A numerical investigation of the performance of an automatic gain-controlled semiconductor optical preamplified receiver for a 4 x 25 Gbits/s wavelength division multiplexing transmission system with a 0-40 km reach is presented. We show that the control scheme acting on the semiconductor optical amplifier (SOA) gain increases the input power dynamic range of the optical receiver, thus allowing the transmission system to operate error free regardless of fiber length. In contrast, a fixed-gain optical receiver shows poor performance that is due to SOA nonlinearity and photodiode overload, which are well captured by the corresponding simulation models. The device represents a practical alternative to the next-generation high-speed Ethernet technology.