A method of enabling transmission of a data service from a content server to a terminal using a data channel in a system in which at least part of the data service is transmitted to the terminal using a broadcast channel, the method including: receiving, using the broadcast channel, first data comprising part of the data service; processing the first data using a signature-generating algorithm to derive first signature data; transmitting the first signature data using the data channel to the content server; validating the first signature data by comparing said first signature data with a signature derived by processing data comprising part of the data service using a corresponding signature-generating algorithm; and enabling the terminal to access the data service using the data channel dependent on validation of the first signature data.