An advanced TDMA based radio access system is described which has been designed to meet the needs of third generation mobile systems such as UMTS. The air interface is based on linear modulation methods with a choice of carrier bitrates, channel codes and radio bearer service types to suit the range of operating environments and applications of UMTS. This transport chain is controlled by a set of functions to set transmit power, link adaptation, ARQ, packet access, channel assignment, handover and admission control. The system is presented in terms of a new modelling method which has the potential to capture the key aspects of a mobile system in terms which are independent of the final network architecture.