Optical network system of bus architecture capable of rapidly detecting collision at each communication station

In an optical network system comprising a plurality of communication stations (21) and a star coupler (22) common to the stations, a collision test signal is produced from a generator (50) included in a transmitter section (31) of each station to be sent to the star coupler prior to transmission of a data sequence and is directly returned from the star coupler to a receiver section (32) thereof. The receiver section comprises a detector (51) for monitoring the collision test signal to detect occurrence of collision. The detector makes the transmitter section intercept the data sequence on occurrence of the collision. Preferably, the collision test signal is encoded by an encoder (37) in the transmitter section to be combined with a snychronizing signal and is decoded by a decoder (42) in the receiver section. Such stations may be coupled to the star coupler through a concentrator and a distributor or may be divided into a plurality of groups each of which is accommodated in a network unit linked through a repeater to the other in a loop.