Exponential growth of the data traffic, high requirements of business users to the security and symmetrical transmission data rate, as well as utilization of access networks for backhaul and fronthaul applications are driving demands on next-generation access networks – fiber-optical access networks with improved parameters (bitrate and transmission distance) in comparison to the solution based on copper cables. However, there is a number of technological issues that have to be solved ensuring high data rate transmission – the choice of the suitable modulation format, the cost and availability of the wideband elements of transmission system, as well as compatibility of the different transmission data rates. In the Thesis, the most suitable data modulation formats for utilization in wavelength division multiplexed passive optical networks (WDM-PONs) are chosen as well as the main performance-affecting factors are investigated. As a result, the model of mixed (with 12.5 Gbit/s and 25 Gbit/s data transmission rates) WDM-PON access telecommunication system is developed (including the approach of modelling of Mach-Zehnder modulator frequency response). The solution of mixed WDM-PON access network with values and limits of the main performance-affecting factors is proposed.