A multiple access communications system for use between a base and a plurality of substations and having a first channel for transmission of data from the base to the substations and a second channel for transmission of data from the substation to the base. The system has, in use, provision at the base and/or substations for assigning each of the substations to one of a plurality of groups, and for allocating, from the base, each of a plurality of slots in the second channel to a particular group. There is provision at the base for reporting to the substation via the first channel the number of substations in each group, and there are analyzing means at each substation for analyzing the number of substations in each of the groups and for making a judgement based on those numbers as to which group to assign each respective substation to in order for that substation to communicate next with the base in the slot or slots allocated to that group.