The invention relates to a method for controlling the flow of data between an RNC (R) and a base station (BS) of a UMTS mobile communications system with HSDPA capability via an lub interface (I2), where an HS-DSCH Frame Protocol mechanism is implemented in the lub interface (I2) 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 (COCU) monitors information about actual lub interface data transport flow (IDF), and when a determined lub interface (I2) transport capacity value is reached, the data transport flow is reduced in the lub interface (I2).