IMS based services can be provided with use of PS bearers and CS bearers for the media. When using CS bearer for media transport of IMS sessions, interworking solutions for IMS Centralized Services as specified in TS 23.292 are used. ICS allows IMS sessions using CS bearers to be treated as standard IMS sessions for the purpose of IMS Service Continuity. ICS defines signalling mechanisms between the UE and IMS for transport of information as needed for service continuity when using CS access for media transport.
Both IMS Centralized Services and IMS Service Continuity specify functions which are provided by a SIP application server.
The present document specifies the architectural requirements and procedures for delivery of IMS Service Continuity.
TS 23.206  is migrated to this specification.
The scope of the specification includes:
Access Transfer related functionality:
PS-CS Access Transfer;
PS-PS Access Transfer;
PS-PS Access Transfer in conjunction with PS-CS Access Transfer;
Adding and/or removing media flows to support service;
MSC Server assisted mid-call feature;
SRVCC session transfer of IMS emergency session;
DRVCC session transfer of IMS emergency session.
Inter UE Transfer related functionality:
Establishment and release of a Collaborative Session;
Addition of media flows to, modification of media flows in, and release of media flows from a Collaborative Session;
Transfer of media flows from one UE to another;
Transfer of the Collaborative Session Control with or without transfer of media flows;
Transfer of all media flows to a target UE without establishing a Collaborative Session;
Inter-UE Transfer initiated by the target UE or by the SCC AS;
Replication of media flows by means of Inter-UE Transfer procedures;
Authorization and other aspects to support Inter-UE Transfer across multiple IMS subscriptions.
The solution is restricted to service continuity using IMS procedures, i.e. mobility mechanisms on the IP-CAN level are not within the scope of this specification.
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.
This is the call control leg between the UE and the SCC AS; also see TS 23.292 for the definition of Access Leg for IMS sessions which use the CS media.
Transfer at the IMS-level of one or more media paths of an ongoing IMS session on one UE between PS to CS access; or transfer at the IMS-level of both the signalling and the media path of an ongoing IMS session on a UE between different IP-CANs.
A set of two or more Access Legs and related media on two or more UEs having IMS subscriptions under the same operator that are presented as one Remote Leg by the SCC AS.
Collaborative Session Control:
The control operations on the Collaborative Session which can only be performed by the Controller UE, e.g. ability to release the Collaborative Session, to invoke supplementary services, and to authorize requests for IUT Media Control Related Procedures from other UEs.
A UE that supports media flows for a Collaborative Session and may request IUT Media Control Related Procedures but is subordinate to the Controller UE for authorization of these procedures.
The UE that controls a Collaborative Session and whose service profile determines the services on the remote leg. The Controller UE may also support media flows for a Collaborative Session and may request IUT Media Control Related Procedures.
An MSISDN used for correlation of sessions. See TS 23.003 for more information.
This refers to the situation where the UE is capable to transmit/receive on two different radio access technologies simultaneously.
A number used in the session transfer procedure for emergency calls, pointing toward the serving (visited if roaming) IMS EATF which handles the IMS emergency session transfer from PS to CS access.
Emergency Session Transfer Number for DRVCC:
A number used by the UE to request the EATF AS to perform Session Transfer from PS to CS access. The E-STN-DR is dynamically assigned by EATF during IMS emergency session procedure with WLAN.
Hosting SCC AS:
The SCC AS that manages the dialog with the remote party for a Collaborative Session.
IMS Service Continuity:
A service of the IMS which supports the use of Session Transfer mechanisms to maintain service continuity in the event of terminal mobility and/or mobility between terminals for the case when such events are not hidden from the IMS session layer and thus service continuity could not otherwise be maintained.
Transfer at the IMS-level of some or all of the media flows and/or service control across a set of having IMS subscriptions under the same operator.
IP Multimedia Routing Number (IMRN):
An IP Multimedia Routeing Number (IMRN) is a routable number that points to an SCC AS in the IM CN subsystem. See TS 23.003 for more information.
IUT Media Control Related Procedures:
The control operations on the media flows of the Collaborative Session which involve multiple UEs or need Controller UE's authorization within the Collaborative Session, e.g. ability to transfer/add/replicate media flows, to remove/modify media flows on a different UE.
Local Operating Environment Information:
This is a set of parameters, which can include access network(s) conditions and other parameters implementation specific, which describe the local environment in which the UE is operating.
Functionality of the MSC Server, the SCC AS and the UE, enabling PS - CS Access Transfer for UEs not using ICS capabilities, while preserving the provision of mid-call services (inactive sessions or sessions using the Conference service).
Replication, at the IMS-level, of a session ongoing in a source UE at a target UE. When the new, independent, session is established at the target UE, the state of the original media is replicated e.g. same playback state; same used media etc. is replicated.
Session State Information:
Information of IMS session state sent by the SCC AS for enablement of PS-CS and CS-PS Access Transfer of IMS multimedia-sessions when ICS UE capabilities can not be used.
Transfer at the IMS-level of one or more of the session signalling paths and/or associated media flow paths of an ongoing IMS session while maintaining service continuity. Session Transfer incorporates Access Transfer and / or Inter-UE Transfer.
Session Transfer Identifier (STI):
An identifier used by the UE to request the SCC AS to perform Session Transfer. The STI is either statically configured on the UE or dynamically assigned. See TS 23.003 for more information.
Session Transfer Identifier for reverse SRVCC (STI-rSR):
A dynamic identifier used by the UE to request the IMS network to perform Session Transfer for CS to PS SRVCC (reverse SRVCC). The STI-rSR is used by the network to correlate two access legs, and is unique for each access transfer control function within an ATCF.
Session Transfer Number (STN):
A number used by the UE to request the SCC AS to perform Session Transfer from PS to CS access. The STN is configured and stored on the UE and can be updated with current address during session establishment. See TS 23.003 for more information.
Session Transfer Number for SRVCC (STN-SR):
A STN used for SRVCC procedures as specified in TS 23.216. STN-SR is a subscription information provided to HSS for each subscriber if SRVCC service is allowed by hPLMN. STN-SR is a routing number indicating the SCC-AS or the ATCF if SRVCC enhanced with ATCF or if 5G-SRVCC is used. See TS 23.003 for more information.
This refers to the situation where the UE is only capable of transmitting/receiving on one radio access technology at a time.
Source Access Leg:
The Access Leg that exists in the transferred-out access before executing Access Transfer procedures.
Target Access Leg:
The Access Leg that is established in the transferred-in access during Access Transfer procedures.
For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.
3rd party call control
Access Transfer Control Function
Access Transfer Gateway
Access Transfer Update - Session Transfer Identifier
Dual Radio Voice Call Continuity
Emergency Access Transfer Function
Emergency Session Transfer Number for DRVCC
Initial Filter Criteria
IP Multimedia Routing Number.
Minimum Set of emergency related Data
Online Charging System
Service Centralization and Continuity Application Server