4 System architecture

23.6323GPPRelease 18Stage 2TSUser data interworking, coexistence and migration

4.1 Architecture for direct UDM-HSS interworking

Figure 4.1-1 shows the reference architecture for direct UDM-HSS interworking.

Figure 4.1-1: Architecture for Direct UDM-HSS interworking

Non-SBA interfaces between the HSS and non-SBA network nodes include interfaces to / from the SMS-GMSC/IWMSC and SMS Router. In the Architecture for direct UDM-HSS interworking , as a deployment option, these Non-SBA interfaces may either be supported by the HSS or by the UDM.

Figure 4.1-2 shows the reference architecture for direct UDM-HSS interworking using the reference point representation.

Figure 4.1-2: Architecture for Direct UDM-HSS interworking in reference point representation

The 5GS-UDR (Unified Data Repository) and EPS-UDR (User Data Repository) may be collocated, forming a common repository.

NOTE: The HSS is only using the NU2 reference point for the purpose of interworking with 5GS, i.e. NU2 is not a replacement of the Ud interface.

4.2 Reference points for direct UDM-HSS interworking

The following reference points are realized by service-based interfaces:

NU1: Reference point between the HSS and the UDM.

NU2: Reference point between the HSS and the 5GS-UDR.

For a list of other SBA reference points supported in 5GC see 3GPP TS 23.501 [2].

For a list of IMS reference points, including SBA IMS reference points, see 3GPP TS 23.228 [7].

For a list of Non-SBA reference points and Network Nodes interfacing the HSS see 3GPP TS 23.002 [3].

The HSS makes use of the Ud reference point to interact with the EPS-UDR. The UDM makes use of the N35 reference point to interact with the 5GS-UDR.

4.3 Service based interfaces for direct UDM-HSS interworking

Nudm: Service-based interface exhibited by UDM.

Nhss: Service-based interface exhibited by HSS.

The HSS shall make use of Nudm services as described in clause 6.2 and may make use of Nudr services as described in clause 6.3.

The UDM shall make use of Nhss services and Nudr services as described in clauses 6.1 and 6.3.

4.4 Subscription Identifiers

As defined in 3GPP TS 23.501 [2], for interworking with the EPC, the SUPI allocated to the 3GPP UE shall always be based on an IMSI to enable the UE to present an IMSI to the EPC.

The subscription identifier used over NU1 reference point in Nhss services shall be an IMSI. The UDM extracts the IMSI from the user’s SUPI.

The subscription identifier used over NU1 reference point in Nudm services shall be a SUPI based on an IMSI. The HSS creates a SUPI from the user’s IMSI or the IMSI associated to the user’s public identifier in the EPS or IMS domain (e.g. user’s MSISDN or IMPU).

4.5 HSS Discovery and Selection

This clause defines the procedures for HSS discovery and selection by the UDM. The procedures for HSS discovery and selection by SBI capable IMS entities is defined in 3GPP TS 23.228 [7].

The UDM performs HSS discovery to discover an HSS that manages the user subscriptions in EPC.

The UDM shall utilize the NRF to discover the HSS instance(s) unless the information about HSS instances is available by other means, e.g. locally configured on the UDM. The HSS selection function in UDM selects an HSS instance based on the available HSS instances (obtained from the NRF or locally configured).

When the NRF is used for HSS discovery, the HSS registers in the NRF using the Nnrf_NFManagement_NFRegister Request message as defined in 3GPP TS 23.502 [5].

Different HSS instances managing different sets of IMSI/MSISDN ranges may be deployed in a given PLMN. In this case, the HSS instances register in NRF using either different ranges of IMSI/MSISDN and/or HSS Group IDs.

NOTE 1: In deployments where simple IMSI/MSISDN ranges are not suitable to describe the IMSI/MSISDN sets served by HSS instances, it is expected the HSS instances only register HSS Group IDs.

When NRF is used for HSS discovery, the UDM sends a Nnrf_NFDiscovery_Request to NRF as defined in 3GPP TS 23.502 [5] to discover HSS instances within a given PLMN. The UDM may store all returned HSS instances and their NF profiles for subsequent use, including, if applicable, supported IMSI/MSISDN ranges, and/or HSS Group IDs.

The UDM may use Nnrf_NFStatusSubscribe/Unsubscribe service operations with NRF as defined in 3GPP TS 23.502 [5] to receive Nnrf_NFStatusNotify service operation for updates to the NF profiles of HSS instances registered in NRF.

The UDM always selects an HSS within its own PLMN. The HSS selection should consider one of the following factors when available to the UDM entity:

1. HSS Group ID of the UE’s IMSI.

2. IMSI; the UDM selects an HSS instance based on the IMSI range the UE’s IMSI belongs to or based on the results of a discovery procedure with NRF using the UE’s IMSI as input for HSS discovery.

NOTE 2: In this release of the specification there is not identified need for the UDM to be able to select the HSS based on IMS identifiers (IMPI, IMPU) neither based on MSISDN or External Group ID.

4.6 UDM Discovery and Selection

The HSS performs UDM discovery and selection as described in 3GPP TS 23.501 [2].

4.7 Subscription Profiles

In the context of Mobility, IMS and SMS use cases, the HSS initiates interworking with the UDM only for UEs which have a 5G subscription that is known to be active in 5GC.

NOTE: The HSS may be aware that a 5G subscription for the UE exists based on the Core Network Type restrictions defined for the UE (i.e. If restriction for Core Network Type indicates that the UE can access to 5GC, it implies that the UE has 5G subscription data).
Additionally, the HSS may be aware that a UE for which 5G subscription data exist is not active in 5GC e.g. when the HSS has not yet received an Nhss_UECM_ SNDeregistration request from the UDM for that UE (see clause 5.3.3 step 3).