This paper presents the design and performance features of a successfully developed optical intraoffice transmission system operating at 100-400 Mbits/s. The keys to the commercial realization of this simple, highly reliable, and low-cost system are the employment of the 1.3 μm LED and graded-index multimode fiber. Additionally important, the system makes use of coded mark inversion (CMI) coding to ensure bit sequence independence (BSI) and good error-monitoring capability. Experimental results have clarified the optimum bandwidth of the low-pass filter at the receiver end and the commercially attainable transmission distance. Furthermore, an available system gain of 15.4 dB is demonstrated through 400 Mbit/s transmission experiments. This value enables transmission over distances in excess of 4 km through multimode fiber (900 MHz\cdotpkm, 0.8 dB/km).