Section: 4.9.2 Disabling and re-enabling of UE's N1 mode capability for 3GPP access
Content: 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: a) select an 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)", 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[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]; b) 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[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15], the UE may select another RAT of the registered PLMN or a PLMN from the list of equivalent PLMNs that the UE supports; c) if another RAT 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[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [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 d) if no other allowed PLMN and RAT combinations are available, then the UE may re-enable the N1 mode capability for 3GPP access and indicate to lower layers to remain camped in NG-RAN of the registered PLMN, and may periodically scan for another PLMN and RAT 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. When the UE is disabling the N1 mode capability for 3GPP access for an SNPN, it should proceed as follows: a) enter the state 5GMM-REGISTERED.PLMN-SEARCH or 5GMM-DEREGISTERED.PLMN-SEARCH and perform SNPN selection as specified in 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [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 b) 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 in NG-RAN 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 subclauses 5.5.1.2.5, 5.5.1.3.5 and 5.6.1.5, it should proceed as follows: a) If the UE is in NB-N1 mode: 1) 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[ Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode ]  [25C]; 2) 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 subclause 4.8.4A.1; or 3) 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 in E-UTRA cell connected to 5GCN, may then start an implementation-specific timer and enter the state 5GMM-REGISTERED.LIMITED-SERVICE. The UE may 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. b) If the UE is in WB-N1 mode: 1) 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[ Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode ]  [25C]; 2) 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 subclause 4.8.4A.1; or 3) 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 in 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 subclause 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: a) shall set the N1mode bit to "N1 mode for 3GPP access not supported" in the UE network capability IE (see 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]) of the ATTACH REQUEST message and the TRACKING AREA UPDATE REQUEST message in EPC; and b) 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 RAT, 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 RAT. If the UE is disabling its N1 mode capability for 3GPP access before selecting E-UTRA or another RAT, the UE shall not perform the UE-initiated de-registration procedure of subclause 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; or - 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 subclauses 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 3GPP 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 subclauses 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[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [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 subclause U.2.2.6.4 in 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [14]). If the UE disabled the N1 mode capability for 3GPP access based on the request from the upper layers (e.g. see subclause U.2.2.6.4 in 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [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 subclause 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's registration attempt counter reaches 5 and the UE disables the N1 mode capability for 3GPP access for cases described in subclauses 5.5.1.2.7 and 5.5.1.3.7. 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[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  [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; or - if the UE is in S1 mode and is in EMM-IDLE mode as specified in 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [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[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [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[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [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 the memorized SNPN as SNPN selection candidates till the timer expires. The UE may disable the N1 mode capability for currently camped PLMN or SNPN over 3GPP access (see 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [5]) if no network slice is available for the camped PLMN or SNPN (see subclauses 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 containing the Network slicing indication IE with the Network slicing subscription change indication set to “Network slicing subscription changed”. 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 subclause 6.5.1.3 of 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [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. If the UE attempts to establish an emergency PDU session in a PLMN 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 subclause 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.