Content for TS 23.280 Word version: 19.4.0

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B.1 Service continuity between on-network MC service and UE-to-network relay MC service
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B Service continuity for MC service p. 375

B.1 Service continuity between on-network MC service and UE-to-network relay MC service p. 375

This Annex describes how TS 23.237 mechanisms for IMS service continuity can be used to provide service continuity between on-network MC service and UE-to-network relay MC service. For illustration, MCPTT AS is considered as the MC service.

Only the procedure for service continuity from on-network MCPTT service to UE-to-network relay MCPTT service is described in Figure B.1-1. The procedure for service continuity in the opposite direction is identical.

Reproduction of 3GPP TS 23.280, Fig. B.1-1: Service continuity from on-network to UE-to-network relay

Figure B.1-1: Service continuity from on-network to UE-to-network relay

As illustrated in Figure B.1-1:

Initially UE-1 has a direct connection to the network (on-network MCPTT service). It is registered with the SIP core and is engaged in a SIP session with the MCPTT Application Server (solid lines SIP-1 and MCPTT-1 in Figure B.1-1).
When UE-1 realises that it is losing connection to the network, or after the connection to the network has been lost, UE-1 discovers a UE-to-network relay (UE-R) and establishes a PC5 connection with UE-R. UE-1 registers with the SIP core over the target access leg and enters UE-to-network relay MCPTT service by transferring the media streams over the target leg (dashed lines SIP-1 and MCPTT-1 in Figure B.1-1).
The SIP session is anchored at a Service Centralisation and Continuity Application Server (SCC AS) before and after the handover, as described in TS 23.237.

Depicted in Figure B.1-2 is the call flow for service continuity when the UE switches from on-network MCPTT service to UE-to-network MCPTT relay service.

Reproduction of 3GPP TS 23.280, Fig. B.1-2: Service continuity when UE switches from on-network MCPTT service to UE-to-network relay MCPTT service

Figure B.1-2: Service continuity when UE switches from on-network MCPTT service to UE-to-network relay MCPTT service

Step 0.

UE-1 has a direct connection to the network and is engaged in a SIP session with the MCPTT AS (on-network MCPTT service). The SIP session is anchored at a Service Centralisation and Continuity Application Server (SCC AS) and a Session transfer Identifier (STI) is assigned for the anchored SIP session, as described in TS 23.237.

Step 1.

UE-1 realises that it is losing connection to the network or has completely lost it.

Step 2.

UE-1 (in the role of remote UE) performs ProSe UE-to-network relay discovery over PC5 and establishes a secure point-to-point link with the relay (UE-R) over PC5. As part of this process the remote UE is mutually authenticated at PC5 layer with either the relay or with the network as specified in TS 23.303. In the process UE-1 is also assigned an IP address/prefix by the relay.

Step 3.

UE-1 registers with the SIP core over the UE-to-network relay leg.

Step 4.

In order to transfer the media streams of the SIP session UE-1 sends an INVITE message on the new access leg towards the SCC AS. The INVITE message includes the STI identifying the session to be transferred. The SCC AS identifies the session based on STI and updates the session over the remote access leg i.e. towards the MCPTT AS.

Step 5.

The procedure is completed when all media streams have been transferred on the access leg relayed via UE-R. At this point UE-1 may deregister the on-network leg if it still has direct network connection (not shown in the Figure).