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Content for  TS 23.122  Word version:  17.1.1

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4.9  SNPN selection process |R16|Word‑p. 47

4.9.1  General

The MS operating in SNPN access mode shall perform the SNPN selection process.
The MS not operating in SNPN access mode shall not perform the SNPN selection process.
There are two SNPN selection modes - automatic SNPN selection mode and manual SNPN selection mode.
In the SNPN selection process, the MS shall consider only the access networks of the NG-RAN access technology.

4.9.2  Registration on an SNPN

The MS shall perform registration on the SNPN if the MS is capable of services which require registration. In both automatic SNPN selection mode and manual SNPN selection mode, the concept of registration on an SNPN is used. An MS successfully registers on an SNPN if:
  1. the MS has found a suitable cell of the SNPN to camp on; and
  2. an LR request from the MS has been accepted in the registration area of the cell on which the MS is camped.

4.9.3  SNPN selection

4.9.3.0  General

The ME is configured with a "list of subscriber data" containing zero or more entries. Each entry of the "list of subscriber data" consists of:
  1. a subscriber identifier in the form of a SUPI with the SUPI format "network specific identifier" containing a network-specific identifier or with the SUPI format "IMSI" containing an IMSI, except when the SNPN uses:
    1. the EAP based primary authentication and key agreement procedure using the EAP-AKA'; or
    2. the 5G AKA based primary authentication and key agreement procedure;
  2. credentials except when the SNPN uses:
    1. the EAP based primary authentication and key agreement procedure using the EAP-AKA'; or
    2. the 5G AKA based primary authentication and key agreement procedure;
  3. an SNPN identity; and
  4. optionally, the unified access control configuration indicating for which access identities (see TS 24.501) the ME is configured in the SNPN.
The MS shall maintain a list of "temporarily forbidden SNPNs" and a list of "permanently forbidden SNPNs" in the ME. Each entry of those lists consists of an SNPN identity.
The MS shall add an SNPN to the list of "temporarily forbidden SNPNs", if a message with cause value #74 "Temporarily not authorized for this SNPN" (see TS 24.501) is received by the MS in response to an LR request from the SNPN. In addition, if:
  • the message is integrity-protected; or
  • the message is not integrity-protected, and the value of the SNPN-specific attempt counter for that SNPN is equal to the MS implementation specific maximum value as defined in TS 24.501;
then the MS shall start an MS implementation specific timer not shorter than 60 minutes.
The MS shall remove an SNPN from the list of "temporarily forbidden SNPNs", if:
  1. there is a successful LR after a subsequent manual selection of the SNPN;
  2. the MS implementation specific timer not shorter than 60 minutes expires;
  3. the MS is configured to use timer T3245 and timer T3245 expires;
  4. the MS is not configured to use timer T3245, the timer T3247 expires and the value of the SNPN-specific attempt counter for that SNPN is less than the MS implementation specific maximum value as defined in TS 24.501;
  5. the MS is switched off; or
  6. an entry of the "list of subscriber data" with the SNPN identity of the SNPN is updated or the USIM is removed if:
    • EAP based primary authentication and key agreement procedure using EAP-AKA'; or
    • 5G AKA based primary authentication and key agreement procedure;
    was performed in the selected SNPN.
If an SNPN is removed from the list of "temporarily forbidden SNPNs" list, the MS shall stop the MS implementation specific timer not shorter than 60 minutes, if running.
The MS shall add an SNPN to the list of "permanently forbidden SNPNs", if a message with cause value #75 "Permanently not authorized for this SNPN" (see TS 24.501) is received by the MS in response to an LR request from the SNPN.
The MS shall remove an SNPN from the list of "permanently forbidden SNPNs", if:
  1. there is a successful LR after a subsequent manual selection of the SNPN;
  2. the MS is configured to use timer T3245 and timer T3245 expires;
  3. the MS is not configured to use timer T3245, the timer T3247 expires and the value of the SNPN-specific attempt counter for that SNPN is less than the MS implementation specific maximum value as defined in TS 24.501 ; or
  4. an entry of the "list of subscriber data" with the SNPN identity of the SNPN is updated or the USIM is removed if:
    • EAP based primary authentication and key agreement procedure using EAP-AKA'; or
    • 5G AKA based primary authentication and key agreement procedure;
    was performed in the selected SNPN.
When the MS reselects to a cell in a shared network, and the cell is a suitable cell for multiple SNPN identities received in the broadcast information as specified in TS 38.331, the AS indicates these multiple SNPN identities to the NAS according to TS 38.304. The MS shall select one of these SNPNs. If the registered SNPN is available among these SNPNs, the MS shall not select a different SNPN.
The MS operating in SNPN access mode shall maintain one or more lists of "5GS forbidden tracking areas for roaming", each associated with an SNPN. The MS shall use the list of "5GS forbidden tracking areas for roaming" associated with the selected SNPN. If the MS selects a new SNPN, the MS shall keep the list of "5GS forbidden tracking areas for roaming" associated with the previously selected SNPN. If the number of the lists to be kept is higher than supported, the MS shall delete the oldest stored list of "5GS forbidden tracking areas for roaming". The MS shall delete all lists of "5GS forbidden tracking areas for roaming", when the MS is switched off and periodically (with period in the range 12 to 24 hours). The MS shall delete the list of "5GS forbidden tracking areas for roaming" associated with an SNPN, when the entry of the SNPN in the list of subscriber data" is updated or when the USIM is removed if:
  • the EAP based primary authentication and key agreement procedure using the EAP-AKA'; or
  • the 5G AKA based primary authentication and key agreement procedure;
was performed in the selected SNPN.
If a message with cause value #15 (see TS 24.501) is received by an MS operating in SNPN access mode, the TA is added to the list of "5GS forbidden tracking areas for roaming" of the selected SNPN. The MS shall then search for a suitable cell in the same SNPN but belonging to a TA which is not in the "5GS forbidden tracking areas for roaming" list of the selected SNPN.
The MS should maintain a list of SNPNs for which the N1 mode capability was disabled due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed". When the MS disables its N1 mode capability due to receipt of a reject from an SNPN with 5GMM cause #27 "N1 mode not allowed":
  • the MS should add the SNPN identity of the SNPN which sent a reject with 5GMM cause #27 "N1 mode not allowed" to the list of SNPNs for which the N1 mode capability was disabled and should start timer TJ if timer TJ is not already running. The number of SNPNs for which the N1 mode capability was disabled that the MS can store is implementation specific, but it shall be at least one. The value of timer TJ is MS implementation specific, but shall not exceed the maximum possible value of background scanning timer T as specified in subclause 4.4.3.3.1;
  • in automatic SNPN selection, the MS shall not select an SNPN for which the N1 mode capability was disabled as SNPN selection candidates, unless no other SNPN is available;
  • if the MS is not configured to use timer T3245, the MS maintains a list of SNPN-specific attempt counters for 3GPP access as specified in TS 24.501, and T3247 expires, then the MS removes for each SNPN-specific attempt counter for 3GPP access that has a value greater than zero and less than the MS implementation-specific maximum value the respective SNPN from the list of SNPNs for which the N1 mode capability was disabled, as specified in subclause 5.3.20.3 in TS 24.501; and
  • the MS shall delete stored information on SNPNs for which the N1 mode capability was disabled when the MS is switched off, the USIM is removed, the entries of the "list of subscriber data" for the SNPNs are updated, or timer TJ expires.
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4.9.3.1  At switch-on or recovery from lack of coverageWord‑p. 49
4.9.3.1.0  General
At switch on, or following recovery from lack of coverage, the MS selects the registered SNPN (if it is available) using NG-RAN access technology and if necessary (in the case of recovery from lack of coverage, see subclause 4.5.2) attempts to perform an LR.
If successful registration is achieved, the MS indicates the selected SNPN.
If there is no registered SNPN, or registration is not possible due to the SNPN being unavailable or registration failure, the MS follows the procedure in subclause 4.9.3.1.1 or subclause 4.9.3.1.2 depending on its SNPN selection mode. At switch on, the MS shall use the SNPN selection mode that was used before switching off.
If registration is not possible on recovery from lack of coverage due to the registered SNPN being unavailable, an MS may, optionally, continue looking for the registered SNPN for an implementation dependent time.
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4.9.3.1.1  Automatic SNPN selection mode procedureWord‑p. 50
The MS selects another SNPN, if available, allowable, and identified by an SNPN identity in an entry of the "list of subscriber data" in the ME. If more than one SNPN are available, allowable, and each of them is identified by an SNPN identity in an entry of the "list of subscriber data" in the ME, how the MS selects one of those SNPNs is MS implementation specific.
The MS shall limit its search for the SNPN to the NG-RAN access technology.
Once the MS selects the SNPN, the MS attempts registrations on the selected SNPN using the NG-RAN access technology, the subscriber identifier and the credentials from an entry of the "list of subscriber data" with the SNPN identity matching the selected SNPN.
If successful registration is achieved, the MS indicates the selected SNPN.
If registration cannot be achieved because no SNPNs are available, allowable, and identified by an SNPN identity in an entry of the "list of subscriber data" in the ME, the MS indicates "no service" to the user, waits until a new SNPN is available, allowable, and identified by an SNPN identity in an entry of the "list of subscriber data" in the ME and then repeats the procedure.
If there were one or more SNPNs which were available, allowable, and identified by an SNPN identity in an entry of the "list of subscriber data" in the ME but an LR failure made registration on those SNPNs unsuccessful, the MS selects one of those SNPNs again and enters a limited service state.
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4.9.3.1.2  Manual SNPN selection mode procedure
The MS indicates to the user one or more SNPNs, which are available and each of them is identified by an SNPN identity in an entry of the "list of subscriber data" in the ME. This includes SNPNs in the list of "permanently forbidden SNPNs", and the list of "temporarily forbidden SNPNs". The MS may indicate to the user whether the available SNPNs are present in the list of "temporarily forbidden SNPNs" or the list of "permanently forbidden SNPNs". The order in which those SNPNs are indicated is MS implementation specific.
For each of the SNPNs indicated to the user, the UE shall forward a human-readable network name along with the SNPN identity to the upper layers if the system information broadcasted for the SNPN includes the human-readable network name for the SNPN.
The MS shall limit its search for the SNPN to the NG-RAN access technology.
The user may select his desired SNPN and the MS then initiates registration on this SNPN using the NG-RAN access technology, the subscriber identifier and the credentials from an entry of the "list of subscriber data", with the SNPN identity matching the selected SNPN (this may take place at any time during the presentation of SNPNs).
Once the MS has registered on an SNPN selected by the user, the MS shall not automatically register on a different SNPN unless the user selects automatic SNPN selection mode.
If the user does not select an SNPN, the selected SNPN shall be the one that was selected either automatically or manually before the SNPN selection procedure started. If no such SNPN was selected or that SNPN is no longer available, then the MS shall attempt to camp on any acceptable cell and enter the limited service state.
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4.9.3.2  User reselectionWord‑p. 51
4.9.3.2.0  General
At any time the user may request the MS to initiate reselection and registration onto an available SNPN, according to the following procedures, dependent upon the SNPN selection mode of the UE.
4.9.3.2.1  Automatic SNPN selection mode
The MS selects an SNPN, if available, allowable, and identified by an SNPN identity in an entry of the "list of subscriber data" in the ME in accordance with the following order:
  1. an SNPN, which is available and identified by an SNPN identity in an entry of the "list of subscriber data" in the ME, excluding the previously selected SNPN. If more than one SNPN different from the previously selected SNPN are available, allowable, and each of them is identified by an SNPN identity in an entry of the "list of subscriber data" in the ME, how the MS selects one of those SNPNs is MS implementation specific.
  2. the previously selected SNPN.
The MS shall limit its search for the SNPN to the NG-RAN access technology.
The previously selected SNPN is the SNPN which the MS has selected prior to the start of the user reselection procedure.
Once the MS selects an SNPN, if the selected SNPN is other than the previously selected SNPN, the MS attempts registrations on the selected SNPN using the NG-RAN access technology, the subscriber identifier and the credentials from an entry of the "list of subscriber data" with the SNPN identity matching the selected SNPN.
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4.9.3.2.2  Manual SNPN selection mode procedure
The manual SNPN selection mode procedure of subclause 4.9.3.1.2 is followed.

4.9.4  Abnormal cases

If:
  1. the "list of subscriber data" is empty; or
  2. for each entry of the "list of subscriber data", such that an SNPN with the SNPN identity of the entry is available:
    1. there has been an authentication failure for the subscriber identifier of the entry on the SNPN; or
    2. the MS has received an "illegal ME" or "illegal UE" response to an LR request for the subscriber identifier of entry on the SNPN;
then effectively there is no selected SNPN ("No SIM" state).
When in automatic SNPN selection mode and the MS is in the "not updated" state with one or more suitable cells to camp on; then after the maximum allowed unsuccessful LR requests (controlled by the specific attempt counters) the MS may continue (or start if it is not running) the user reselection procedure in subclause 4.9.3.2 1.
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