An LTE mobile communication system in which relays RN (14), each in communication with a donor eNB (13) are used to improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput. The problem addressed is that in the RN←→DeNB (13) Un interface, all the S1-AP and/or X2-AP messages with different QoS requirements (for example different S1-AP/X2-AP messages that have different time criticalities) have to be delivered in a limited number of Data or Signalling Radio Bearers. A multiplexing mechanism enables the categorization of different types of S1-AP and/or X2-AP messages and the mapping of these to a limited number of Radio Bearers over the Un radio interface.