3GPP 23.501 v20.2.0 — the document's own text
5.16.3.10 IMS Voice Service via EPS Fallback or RAT fallback in 5GS
Taught in 21. Emergency calls, messages, voice and location (The 5G system architecture, in depth).
In order to support various deployment scenarios for obtaining IMS voice service, the UE and NG-RAN may support the mechanism to direct or redirect the UE from NG-RAN either towards E-UTRA connected to 5GC (RAT fallback) or towards EPS (E-UTRAN connected to EPC System fallback).
Following principles apply for IMS Voice Service:
- The serving AMF indicates toward the UE during the Registration procedure that IMS voice over PS session is supported.
- If a request for establishing the QoS Flow for IMS voice reaches the NG-RAN, the NG-RAN responds indicating rejection of the establishment request and the NG-RAN may trigger one of the following procedures depending on UE capabilities, N26 availability, network configuration and radio conditions:
- Redirection to EPS;
- Handover procedure to EPS;
- Redirection to E-UTRA connected to 5GC; or
- Handover to E-UTRA connected to 5GC.
- If needed, Network Provided Location Information is provided as described in clauses 4.13.6.1 and 4.13.6.2 of TS 23.502 [3].
- The ongoing IMS voice session is not impacted by a change of the IMS voice over PS session indicator from supported to unsupported (e.g. the UE receives during RAT Fallback or EPS Fallback the IMS voice over PS session indicator indicating that IMS voice over PS sessions are not supported).
NOTE: Any change in IMS voice over PS session indicator applies to new IMS sessions initiated only after the ongoing IMS voice session is terminated.
During any release of RRC connection including after EPS/RAT fallback is performed, the eNB or NG-RAN node may provide to the UE dedicated idle mode priorities for NR as defined in TS 36.331 [51] taking into account RFSP, PLMNs contained in Handover Restriction List and local operator policy. If the UE remains ECM/CM-CONNECTED after the voice call has ended, the eNB or NG-RAN node may trigger handover to NR connected to 5GC, if configured to do so, taking into account local operator policy and Handover Restriction List.