Abstract: A slotted CSMA contention technique and arrangement is disclosed for use in a radio data communications systems having a central base station/controller and a plurality of remote data terminals operating over inbound and outbound channels. The channels are divided into major frames, which are further divided into minor frames, which in turn are divided into microslots. An arrangement is devised whereby channel access may be prioritized as between periodic and aperiodic messages by advantageously utilizing the occurrence of these microslots so as to give priority to the former while completely eliminating collisions therebetween and, at the same time, optimally controlling the efficiency of the system as a whole.