A method and a communication system which includes a first network element, e.g. portable terminal, connectable to a second network element. One of selectable modes is used for communication. A network element is adapted to perform a mode selection procedure for selecting the same mode for bidirectional communication between the network elements. The mode selection ensures the use of one and the same mode in uplink and downlink direction and thus enables e.g. IP telephony in UMTS using SIP protocol. The invention provides for facilitating a VoIP communication session by way of a radio link with a mobile station. The mobile station forms a QoS (Quality of Service) information element for communication to a radio access network portion. The QoS information element is indicating whether to remove packet header information of the data packets to be communicated upon the radio link pursuant to the communication session.