A transmission frame for controlling data transmission includes a plurality of slots, each slot used for the transmission of data over an interconnect that couples master units to slave units. Slots in the transmission frame are in three groups: (a) a first group in which slots are each allocated to one predetermined master unit for data transmission with any slave unit; (b) a second group of slots each being allocatable to any master unit for data transmission with any slave unit; and (c) a third group of slots each allocated to a predetermined one of the master units and slave units. Where a slot of the third group is not needed by the allocated master unit or slave unit, the slot can then be reallocated to another master unit. Hence, more slots can be used and the efficiency of data transfers over the interconnect can be improved.