a data communications system wherein a plurality of stations each transmits data messages through a time - shared channel . in one embodiment of the invention , each data message is transmitted in either one of two alternative modes . in the first mode , a station transmits a data message at any time while in the second mode each station is assigned different times to transmit their data messages and transmission by that station is inhibited at other times . the mode of transmission is determined by a number of events . one such event is when a data message is successfully transmitted by a station and within a predetermined time interval thereafter that station has another data message to transmit . in another embodiment of the invention , a station is operative in the above - described first or second modes . the operative mode is modified to maintain a waiting or idle period between two successively transmitted data messages when the first of these two data messages has been successfully transmitted . in the first mode , this waiting period causes transmission of the second of the two successive data messages to be delayed until at least the expiration of the waiting period . in the second mode , this waiting period causes a station to delay transmission at one of its assigned times if such transmission does not maintain the waiting period .