A collision signal detection system for shared, multi-user, frequency-division-multiplexed transmission media includes a plurality of listening stations connected to the shared, multi-user, frequency-division-multiplexed transmission media. The listening stations receive and transmit signals including data signals within a databand. A collision signal means is associated with each of the plurality of listening stations for sending a collision enforcement signal from one of the plurality of listening stations to the other ones of the plurality of stations after a data collision has been detected at the first station, the collision enforcement signal having a frequency within the data band. A central station is provided with an input connected with the output of each of the listening stations and an output connected with the input of each of the listening stations with all signals passing through the central station. Signal detection means is provided in the central station for receiving an input signal and for detecting the collision enforcement signal.