Abstract: A radio based communication system comprises a base station (1) and one or more remote stations (2) each incorporating a radio transmitter and receiver to support communication between the base station and each remote station on a down link and between each remote station and the base station on an up link. Each link comprises a plurality of frames of fixed length divided into a fixed number of slots. The base station includes a base control which transmits control data in a general control slot (GC) in each frame of the down link so as to identify to each remote station a down-setup slot (DSU) in each frame of the down link in which the base station (1) is to announce the transmission of data for it and identify at least one down-transfer slot (DTR) in the frame of the down link in which the data is to be transmitted. The control data further identifies an acknowledgement slot (ACK) in each frame of the down link and an up-setup slot (USU) divided into sub-slots (1/4) in each frame of the up link.