"As Next Generation Networks (NGNs) evolve and are deployed@ there will increasingly exist a need to provide continuity of service between different types of access networks and a consistent user perspective. To support the needed equipment interworking@ standards are needed that specify service requirements@ architecture@ and signaling interfaces. The requirements of all forms of access should be considered@ including ""wired"" and ""wireless"". Fixed@ nomadic@ and mobile oriented service -- including voice@ video@ and data -- need to be taken into consideration. However@ the initial focus may be constrained to dual mode user equipment (e.g.@ WLAN/GSM) for Voice services (due to pressing market needs). NOTE -- The use cases described in this document are illustrative and not exhaustive. They may be subsequently modified through additions@ deletions@ and/or other modifications as the standardization process continues through requirements generation and the follow-on architecture and protocol work. These use cases are intended to be an initial list intended solely to progress the initial work in this standardization area. Areas for consideration include@ but are not limited to@ the following: ? Seamless handover between different access networks (e.g.@ Circuit Switched Cellular and IP WLAN) and associated core network interworking (e.g.@ Circuit Switched and IMS). ? Interoperability between independent IMS and CS core networks. ? Delivery of services needs to be access independent. ? Coordination of service data in and between user equipment@ access networks@ and core networks for a consistent user experience. ? Flexible subscriber addressing@ including multiple directory numbers (e.g.@ two E.164 numbers). ? Network/Domain selection for originating and terminating calls. ? Supplementary Services support (e.g.@ Call Forwarding@ Call Waiting@ Call Hold@ etc.). ? Security. ? Quality of Service. ? Multimedia support (e.g.@ Voice@ Video@ Data). ? Emergency Call. ? Call Transfer. ? Charging (For the purpose of this document charging is only mentioned to call attention to specific use cases. The future work should consider charging implications in all cases. ? Legal Intercept. These areas are not all described in this document but should be considered in the future. Purpose The identification of Use Cases is a valuable first step in the specification of standards because they provide a basis for providing guidance in making decisions@ determining requirements@ and ensuring that the subsequently developed specifications sufficiently enable the implementation of capabilities needed to deploy the envisioned services. This document identifies Use Cases for service continuity between access networks. Additionally@ the identification of Requirements based on Use Cases is important for the design and implementation of interoperable core network@ access network@ and user equipment. This document is intended to assist in the identification of requirements for service continuity between access networks."