Section: 4.3.12.4 PDN GW selection function (3GPP accesses) for Emergency Services
Content: When a PDN GW is selected for IMS emergency services support, the PDN GW selection function described in clause 4.3.8.1 for normal bearer services is applied to the Emergency APN or the MME selects the PDN GW directly from the MME Emergency Configuration Data. If the PDN GW selection function described in clause 4.3.8.1 is used it shall always derive a PDN GW in the visited PLMN, which guarantees that also the IP address is allocated by the visited PLMN. In networks that support handover between E-UTRAN and HRPD accesses, the MME selects a PDN GW that is statically configured in the MME Emergency Configuration Data. In networks that support handover between E-UTRAN and WLAN accesses, when the UE has been authorized but has not been authenticated, the MME selects the PDN GW that is statically configured in the MME Emergency Configuration Data. The PDN GW selection does not depend on subscriber information in the HSS since emergency service support is a local, not subscribed service. The MME Emergency Configuration Data contains the Emergency APN which is used to derive a PDN GW, or the MME Emergency Configuration Data may also contain the statically configured PDN GW for the Emergency APN. In the case of GateWay Core Network sharing, the MME shall support separate MME Emergency Configuration Data for each of the sharing PLMNs. In the case of GateWay Core Network sharing and PDN GW selection for the Emergency APN, the MME shall be able to take the selected PLMN ID into account to derive a PDN GW. This functionality is used by the Attach procedure and by the UE Requested PDN Connectivity procedure, in both cases when establishing emergency bearer services. NOTE: It is assumed that the PDN GW that is statically configured in the MME Emergency Configuration Data is the same as the PDN GW configured in WLAN and HRPD accesses.