Transmissions on a composite carrier comprising at least two component carriers can be controlled by means of a message by the provider of the carrier. The provider may receive a message from a device attempting to transmit on the composite carrier, and include in a response thereof an indication of at least one component carrier to be used by the device for a subsequent transmission. The device then received the message from the provider, and can determine based thereon at least one component carrier to be used by the device for at least one subsequent transmission, and transmit on the determined at least one component carrier.