Section: 4.3.8.3 MME selection function
Content: The MME selection function selects an available MME for serving a UE. The selection is based on network topology, i.e. the selected MME serves the UE's location and for overlapping MME service areas, the selection may prefer MMEs with service areas that reduce the probability of changing the MME. When a MME/SGSN selects a target MME, the selection function performs a simple load balancing between the possible target MMEs. In networks that deploy dedicated MMEs/SGSNs for UEs configured for low access priority, the possible target MME selected by source MME/SGSN is typically restricted to MMEs with the same dedication. When a MME/SGSN supporting DCNs selects a target MME, the selected target MME should be restricted to MMEs that belong to the same DCN. The DNS procedure may be used by the source CN node to select the target MME from a given DCN. If both low access priority and UE Usage Type parameter are used for MME selection, selection based on UE Usage type parameter overrides selection based on the low access priority indication. When a MME supporting CIoT EPS Optimisation(s) selects a target MME, the selected MME should all support the CIoT EPS Optimisations applicable to the given UE's attachment. if the source MME is unable to find a target MME matching all CIoT EPS Optimisation(s) applicable to a given UE's attachment, then the source MME, based on implementation, selects a target MME which provides the CIoT EPS Optimisation(s) best applicable to that UE's attachment. When an eNodeB selects an MME, the eNodeB may use a selection function which distinguishes if the GUMMEI is mapped from P-TMSI/RAI or is a native GUMMEI. The indication of mapped or native GUMMEI shall be signalled by the UE to the eNodeB as an explicit indication. The eNodeB may differentiate between a GUMMEI mapped from P-TMSI/RAI and a native GUMMEI based on the indication signalled by the UE. Alternatively, the differentiation between a GUMMEI mapped from P-TMSI/RAI and a native GUMMEI may be performed based on the value of most significant bit of the MME Group ID, for PLMNs that deploy such mechanism. In this case, if the MSB is set to "0" then the GUMMEI is mapped from P-TMSI/RAI and if MSB is set to "1", the GUMMEI is a native one. Alternatively the eNodeB makes the selection of MME only based on the GUMMEI without distinguishing on mapped or native. When an eNodeB selects an MME, the selection shall achieve load balancing as specified in clause 4.3.7.2. When an eNodeB selects an MME, the selection shall consider the IAB support capability if the UE includes an IAB-Indication in the RRC connection establishment signalling as defined in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]. When the UE attempts to establish a signalling connection and the following conditions are met: - the eNodeB serves more than one country (e.g. it supports E-UTRA satellite access); and - the eNodeB knows in what country the UE is located; and - the eNodeB is connected to MMEs serving different PLMNs of different countries; and - the UE provides an S-TMSI or GUMMEI, which indicates an MME serving a different country to where the UE is currently located; and - the eNodeB is configured to enforce selection of the MME based on the country the UE is currently located; then the eNodeB shall select an MME serving a PLMN corresponding to the UE's current location. How the eNodeB selects the MME in this case is defined in TS 36.410[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 general aspects and principles ]  [92]. When DCNs are deployed, to maintain a UE in the same DCN when the UE enters a new MME pool area, the eNodeB's NNSF should have configuration that selects, based on the MMEGIs or NRIs of neighbouring pool areas, a connected MME from the same DCN. Alternately, for PLMN wide inter-pool intra-RAT mobility, the operator may divide up the entire MMEGI and NRI value space into non-overlapping sets with each set allocated to a particular DCN. In this case all eNodeBs may be configured with the same MME selection configuration. If UE assisted DCN selection feature is supported and a DCN-ID is provided by the UE, the DCN-ID shall be used in the eNodeB for MME selection to maintain the same DCN when the serving MME is not available. When selecting an MME for a UE that is using the NB-IoT RAT, and/or for a UE that signals support for CIoT EPS Optimisations in RRC signalling (as specified in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37], for NB-IoT, UE indicates whether it supports "User Plane CIoT EPS Optimisation" and "EPS Attach without PDN Connectivity". And for WB-E-UTRAN, UE indicates whether it supports "Control Plane CIoT EPS Optimisation", "User Plane CIoT EPS Optimisation" and "EPS Attach without PDN Connectivity"), the eNodeB's MME selection algorithm shall select an MME taking into account the MME's support (or non-support) for the Release 13 NAS signalling protocol. When DCN are deployed for the purpose of CIoT EPS Optimisation, UE included CIoT EPS Optimisation information in the RRC signalling, may depending on eNodeB configuration, be used to perform initial DCN selection. When Restricted Local Operator Services feature is supported, a UE initiates access to Restricted Local Operator Services via RRC signalling as defined in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]. The UE included RLOS indication in RRC signalling may be used by the eNodeB to select an appropriate MME.