Packet switched real time applications are receiving considerable interest in mobile networks, in particular the General Packet Radio Service (GPRS) is currently undergoing study into its evolution for the support of such applications. Telephony is expected to be one of the first real time applications such as voice over IP (VoIP) to be made available and support for existing telephony services is essential to provide users with a seamless transition from circuit to packet based networks. A great deal of mobile telephony services are currently provided by a combination of switched supplementary services and intelligent network (IN) based services. Operators who already provide such services would seek to maximise return on their current network investment by reusing the existing portfolio of IN services for VoIP. In order to achieve this, it is necessary for the Internet call control functions within say a GPRS to interwork with a service control point (SCP). This paper proposes an architecture to support CAMEL (Customised Application for Mobile Enhanced Logic) based operator specific services for VoIP users in a GSM/UMTS based mobile network environment to provide seamless service portability between 2G and 3G networks. The proposals put forward are suitable for the two popular Internet telephony call control protocols currently available, namely ITU-T's H.323 and IETF's Session Initiation Protocol (SIP). The work presented in this paper is an extension and generalisation of our contributions to a related feasibility study within the standardisation efforts of the 3/sup rd/ Generation Partnership Project core network group in collaboration with BTCellnet.
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