Y.0 General

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

This clause describes the main IP-Connectivity Access Network specific concepts that are used for the provisioning of IMS services over 5GS.

HSS is used to store IMS related subscription and context as shown in the Figure 4.0 "Reference Architecture" specified in clause 4.0. For 5GS, HSS functionality for IMS shall continue to be as standalone regardless if it is co-located or implemented as part of the UDM. When the HSS and UDM are deployed as separate network functions, their interaction is defined by TS 23.632 [97] and TS 29.563 [98], or it may be implementation specific. A single IMS subscription profile is used regardless of UE accessing IMS via different IP-CANs.

Figure Y.0-1: UDM and HSS collocated or HSS as part of UDM

NOTE: The HSS shown in Figure Y.0-1 is only considering the functionality required for IMS.

From IMS perspective, either the N5 interface as specified in TS 23.503 [95] or the Rx interface as specified in TS 23.203 [54] are used between the P‑CSCF and the Policy Control Function (PCF).

NOTE: In PLMNs where both Rx and N5 are used it is implementation specific how the P-CSCF determins the applicable interface/protocol used for a particular interaction with the PCF/PCRF – e.g. Separate P-CSCF’s used for Rx and N5, local routing configuration in the P-CSCF.

In 5G System the Gm reference point defined in clause 4.0 and clause 4.4 is supported for the communication between UE and IMS, e.g. related to registration and session control. Therefore, the same interface is used, i.e. Gm, from P-CSCF perspective towards the UE, when UE is in 5G System or any other IP-CAN.