During the course of Release 6, TS 23.234 (3GPP system to Wireless Local Area Network (WLAN) interworking: System description) was developed that provides the possibility to offer VoIP over WLAN interworking with IMS. Thus there is the possibility to support the most prevalent GSM service (voice calls) over I-WLAN when there is coverage. By developing the capability to support seamless voice call continuity between the CS Domain and an I-WLAN, or other IP-CANs, an operator would be able to provide relief to the GSM/UMTS radio resources and increase service revenue. In addition, wireline operators with VoIP offerings should be able to use the 3GPP IMS architecture to offer converged services. This TR documents alternatives for how to provide such seamless voice call continuity between the CS Domain and IP-CANs. During the course of this study, a number of alternative architectural solutions that enable Voice Call Continuity between CS and IMS domains have been proposed and documented in this report. As a result of analysis, some options have been removed or placed in annexes of this document (thereby ensuring that the work is not completely lost). The outcome is that two alternatives remained: Original Domain Controlled (clause 6.4) and IMS Controlled Static Anchoring (clause 6.3). Work has been undertaken to identify and separately document any common aspects of the two alternatives. In working on the study, it was determined that the VCC solution impacts Registration, Origination, Termination, Network Domain Selection as well as mid-call supplementary services. When these areas were deemed common or only had minor differences, they were placed in common sections. The common aspects have been captured in clauses 6.2 and 6.2a. These clauses retain some solution-specific details where necessary.

This document contains the results of the feasibility study into the architectural requirements and alternatives for the active voice call continuity between Circuit Switched (CS) domain and the IP Multimedia Subsystem (IMS). Considerations include overall requirements, architectural requirements, evaluation of potential architectural solutions and alternative architectures. The Feasibility Study considers different solutions for offering real-time voice call continuity when users move between the GSM/UMTS CS Domain and the IP Connectivity Access Network (e.g., WLAN interworking) with home IMS functionality. Voice call related functionality, including the need for Regulatory issues (e.g. Text Telephone (TTY as defined in TS 26.226)), Emergency Call and support for supplementary services are taken into consideration. The objective is to identify an architectural solution that allows completely automatic connectivity from the end-user point of view, while minimizing the additional complexity and impacts to the existing system. The feasibility study shall also investigate mechanisms for selecting the most appropriate network domain to serve the user. Existing solutions developed by the 3GPP (e.g. 3GPP system to Wireless Local Area Network Interworking (I-WLAN)) should be reused as much as possible. The impact to, and support of service continuity for sessions/calls established following the principles outlined in the combining of CS and IMS sessions (CSI) will also be considered in this study.