Abstract: In this study, a system consist of optic fiber communication downstream analysis has been utilized by employing co-existing GPON-FTTX architecture. In this system by changing the input power and the length of the fiber, gave us ability to analysis and execute this design performance by inspecting the two type of data formats Return-to-Zero (RZ) and non-return-to-zero working at constant bit rates 2.5 G. The simulative analysis has shown how outstanding the RZ modulation format as compare to the traditional NRZ modulation design due to preferable inviolability to average peak power and non-linearities fiber, it is also evident from the results that RZ can reach up to 90 km with 5 dB power and with any wavelength (1557 or 1490 nm) and beyond which BER is above 10–9 (bad signal) in contrast the NRZ with power 5 dB has reach 40 km with 1557 nm and 70 km with 1490 nm.