IMS to PSTN/CS interworking

Internet protocol (IP) multimedia subsystem (IMS) has been added to the universal mobile telecommunications system (UMTS) core network starting from 3GPP UMTS Release S (see [13]). Now, UMTS network consists of the following three domains: Circuit Switched (CS), Packet Switched (PS) and IP multimedia subsystem (IMS). IMS needs connectivity to old networks like CS networks and public switching telephone networks (PSTN). For that purposes, some interworking must be done. The IMS provides services for any type of IP communication whether it is voice or video telephony, video streaming, instant and multimedia messaging, multimedia gaming or virtual-reality. This paper describes connectivity between user in the IMS network and user in the PSTN/CS network.