Content for  TS 23.402  Word version:  18.3.0

This clause provides an overview about the EPC functionality for emergency PDN connections used to support IMS Emergency Session over WLAN untrusted or trusted access to EPC defined in TS 23.167. The specific functionality is described in the affected procedures and functions of this specification. For discrepancies between this overview clause and the detailed procedure and function descriptions the latter take precedence. UEs request a PDN Connection for emergency services (also called an emergency PDN connection) when they are aware they need to establish an IMS emergency session. The UE shall not issue an emergency session over WLAN access to EPC if the emergency session can be established via 3GPP access. In this Release of the specification, the same four behaviours of IMS emergency session support as identified in clause 4.3.12 of TS 23.401 are applicable. To get EPC access for emergency services in case of untrusted WLAN, the UE shall select an ePDG that supports emergency services as specified in clause 4.5.4a. Then, if a new ePDG is selected, the UE shall execute the procedure of Initial attach for S2b emergency services described in clause 7.2.5. Otherwise, if an existing ePDG connection is reused, the UE shall perform the UE-initiated Connectivity to Additional PDN to Emergency Service PDN connectin described in clause 7.6.3. An ePDG/TWAG that supports emergency services is configured with Emergency Configuration Data that are applied to all PDN Connections for emergency services. The Emergency Configuration Data contain the Emergency APN which is used to derive a PDN-GW, the statically configured PDN-GW for the Emergency APN and optionally a fallback statically configured PDN-GW, and may also contain information on the default QoS to apply to a PDN Connection for emergency services (as defined in clause 4.5.7.2.4). The following procedures apply for emergency PDN connections for untrusted WLAN case:

• procedures defined in clause 7.4.3 ("UE/ePDG-initiated Detach Procedure and UE-Requested PDN Disconnection");
• procedures defined in clause 7.6 ("UE-initiated Connectivity to Additional PDN");
• procedures defined in clause 7.9.2 ("PDN-GW initiated Resource Allocation Deactivation");
• procedures defined in clause 7.10 ("Dedicated S2b bearer activation");
• procedures defined in clause 7.11.1 ("PDN-GW initiated bearer modification");
• procedures defined in clause 8 ("Handovers without Optimizations Between 3GPP Accesses and Non-3GPP IP Accesses").

As part of these procedures, the UE local IP address and optionally UDP or TCP source port number (if NAT is detected) are reported from ePDG to the PDN-GW. When Access Network Information reporting has been set by the PCRF, the UE local IP address and optionally UDP or TCP source port number (if NAT is detected) is reported to the PCRF. To get EPC access for emergency services in case of trusted WLAN, the UE shall select a Trusted WLAN that supports emergency services. This is defined in clause 4.8.2b. Then the UE executes the procedure of Initial attach for S2a emergency services described in clause 16.2.1a The following procedures apply for emergency PDN connections for trusted WLAN case:

• procedures defined in clause 16.3 ("Detach and PDN disconnection in WLAN on S2a");
• procedures defined in clause 16.4 ("PDN-GW initiated Resource Allocation Deactivation in WLAN on S2a");
• procedures defined in clause 16.5 ("Dedicated bearer activation in WLAN on GTP S2a");
• procedures defined in clause 16.6.1 ("PDN-GW Initiated Bearer Modification");
• procedures defined in clause 16.7.1.1 ("UE/TWAN Initiated Detach Procedure in WLAN on GTP S2a");
• procedures defined in clause 16.7.2.1 ("UE/TWAN Initiated Detach Procedure in WLAN on PMIP S2a");
• procedures defined in 16.8 ("UE Initiated PDN connectivity request procedure in WLAN on S2a for Multi-connection Mode");
• procedures defined in clause 16.9 ("UE/TWAN Initiated PDN disconnection for Multi-connection Mode");
• procedures defined in clause 16.10 ("Handover procedure from 3GPP access to WLAN on S2a");

The emergency PDN connection is not a subscribed service. Thus procedures related with HSS Initiated Subscribed QoS Modification in clause 7.11.2, procedures related with HSS Initiated Bearer Modification in clauses 16.6.2 and 16.7.2.2 or procedures related with "HSS/AAA-initiated Detach Procedure" in clauses 7.4.4, 16.3.1.2 and 16.3.2.2 do not apply to emergency PDN connections. Procedures related with S2c do not apply to emergency PDN connections.

In this Release of the specification, both the non-roaming architecture defined in Figure 4.2.2-1 and the roaming architecture defined in Figure 4.2.3-1 apply for emergency services.

The PDN-GW selection does not depend on subscriber information in the HSS since emergency service support is not a subscribed service but a local service. Upon reception from the UE indication that a PDN connection for emergency services needs to be established, the ePDG/TWAG looks up its configured Emergency Configuration Data. The Emergency Configuration Data contains the Emergency APN to be used to derive a PDN-GW, or may also contain the statically configured PDN-GW for the Emergency APN. When a PDN-GW is selected based on the Emergency APN, the PDN-GW selection function described in clause 4.3.8.1 of TS 23.401 for normal bearer services is applied to the Emergency APN. The PDN-GW selection function shall always derive a PDN-GW in the local PLMN. This functionality is used by the Initial Attach procedure for emergency services as described in clause 7.2.5 for untrusted WLAN and clause 16.2.1a for trusted WLAN.

The Default QoS values used over S2b/S2a for establishing emergency PDN connections are configured in the Emergency Configuration Data.

The same mechanisms than defined for 3GPP access in clause 4.3.12.6 of TS 23.401 apply.

The same mechanisms than defined for 3GPP access in clause 4.3.12.8 of TS 23.401 apply.

The same mechanisms as those defined for 3GPP access in clause 4.3.12.9 of TS 23.401 apply with the only difference being that it is the ePDG/TWAG (and not the MME) that shall reject any additional emergency PDN Connection requests.

In the case of trusted WLAN Access, WLAN Location Information is provided as specified in clause 16.1.7. In the case of untrusted WLAN access, WLAN Location Information is provided as follows:

If the UE needs to provide location information when requesting an emergency session, the UE may determine its location by using its own implementation-specific means (e.g. by using its GPS receiver). If, however, the UE cannot determine its location by its own means, the UE may retrieve its location from a WLAN AP, prior or after association with the AP, by requesting the Civic Location ANQP element, the Geospatial Location ANQP element or both as specified in IEEE Std 802.11-2012 [64]), using ANQP procedures described in HS2.0 Rel-2 specification [75].

Seamless PS handover between WLAN and 3GPP EPC is supported if the following conditions are satisfied:

• For UEs without UICC or with an unauthenticated IMSI and for authenticated roaming UEs, the ePDG/TWAG is configured with a static PDN-GW identity and optionally a fallback PDN-GW identity as part of the Emergency Configuration Data.
  • When the UE initiates an Emergency Attach with handover indication to the MME, the MME uses the static PDN-GW identity locally configured instead of querying the HSS as described in TS 23.401.
• For authenticated non-roaming UEs, based on operator policy, the ePDG/TWAG may select a PDN-GW based on DNS look up for the emergency APN, which is configured in the Emergency Configuration Data. In such case:
  • During the initial establishment of the PDN connection for emergency services, the "PDN-GW currently in use for emergency services", which comprises the PDN-GW address and an indication that the PDN connection is for emergency services is provided to the HSS by the PDN-GW via the 3GPP AAA server over S6b and SWx as described in clause 7.2.5 for untrusted WLAN and clauses 16.2.1a and 16.8.1 for trusted WLAN and by the MME over S6a as described in TS 23.401. The HSS stores it as a specific data "PDN-GW currently in use for emergency services", not associated with any APN. If the HSS detects that the UE is attached to the other access (3GPP or WLAN), the HSS updates the corresponding serving node (MME, ePDG or TWAN) with the "PDN-GW currently in use for emergency services".
  • During the handover procedure from WLAN to 3GPP EPC, the "PDN-GW currently in use for emergency services" information is provided by the HSS as part of the subscription information sent to the MME over S6a as described in clause 8.6.1.1 for untrusted WLAN and clause 16.11 for trusted WLAN.
  • During the handover procedure from 3GPP EPC to WLAN, the "PDN-GW currently in use for emergency services" information is provided by the HSS as part of the subscription information sent to the ePDG / TWAG via the 3GPP AAA server over SWx and SWm / STa as described in clause 8.6.2.1 for untrusted WLAN and clauses 16.10.1.1 and 16.10.2.1 for trusted WLAN.
• Alternatively for non-roaming authenticated UEs, based on operator policy (e.g. the network supports handovers to/from HRPD), the ePDG/TWAG may be configured with a static PDN-GW identity as part of the Emergency Configuration Data. In such case, when the UE initiates an Emergency Attach with handover indication to the MME, the MME uses the static PDN-GW identity locally configured instead of querying the HSS as described in TS 23.401.