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Content for  TR 23.892  Word version:  8.0.1

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0  Introductionp. 9

Development of the architecture for Voice Call Continuity has identified that supporting domain transfer of active mid-call services by implementing these services in both the CS domain and IMS is not a viable solution in the 3GPP Rel 7 timeframe. Therefore it has been proposed that an architecture is necessary that allows implementation of such services in IMS while also allowing control of the services when the serving access network is the CS domain. In addition to the VCC scenario, the increased deployment of VoIP capable access technologies will encourage further service development on IMS also increasing the importance of being able to access these services via CS domain access independently of the support of VCC.
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1  Scopep. 10

1.1  Generalp. 10

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.
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1.2  Motivation and backgroundp. 10

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.
Copy of original 3GPP image for 3GPP TS 23.892, Fig. 1.2-1: IMS based services across a variety of PS and/or CS domain access networks
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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.
Copy of original 3GPP image for 3GPP TS 23.892, Fig. 1.2-2: Scenarios for Centralized IMS Service Control
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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:
Copy of original 3GPP image for 3GPP TS 23.892, Fig. 1.2-3: Service continuity between scenario A and B
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Service Continuity Scenario 2: Access Scenarios A & C
Copy of original 3GPP image for 3GPP TS 23.892, Fig. 1.2-4: Service continuity between scenario A and C
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2  Referencesp. 13

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1]
TS 22.101: "Service principles".
[2]
TS 22.173: "IP Multimedia Core Network Subsystem (IMS) Multimedia Telephony Service and supplementary services".
[3]
TS 23.206: "Voice Call Continuity between CS and IMS".
[4]
TS 23.228: "IP Multimedia Subsystem (IMS)".
[5]
TS 24.173: "IMS Multimedia telephony service and supplementary services".
[6]
TS 24.229: "Internet Protocol (IP) multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)".
[7]
TS 24.008: "Mobile radio interface Layer 3 specification; Core network protocols".
[8]
TR 23.806: "Voice call continuity between Circuit Switched (CS) and IP Multimedia Subsystem (IMS) Study".
[9]
TR 23.826: "Feasibility study on Voice Call Continuity (VCC) support for emergency calls".
[10]
TS 25.413: "UTRAN Iu interface Radio Access Network Application Part (RANAP) signalling".
[11]
TS 23.009: "Handover procedures".
[12]
TS 43.055: "Dual Transfer Mode (DTM)".
[13]
ITU-T Recommendation H.248.18 (11/2002): "Gateway control protocol: Package for support of multiple profiles".
[14]
TS 29.232: "Media Gateway Controller (MGC) - Media Gateway (MGW) interface; Stage 3".
[15]
TS 29.332: "Media Gateway Control Function (MGCF) - IM Media Gateway (IM-MGW); Mn interface".
[16]
RFC 3680  (March 2004): "A Session Initiation Protocol (SIP) Event Package for Registrations".
[17]
TS 23.204: "Support of Short Message Service (SMS) over generic 3GPP Internet Protocol (IP) access".
[18]
TS 29.163: "Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks".
[19]
TS 23.893: "Feasibility study on multimedia session continuity".
[20]
TS 33.210: "3G Security; Network Domain Security; IP network layer security".
[21]
TS 23.292: "IP Multimedia Subsystem (IMS) Centralized Services; Stage 2".
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3  Definitions and abbreviationsp. 14

3.1  Definitionsp. 14

For the purposes of the present document, the [following] terms and definitions [given in ... and the following] apply.
Bearer Control Signalling Path:
Standard CS signalling path used to control the call established to setup CS voice bearer between the UE and IMS.
CS Access Signalling:
Standard CS signalling used between the UE and the CS network.
ICS UE:
The ICS UE is a User Equipment that contains the new UE capabilities defined in this document and that is capable of receiving telephony services and other services offered by IMS while the voice bearer is established via CS. An ICS UE can also be a UE which can access IMS via an IP-CAN that supports the full duplex speech component of the IMS multimedia telephony service, and follows the procedure defined in TS 22.101, TS 22.173, TS 23.228, TS 24.229 and TS 24.173. An ICS UE is not necessarily capable of VCC.
ICS Enhanced MSC Server:
An MSC Server which has been enhanced with ICS capability.
Non-ICS UE:
The non-ICS UE is a User Equipment that does not contain the new capabilities defined in this document. A non-ICS UE is not necessarily capable of VCC.
RUA Leg:
The call leg between the ICCF and the remote end. It is formed at the ICCF for presentation of the SIP UA behaviour to IMS on behalf of the UE. The TAS, VCC AS and other Application Servers are invoked on the RUA Leg.
Session Control Signalling Path:
Signalling path established between the UE and the ICCF, either directly via PS through an IP-CAN or via CS network elements such as the VMSC and the HSS for enablement of IMS control of user sessions at the ICCF when using CS voice bearers.
Session Transfer Identifier:
Session Transfer Identifier is an identifier used by Domain Transfer Function to identify the anchored sessions destined to or originated from a specific UE. It is used by the UE or the network to identify the specific anchored session to perform Domain Transfer.
UE Leg:
The call leg between the ICCF and the UE. It is formed at the ICCF by combining of the CS call established between the UE and the ICCF to set up the voice bearer, with the control information communicated between the UE and ICCF to enable the completion of the call towards the remote end.
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3.2  Abbreviationsp. 15

For the purposes of the present document, the following abbreviations apply:
CAAF
CS Access Adaptation Function
ICCC
IMS CS Control Channel
I1-cs
IMS CS Control Channel implemented over the CS domain
I1-ps
IMS CS Control Channel implemented over the PS domain
ICS
IMS Centralized Services
ICCP
IMS CS Control Protocol
ICCF
IMS CS Control Function
IMSC
ICS Enhanced MSC Server
L-CAAF
Local CS Access Adaptation Function
L-CAAF-n
Local CS Access Adaptation Function-network equivalent
RUA
Remote User Agent
R-CAAF
Remote CS Access Adaptation Function
VCC
Voice Call Continuity

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