Architecture
This section describes the architecture for the LBO roaming feature.
EPC LBO Scenario
The following diagram shows the LBO roaming architecture for the 4G sessions connected to the SMF and PGW-C in EPC.

During LBO roaming for 4G sessions, the SGW and the SMF with PGW-C both reside in VPLMN. The SGW and SMF exchange messages through S5-C interface. All northbound SBI interfaces are common for 4G and 5G. The SMF interacts with vPCF, vCHF, and UDM.
ePDG LBO Scenario
The following diagram shows the LBO roaming architecture for the Wi-Fi sessions connected to the SMF and PGW-C in EPC.

SMF resides in VPLMN and interacts with vPCF, vCHF, and UDM. SMF doesn’t support S6b toward the 3GPP AAA server, but uses the N10 interface.
5G NR LBO Scenario
The following diagram shows the LBO roaming architecture for the 5G sessions connected through NR.

As shown in the preceding diagram, the SMF resides in the VPLMN. Only AUSF and UDM are the NFs in the HPLMN. The PCF in the VPLMN communicates with PCF in the HPLMN over N24 interface. The PCFs communicate with each other to get the policies related to the subscriber session and pass them to SMF.
SMF Functionalities During LBO
The SMF supports the following functionalities related to LBO for in-roamers.
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Detection of in-roamers based on local configuration and MCC and MNC in the SUPI received
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N11
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Determination of LBO for the in-roamers
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If the SMF receives the session setup request for a visitor without the support for LBO, then the SMF sends an error to the AMF. Then, the AMF reattempts the session setup with the SMF that supports Home Routed (HR) roaming.
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Support of PCF ID that is vPCF from the AMF
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N2
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The SMF provides Single Network Slice Selection Assistance Information (S-NSSAI) of VPLMN in the N2 SM Information.
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N7
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Selection of PCF in VPLMN
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vPCF interacts with AF in HPLMN for PCC rule generation (for example, IMS). However, PCC rules are generated using roaming policies and the subscribed policies in HPLMN are inaccessible by vPCF. Also, vPCF doesn’t interact with CHF for spending limits. The PCC rules in LBO have limited capabilities.
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N40
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Selection of CHF in VPLMN. vSMF considers additional parameters of the HPLMN ID that CHF has to service the roamer status (in-roamer) of the UE.
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N10
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Selection of UDM in HPLMN
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NRF
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The SMF uses the chf-supported-plmn query parameter while discovering the vCHF servicing HPLMN.
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During EPS procedures, if the SMF supports more than one S-NSSAI and the APN is valid for more than one S-NSSAI, then it performs the Nnssf_NSSelection_Get service operation. This operation is in effect before the SMF provides an S-NSSAI to the UE. This operation helps to retrieve the mapping of the subscribed S-NSSAIs to the serving PLMN S-NSSAI values.
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Emergency services on SMF are supported only in LBO model. For LBO roaming, the SMF does not register with UDM for an emergency session.
For emergency calls, the SMF ignores the UE PLMN ID and relays the serving PLMN ID across all the interfaces.
Network Slicing
The SMF supports the following functionalities related to network slicing:
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The SMF can be configured with a list of allowed NSSAI.
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When the SMF acts as a vSMF during roaming, the S-NSSAI of the UE used in the VPLMN must be the value that is configured on the SMF.
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In the case of LBO, the SMF performs mapping of S-NSSAI received from UDM to the NSSAI of HPLMN received during PDU connection setup. The received NSSAI must be configured on vSMF as the supported NSSAI.
Node Selection Considerations
The following criteria are applied for selecting the nodes in the LBO roaming:
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When roaming is enabled, each SMF registers the inter-PLMN FQDN value with the NRF. This operation helps the AMF to select the hSMF in a different PLMN.
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The SMF treats target-plmn-list and requester-plmn-list as the query parameters.
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The NRF in the serving PLMN handles all the discovery requests from the NFs.
PDU Establishment During LBO
The following conditions are considered for PDU session establishment in LBO roaming case:
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If the SMF receives the session setup request for a visitor without support for LBO, then the SMF sends SM Context Create error to the AMF with the cause HOME_ROUTED_ROAMING_REQUIRED. Then, the AMF reattempts the session setup with the SMF that supports Home Routed (HR) roaming. An example scenario is when the NAS PDU Session Establishment Request has requested SSC mode as 3 and the allowed SSC mode in vSMF does not support the SSC mode 3.
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The SMF receives both HPLMN S-NSSAI and S-NSSAI. The SMF uses S-NSSAI to validate NSSAI against the vSMF supported NSSAI.
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On N40 interface:
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vSMF sends the roamerInOut attribute to CHF through the CDR message. The roamerInOut attribute includes PDUSessionChargingInformation and userInformation. This attribute value is either IN_BOUND for in-roamers or OUT_BOUND for out-roamers.
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vSMF sends the PDUSessionInformation and chargingCharacteristicsSelectionMode IE with appropriate value (HOME_DEFAULT, ROAMING_DEFAULT, and VISITING_DEFAULT) for non-roaming and roaming cases.
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The hPlmnId and servingCNPlmnId fields in the PDUSessionInformation IE carry the value as per the roaming status of the UE.
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During N1N2 Message Transfer, the S-NSSAI provided in N2 content should be the same as the VPLMN S-NSSAI.
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For LBO roaming scenario, the PDU Session Establishment Accept message includes the S-NSSAI from the allowed NSSAI for the VPLMN. It also includes the corresponding S-NSSAI of the HPLMN from the mapping of allowed NSSAI that the SMF received from AMF.
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The SMF uses HPLMN for UDM discovery during LBO roaming.
PDN Establishment During LBO
The S-GW sends Serving Network IE to the PGW-C with the PLMN ID where the S-GW belongs. The SMF uses that PLMN as VPLMN for validation, node selection, and passing on the VPLMN to other north bound interfaces.
The N40 interface related requirements and the emergency session-related requirements applicable for 5G session creation, also apply for the 4G and Wi-Fi sessions.
PLMN Usage
The following table shows an example of how the PLMN values configured in SMF service profile are relayed across all the interfaces.
Interface |
Attribute |
Homer |
In-roamer (LBO) |
Out-roamer (HR) |
In-roamer (HR) |
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UE PLMN |
310-240 |
262-06 |
310-310 |
302-610 |
|
NRF |
plmn-list in nrf Discover to discover UDM (queryParam = target-plmn) |
UE PLMN |
UE PLMN |
UE PLMN |
Not applicable |
NRF |
plmn-list in nrf Discover to discover PCF/CHF(queryParam = target-plmn) |
UE PLMN |
Serving PLMN |
UE PLMN |
Serving PLMN |
N10 |
PLMN in smfRegistration IE in N10 registration |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Not applicable |
N10 |
PLMN in GET subscription URI |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Not applicable |
N10 |
PLMN in sdmSubscription IE in N10 subscribe ToNotification |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Not applicable |
N40 |
PLMN in NfConsumer identification IE in N40 charging data request |
Primary home PLMN |
Primary home PLMN |
Primary home PLMN |
Primary home PLMN |
N40 |
hPlmnId in PDUSession Information IE in pduSession Charging Information in chargingData Request |
UE PLMN |
UE PLMN |
UE PLMN |
UE PLMN |
N40 |
Serving PLMN in PDU Session Information IE in pduSession ChargingInformation in chargingData Request |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Serving PLMN |
N7 |
PLMN in PCF notify for AC_TY_CH/ SAREA_CH/ RAT_TY_CH trigger |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Not applicable |
N7 |
PLMN in create request to PCF |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Not applicable |
RADIUS |
PLMN in 3GPP UE location IE RADIUS authentication |
Serving PLMN |
Serving plmn |
Not applicable |
|
RADIUS |
PLMN in 3GPP GGSN MCC MNC in RADIUS authentication |
Primary home PLMN |
Primary home PLMN |
Not applicable |
|
N4 |
PLMN in X-header of N4 requests |
Serving PLMN |
Serving PLMN |
Primary home PLMN |
Not applicable |
Roaming Status Determination
The SMF extracts the UE PLMN from SUPI. The SMF compares the UE PLMN and the serving PLMN with the configured PLMN list. The SMF determines the roaming status of subscribers based on the HPLMN values.
If the UE PLMN and the serving PLMN both belong to the PLMN list configured in SMF, then it is a home subscriber. If the UE PLMN does not belong to the configured PLMN list and the serving PLMN belongs to the configured PLMN list, then it is a visitor. If the UE PLMN belongs to the configured PLMN list and the serving PLMN does not belong to the configured PLMN list, then it is a roamer.
Handover Scenarios
Once the roaming status is determined, there will be no change to the status even if the configuration of PLMN values changes after the handover (HO).
Local Policies
In HO scenarios, vSMF supports local policy to enable vPLMN operators to override the signaled parameter from hPLMN domain as per the roaming agreements. The SMF uses the local policies to—
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allow always-on session requests.
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perform paging policy differentiation
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allow PDU session setup in HR or LBO mode
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support subscriber QoS as per the roaming agreement
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allow ARP priority levels 1-8 for HO roaming sessions.
Other Procedures
Paging Policy Differentiation (PPD)
The SMF needs a configuration per PLMN to allow different PPD profiles for different roaming partners. The vSMF picks the appropriate configuration for the HPLMN and applies the same for the roaming session.
PCF and UDM Selection
During roaming, the AMF selects both vPCF and hPCF and sends the vPCF ID and hPCF ID to the SMF and vPCF respectively during policy association. The SMF selects the PCF using the received vPCF ID. During AMF relocation, target AMF selects a new vPCF and hPCF. The SMF receives a redirection indication with PCF ID from the existing PCF for the PDU session. The SMF terminates the current SM Policy Control association and reselects a PCF based on the received PCF ID. The SMF then establishes an SM Policy Control association with the reselected PCF.
For selection of PCF and UDM based on local configuration, the locally configured addresses map to the VPLMN and HPLMN respectively since the PCF is in VPLMN and the UDM is in HPLMN for roaming with LBO case.
For NRF-based discovery of PCF and UDM, the query criteria includes VPLMN for PCF discovery and HPLMN for UDM discovery. The AMF sends the UDM group ID to enable the SMF to select UDM. The S-NSSAI used by SMF to select PCF should be the VPLMN S-NSSAI received from AMF.
Lawful Interception
During roaming scenario, the SMF uses S-NSSAI of the VPLMN to generate IRI events. The S-NSSAI information is sent to the mediation device through the IRI event message.