7.5 Service continuity

23.2923GPPIP Multimedia Subsystem (IMS) centralized servicesRelease 17Stage 2TS

7.5.1 Service continuity for ICS UE

7.5.1.1 Service continuity while maintaining the use of CS access for the media

7.5.1.1.1 IMS sessions established using Gm reference point

7.5.1.1.1.1 Overview

When the CS bearer is used for the media of the IMS session, the Gm reference point may be used for communication of service control signalling, contingent upon the VPLMN support of the Gm reference point. A change of access network due to handover (e.g. as described in TS 23.009 [22] and TS 25.413 [23]), may result in an inability to use the PS access for the Gm reference point while the use of CS access for the media of the IMS session is still possible; under such circumstance, the service continuity is maintained by switching the signalling transport over to the CS access, e.g. by switching the use of Gm with the I1 reference point or to not using Gm or I1.

7.5.1.1.1.2 Use of Gm reference point possible after handover

Standard Handover procedures, that are specific to different access networks (e.g. as described in TS 23.009 [22] and TS 25.413 [23]), are used for handover of the Service Control Signalling Path and the CS Bearer Control Signalling Path along with the associated circuit bearer to the target access network.

The use of Gm reference point for Service Control Signalling Path is maintained upon handover.

7.5.1.1.1.3 Use of Gm reference point not possible after Handover

Standard CS handover procedures, that are specific to different access networks (e.g. as described in TS 23.009 [22] and TS 25.413 [23]), are used to relocate the CS Bearer Control Signalling Path and the associated circuit bearer to the target access network. Upon completion of the handover of the CS Bearer Control Signalling Path and the associated circuit bearer to the target access, the UE sends a handover notification message to the SCC AS to indicate use of I1 for the Service Control Signalling Path if the I1 reference point is available in the target access network or to fallback to not using Gm or I1.

After handover, the I1 reference point is used for service control signalling in networks supporting the I1 reference point; the non ICS UE procedures apply in networks not supporting the I1 reference point.

7.5.1.1.1.3a Use of Service Control Signalling Path not possible after Handover

7.5.1.1.1.3a.1 MSC Server assisted mid-call feature not used

If the SCC AS detects that the ICS UE is not reachable over the Service Control Signalling Path and if the UE or the network does not support the MSC Server assisted mid-call feature as specified in TS 23.237 [12], the SCC AS shall:

– clear any held sessions related to the user and update the remote leg if necessary; and

– clear any active session that is a conference initiated by the user and hosted in the IMS.

NOTE 1: Detecting the loss of the Service Control Signalling Path is an implementation issue.

NOTE 2: In order to avoid an unintentional release of an ongoing ICS session, an ICS UE can re-register its Public User Identity with the IMS at a time chosen to minimize the probability of its IMS registration expiring during an ongoing ICS session when Gm is not available. To prevent the ICS UE requesting re-registration with unacceptable frequency, the Registrar in a network implementing ICS would need to set the IMS registration expiry timer to an appropriate value.

Figure 7.5.1.1.1.3a-1 is an example call flow of releasing the aforementioned session at the SCC AS when the SCC AS detects that the ICS UE is not reachable over Service Control Signalling Path and the UE or the network does not support the MSC Server assisted mid-call feature as specified in TS 23.237 [12].

Figure 7.5.1.1.1.3a-1: release of held sessions at SCC AS when Service Control Signalling Path unavailable

1. The SCC AS detects that the ICS UE is not reachable over Service Control Signalling Path.

2. The SCC AS releases the held session between UE A and UE C and/or the active session that is a conference initiated by UE A and hosted in the IMS.

3. The SCC AS sends the UPDATE to the S‑CSCF.

4. The S‑CSCF forwards the UPDATE to the UE B to update the remote leg.

7.5.1.1.1.3a.2 MSC Server assisted mid-call feature used and MSC Server supports ICS

See TS 23.237 [12] IMS Service Continuity.

7.5.1.2 Service continuity when transferring the media of IMS sessions between PS and CS access

See TS 23.237 [12] IMS Service Continuity.