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 and TS 29.563, or it may be implementation specific. A single IMS subscription profile is used regardless of UE accessing IMS via different IP-CANs.

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

A PDU session is associated with either an IPv4 or an IPv6 address. For communication with the IMS, the UE shall acquire an IP address according to clause 5.8.2.2.1 of TS 23.501 for the PDU session. The IP address will be either an IPv4 address or and IPv6 address. Hence, a UE will register to the IMS with either an IPv4 or an IPv6 address. Here the P-CSCF and IMS-AGW may support both IP versions and/or may do interworking depending IP version used within the IMS. If the P-CSCF and IMS-AGW do not support both versions, then network design is expected to ensure that IP address incompatibility does not occur.

There should be no NAT (or its existence should be kept transparent towards the UE) located between the UPF and the P-CSCF, which is possible as they are either located within the same private network and share same address space, or both the UE and the P-CSCF are assigned globally unique IP addresses (see Annex M).

Information related to the location of the user provided by the access network may be required in IMS in order to comply with regulatory requirements (e.g. data retention, lawful interception) and/or in order to enable certain types of added value services based on the user's location. Depending on usage scenario, the following mechanisms are defined and can be used to retrieve the user location and/or UE Time Zone information from the access network when using 5GS to access IMS:

• The P-CSCF can retrieve the user location and/or UE Time Zone information using PCC mechanisms as specified in TS 23.203 / TS 23.503 and in TS 29.214. Operator policy determines whether to provide the user location and/or UE Time Zone information from the access network in the INVITE request, a MESSAGE request, or within a subsequent message of the dialog.
• When the user location and/or UE Time Zone information is required from the access network but not already available (e.g. when required in an INVITE request, when it is needed prior to session delivery, or when call is broken out to a MGCF), an IMS AS can trigger the retrieval of the user location and/or UE Time Zone information from the AMF via the HSS/UDM as specified in TS 29.328 and as described in clause 4.2.4a.

Information flows on how user location and/or UE Time Zone information can be further distributed within IMS depending on the alternative mechanism used can be found in, Annex R, where the terms HSS and HSS/UDM shall be understood as in clause Y.0. The level of granularity of user location information may be changed at network/trust boundaries. Thus, the level of user location information granularity that can be retrieved by an IMS AS via the HSS/UDM-based procedures in roaming scenarios depends on inter-operator agreement, and needs to be aligned with policies in the P-CSCF. Information related to the location of the user provided by the access network may be required in IMS in order to comply with regulatory requirements for SMS over IP. The P-CSCF applies the above mechanisms upon reception of a MESSAGE including the distribution of received information to other IMS entities.

A network which requires the Geographical Identifier to be generated in the IMS may implement a mapping table between a NG-RAN cell identifier received as part of Access Network Information and a Geographical Identifier. The P-CSCF or an IMS AS may then, based on operator policy, use this mapping table to convert the user location into a Geographical Identifier, and insert the Geographical Identifier in the SIP signalling, thus enabling routing decision in downstream IMS entities or interconnected network. In the case that a given cell belongs to more than one area identified by a Geographical Identifier, based on operator policy, the P-CSCF or an IMS AS may use the UE mobility analytics provided by NWDAF as specified in TS 23.288.

P-CSCF may support Paging Policy Differentiation (as defined in TS 23.501) for a specific IMS service by marking packet(s) to be send towards the UE related to that IMS service. For such an IMS service, a specific DSCP (IPv4) value and/or a specific Traffic Class (IPv6) value are assigned by local configuration in the P-CSCF. When Paging Policy Differentiation is deployed in a PLMN, all P-CSCF entities of that PLMN shall homogeneously support it and shall be configured with the same policy for setting the specific DSCP (IPv4) and/or Traffic Class (IPv6) values used by P-CSCF for that feature.

RAN assisted codec adaptation is supported as described in clause E.10 with the addition that RAN assisted codec adaptation needs to be supported on NR RAT as specified in TS 38.300 and TS 38.321, in addition to E-UTRA RAT.

In order to support service based SMF discovery of the P-CSCF using the NRF (as described in clause 5.16.3 of TS 23.501) the P-CSCF's in a network will need to register with an applicable NRF. When a network uses other P-CSCF discovery methods (as described in clause 5.1.1) the P-CSCF does not need to register with the NRF. Local configuration of the P-CSCF is used to determine if the P-CSCF shall perform registration with the NRF. When the P-CSCF is configured to support SMF discovery of the P-CSCF, P-CSCF shall register in NRF their capabilities using the Nnrf_NFManagement_NFRegister Request message. The NF profile of the P-CSCF registered in NRF shall include the IP address and may include an FQDN if available. The same applies to the Nnrf_NFManagement_NFUpdate Request. Based on the same configuration, a P-CSCF taken out of service will deregister itself using the Nnrf_NFManagement_NFDeregister Request.

Based on local configuration in the case of an originating or a terminating session, the P-CSCF may subscribe to the PCF for notification for the EPS Fallback event using existing procedures defined in TS 23.503. If the PCF reports that an EPS fallback occurred, based on local configuration, the P-CSCF may include this information in outgoing SIP messages towards other IMS nodes.