Method of checking amount of transmitted data

During a connection between a network infrastructure and user equipment a first indication of the amount of data transmitted over the connection is maintained in the network infrastructure and a second indication of the amount data transmitted is maintained in the user equipment. A checking procedure is triggered in response to encountering a predetermined checking value. The checking procedure utilizes integrity protected signalling. During the checking procedure the first indication is compared with the second indication. This checking procedure enables easy discovery of an intruder who either sends and/or receives data on an authorized connection between a network infrastructure and a mobile station, the data transmission being charged from the mobile station.