Recently, IP-related protocol groups such as not only IP (Internet Protocol) but also SIP (Session Initiation Protocol; RFC3261) to execute call control among radio communication terminals have been used also in a radio communication system such as a mobile communication system.
In a radio communication system using the protocol groups, a group call is provided in which a call within a group consisting of multiple radio communication terminals is established when one of the radio communication terminals (for example, a cellular telephone terminal) calls up the other radio communication terminals. Each of the radio communication terminals participating in the group call can transmit and receive voice data and video data to and from the other radio communication terminals participating in the group call.
Meanwhile, there is proposed a radio communication system which allows a radio communication terminal to participate halfway in a group call (hereinafter such participation being referred to as halfway participation) (see Patent Document 1). Specifically, in this radio communication system, a control station device periodically broadcasts notification information including information on the group call to radio communication terminals through a radio base station.
In this way, a radio communication terminal which has failed to receive a call-up signal can receive the notification information, and participate halfway in the group call by transmitting a request for participation in the group call. Patent Document 1: JP-A 2001-168793 (pages 5 to 7, FIGS. 1 and 3)