5.16.3.5 Domain selection for UE originating sessions / calls

For UE originating calls, the 5GC capable UE performs access domain selection. The UE shall be able to take following factors into account for access domain selection decision:

• The state of the UE in the IMS. The state information shall include: Registered, Unregistered.
• The "IMS voice over PS session supported indication" as defined in clause 5.16.3.2.
• Whether the UE is expected to behave in a "voice centric" or "data centric" way for 5GS.
• UE capability of supporting IMS PS voice.
• UE capability for operating in dual-registration mode with selective PDU Session transfer as defined in clause 5.17.2.3.3.
• Whether 3GPP PS Data Off is active or not and whether IMS voice is included in 3GPP PS Data Off Exempt Services or not as defined in clause 5.24.

NOTE 1: In this release of the specification, the exact logic of which PDU sessions are kept in which system for Dual Registration UE with selective transfer of certain PDU Sessions as defined in clause 5.17.2.3.3, is left up to UE implementation. The voice centric UE will keep the PDU Session used for IMS services to a system that supports voice over IMS. The voice centric UE can re-register with the IMS (if needed) when the IMS PDU session is transferred between 5GS and EPS.

To allow for appropriate domain selection for originating voice calls, the UE shall attempt Initial Registration in 5GC. If the UE fails to use IMS for voice, e.g. due to "IMS voice over PS session supported indication" indicates voice is not supported in 5G System, the UE behaves as described below for "voice centric" for 5GS or "data centric" for 5GS:

• A UE set to "voice centric" for 5GS shall always try to ensure that Voice service is possible. A voice centric 5GC capable and EPC capable UE unable to obtain voice service in 5GS shall not select a cell connected only to 5GC. By disabling capabilities to access 5GS, the UE re-selects to E-UTRAN connected to EPC first (if available). When the UE selects E-UTRAN connected to EPC, the UE performs Voice Domain Selection procedures as defined in TS 23.221 [23].
• A UE set to "data centric" for 5GS does not need to perform any reselection if voice services cannot be obtained.

NOTE 2: The related radio capabilities in order for the voice centric UE to not reselect to NR or E-UTRA cell connected to 5GC (i.e. avoid ping pong) will be defined by RAN WGs.