Section: 4.8.2.3 Connection Resume in CM-IDLE with Suspend procedure
Content: For the UE supporting User Plane CIoT 5GS Optimisation, the Connection Resume procedure is used by the UE to perform RRC_IDLE with Suspend to RRC_CONNECTED state transition. Triggers for the UE to initiate this procedure are defined in clause 5.31.18 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. Figure 4.8.2.3-1: Connection Resume in CM-IDLE with Suspend 1. UE to NG-RAN: RRC message (Resume ID). The UE initiates the transition from CM-IDLE and RRC_IDLE state with Suspend to CM-CONNECTED and RRC_CONNECTED state, see TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [46]. The UE provides its Resume ID needed by the NG-RAN to access the UE's stored Context. 2. [Conditional] NG-RAN performs UE Context Retrieval. UE Context Retrieval may be performed when the UE Context associated with the UE attempting to resume its connection is not locally available at the accessed NG-RAN. The UE Context Retrieval procedure via NG-RAN is specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. 3. NG-RAN and UE continues the resume procedure and access stratum configuration synchronization is performed between the UE and the network. UE enters CM-CONNECTED and RRC_CONNECTED. 4. NG-RAN to AMF: a) If the NG-RAN is the same as the NG-RAN when UE is suspended, the NG-RAN sends N2 Resume Request to AMF including Resume cause and N2 SM information which indicates the PDU sessions successfully resumed and PDU sessions failed or partially failed to resume the user plane resource. b) If the NG-RAN is different from the NG-RAN when UE is suspended and the new NG-RAN is able to retrieve the UE Context from the old NG-RAN, the new NG-RAN node initiates N2 Path Switch Request towards AMF, i.e. Steps 1b of clause 4.9.1.2.2. 5. AMF to SMF: For each of the PDU Sessions indicated in step 4, the AMF invokes Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, Cause, Operation type, User Location Information, Age of Location Information, N2 SM Information). The Operation Type is set to "UP Resume" to indicate resume of user plane resources for the PDU Session. For PDU Session(s) to be switched to the new NG-RAN, upon receipt of the Nsmf_PDUSession_UpdateSMContext request, the SMF determines whether the existing UPF can continue to serve the UE. If the existing UPF cannot continue to serve the UE, steps 3 to 7 of clause 4.9.1.2.3 or clause 4.9.1.2.4 are performed depending on whether the existing UPF is a PDU Session Anchor and flow continues in step 7. Otherwise, step 6 is performed if the existing UPF can continue to serve the PDU Session. If the RRC connection is resumed and the UE is accessing via the NB-IoT RAT with the RRC resume cause set to "MO exception data" the AMF updates all (H-)SMFs. If AMF indicates "MO exception data" then it includes the MO Exception Data Counter. The AMF maintains the MO Exception Data Counter for Small Data Rate Control purposes as described in clause 5.31.14.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. Each (H-)SMF should be updated for every RRC Connection which is triggered for "MO exception data". In the home-routed roaming case, if Small Data Rate Control applies, the V-SMF waits for H-SMF response before proceeding with the N3 establishment. 6. SMF to UPF: N4 Session Modification Request (AN Tunnel Info to be resumed, Buffering on/off). The SMF initiates an N4 Session Modification procedure indicating the resume of AN tunnel. Buffering on/off indicates whether the UPF shall buffer incoming DL PDU or not. If step 4a) is performed, the AN tunnel info is the one maintained by the SMF during Connection Suspend procedure, step 3 of clause 4.8.1.2. If step 4b) is performed, the AN tunnel info is part of the N2 SM information received by SMF in step 5. The UPF sends N4 Session Modification Response to acknowledge the SMF request. 7. SMF to AMF: The SMF sends Nsmf_PDUSession_UpdateSMContext response to the AMF. If new CN tunnel information is allocated for the PDU session, i.e. in the case of new AN tunnel is received in step 6, the SMF includes the new CN tunnel information as part of the N2 SM information. If the resume for PDU session is unsuccessful, the SMF shall include the resume failure as part of the N2 SM information. 8. AMF to NG-RAN: After response for each PDU session in step 7, the AMF sends N2 Resume Response to NG-RAN and indicates success, including N2 SM information for PDU session received in step 7, if at least one PDU session is resumed successfully. If none of the PDU sessions is resumed successfully, AMF indicates failure to NG-RAN. The AMF sends N2 Path Switch Acknowledge with PDU session resume information, if Path Switch Request is received in step 4. The AMF may provide Extended Connected Time value to the NG-RAN. If the NG-RAN receives the Extended Connected Time value, the NG-RAN may take this information into account when determining user inactivity. 9. [Conditional] NG-RAN to UE: RRC message. The NG-RAN may reconfigure the RRC connection based on resume result received from AMF. 10. If the AMF has paged the UE to trigger the Connection Resume procedure, the AMF shall initiate the UE configuration update procedure as defined in clause 4.2.4.2 to assign a new 5G-GUTI.