration of mixed-HDWDM system that is offered as model for the designing of next generation of an optical transport networks. In general, the investigated mixed-WDM FOTS can be represented using the following configuration scheme: [1st, 4th and 7th channels: NRZ-OOK, R= 10 Gbit/s] – [2nd (fc = 193.1 THz), 5th and 8th channels: 2-POLSK, R=40 Gbit/s] – [3rd, 6th and 9th channels: NRZ-DPSK, R=40 Gbit/s]. This is the “pessimistic scenario” since secures the highest in system’s channel detected signals bit-error-rate as compared with other possible bitrate distribution among the channels. The maximum distance reach was evaluated using iteration loops consisted of chromatic dispersion compensation module (DCM), optical signal inline amplifier (based on EDFA) and one span of standard single mode fiber (SSMF, ITU-T Rec. G.652 D). One such loop emulates one sector of a fiber-optic transmission line (FOTL) 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. It is found out and analysed the correlation between the maximum distance reach and the investigated system’s average spectral efficiency and the length of used SSMF span.