Section: 5.31.12 Restriction of use of Enhanced Coverage
Content: Support of UEs in E-UTRA Enhanced Coverage is specified in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [30]. The usage of Enhanced Coverage requires use of extensive resources (e.g. radio and signalling resources). Specific subscribers can be restricted to use the Enhanced Coverage feature through Enhanced Coverage Restricted information that is stored in the UDM as part of subscription data and specifies per PLMN whether the Enhanced Coverage functionality is restricted or not for the UE. For eMTC, the Enhanced Coverage Restricted information indicates whether CE mode B is restricted for the UE, or both CE mode A and CE mode B are restricted for the UE, or both CE mode A and CE mode B are not restricted for the UE. For NB-IoT, the NB-IoT Enhanced Coverage Restricted information indicates whether the Enhanced Coverage is restricted or not for the UE. The AMF receives Enhanced Coverage Restricted information from the UDM during the Registration procedure. If the UE includes the support for restriction of use of Enhanced Coverage in the Registration Request message, the AMF based on local configuration, UE Usage setting, UE subscription information and network policies, or any combination of them, determines whether Enhanced Coverage is restricted for the UE and stores updated Enhanced Coverage Restriction information in the UE context in the AMF. If the UE usage setting indicated that UE is "voice centric", then the AMF shall set CE mode B restricted for the UE in Enhanced Coverage Restriction information. The AMF sends Enhanced Coverage Restricted information to the UE in the Registration Accept message. The UE shall use the value of Enhanced Coverage Restricted information to determine if enhanced coverage feature is restricted or not. The AMF provides an Enhanced Coverage Restricted information to the RAN via N2 signalling whenever the UE context is established in the RAN, e.g. during N2 Paging procedure, Service Request procedure, Initial Registration and Periodic Registration procedure. For roaming UEs, if the UDM doesn't provide any Enhanced Coverage Restricted information or the provided Enhanced Coverage Restricted information is in conflict with the roaming agreement, the AMF uses default Enhanced Coverage Restricted information locally configured in the VPLMN based on the roaming agreement with the subscriber's HPLMN. The UE indicates its capability of support for restriction of use of Enhanced Coverage to the AMF in the Registration procedure for the RAT it is camping on. A UE that supports Enhanced Coverage shall also support restriction of the Enhanced Coverage. The UE shall assume that restriction for use of Enhanced Coverage indicated by Enhanced Coverage Restricted information is the same in the equivalent PLMNs. NB-IoT cells also broadcast the support of restriction of use of Enhanced Coverage as defined in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [51]. If the UE supports CE mode B and use of CE mode B is not restricted according to the Enhanced Coverage Restriction information in the UE context in the AMF, then the AMF shall use the extended NAS-MM timer setting for the UE as specified in TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [47] and shall send the extended NAS-SM timer indication during PDU session establishment to the SMF. If the UE supports CE mode B and use of CE mode B changes from restricted to unrestricted or vice versa in the Enhanced Coverage Restriction information in the UE context in the AMF (e.g. due to a subscription change) then: - The AMF determines when to enforce the change of restriction of use of Enhanced Coverage. - When the UE is in CM-CONNECTED mode, the AMF can use the UE Configuration Update procedure, as specified in step 3a of clause 4.2.4.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], to trigger a mobility registration update procedure in CM-CONNECTED mode for the AMF to inform the change of restriction of Enhanced Coverage towards the UE. - If the UE has already established PDU sessions, then the AMF shall trigger a PDU session modification to the SMFs serving the UE's PDU sessions to update the use of the extended NAS-SM timer setting as described in step 1f of clause 4.3.3.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3] when the AMF determines that NAS-SM timer shall be updated due to the change of Enhanced Coverage Restriction. - The UE and network applies the new Enhanced Coverage Restriction information after mobility registration procedure is completed. Based on the extended NAS-SM timer indication, the SMF shall use the extended NAS-SM timer setting for the UE as specified in TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [47]. The support for Enhanced Coverage Restriction Control via NEF enables AF to query status of Enhanced Coverage Restriction or enable/disable Enhanced Coverage Restriction per individual UEs. The procedure for Enhanced Coverage Restriction Control via NEF is described in clause 4.27 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3].