The present invention relates to an architectural model of a radio network unit, e.g. a Radio Base Station, that separates functionality in such a way that it is possible to easily adapt the unit to various requirements and conditions. The radio network unit comprises an internal RBS-interface that subdivides the its functionality into a first part, which solely relates to the functionality of the radio network, and a second part, which solely relates to the radio part, i.e. the airborne part of the transmission. The internal interface comprises at least a link that handles the necessary additional communication that arises due to the subdivision and a link that handles the user data that is to be processed, i.e. transmitted or received, by said unit.