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Content for  TS 23.122  Word version:  17.1.1

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C (Normative)  Control plane solution for steering of roaming in 5GS |R15|Word‑p. 65

C.0  Requirements for 5G steering of roaming over the control plane

In addition to the requirements specified in TS 22.011, subclause 3.2.2.8 and in TS 22.261, subclause 6.30, the requirements in this subclause apply.
The UE supporting N1 mode shall support the control plane solution for steering of roaming in 5GS. If the HPLMN supports and wants to use the control plane solution for steering of roaming in 5GS, then the HPLMN shall provide the steering of roaming information to the UE using the control plane mechanism defined in this annex.
The VPLMN shall transparently relay the steering of roaming information received from the HPLMN to the UE. The UE shall be able to detect whether the VPLMN removed the steering of roaming information during the initial registration procedure in the VPLMN. The UE shall be able to detect whether the VPLMN altered the steering of roaming information. If the UE detects that the VPLMN altered or removed the steering of roaming information then the UE shall consider the current VPLMN as the lowest priority PLMN and perform PLMN selection as defined in this annex.
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C.1  General

The purpose of the control plane solution for steering of roaming in 5GS procedure is to allow the HPLMN to update the "Operator Controlled PLMN Selector with Access Technology" list in the UE by providing the HPLMN protected list of preferred PLMN/access technology combinations via NAS signalling. If the selected PLMN is a VPLMN, the HPLMN can provide the steering of roaming information to the UE using the control plane mechanism during and after registration. If the selected PLMN is the HPLMN, the HPLMN can provide the steering of roaming information to the UE using the control plane mechanism after registration only. The HPLMN updates the "Operator Controlled PLMN Selector with Access Technology" based on the operator policies, which can be based on the registered VPLMN, the location of the UE, etc.
The HPLMN can configure their subscribed UE's USIM to indicate that the UE is expected to receive the steering of roaming information due to initial registration in a VPLMN. At the same time the HPLMN will mark the UE is expected to receive the steering of roaming information due to initial registration in a VPLMN, in the subscription information in the UDM. In this case, it is mandatory for the HPLMN to provide the steering of roaming information to the UE during initial registration in a VPLMN. Otherwise if such configuration is not provided in the USIM, it is optional for the HPLMN to provide the steering of roaming information to the UE during initial registration (based on operator policy). The HPLMN can provide the steering of roaming information to the UE during the registration procedure for mobility registration update and initial registration procedure for emergency services. In addition, the HPLMN can request the UE to provide an acknowledgement of successful reception of the steering of roaming information.
As the HPLMN needs to consider certain criteria including the number of customers distributed through multiple VPLMNs in the same country or region, the list of the preferred PLMN/access technology combinations is not necessarily the same at all times and for all users. The list of the preferred PLMN/access technology combinations needs to be dynamically generated, e.g. generated on demand, by a dedicated steering of roaming application function (SOR-AF) providing operator specific data analytics solutions.
In order to support various deployment scenarios, the UDM may support:
  • using a list of preferred PLMN/access technology combinations or a secured packet which is or becomes available in the UDM (i.e. retrieved from the UDR);
  • obtaining a list of preferred PLMN/access technology combinations or a secured packet from the SOR-AF; or
  • both of the above.
The HPLMN policy for the SOR-AF invocation can be present in the UDM only if the UDM supports obtaining a list of preferred PLMN/access technology combinations or a secured packet from the SOR-AF.
The UDM discards any list of preferred PLMN/access technology combinations or any secured packet obtained from the SOR-AF or which is or becomes available in the UDM (i.e. retrieved from the UDR), either during registration (as specified in annex C.2) or after registration (as specified in annex C.3), when the UDM cannot successfully forward the SOR information to the AMF (e.g. in case the UDM receives the response from the SOR-AF with the list of preferred PLMN/access technology combinations or the secured packet after the expiration of the operator specific timer, or if there is no AMF registered for the UE).
The UE maintains a list of "PLMNs where registration was aborted due to SOR". If the UE receives steering of roaming information in the REGISTRATION ACCEPT or DL NAS TRANSPORT message and the security check to verify that the steering of roaming information is provided by HPLMN is successful, the UE shall remove the current selected PLMN from the list of "PLMNs where registration was aborted due to SOR". The UE shall delete the list of "PLMNs where registration was aborted due to SOR" when the MS is switched off or the USIM is removed.
If:
  • the UE's USIM is configured to indicate that the UE shall expect to receive the steering of roaming information during initial registration procedure but did not receive it or security check on the steering of roaming information fails;
  • the current chosen VPLMN is not contained in the list of "PLMNs where registration was aborted due to SOR";
  • the current chosen VPLMN is not part of "User Controlled PLMN Selector with Access Technology" list; and
  • the UE is not in manual mode of operation;
then the UE will perform PLMN selection with the current VPLMN considered as lowest priority.
It is mandatory for the VPLMN to transparently forward to the UE the steering of roaming information received from HPLMN and to transparently forward to the HPLMN the acknowledgement of successful reception of the steering of roaming information received from UE, both while the UE is trying to register onto the VPLMN as described in subclause C.2, and after the UE has registered onto the VPLMN as described in subclause C.3.
If the last received steering of roaming information contains the list of preferred PLMN/access technology combinations then the ME shall not delete the "Operator Controlled PLMN Selector with Access Technology" list stored in the ME when the UE is switched off.
The ME shall delete the "Operator Controlled PLMN Selector with Access Technology" list stored in the ME when a new USIM is inserted.
The procedure in this annex for steering of UE in VPLMN can be initiated by the network while the UE is trying to register onto the VPLMN as described in subclause C.2, or after the UE has registered onto the HPLMN or the VPLMN as described in subclause C.3.
The steering of roaming connected mode control information (SOR-CMCI) enables the HPLMN to control the timing of a UE in connected mode to move to idle mode to perform the steering of roaming. The UE shall support the SOR-CMCI. The support and use of SOR-CMCI by the HPLMN is based on the HPLMN's operator policy.
The UDM may support providing the UE with the SOR-CMCI:
  • which becomes available in the UDM (i.e. retrieved from the UDR);
  • received from the SOR-AF using the Nudm_ParameterProvision_Update service operation; or
  • both of the above.
The following requirements are applicable for the SOR-CMCI:
  • The HPLMN may configure SOR-CMCI in the UE and may also send SOR-CMCI over N1 NAS signalling. The SOR-CMCI received over N1 NAS signalling has precedence over the SOR-CMCI configured in the UE.
  • The UE shall indicate to the HPLMN its support for SOR-CMCI; and
  • While performing SOR, the UE shall consider the list of preferred PLMN/access technology combinations or secured packet received in the SOR information together with the available SOR-CMCI.
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