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We investigate the effects of the discrete reflection on the performances of upstream and downstream signals in the wavelength-division-multiplexed passive optical network (WDM PON) implemented in a single-fiber loopback configuration using the reflective semiconductor optical amplifiers (RSOAs). We first analyze the optical beat interference (OBI) noise caused by the discrete reflection, and clarify the relation between the reflection tolerance and the network's operating conditions such as the RSOA gain, the link loss, and the location of the reflection point, etc. The results show that the impact of the reflection can be expressed by using the effective crosstalk level. We then measured the reflection tolerance of the RSOA-based WDM PON, in which the downstream signal operating at 1.25 Gb/s is remodulated by the RSOA at the subscriber's site for the transmission of 155-Mb/s upstream signal. The reflection tolerances are measured to be in the range of -42 to - 35 dB for the downstream signals and -29 to -19 dB for the upstream signals, depending on the RSOA gain. These small reflection tolerances are caused by the fact that the reflected light is re-amplified by the RSOA. We also show that the dependence of the reflection tolerance on the RSOA gain can be explained by using the effective crosstalk level. These results are used to evaluate the impacts of the unwanted discrete reflections on the RSOA-based WDM PON.