A method for controlling the flow of data between an RNC and a base station of a UMTS mobile communications system with HSDPA capability via an Iub interface, where an HS-DSCH Frame Protocol mechanism is implemented in the Iub interface to provide control and data services for the communication of HSDPA user data between the RNC and the base station, wherein a capacity overload control entity monitors information about actual Iub interface data transport flow, and when a determined Iub interface transport capacity value is reached, the data transport flow is reduced in the Iub interface.