Currently, regarding mobile phone terminals and mobile networks using W-CDMA (Wideband Code Division Multiple Access) technology of the third generation, voice calls are realized based on a CS (Circuit-Switched) system. Meanwhile, along with the trend of mobile core networks toward use of an IP (Internet Protocol), CSIP (Circuit Switched over IP Network), in which a circuit-switched function is realized with an IP and connection to an IMS (IP Multimedia Subsystem) network is realized, is appearing.
Based on CSIP, on the circuit-switched network, an ISUP (ISDN User Part) call control signal and a voice compressed-and-encoded bit stream are transmitted and received by an ATM bearer. However, on the IMS network, a SIP (Session Initiation Protocol) signal is used for session control, and a voice compressed-and-encoded bit stream is transmitted and received after stored in an RTP (Real-time Transport Protocol) packet.
It is conceivable that, through research and development, mobile networks will be further improved in speed and capacity based on IP. In fact, currently, HSDPA (High Speed Downlink Packet Access), HSPA (High Speed Packet Access), HSPA+, EVDO (Evolved Data Optimized (Only)) Rev.A, and EVDO Rev.B have already been in practical use as mobile access networks.
In addition, along with a further increase in the speed of mobile access networks, LTE (Long Term Evolution) aiming at 100 Mbps (Megabit per second) or greater in the downlink direction and 50 Mbps or greater in the uplink direction and EPC (Evolved Packet Core) as an IP backbone network supporting this LTE are expected to be introduced.
Voice communication needs to be continuously supported for terminals connected to these LTE and EPC, which are expected to appear. In LTE and EPC, VoIP (Voice over Internet Protocol) voice communication is executed by using RTP packets. In addition, the 3GPP (Third Generation Partnership Project) is discussing including a function of changing a voice encoding bit rate when network congestion or the like is detected in the LTE section.
Patent Literature PTL 1 discloses a gateway apparatus interconnecting a mobile circuit-switched network and a mobile IMS network and realizing a video phone.
[PTL 1]
    Japanese Patent Kokai Publication No. JP2009-267669A