23.1673GPPIP Multimedia Subsystem (IMS) emergency sessionsRelease 17TS
For the specific case where the UE has selected to make an emergency call over EPS, GPRS or 5GS the UE shall use the following procedures:
– A UE shall establish an emergency PDN connection or emergency PDU session and perform an IMS emergency registration before initiating a UE detectable emergency session via UTRAN, E-UTRAN or NG-RAN.
– A UE shall not establish an emergency PDN connection or emergency PDU session if the UE initiated a non UE detectable emergency session and is subsequently informed by the network that the ongoing session is an emergency session.
NOTE 1: If SRVCC is required in the network, an operator could download the local emergency numbers to avoid non UE detectable emergency sessions since UTRAN and E-UTRAN will not be able to identify emergency SRVCC handling without an emergency PDN connection.
– If the UE initiates a non UE detectable emergency session, and the session initiation request is rejected by the P‑CSCF with an indication that this is an emergency session, the UE shall select a domain according to the requirements for domain priority and selection in clause H.5 when applying the requirements in clause 7.1.2.
– A UE in 5GS shall not initiate establishment of an IMS emergency session over Non-3GPP access unless the emergency session could not be established via 3GPP access.
– If the UE has not been authenticated in the PS domain, the UE shall initiate an IMS emergency session establishment without registration according to clause 7.4.
– The UE shall include the latest available Cell Global Identification (CGI) in the IMS emergency request establishing the emergency call.
– When the UE is using NR satellite access to 5GC, the UE shall include an indication of NR satellite access in the IMS request.
NOTE 2: When using UTRAN, the UE is not always able to read the current cell identity and in some cases the UE can be connected to several cells simultaneously.
NOTE 3: For NR satellite access to 5GC the NR CGI (NCGI) available to the UE and the NCGI provided from NG-RAN to 5GC might be different. The NCGI provided by the UE might not relate to the UE’s geographic location. A P-CSCF relying only on the UE provided NCGI could then lead to errors.
– If the UE is required to include an equipment identifier (according to clauses 4.1 and 6.1) the equipment identifier shall be the IMEI.
– For the media supported during IMS emergency sessions in clause 10.4 of TS 22.101 , media codec and format support is specified in TS 26.114 .
– UE shall only perform eCall procedure over IMS as specified in clause 7.7.1 when it detects the "eCall supported" indication as defined in TS 23.401  and TS 23.501 .
– When PS access is available, but the UE does not detect the "eCall supported" indication as defined in TS 23.401  and TS 23.501 , and there is no CS access available, the UE shall establish a regular IMS emergency call.
– If the broadcast indicators in an E-UTRA cell connected to 5G and EPC as defined in TS 23.501  indicate only one core network (EPC or 5GC) supports emergency services then the UE shall register to the core network which supports emergency services when the UE initiates emergency services. If broadcast indicators indicate that both 5GC and EPC support emergency services then the UE initiates emergency services to either EPC or 5GC according to the UE implementation.
– If the broadcast indicator in an NR cell indicates that the cell provides access to SNPNs and that 5GC supports emergency services, the UE shall only register to the core network of the SNPN that indicates support for emergency services. If broadcast indicators indicate that one or more SNPNs and/or PLMNs support emergency services, then the UE initiates emergency services to either a SNPN or a PLMN according to UE implementation.