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Content for  TS 23.122  Word version:  17.1.1

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C.2  Stage-2 flow for steering of UE in VPLMN during registrationWord‑p. 67
The stage-2 flow for the case when the UE registers with VPLMN AMF is described below in figure C.2.1. The selected PLMN is the VPLMN. The AMF is located in the selected VPLMN.
Reproduction of 3GPP TS 23.122, Figure C.2.1: Procedure for providing list of preferred PLMN/access technology combinations
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For the steps below, security protection is described in TS 33.501.
Step 1.
The UE to the VPLMN AMF: The UE initiates initial registration, emergency registration or mobility registration update procedure to the VPLMN AMF by sending REGISTRATION REQUEST message with the 5GS registration type IE indicating "initial registration", "emergency registration" or "mobility registration updating";
Step 2.
Upon receiving REGISTRATION REQUEST message, the VPLMN AMF executes the registration procedure as defined in subclause 4.2.2.2.2 of TS 23.502. As part of the registration procedure;
  1. if the VPLMN AMF does not have subscription data for the UE, the VPLMN AMF invokes Nudm_SDM_Get service operation to the HPLMN UDM to get amongst other information the Access and Mobility Subscription data for the UE (see step 14b in subclause 4.2.2.2.2 of TS 23.502); or
  2. if the VPLMN AMF already has subscription data for the UE and
    1. the 5GS registration type IE in the received REGISTRATION REQUEST message indicates "initial registration" and the "SoR Update Indicator for Initial Registration" field in the UE context is set to 'the UDM requests the AMF to retrieve SoR information when the UE performs NAS registration type "initial registration"' as specified in Table 5.2.2.2.2-1 of TS 23.502); or
    2. the 5GS registration type IE in the received REGISTRATION REQUEST message indicates "emergency registration" and the "SoR Update Indicator for Emergency Registration" field in the UE context is set to 'the UDM requests the AMF to retrieve SoR information when the UE performs NAS registration type "emergency registration"' as specified in Table 5.2.2.2.2-1 of TS 23.502);
    then the VPLMN AMF invokes Nudm_SDM_Get service operation message to the HPLMN UDM to retrieve the steering of roaming information (see step 14b in subclause 4.2.2.2.2 of TS 23.502);
    otherwise the VPLMN AMF sends a REGISTRATION ACCEPT message without the steering of roaming information to the UE and steps 3a, 3b, 3c, 3d, 4, 5, 6 are skipped;
Step 3a.
If the user subscription information indicates to send the steering of roaming information due to initial registration in a VPLMN, then the HPLMN UDM shall provide the steering of roaming information to the UE when the UE performs initial registration in a VPLMN, otherwise the HPLMN UDM may provide the steering of roaming information to the UE, based on operator policy.
If the HPLMN UDM is to provide the steering of roaming information to the UE when the UE performs the registration in a VPLMN, and the HPLMN policy for the SOR-AF invocation is absent then steps 3b and 3c are not performed and the HPLMN UDM obtains the available list of preferred PLMN/access technology combinations and the available SOR-CMCI, if any, or the available secured packet (i.e. all retrieved from the UDR).
If the HPLMN UDM is to provide the steering of roaming information to the UE when the UE performs the registration in a VPLMN, and the HPLMN policy for the SOR-AF invocation is present, then the HPLMN UDM obtains the list of preferred PLMN/access technology combinations or the secured packet from the SOR-AF using steps 3b and 3c.
Step 3b.
The HPLMN UDM to the SOR-AF: Nsoraf_SoR_Get request (VPLMN ID, SUPI of the UE, access type (see TS 29.571). The VPLMN ID and the access type parameters, indicating where the UE is registering, are stored in the HPLMN UDM.
Step 3c.
The SOR-AF to the HPLMN UDM: Nsoraf_SoR_Get response (the list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or the secured packet, or neither of them).
Based on the information received in step 3b and any operator specific criteria, the SOR-AF may include the list of preferred PLMN/access technology combinations or the secured packet in the Nsoraf_SoR_Get response or may provide the Nsoraf_SoR_Get response with neither a list of preferred PLMN/access technology combinations nor a secured packet.
Step 3d.
The HPLMN UDM forms the steering of roaming information as specified in TS 33.501 from the list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or the secured packet obtained in step 3a or the list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or the secured packet, obtained in step 3c. If:
  • neither the list of preferred PLMN/access technology combinations nor the secured packet was obtained in steps 3a or 3c; or
  • the SOR-AF has not sent to the HPLMN UDM an Nsoraf_SoR_Get response (step 3c) within an operator defined time after the HPLMN UDM sending to the SOR-AF an Nsoraf_SoR_Get request (step 3b);
and the UE is performing initial registration in a VPLMN and the user subscription information indicates to send the steering of roaming information due to initial registration in a VPLMN, then the HPLMN UDM forms the steering of roaming information as specified in TS 33.501 from the HPLMN indication that 'no change of the "Operator Controlled PLMN Selector with Access Technology" list stored in the UE is needed and thus no list of preferred PLMN/access technology combinations is provided'.
Step 4.
The HPLMN UDM to the VPLMN AMF: The HPLMN UDM sends a response to the Nudm_SDM_Get service operation to the VPLMN AMF, which includes the steering of roaming information within the Access and Mobility Subscription data. The Access and Mobility Subscription data type is defined in subclause 5.2.3.3.1 of TS 23.502). The HPLMN may also request the UE to acknowledge the successful security check of the received steering of roaming information, by providing the indication as part of the steering of roaming information in the Nudm_SDM_Get response service operation;
Step 5.
The VPLMN AMF to the HPLMN UDM: As part of the registration procedure, the VPLMN AMF also invokes Nudm_SDM_Subscribe service operation to the HPLMN UDM to subscribe to notification of changes of the subscription data (e.g. received in step 4) including notification of updates of the steering of roaming information included in the Access and Mobility Subscription data (see step 14c in subclause 4.2.2.2.2 of TS 23.502);
Step 6.
The VPLMN AMF to the UE: The VPLMN AMF shall transparently send the received steering of roaming information to the UE in the REGISTRATION ACCEPT message;
Step 7.
If the steering of roaming information is received and the security check is successful, then:
  1. if the steering of roaming information contains a secured packet (see TS 31.115):
    • if the UDM has not requested an acknowledgement from the UE the UE shall send the REGISTRATION COMPLETE message to the serving AMF without including an SOR transparent container;
    • the ME shall upload the secured packet to the USIM using procedures in TS 31.111, if the service "data download via SMS Point-to-point" is allocated and activated in the USIM Service Table (see TS 31.102);
    • if the UDM has not requested an acknowledgement from the UE and:
      1. the UE receives SOR-CMCI in the USAT REFRESH with command qualifier of type "Steering of Roaming", the UE shall perform items a), b) and c) of the procedure for steering of roaming in subclause 4.4.6, and if the UE is in automatic network selection mode then it shall apply the actions in subclause C.4.2. In this case steps 8 to 11 are skipped; or
      2. the ME receives a USAT REFRESH command qualifier (TS 31.111) of type "Steering of Roaming" it shall perform items a), b) and c) of the procedure for steering of roaming in subclause 4.4.6 and if:
        1. the UE has a list of available and allowable PLMNs in the area and based on this list or any other implementation specific means the UE determines that there is a higher priority PLMN than the selected VPLMN; or
        2. the UE does not have a list of available and allowable PLMNs in the area and is unable to determine whether there is a higher priority PLMN than the selected VPLMN using any other implementation specific means;
        and the UE is in automatic network selection mode, then the UE shall either:
        1. release the current N1 NAS signalling connection locally and then attempt to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3 by acting as if timer T that controls periodic attempts has expired. In this case, steps 8 to 11 are skipped. The UE shall suspend the transmission of 5GSM messages until the N1 NAS signalling is released. If the UE has an established emergency PDU session (see TS 24.501), the receipt of the steering of roaming information shall not trigger the release of the N1 NAS signalling connection. The UE shall release the current N1 NAS signalling connection locally subsequently after the emergency PDU session is released; or
        2. not release the current N1 NAS signalling connection locally (e.g. if the UE has established PDU session(s)) and skip steps 8 to 10.
  2. if the steering of roaming information contains the list of preferred PLMN/access technology combinations, the ME shall replace the highest priority entries in the "Operator Controlled PLMN Selector with Access Technology" list stored in the ME with the received list of preferred PLMN/access technology combinations, and delete the PLMNs identified by the list of preferred PLMN/access technology combinations from the Forbidden PLMN list and from the Forbidden PLMNs for GPRS service list, if they are present in these lists. Additionally, if the UDM has not requested an acknowledgement from the UE, the UE shall send the REGISTRATION COMPLETE message to the serving AMF without including an SOR transparent container, and if:
    1. the UE has a list of available and allowable PLMNs in the area and based on this list or any other implementation specific means the UE determines that there is a higher priority PLMN than the selected VPLMN; or
    2. the UE does not have a list of available and allowable PLMNs in the area and is unable to determine whether there is a higher priority PLMN than the selected VPLMN using any other implementation specific means;
    and the UE is in automatic network selection mode:
    1. if the UE is configured with the SOR-CMCI or received the SOR-CMCI over N1 NAS signalling, the UE shall apply the actions in subclause C.4.2. In this case steps 8 to 11 are skipped;
    2. otherwise, the UE shall:
      1. release the current N1 NAS signalling connection locally and then attempt to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3 by acting as if timer T that controls periodic attempts has expired. In this case, steps 8 to 11 are skipped. The UE shall suspend the transmission of 5GSM messages until the N1 NAS signalling is released. If the UE has an established emergency PDU session (see TS 24.501), the receipt of the steering of roaming information shall not trigger the release of the N1 NAS signalling connection. The UE shall release the current N1 NAS signalling connection locally subsequently after the emergency PDU session is released. If the UE needs to disable the N1 mode capability (see TS 24.501) and there is no emergency service pending, the UE shall first attempt to obtain service on a higher priority PLMN as described in this step, and if no higher priority PLMN can be selected but the last registered PLMN is selected, then the UE shall disable the N1 mode capability; or
      2. not release the current N1 NAS signalling connection locally (e.g. if the UE has established PDU session(s)) and skip steps 8 to 10.
Step 8.
If the UE's USIM is configured with indication that the UE is to receive the steering of roaming information due to initial registration in a VPLMN, but neither the list of preferred PLMN/access technology combinations nor the secured packet nor the HPLMN indication that 'no change of the "Operator Controlled PLMN Selector with Access Technology" list stored in the UE is needed and thus no list of preferred PLMN/access technology combinations is provided' is received in the REGISTRATION ACCEPT message, when the UE performs initial registration in a VPLMN or if the steering of roaming information is received but the security check is not successful, then the UE shall:
  1. send the REGISTRATION COMPLETE message to the serving AMF without including an SOR transparent container;
  2. if the current chosen VPLMN is not contained in the list of "PLMNs where registration was aborted due to SOR", and is not part of "User Controlled PLMN Selector with Access Technology" list and the UE is not in manual mode of operation, release the current N1 NAS signalling connection locally and attempt to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3 by acting as if timer T that controls periodic attempts has expired, with an exception that the current PLMN is considered as lowest priority, and skip steps 9 to 11. The UE shall suspend the transmission of 5GSM messages until the N1 NAS signalling is released. If the UE has an established emergency PDU session (see TS 24.501), the UE shall release the current N1 NAS signalling connection locally after the release of the emergency PDU session. If the UE needs to disable the N1 mode capability (see TS 24.501) and there is no emergency service pending, the UE shall first attempt to obtain service on a higher priority PLMN as described in this step, and if no higher priority PLMN can be selected but the last registered PLMN is selected, then the UE shall disable the N1 mode capability; and
  3. store the PLMN identity in the list of "PLMNs where registration was aborted due to SOR".
Step 9.
The UE to the VPLMN AMF: If the UDM has requested an acknowledgement from the UE:
  • the UE verified that the steering of roaming information has been provided by the HPLMN in step 7, the UE sends the REGISTRATION COMPLETE message to the serving AMF with an SOR transparent container including the UE acknowledgement; and
    • if the steering of roaming information contained a secured packet, then when the UE receives the USAT REFRESH command qualifier of type "Steering of Roaming", it performs items a), b) and c) of the procedure for steering of roaming in subclause 4.4.6;
    • if the steering of roaming information contained a secured packet, then when the UE receives SOR-CMCI in the USAT REFRESH with command qualifier of type "Steering of Roaming", the UE shall perform items a), b) and c) of the procedure for steering of roaming in subclause 4.4.6 and if the UE is in automatic network selection mode then it shall apply the actions in subclause C.4.2, and step 11 is skipped; or
    • if the steering of roaming information contains the list of preferred PLMN/access technology combinations, the UE is configured with the SOR-CMCI or received the SOR-CMCI over N1 NAS signalling, if the UE is in automatic network selection mode, then the UE shall apply the actions in subclause C.4.2, and step 11 is skipped;
Step 10.
The VPLMN AMF to the HPLMN UDM: If an SOR transparent container is received in the REGISTRATION COMPLETE message, the AMF uses the Nudm_SDM_Info service operation to provide the received SOR transparent container to the UDM. If the HPLMN decided that the UE is to acknowledge the successful security check of the received steering of roaming information in step 4, the UDM verifies that the acknowledgement is provided by the UE as specified in TS 33.501;
Step 10a.
The HPLMN UDM to the SOR-AF: Nsoraf_SoR_Info (SUPI of the UE, successful delivery). If the HPLMN policy for the SOR-AF invocation is present and the HPLMN UDM received and verified the UE acknowledgement in step 10, then the HPLMN UDM informs the SOR-AF about successful delivery of the list of preferred PLMN/access technology combinations, or of the secured packet to the UE; and
Step 11.
If the UE has a list of available PLMNs in the area and based on this list the UE determines that there is a higher priority PLMN than the selected VPLMN and the UE is in automatic network selection mode, then the UE shall attempt to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3 by acting as if timer T that controls periodic attempts has expired after the release of the N1 NAS signalling connection. If the N1 NAS signaling connection is not released after implementation dependent time, the UE may locally release the N1 signaling connection except when the UE has an established emergency PDU session (see TS 24.501).
When the UE performs initial registration for emergency services (see TS 24.501 and TS 23.502) while the UE has a valid USIM and the AMF performs the authentication procedure, then based on HPLMN policy, the SOR procedure described in this subclause may apply.
If:
  • the UE in manual mode of operation encounters scenario mentioned in subclause 8 above; and
  • upon switching to automatic network selection mode the UE remembers that it is still registered on the PLMN where the missing or security check failure of SOR information was encountered as described in subclause 8;
the UE shall wait until it moves to idle mode or 5GMM-CONNECTED mode with RRC inactive indication (see TS 24.501) before attempting to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3, by acting as if timer T that controls periodic attempts has expired, with an exception that the current registered PLMN is considered as lowest priority. If the UE has an established emergency PDU session then the UE shall attempt to perform the PLMN selection subsequently after the emergency PDU session is released.
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