Apparatus arranged to perform data multiplexing or demultiplexing or a non-transitory computer readable medium comprising a computer program which, when executed by a computer, carries out a method of multiplexing or demultiplexing data, the apparatus comprising a memory, an interface, and a processing unit arranged to determine locations of corresponding regions of the memory for corresponding channels based on data sizes and time-unit-numbers that have passed since a previous corresponding quantity of channel data for a corresponding channel was last stored in the memory or processed, wherein the data sizes and time-unit numbers correspond to one or more channels of the plurality of channels, such that the processing unit will not store the corresponding quantity of the channel data, for each corresponding channel of the plurality of channels, at a location in the memory that is currently storing any channel data that has not yet been output or processed.