23.1673GPPIP Multimedia Subsystem (IMS) emergency sessionsRelease 17TS
For the high-level procedures (as described in clause 7.1.1) the following statements apply for UE detected emergency calls when 5GC access over non-3GPP access is used:
– the IP-CAN bearer resource is a PDU session dedicated for emergency services. TS 23.502  describes how to indicate that a PDU Session Request is for emergency.
– For registration requests received from an emergency PDU session, the P-CSCF shall reject any IMS registration which is not for the emergency purpose.
NOTE 1: When the IMS network detects that the UE is establishing an emergency session over non-3GPP access to 5GC while the UE is not in its Home country, local policies in the Home IMS network may determine whether to nevertheless handle the emergency session.
For the high-level procedures (as described in clause 7.1.2) the following statements apply for Non UE detectable emergency calls when non-3GPP access to 5GC is used:
– The P-CSCF may reject the session initiation request with an indication that this is an emergency session or may allow the session initiation request to continue.
The following flow contains a high-level description of the emergency service procedures performed over non-3GPP access to 5GC when the UE can detect that an emergency PDU session is being needed. This flow provides details (or references) related with non-3GPP access to 5GC on top of the general procedure described in clause 7.1.
Figure L.3: Terminal Detected Emergency Calls (non-3GPP access to 5GC)
The following steps (numbering being the same as in figure 7.1) are performed:
1. Same as step 1 of Figure 7.1.
2. In some exceptional cases (e.g. in congestion situation), the UE may release some PDU session it may have over non-3GPP access to 5GC.
3. Void (no "bearer registration" as described in step 3 of Figure 7.1).
4. As described in TS 23.502 :
– in the case of untrusted access, the UE selects an N3IWF as for accessing regular services and sets-up a PDU session for emergency services;
– in the case of trusted access, the UE sets-up a PDU session for emergency services.
The UE is assigned an IP address at this stage.
NOTE 2: When non-3GPP access is WLAN, no specific WLAN AP selection is carried out to support emergency services.
5. Same as step 5 of Figure 7.1.
6. Same as step 6 of Figure 7.1.
7. Same as step 7 of Figure 7.1, except that eCall is not supported via untrusted non-3GPP access.