This document contains the results of the feasibility study into the architectural requirements and alternatives for the delivery of consistent services to the user mainly via IMS centralized services regardless of the attached access type; e.g. CS domain access or IP-CAN. Considerations include overall requirements, architectural requirements, alternative architectures and evaluation of potential architectural solutions. The study shall consider how to access the IMS-based multimedia telephony services while still allowing innovative services. Specifically, it shall provide consideration for the handling of the multiple media types that are enabled by the IMS multimedia telephony communication service (MMTel) utilizing CS services of TS11 and CS video telephony reliant upon BS30. Emergency calls that utilise TS12 are outside the scope of this work item in this release. It shall include an investigation into call/session establishment via CS domain access and IP-CAN and for calls/sessions transferred across CS domain access and IP-CAN, including the interactions with domain selection. The solution should be applicable for terminals with or without VCC capabilities. Impact on legacy terminals with the same subscription (e.g. SIM swapping) should be studied. The study shall also investigate the means to support and the need of the evolution of a network towards the IMS centralized services architecture. The assumption for this evolution is that some networks may not immediately migrate all services to the IMS centralized services architecture. In addition, given that some calls may not be rerouted to IMS during the migratory period, the study shall investigate how to ensure that equivalent services are implemented in IMS and the CS domain. Overall, Centralized IMS Services Control supports the introduction of MMTel by enabling support of IMS bi directional speech media services when no bi-directional speech media capable IP-CAN is useable. Combined with the domain transfer capability of VCC, service continuity for bi-directional speech media is enabled for MMTel between a bi-directional speech media capable IP-CAN and CS access. Furthermore it supports the introduction of MMTel by enabling terminals without IMS to support receiving IMS based speech services.

Communication networks are evolving towards packet-based infrastructures. A single common consolidated core network offers service providers the possibility of reduced core network complexity and maintenance. As service providers shift their core network infrastructure from the CS domain to a consolidated common IMS infrastructure the need will exist to enable the consistent provision of services to subscribers over a variety of accesses, including CS domain and PS domain accesses. Initially it can be expected that the coverage of IP-CANs capable of transporting bi-directional speech media will be limited compared to CS domain access networks at least during the introduction period of IP-CANs capable of transporting bi-directional speech media. Therefore a need exists to specify an architecture that supports the provision of IMS based services across a variety of PS domain or CS domain access networks (see Figure 1.2-1). This thereby enables a consistent user-experience with bi-directional speech services of IMS subscribers irrespective when being inside or outside the coverage of an IP-CAN capable of transporting bi-directional speech media.

In order to take advantage of all the new capabilities IMS has to offer, it is assumed that new terminals are required. However, there is also an interest to analyze whether IMS bi-directional speech services can be supported to terminals without any IMS capabilities in order to enable centralization of service management in IMS. Already TR 23.806, introduced the concept of IMS controlled service continuity in clause 6.3 "Service Continuity Model: IMS Controlled Alternative". The Voice Call Continuity (VCC) Application is described in TS 23.206,which implements a distributed service model where both CS and IMS services can be used by a VCC subscriber as follows:

• When using a PS domain access, only IMS services are offered.
• When using a CS domain access, call control and supplementary services are active in IMS with the exception of mid call and presentation services, which are active in the CS domain.

Moreover, the development of the architecture for Voice Call Continuity has identified that supporting domain transfer of active mid-call services by implementing such services in both the CS domain and IMS is not a viable solution in the 3GPP Rel 7 timeframe. When mid-call services are active, the domain transfer cannot be executed unless the mid-call service is finalized first. Therefore it is proposed to develop an architecture that allows implementation of such services in IMS while also allowing IMS control when the serving access network is in the CS domain. In addition to the VCC scenario, the increased deployment of bi-directional speech media capable access technologies will encourage further service development on IMS. This therefore increases the importance of being able to access these services via CS domain access independently of the support of VCC and also independently of having an IMS enabled terminal. The solution for centralized IMS service control should also support call independent IMS supplementary services management. Different access network scenarios for centralized IMS service control can be envisioned as depicted in Figure 1.2-2:

• Scenario A: The serving access network is an IP-CAN capable of transporting bi-directional speech media. Here both media transport and session control signalling is carried over the IP CAN.
• Scenario B: The serving access network is a CS access: Here both media transport and session control signalling is carried over CS domain access.
• Scenario C: Both CS domain access and an IP-CAN are serving access networks (possibly, it has been determined that the IP-CAN is not bi-directional speech media capable or operator policy/user preference prevent usage of IP-CAN or usage of IP-CAN for bi-directional speech media communications): Here, media transport is carried over CS access and session and media control signalling is carried either over the CS domain access or over the IP-CAN not capable of transporting bi-directional speech media.

Within the study of Centralized IMS Service Control both Scenario B and Scenario C need to be investigated for bi-directional speech services. Scenario A shall not be impacted by the study of Centralized IMS Service Control. In Scenario C, depending on operator policy/user preference, access and terminal capabilities, additional IMS controlled media (e.g. video, text) can be carried over an IP-CAN that is not capable of transporting bi-directional speech media. For the above-introduced scenarios, the following main cases for session continuity for bi-directional speech can be distinguished (see also Figure 1.2-3 and Figure 1.2-4), both requiring domain transfer for bi-directional speech between bi-directional speech media capable IP-CAN and CS access (Voice Call Continuity as specified in TS 23.206). Service Continuity Scenario 1: Access Scenarios A & B:

Service Continuity Scenario 2: Access Scenarios A & C

For the purposes of the present document, the [following] terms and definitions [given in ... and the following] apply. Bearer Control Signalling Path: CS Access Signalling: ICS UE: ICS Enhanced MSC Server: Non-ICS UE: RUA Leg: Session Control Signalling Path: Session Transfer Identifier: UE Leg:

For the purposes of the present document, the following abbreviations apply: CAAF ICCC I1-cs I1-ps ICS ICCP ICCF IMSC L-CAAF L-CAAF-n RUA R-CAAF VCC