5.9 P-CSCF Restoration in EPC with 5GC interworking

23.3803GPPIMS Restoration ProceduresRelease 17TS

If IMS service is required in EPC with 5GC interworking as specified in 3GPP TS 23.501 [28], HSS+UDM and PGW-C+SMF are involved in the P-CSCF procedures.

After mobility from 5GC to EPC happens for a UE as specified in 3GPP TS 23.502 [29], the MME via S6a (or AAA for WLAN via SWx) registers itself to HSS+UDM and PGW-C+SMF registered itself via N10 in the HSS+UDM when the UE was in 5GC. In this case, when P-CSCF restoration indication is received from S-CSCF, the HSS+UDM shall, either:

– send a P-CSCF restoration indication to the MME serving the UE as specified in clause 5.4 "HSS-based P-CSCF restoration for 3GPP access", or to the AAA for WLAN as specified in clause 5.6.2 "Basic mechanism for the HSS-based solution", or

– send a P-CSCF restoration indication via N10 interface to the PGW-C+SMF serving IMS PDN connection as described in clause 5.8.4.2 step 6, and the PGW-C+SMF thus initiates IMS PDN connection update procedure in EPC or the IMS PDN connection deactivation with reactivation required, to trigger the UE re-selects P-CSCF and registers again to IMS as described in clause 5.4.3.2 from step 8 to 10, in clause 5.6.2.2 from step 8 to 10, or in clause 5.6.5.2 step 8 to 11.