Architecture

This section describes the architecture for the HR roaming support feature.

EPC HR Roaming Scenario

The following diagram shows the architecture for an EPC HR roaming scenario.

EPC HR Roaming Architecture

The 3GPP reference point for nodes in the VPLMN and HPLMN in an EPC HR roaming scenario, are as follows:

  • SMF+IWK resides in the HPLMN.

  • SMF-IWK interacts with the hPCF, hCHF and UDM.

  • SMF-IWK supports S8-C with the S-GW (in VPLMN).

  • vSMF interacts with the vCHF.

5G NR HR Roaming Scenario

The following diagram shows the architecture for a 5G NR HR roaming scenario.

5G NR HR Roaming Architecture

The 3GPP reference point for nodes in the VPLMN and HPLMN in a 5G NR HR roaming scenario, are as follows:

  • SMF resides in both the VPLMN and HPLMN.

  • vSMF and hSMF support the N16 interface.

  • hSMF interacts with UDM, h-PCF and h-CHF.

  • vSMF interacts with the vCHF.

  • When SEPP appears in the network, vSMF communicates to cSEPP for hSMF messaging.

  • When SCP appears in the home network, hSMF communicates to SCP for UDM, hPCF and hCHF messaging.

vSMF

The SMF supports the following functionalities related to HR roaming for visitors:

  • N1

    • The NAS SM information is of two parts, one is visible to vSMF (for example, PDU session type, Session AMBR, UE address). The other one that is not visible to vSMF (for example, SSC Mode, PCO, QoS rules, and so on), which it transparently relays to the hSMF.

    • The vSMF transfers the NAS signalling messages information, which is not visible to the vSMF, in a container toward the hSMF.

    • The vSMF transfers the NAS signalling messages information, which it does not comprehends, these are unknown IEs or IEs with an unknown value not set to "reserved" according to the release to which the vSMF complies, in a different container toward the hSMF.

    • The vSMF appends unknown NAS signalling messages information received in the N16 container at the end of the NAS signalling message it sends to the UE.

  • N40

    • Assignment and transfer of Charging ID of VPLMN to the hSMF.

    • Negotiation of roaming charging profile.

    • When NRF is used, the vCHF is selected based on the UE identified as an in-bound roamer and the PLMN id of the HPLMN.

  • N4

    • V-CN-Tunnel lifecycle management.

  • N16

    • Support for the N16 interface (between vSMF and hSMF).

    • Support for Always-on PDU Session Granted indication.

  • EPS interworking procedures for home routed roaming, are as follows:

    • Caching of EPS bearer IDs and mapped QoS parameters received in hSMF. AMF retrieves the PDN contexts from the vSMF during the 5G to 4G handover. Also, the vSMF supports the release PDN context and not the forward to hSMF context.

    • During the 4G to 5G handover.

    • Support for indirect data forwarding tunnels.

  • Does not interact with PCF or UDM.

hSMF

The SMF supports the following functionalities related to HR roaming for roamers:

  • N1

    • The entire NAS SM information must be interpreted by the hSMF.

    • The hSMF transfers NAS SM information which the vSMF does not need to interpret in one container toward the vSMF.

  • N10

    • Registers with UDM with the S-NSSAI value defined in the HPLMN.

  • N4

    • User-plane inactivity detection is not performed during roaming (does not provide inactivity timer to hUPF).

    • H-CN-tunnel lifecycle management.

  • N40

    • Negotiation of “roaming charging profile.

    • Generate a "home provided charging identifier"

    • When the NRF is used, the hCHF is selected based on a UE identified as an out-bound roamer and the PLMN ID of the VPLMN.

  • N7

    • N7 interaction for the hSMF is similar to the non-roaming case.

  • N16

    • Support for the N16 interface (between vSMF and hSMF).

  • If the UE uses IPv6, IPv4v6, it generates router advertisements Secondary authorization or authentication.

Network Slicing

The SMF supports the following functionalities related to network slicing:

  • The SMF can be configured with a list of allowed NSSAI.

  • When the SMF acts as a vSMF during roaming, the S-NSSAI sent by the UE used in the VPLMN must be the value that is configured on the SMF.

  • In HR, the vSMF sends the PDU Session Establishment Request message to the hSMF along with NSSAI valued used in HPLMN.

Node Selection Considerations

The following criteria are applied for selecting the nodes in the HR roaming:

  • When roaming is enabled, each SMF registers the interPlmnFqdn value with the NRF. This helps the AMF to select the hSMF in a different PLMN.

  • The SMF supports target-plmn-list and requester-plmn-list query parameters.

  • The NRF in the serving PLMN handles all the discovery requests from the NFs.

Lawful Intercept

The SMF provides the IRI-POI functions in the following network topology cases:

  • Non-roaming case.

  • Roaming case, in VPLMN.

  • Roaming case, in HPLMN.

The SMF generates the following IRI events during the roaming scenarios:

  • PDU session establishment

  • PDU session modification

  • PDU session release

  • Start of interception with an established PDU session

Session Management

With roaming considerations, the SMF sessions are categorized into the following flavors:

  • Non-roaming

  • LBO

  • vSMF-HO

  • hSMF-HO