In the Universal Mobile Telecommunications System, a method of transport format combination selection is provided for directing data units for transmission from a buffer in the RLC layer to transport channels in the MAC layer. The method includes providing a set of information channels in the RLC layer for connection to the buffer and a set of transport channels in the MAC layer for transmission of the data units, selecting a subset of the information channels and a subset of the transport channels for interconnection, and selecting for each transport channel one or more information channels to be multiplexed on the transport channel. A mapping table and a transport block combination table are created as a result of prioritizing the selected transport channels according to a predetermined scheme in dependence on the characteristics of the information channels and then there from, determining the number of data units which may be transmitted in a timing interval.