A method of operating a mobile communication system supporting data transmission between a mobile station (MS) and a network in a number of a different packet data protocols (PDPs) including a point-to-multipoint-multicast (PTM-M) protocol. PDP data is formatted and unformatted by a subnetwork dependent convergence protocol (SNDCP) according to the PDP of data. The PDP is identified to the SNDCP by a protocol identifier transmitted between the network and the mobile station. In order to allow a MS to receive a PTM-M in an IDLE state, a unique protocol identifier is permanently assigned to PTM-M transmissions whilst other identifiers are dynamically assigned to other PDPs by the network.