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Content for  TS 23.122  Word version:  18.1.0

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C (Normative)  Control plane solution for steering of roaming in 5GS |R15|p. 84

C.0  Requirements for 5G steering of roaming over the control planep. 84

In addition to the requirements specified in TS 22.011, clause 3.2.2.8, TS 22.261, clause 6.30 and TS 23.501, clause 5.30.2.2, the requirements in this clause apply.
The UE supporting N1 mode shall support the control plane solution for steering of roaming in 5GS. If the HPLMN or subscribed SNPN supports and wants to use the control plane solution for steering of roaming in 5GS, then the HPLMN or subscribed SNPN 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.
The non-subscribed SNPN shall transparently relay the steering of roaming information received from the HPLMN or subscribed SNPN to the UE. The UE shall be able to detect whether the non-subscribed SNPN removed the steering of roaming information during the initial registration procedure in the non-subscribed SNPN. The UE shall be able to detect whether the non-subcribed SNPN altered the steering of roaming information. If the UE detects that the non-subscribed SNPN altered or removed the steering of roaming information then the UE shall consider the current SNPN as the lowest priority SNPN and perform SNPN selection as defined in this Annex.
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C.1  Generalp. 84

C.1.1  Steering of roaming over the control plane in a PLMN |R17|p. 84

The purpose of the control plane solution for steering of roaming in 5GS procedure in a PLMN is to allow the HPLMN to update one or more of the following via NAS signalling:
  1. the "Operator Controlled PLMN Selector with Access Technology" list in the UE by providing the HPLMN protected list of preferred PLMN/access technology combinations or a secured packet;
  2. the SOR-CMCI; and
  3. the SOR-SNPN-SI associated with the selected PLMN subscription in the ME.
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 5GS 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 5GS 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 and periodic registration update (see TS 24.501) 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.
The steering of roaming connected mode control information (SOR-CMCI) enables the HPLMN to control the timing of a UE in 5GS connected mode to move to idle mode to perform the steering of roaming. If the UE selects a cell of any access technology other than NG-RAN, the SOR procedure is terminated (see clause C.4.2). 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 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 ME's support for SOR-CMCI to the HPLMN.
  • 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.
  • The HPLMN may provision the SOR-CMCI in the UE over N1 NAS signalling. The UE shall store the configured SOR-CMCI in the non-volatile memory of the ME or in the USIM as described in clause C.4.
The following requirements are applicable for the SOR-SNPN-SI:
  • If the UE supports access to an SNPN using credentials from a credentials holder, the UE shall indicate ME's support for SOR-SNPN-SI to the HPLMN.
In order to support various deployment scenarios, the UDM may support:
  • obtaining a list of preferred PLMN/access technology combinations, and SOR-CMCI, if any (if supported by the UDM and required by the HPLMN), 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 and SOR-CMCI, if any (if supported by the UDM and required by the HPLMN), 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 and SOR-CMCI, if any, or a secured packet from the SOR-AF.
The UDM discards any list of preferred PLMN/access technology combinations, SOR-CMCI, if any, 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 and C.4.3), 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, the SOR-CMCI, if any, 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, the USIM is removed or after a UE implementation dependent time.
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 clause C.2, and after the UE has registered onto the VPLMN as described in clause C.3 and C.4.3.
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 non-volatile memory of the ME when the UE is switched off.
The "Operator Controlled PLMN Selector with Access Technology" list shall be stored in the non-volatile memory of the ME together with the SUPI from the USIM. 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 clause C.2, or after the UE has registered onto the HPLMN or the VPLMN as described in clause C.3, C.7 and C.4.3.
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C.1.2  Steering of roaming over the control plane in an SNPN |R17|p. 86

The purpose of the control plane solution for steering of roaming in 5GS procedure in an SNPN is to allow the HPLMN or subscribed SNPN to update one or more of the following via NAS signalling:
  1. the SOR-SNPN-SI associated with the selected entry of "list of subscriber data" or the selected PLMN subscription in the ME, for a UE which supports access to an SNPN using credentials from a credential holder; and
  2. the SOR-CMCI.
The control plane solution for steering of roaming in 5GS procedure in an SNPN can also be used by 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 or a secured packet via NAS signalling.
The HPLMN or subscribed SNPN can provide the steering of roaming information to the UE using the control plane mechanism during and after registration. The HPLMN or subscribed SNPN updates the SOR-SNPN-SI based on the HPLMN or subscribed SNPN policies, which can be based on the registered SNPN, the location of the UE, etc. The control plane solution for steering of roaming in 5GS procedure in an SNPN is not applicable for credentials holder with AAA server.
If the UE supports access to an SNPN using credentials from a credentials holder:
  1. the UE shall indicate ME's support for SOR-SNPN-SI when registering in a subscribed SNPN or in the HPLMN; and
  2. the UE may indicate ME's support for SOR-SNPN-SI when sending an SOR transparent container including a UE acknowledgement in a PLMN.
When the UE indicates ME's support for SOR-SNPN-SI in the 5GMM capability in initial registration or emergency registration or when ME's support for SOR-SNPN-SI changes in mobility registration update, the AMF shall inform the UDM.
The HPLMN or subscribed SNPN can configure their subscribed UEs' SNPN configuration parameters associated with the PLMN subscription or the selected entry of the "list of subscriber data", respectively, to expect to receive the steering of roaming information due to initial registration in a non-subscribed SNPN. At the same time the HPLMN or subscribed SNPN will mark the UE as expecting to receive the steering of roaming information due to initial registration in a non-subscribed SNPN, in the subscription information in the UDM. In this case, it is mandatory for the HPLMN or subscribed SNPN to provide the steering of roaming information to the UE during initial registration in a non-subscribed SNPN. Otherwise if such configuration is not provided in the ME, it is optional for the HPLMN or subscribed SNPN to provide the steering of roaming information to the UE during initial registration (based on HPLMN or subscribed SNPN policy). The HPLMN or subscribed SNPN can provide the steering of roaming information to the UE during the registration procedure for mobility and periodic registration update (see TS 24.501) and initial registration procedure for emergency services. In addition, the HPLMN or subscribed SNPN can request the UE to provide an acknowledgement of successful reception of the steering of roaming information.
As the HPLMN or subscribed SNPN needs to consider certain criteria including the number of customers distributed through multiple SNPNs in the same country or region, the SOR-SNPN-SI is not necessarily the same at all times and for all users.
The steering of roaming connected mode control information (SOR-CMCI) enables the HPLMN or subscribed SNPN to control the timing of a UE in connected mode to move to idle mode, if the UE decides to perform SNPN selection upon receiving the steering of roaming information. The UE shall support the SOR-CMCI. The support and use of SOR-CMCI by the HPLMN or subscribed SNPN is based on the HPLMN or subscribed SNPN policy.
The following requirements are applicable for the SOR-CMCI:
  • The HPLMN or subscribed SNPN 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 ME's support for SOR-CMCI to the HPLMN or subscribed SNPN.
  • While performing SOR, the UE shall consider the SOR-SNPN-SI received in the SOR information together with the available SOR-CMCI.
  • The HPLMN or subscribed SNPN may provision the SOR-CMCI in the UE over N1 NAS signalling. The UE shall store the configured SOR-CMCI in the non-volatile memory of the ME or in the USIM as described in clause C.4.
In order to support various deployment scenarios, the UDM may support:
  • obtaining the SOR-SNPN-SI which is or becomes available in the UDM (i.e. retrieved from the UDR);
  • obtaining the SOR-SNPN-SI from the SOR-AF; or
  • both of the above.
The HPLMN or subscribed SNPN policy for the SOR-AF invocation can be present in the UDM only if the UDM supports obtaining the SOR-SNPN-SI from the SOR-AF.
The UDM discards any SOR-SNPN-SI 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.5) or after registration (as specified in Annex C.6), 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 SOR-SNPN-SI after the expiration of the HPLMN or subscribed SNPN specific timer, or if there is no AMF registered for the UE).
The UE maintains a list of "SNPNs where registration was aborted due to SOR" per entry of the "list of subscriber data" or the PLMN subscription. If the UE receives steering of roaming information in the REGISTRATION ACCEPT or DL NAS TRANSPORT message in an SNPN and the security check to verify that the steering of roaming information is provided by the HPLMN or subscribed SNPN is successful, the UE shall remove the current selected SNPN from the list of "SNPNs where registration was aborted due to SOR" for the selected entry of the "list of subscriber data" or the selected PLMN subscription. The UE shall delete the list of "SNPNs where registration was aborted due to SOR" when the selected entry of the "list of subscriber data" is updated or the UICC containind the USIM is removed.
If:
  • the UE's ME is configured to indicate that the UE shall expect to receive the steering of roaming information during initial registration procedure for the selected entry of the "list of subscriber data" or the selected PLMN subscription but did not receive it or security check on the steering of roaming information fails;
  • the current chosen non-subscribed SNPN is not contained in the list of "SNPNs where registration was aborted due to SOR" for the selected entry of the "list of subscriber data" or the selected PLMN subscription;
  • the current chosen non-subscribed SNPN is not part of the user controlled prioritized list of preferred SNPNs for the selected entry of the "list of subscriber data" or the selected PLMN subscription; and
  • the UE is not in manual mode of operation;
then the UE will perform SNPN selection with the current SNPN considered as lowest priority.
It is mandatory for the non-subscribed SNPN to transparently forward to the UE the steering of roaming information received from the HPLMN or subscribed SNPN and to transparently forward to the HPLMN or subscribed SNPN the acknowledgement of successful reception of the steering of roaming information received from the UE, both while the UE is trying to register onto the non-subscribed SNPN as described in clause C.5, and after the UE has registered onto the non-subscribed SNPN as described in clause C.6.
The ME shall delete the SOR-SNPN-SI stored in the ME when the subscriber identifier, the SNPN identity of the subscribed SNPN, or both, of the selected entry of the "list of subscriber data" are updated or the UICC containing the USIM is removed.
The procedure in this Annex for steering of UE in an SNPN can be initiated by the network while the UE is trying to register onto a non-subscribed SNPN as described in clause C.5, or after the UE has registered onto the subscribed SNPN or a non-subscribed SNPN as described in clause C.6.
The procedure in this annex for steering of UE in an SNPN can be initiated by the network while the UE is trying to register onto a non-subscribed SNPN as described in clause C.5, or after the UE has registered onto the subscribed SNPN or a non-subscribed SNPN as described in clause C.6 and C.8.
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