Section: 4.8.2.2 UE Triggered Connection Resume in RRC_INACTIVE procedure
Content: The Connection Resume procedure is used by the UE in RRC_INACTIVE state, e.g. to transition to RRC_CONNECTED state or for Small Data Transmission while in RRC_INACTIVE as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. Triggers for the UE to initiate this procedure are defined in clause 5.3.3.2.5 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. Figure 4.8.2.2-1: Connection Resume in RRC_INACTIVE 1. UE to NG-RAN: RRC message (Resume ID). The UE initiates connection resume from RRC_INACTIVE state, see TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. 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 is 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 to UE: RRC messages. NG-RAN determines whether the UE shall be transitioned to RRC_CONNECTED state or kept in RRC_INACTIVE (e.g. the latter in the case of Small Data Transmission as defined in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]). 4a. [Conditional] N2 Path switch procedure. If the accessed NG-RAN is able to retrieve the UE Context, the accessed NG-RAN node initiates N2 Path Switch procedure, i.e. steps 1 to 8 of clause 4.9.1.2.2 and including Xn data forwarding. If the Connection Resume procedure is a response to RAN paging which is triggered by 5GC due to an N2 interface procedure, NG-RAN and 5GC handle the N2 interface procedure as a collision described in clause 4.9.1.2. If Connection Inactive procedure with CN based MT communication handling (see clause 4.8.1.1a) has been performed previously then when the path switch procedure is performed downlink data or signalling delivery is triggered, if there is any. 4b. [Conditional] N2 Notification, 4b.1 If the accessed NG-RAN is the same as the NG-RAN that configured RRC_INACTIVE and still has the UE context, NG-RAN sends: - an N2 Notification to the AMF indicating the UE is in RRC_CONNECTED, if an AMF requested N2 Notification (see clause 4.8.3); or - an MT Communication Handling request to the AMF indicating the UE is now reachable for downlink data and/or signalling if Connection Inactive procedure with CN based MT communication handling (see clause 4.8.1.1a) has been performed previously. 4b.2 The AMF invokes Nsmf_PDUSession_UpdateSMContext Request towards SMF indicating the Downlink data delivery for each PDU session with active user plane, if the AMF has requested data buffering as described in clause 4.8.1.1a. 4b.3 N4 session modification procedure is triggered by the SMF. If data buffering is handled in the UPF, the SMF updates the UPF with appropriate rules to trigger data delivery. 4b.4 The SMF sends the Nsmf_PDUSession_UpdateSMContext response. 4b.5 The AMF sends the N2 MT Communication Handling response message to NG-RAN. If NG-RAN determines that the connection resume is for Small Data Transmission as defined in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9] and step 4a or steps 4b.1 to 4b.5 have been performed, then NG-RAN keeps the UE in RRC_INACTIVE state and the UL/DL Small Data are transferred via the NG-RAN. Based on the procedures defined in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9], if the UE is re-configured with RRC Inactive with eDRX>10.24s, the NG-RAN may send an N2 message to 5GC as described in step 2 in clause 4.8.1.1a so the CN can then handle mobile terminated (MT) communication.