There have been numerous efforts to develop a practical wavelength-division-multiplexed passive optical network (WDM PON) by utilizing the reflective semiconductor optical amplifier (RSOA). However, the ultimate operating speed of this WDM PUN is primarily limited by the narrow modulation bandwidth of the RSOA. We estimate the maximum operable speed of such a bandwidth-limited RSOA by using the Shannon's capacity theorem. The results show that the operating speed of the RSOA can be increased to similar to 60 Gb/s when its modulation bandwidth is similar to 3 GHz. We also find that, if the modulation bandwidth of the RSOA is less than similar to 3 GHz, the use of the high-level (i.e., >4 levels per symbol) modulation formats is not very effective for increasing its operating speed due to their requirements of the high signal-to-noise ratio (SNR). To operate the RSOA at 100 Gb/s, its modulation bandwidth should be > 5.8 GHz. (C) 2013 Elsevier B.V. All rights reserved.