14A MCPTT Service Continuity

24.3793GPPMission Critical Push To Talk (MCPTT) call controlProtocol specificationRelease 18TS

14A.1 General

This clause describes the procedures for service continuity of an ongoing SIP session supporting an MCPTT private call or MCPTT group call when:

– the MCPTT UE (referred to as the remote UE) is connected to the network via E-UTRAN and decides to connect to a UE-to-network relay, e.g. because it realises that it is losing connection to the network and wants to ensure seamless service; and

– the remote UE is connected to the network via the UE-to-network-relay and decides to disconnect from the UE-to-network relay, e.g. because the remote UE realises that it is losing connection to UE-to-network relay or because the LTE-Uu link quality goes above a certain threshold, and decides to connect to the network via E-UTRAN for seamless service.

MCPTT service continuity follows the principles of 3GPP TS 24.237 [58] for PS-PS service continuity. In particular:

1) the SIP session is anchored at a Service Centralisation and Continuity Application Server (SCC AS) before and after the handover. This requires that initial filter criteria is configured to ensure that the SCC AS is in the registration path, is the first application server in the path of an originating session, and the last AS in the path of a terminating session;

2) the remote UE is an SC UE that supports PS-PS access transfer as per 3GPP TS 24.237 [58]; and

3) the remote UE is either configured with a static PS to PS STI as specified in 3GPP TS 24.216 [66] that it uses when initiating the session transfer request, or it uses a dynamic PS to PS STI which is the URI contained in the Contact header field returned at the creation of the dialog over the Source Access Leg, as specified in 3GPP TS 24.237 [58].

14A.2 Service continuity from on-network MCPTT service to UE-to-network relay MCPTT service

14A.2.1 Remote UE

When performing service continuity from on-network MCPTT service to UE-to-network relay MCPTT service, the remote UE:

1) shall perform ProSe UE-to-network relay discovery over PC5 as specified in clause 10A of 3GPP TS 24.334 [28];

NOTE 1: Depending on the model (A or B) used for discovery as specified in 3GPP TS 24.334 [28], the remote UE can perform UE-to-network relay discovery while still in coverage (when model A is used), or while still in coverage if the LTE-Uu link quality drops below a certain threshold (when model B is used).

NOTE 2: As part of the discovery process, service authorisation is performed as specified in 3GPP TS 24.334 [28]. The UE-to-network relay is provisioned with relay service code(s) associated with allowed MCPTT group(s) as specified in 3GPP TS 24.483 [45] and 3GPP TS 24.484 [50]. To find a permitted UE-to-network relay for group communications, a remote UE is provisioned with the relay service code(s) associated with the MCPTT group(s) which the MCPTT user is part of, in the MCPTT group configuration MO as specified in 3GPP TS 24.483 [45].

2) shall select a suitable UE-to-network relay by performing the UE-to-network relay selection procedure specified in clause 10A.2.12 of 3GPP TS 24.334 [28];

3) shall establish a direct link to the relay as specified in clause 10.4.2 of 3GPP TS 24.334 [28];

NOTE 3: As part of this process the remote UE is assigned a /64 IPv6 Prefix by the relay.

4) shall initiate IMS registration over the UE-to-network relay target access leg by following the procedures in clause 10.2.0 of 3GPP TS 24.237 [58];

NOTE 4: As part of this process the remote UE needs to discover the P-CSCF address to connect to via the UE-to-network relay. The remote UE either uses mechanism I or mechanism III of clause 9.2.1 in 3GPP TS 24.229 [4] to discover the P-CSCF address. The details of how mechanism I or mechanism III are used to discover the P-CSCF address are not covered by the present document.

5) shall initiate session transfer by following the procedures specified in clause 10.2.1 of 3GPP TS 24.237 [58];

6) after successful session transfer if MCPTT content is being distributed on the target side using MBMS bearers, shall send a MBMS bearer listening status report procedure to the participating MCPTT function by performing the procedures in clause 14.3.3; and

NOTE 5: Upon receiving the MBMS bearer listening status from an MCPTT client indicating that the MCPTT UE is now listening to a MBMS subchannel, the participating MCPTT function performs the procedures in clause 14.2.3 to switch to MBMS bearer.

7) after successful session transfer if the remote UE still has an connection in the source access, may perform IMS de-registration of the contact address of the IMS public user identity registered on the source access leg by following the procedures in 3GPP TS 24.229 [4];

14A.2.2 SCC AS

The SCC AS follows the procedures in clause 10.3.2 of 3GPP TS 24.237 [58].

14A.3 Service continuity from UE-to-network relay MCPTT service to on-network MCPTT service

14A.3.1 Remote UE

When performing access transfer between UE-to-network relay MCPTT service and on-network MCPTT service, the remote UE:

1) shall initiate IMS registration over the on-network target access leg by following the procedures in clause 10.2.0 of 3GPP TS 24.237 [58]; and

NOTE: The remote UE uses option II procedures for P-CSCF discovery as defined in clause L.2.2.1 of 3GPP TS 24.229 [4] to discover the P-CSCF address when connecting to EPC.

2) follows the procedures in steps 5), 6) and 7) of clause 14A.2.1.

14A.3.2 SCC AS

The SCC AS follows the procedures in clause 14A.2.2.