4.9.2 Disabling and re-enabling of UE's N1 mode capability for 3GPP access

The UE shall only disable the N1 mode capability for 3GPP access when in 5GMM-IDLE mode.

When the UE is disabling the N1 mode capability for 3GPP access for a PLMN not due to redirection to EPC, it should proceed as follows:

• select an E-UTRA cell, which is not a restricted access technology as specified in clauses 4.2A, connected to EPC, or for the UE which supports CIoT EPS optimization select a satellite E-UTRA cell connected to EPC via "WB-E-UTRAN(LEO)", "WB-E-UTRAN(MEO)", or "WB-E-UTRAN(GEO)", of the registered PLMN or a PLMN from the list of equivalent PLMNs, if the UE supports S1 mode and the UE has not disabled its E-UTRA capability as specified in 3GPP TS 24.301 [15];
• if an E-UTRA cell, connected to EPC, or for the UE which supports CIoT EPS optimization if a satellite E-UTRA cell connected to EPC via "WB-E-UTRAN(LEO)", "WB-E-UTRAN(MEO)", or "WB-E-UTRAN(GEO)", of the registered PLMN or a PLMN from the list of equivalent PLMNs cannot be found, the UE does not support S1 mode or the UE has disabled its E-UTRA capability as specified in 3GPP TS 24.301 [15], the UE may select another access technology, which is not a restricted access technology as specified in clauses 4.2A, of the registered PLMN or a PLMN from the list of equivalent PLMNs that the UE supports;
• if another access technology of the registered PLMN or a PLMN from the list of equivalent PLMNs cannot be found, then enter the state 5GMM-REGISTERED.PLMN-SEARCH or 5GMM-DEREGISTERED.PLMN-SEARCH, or the UE does not have a registered PLMN, then enter the state 5GMM-DEREGISTERED.PLMN-SEARCH and perform PLMN selection as specified in 3GPP TS 23.122 [5]. If disabling of the N1 mode capability for 3GPP access was not due to a UE-initiated de-registration procedure for 5GS services over 3GPP access not due to switch-off, the UE may re-enable the N1 capability for this PLMN selection. As an implementation option, if the UE does not have a registered PLMN, instead of performing PLMN selection, the UE may select another RAT of the selected PLMN if the UE has chosen a PLMN and the RAT is supported by the UE; or
• if no other allowed PLMN and access technology combinations are available, then:
• if the N1 mode capability for 3GPP access was disabled due to IMS voice not available for NG-RAN access:
• if the UE has disabled its E-UTRA capability as specified in 3GPP TS 24.301 [15], the UE shall re-enable both the N1 mode capability for 3GPP access and the E-UTRA capability in order to remain registered for 5GS services or EPS services in the registered PLMN, and how this is done is UE implementation dependent; or
• if the UE does not support S1 mode, the UE shall re-enable the N1 mode capability for 3GPP access and remain registered for 5GS services in NG-RAN or satellite NG-RAN of the registered PLMN; or
• otherwise, the UE may re-enable the N1 mode capability for 3GPP access and indicate to lower layers to remain camped on non-satellite NG-RAN cell or satellite NG-RAN cell of the registered PLMN.

If the UE re-enables the N1 mode capability for 3GPP access, then it may periodically scan for another PLMN and access technology combination which can provide EPS services or non-EPS services (if the UE supports EPS services or non-EPS services). How this periodic scanning is done, is UE implementation dependent.

NOTE 0: If EPS services or non-EPS services can be obtained from another PLMN and access technology combination, to avoid ping pong, the UE can disable the N1 mode capability again.

When the UE is disabling the N1 mode capability for 3GPP access for an SNPN, it should proceed as follows:

• enter the state 5GMM-REGISTERED.PLMN-SEARCH or 5GMM-DEREGISTERED.PLMN-SEARCH and perform SNPN selection as specified in 3GPP TS 23.122 [5]. If disabling of the N1 mode capability for 3GPP access was not due to a UE-initiated de-registration procedure for 5GS services over 3GPP access not due to switch-off, the UE may re-enable the N1 capability for this SNPN selection; or
• if no other SNPN is available, then the UE may re-enable the N1 mode capability for 3GPP access and indicate to lower layers to remain camped on NG-RAN cell of the registered SNPN.

When the UE is disabling the N1 mode capability upon receiving cause value #31 "Redirection to EPC required" as specified in clauses 5.5.1.2.5, 5.5.1.3.5 and 5.6.1.5, it should proceed as follows:

• If the UE is in NB-N1 mode:
• if lower layers do not provide an indication that the current E-UTRA cell is connected to EPC or lower layers do not provide an indication that the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, search for a suitable NB-IoT cell connected to EPC according to 3GPP TS 36.304 [25C];
• if lower layers provide an indication that the current E-UTRA cell is connected to EPC and the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, perform a core network selection to select EPC as specified in clause 4.8.4A.1; or
• if lower layers cannot find a suitable NB-IoT cell connected to EPC or there is no suitable NB-IoT cell connected to EPC which supports CIoT EPS optimizations that are supported by the UE, the UE, as an implementation option, may indicate to lower layers to remain camped on E-UTRA cell connected to 5GCN, may then start an implementation-specific timer and enter the state 5GMM-REGISTERED.LIMITED-SERVICE. The UE may re-enable the N1 mode capability for 3GPP access at expiry of the implementation-specific timer, if the timer had been started, and may then proceed with the appropriate 5GMM procedure.
• If the UE is in WB-N1 mode:
• if lower layers do not provide an indication that the current E-UTRA cell is connected to EPC or lower layers do not provide an indication that the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, search for a suitable E-UTRA cell connected to EPC, or for the UE which supports CIoT EPS optimization select a satellite E-UTRA cell connected to EPC via "WB-E-UTRAN(LEO)", "WB-E-UTRAN(MEO)", or "WB-E-UTRAN(GEO)", according to 3GPP TS 36.304 [25C];
• if lower layers provide an indication that the current E-UTRA cell is connected to EPC and the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, then perform a core network selection to select EPC as specified in clause 4.8.4A.1; or
• if lower layers cannot find a suitable E-UTRA cell connected to EPC, or if the lower layers cannot find a suitable satellite E-UTRA cell via "WB-E-UTRAN(LEO)", "WB-E-UTRAN(MEO)", or "WB-E-UTRAN(GEO)", or there is no suitable E-UTRA cell connected to EPC, or there is no suitable satellite E-UTRA cell connected to EPC via "WB-E-UTRAN(LEO)", "WB-E-UTRAN(MEO)", or "WB-E-UTRAN(GEO)", which supports CIoT EPS optimizations that are supported by the UE, the UE, as an implementation option, may indicate to lower layers to remain camped on E-UTRA cell connected to 5GCN, may then start an implementation-specific timer and enter the state 5GMM-REGISTERED.LIMITED-SERVICE. The UE may re-enable the N1 mode capability for 3GPP access at expiry of the implementation-specific timer, if the timer had been started, and may then proceed with the appropriate 5GMM procedure.

When the UE supporting both N1 mode and S1 mode needs to stay in E-UTRA connected to EPC (e.g. due to the domain selection for UE originating sessions as specified in clause 4.3.2), in order to prevent unintentional handover or cell reselection from E-UTRA connected to EPC to NG-RAN connected to 5GCN, the UE operating in single-registration mode shall disable the N1 mode capability for 3GPP access and:

• shall set the N1mode bit to "N1 mode for 3GPP access not supported" in the UE network capability IE (see 3GPP TS 24.301 [15]) of the ATTACH REQUEST message and the TRACKING AREA UPDATE REQUEST message in EPC; and
• the UE NAS layer shall indicate the access stratum layer(s) of disabling of the N1 mode capability for 3GPP access.

If the UE is required to disable the N1 mode capability for 3GPP access and select E-UTRA or another access technology, and the UE is in the 5GMM-CONNECTED mode,

• if the UE has a persistent PDU session, then the UE waits until the radio bearer associated with the persistent PDU session has been released;
• otherwise the UE shall locally release the established NAS signalling connection;

and enter the 5GMM-IDLE mode before selecting E-UTRA or another access technology.

If the UE is disabling its N1 mode capability for 3GPP access before selecting E-UTRA or another access technology, the UE shall not perform the UE-initiated de-registration procedure of clause 5.5.2.2.

The UE shall re-enable the N1 mode capability for 3GPP access when the UE performs PLMN selection, SNPN selection or SNPN selection for onboarding services over 3GPP access, unless:

• disabling of the N1 mode capability for 3GPP access was due to a UE-initiated de-registration procedure for 5GS services over 3GPP access not due to switch-off;
• the UE has already re-enabled the N1 mode capability for 3GPP access when performing items c) or d) above; or
• the UE disables the N1 mode capability for 3GPP access for cases described in clauses 5.5.1.2.7 and 5.5.1.3.7.

If the disabling of N1 mode capability for 3GPP access was due to IMS voice is not available over NG-RAN access and the UE's usage setting is "voice centric", the UE shall re-enable the N1 mode capability for 3GPP access when the UE's usage setting is changed from "voice centric" to "data centric", as specified in clauses 4.3.3.

The UE should memorize the identity of the PLMN or SNPN where N1 mode capability for 3GPP access was disabled and should use that stored information in subsequent PLMN or SNPN selections as specified in 3GPP TS 23.122 [5].

If the disabling of N1 mode capability for 3GPP access was due to successful completion of an emergency services fallback, the criteria to enable the N1 mode capability again are UE implementation specific.

The UE shall disable the N1 mode capability for 3GPP access if requested by the upper layers (e.g. see clause U.2.2.6.4 in 3GPP TS 24.229 [14]). If the UE disabled the N1 mode capability for 3GPP access based on the request from the upper layers (e.g. see clause U.2.2.6.4 in 3GPP TS 24.229 [14]), the criteria to re-enable the N1 mode capability for 3GPP access after the completion of an emergency service are UE implementation specific.

If the N1 mode capability for 3GPP access was disabled due to the UE initiated de-registration procedure for 3GPP access or for 3GPP access and non-3GPP access and the UE is operating in single-registration mode (see clause 5.5.2.2.3), upon request of the upper layers to re-register for 5GS services over 3GPP access or the UE needs to come out of unavailability period and resume normal services, the UE shall enable the N1 mode capability for 3GPP access again.

As an implementation option, the UE may start a timer for enabling the N1 mode capability for 3GPP access when the UE disables the N1 mode capability for 3GPP access. The UE should memorize the identity of the PLMNs or SNPNs where N1 mode capability for 3GPP access was disabled. On expiry of this timer:

• if the UE is in Iu mode or A/Gb mode and is in idle mode as specified in 3GPP TS 24.008 [13], the UE should enable the N1 mode capability for 3GPP access;
• if the UE is in Iu mode and a PS signalling connection exists, but no RR connection exists, the UE may abort the PS signalling connection before enabling the N1 mode capability for 3GPP access;
• if the UE is in S1 mode and is in EMM-IDLE mode as specified in 3GPP TS 24.301 [15], the UE should enable the N1 mode capability for 3GPP access; and
• if the UE is in Iu mode or A/Gb mode and an RR connection exists, the UE should delay enabling the N1 mode capability for 3GPP access until the RR connection is released. If the UE is in S1 mode and is in EMM-CONNECTED mode as specified in 3GPP TS 24.301 [15], the UE should delay enabling the N1 mode capability for 3GPP access until the NAS signalling connection in S1 mode is released.

When the UE enables the N1 mode capability for 3GPP access, the UE shall remove the PLMN or SNPN from the memorized identity of the PLMNs or SNPNs where N1 mode capability for 3GPP access was disabled.

NOTE 1: As described in 3GPP TS 23.122 [5], if the UE is in automatic PLMN selection mode or automatic SNPN selection mode, the UE does not consider the memorized PLMNs as PLMN selection candidates for NG-RAN access technology or satellite NG-RAN access technology or the memorized SNPN as SNPN selection candidates till the timer expires.

If the N1 mode was disabled for a particular PLMN due to no available slice in that PLMN and the timer T3526 has expired, then the UE may re-enable the N1 mode for the PLMN and start a registration procedure for mobility and periodic registration update with a requested NSSAI containing the S-NSSAI for which the associated T3526 has expired. In this case, the UE shall stop the timer TNSU for the PLMN and remove the PLMN ID from the memorized identity of the PLMNs where N1 mode is disabled due to no available network slices.

If the N1 mode was disabled for a particular SNPN due to no available slice in that SNPN and the timer T3526 has expired, then the UE may re-enable the N1 mode for the SNPN for the selected entry of the "list of subscriber data" or the selected PLMN subscription, and start a registration procedure for mobility and periodic registration update with a requested NSSAI containing the S-NSSAI for which the associated T3526 has expired. In this case, the UE shall stop the timer TNSU for the SNPN and remove the SNPN identity from the memorized identity of the SNPNs where N1 mode is disabled due to no available network slices, for the selected entry of the "list of subscriber data" or the selected PLMN subscription.

The UE may disable the N1 mode capability for the current PLMN or SNPN over 3GPP access (see 3GPP TS 23.122 [5]) if no network slice is available for the current PLMN or SNPN (see clauses 5.5.1.2.5 and 5.5.1.3.5). If the disabling of N1 mode capability for 3GPP access was due to no network slices available, the UE should memorize the identity of the PLMN or SNPN where N1 mode is disabled due to no available network slices or the list of SNPNs where N1 mode is disabled due to no available network slices, respectively. As an implementation option, the UE may start a timer TNSU for enabling the N1 mode capability that was disabled due to no available network slices for the 3GPP access. The value of timer TNSU is UE implementation specific. The UE should remove the memorized identity of the PLMNs or SNPNs where N1 mode is disabled due to no available network slice upon:

• the expiry of the timer TNSU; or
• receiving REGISTRATION ACCEPT message or a CONFIGURATION UPDATE COMMAND message containing the Network slicing indication IE with the Network slicing subscription change indication set to "Network slicing subscription changed".

NOTE 1A: If a network slice is temporarily unavailable to the UE due to the S-NSSAI time validity information or the S-NSSAI location validity information as described in clauses 4.6.2.8 and 4.6.2.10 respectively, and no other network slice is available to the UE for the current PLMN or SNPN, as an implementation option the UE can keep the N1 mode capability enabled for the current PLMN or SNPN over 3GPP access.

If the UE receives ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message provided with S-NSSAI and the PLMN ID in the Protocol configuration options IE or Extended protocol configuration options IE (see clause 6.5.1.3 of 3GPP TS 24.301 [15]), the UE may remove the PLMN ID from the memorized identity of the PLMNs where N1 mode is disabled due to no available network slices and enable the N1 mode capability for that PLMN ID. Additionally, the UE shall stop timer TNSU if the disabling of N1 mode capability for 3GPP access was due to no network slices available.

If the UE attempts to establish an emergency PDU session in a PLMN or SNPN where N1 mode capability was disabled due to the UE's registration attempt counter have reached 5, the UE may enable N1 mode capability for that PLMN memorized by the UE.

NOTE 2: If N1 mode capability is disabled due to the UE's registration attempt counter reaches 5, the value of the timer for re-enabling N1 mode capability is recommended to be the same as the value of T3502 which follows the handling specified in clause 5.3.8. If the value of T3502 is indicated as zero by the network, an implementation specific non-zero value can be used for the timer for re-enabling N1 mode capability.

If the UE supports access to an SNPN providing access for localized services in SNPN and access for localized services in SNPN is enabled, then:

• the UE may re-enable the N1 mode capability for 3GPP access if disabled for that SNPN when:
• the validity information of the SNPN contained in the "credentials holder controlled prioritized list of preferred SNPNs for access for localized services in SNPN" changes from not met to met; or
• the validity information of a GIN broadcasted by an SNPN contained in the "credentials holder controlled prioritized list of preferred GINs for access for localized services in SNPN" changes from not met to met; and
• the UE need not re-enable N1 mode capability for 3GPP access for that SNPN if the N1 mode capability for 3GPP access for that SNPN was disabled due to 5GMM cause value #27 (N1 mode not allowed).

NOTE 3: If the UE receives a reject message with a 5GMM cause value and the N1 mode capability is disabled again for the SNPN, it is up to UE implementation whether to re-enable N1 mode capability for the SNPN if the validity information of the SNPN is still met.

If the N1 mode of the current PLMN is disabled due to the reception of 5GMM cause #27 (N1 mode not allowed) and the UE receives an access technology utilization control information indicating that NG-RAN is not restricted, the UE may enable the N1 mode capability for the current PLMN after the ongoing NAS signalling connection is released.