Section: 4.24.1 UPF anchored Mobile Originated Data Transport in Control Plane CIoT 5GS Optimisation
Content: This clause describes the procedures for Mobile Originated Transport in Control Plane CIoT 5GS Optimisation where the PDU Session is terminated at a UPF. Figure 4.24.1-1: UPF anchored Mobile Originated Data Transport in Control Plane CIoT 5GS Optimisation 1. If the UE is CM-CONNECTED it sends a NAS message carrying the ciphered PDU session ID and ciphered uplink data as payload. If the UE is in CM-IDLE, the UE first establishes an RRC connection or sends the RRCEarlyDataRequest message and sends a NAS message as part of this. The UE may also send NAS Release Assistance Information (NAS RAI) included in the NAS message. NAS RAI indicates that no further Uplink and Downlink Data transmissions are expected, or that only a single Downlink data transmission (e.g. Acknowledgement or response to Uplink data) subsequent to this Uplink Data transmission is expected. 1a. In the NB-IoT case, during step 1 the NG-RAN, based on configuration, may retrieve the NB-IoT UE Priority and the Expected UE Behaviour Parameters from the AMF, if not previously retrieved. Based on such parameters, the NG-RAN may apply prioritisation between requests from different UEs before triggering step 2 and throughout the RRC connection. The NG-RAN may retrieve additional parameters (e.g. UE Radio Capabilities). 2. NG-RAN forwards the NAS message to the AMF using the Initial NAS message procedure (if the UE was in CM-IDLE before step 1) or using the Uplink NAS transport procedure (if the UE was in CM-CONNECTED before step 1). If RRCEarlyDataRequest message was received in step 1, the NG-RAN includes "EDT Session" indication in the N2 Initial UE message. The RAI signalled by MAC based on the Buffer Status Report (BSR), see TS 36.321[ Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification ]  [56], shall not be used when using Control Plane CIoT 5GS Optimisations. 3. AMF checks the integrity of the incoming NAS message and deciphers the PDU session ID and uplink data. If a NAS RAI is received from the UE and it conflicts with the Expected UE Behaviour, the NAS RAI takes precedence. 3a. If the AMF received "EDT Session" indication from the NG-RAN in step 2, the AMF sends an N2 message to the NG-RAN. a) In the case of NAS RAI with Uplink data and it indicated that Downlink data was not expected and the AMF does not expect any other signalling with the UE, the AMF shall - either send a NAS service accept in the N2 Downlink NAS Transport message and include End Indication to indicate that no further data or signalling is expected with the UE; or - alternatively, the AMF sends an N2 Connection Establishment Indication message including End Indication to indicate that no further data or signalling is expected with the UE. b) If the AMF determines more data or signalling may be pending, the AMF sends an N2 Downlink NAS Transport message or Initial Context Setup Request message without End Indication. 3b. If 3a was executed, the NG-RAN completes the RRC early data procedure as follows. a) For the case of 3a.a) the NG-RAN proceeds with RRCEarlyDataComplete message. The procedure is completed in step 5. b) For the case of 3a.b) the NG-RAN proceeds with RRC connection establishment procedure. In that case, all steps up to step 13 apply. 4. AMF determines the (V-)SMF handling the PDU session based on the PDU session ID contained in the NAS message and passes the PDU Session ID and the data to the (V-)SMF by invoking Nsmf_PDUSession_SendMOData service operation. If NG-RAN forwarded the NAS message to the AMF using the Initial NAS message procedure in step 2 and the UE is accessing via NB-IoT RAT then the AMF may inform the (H-)SMFs whether the RRC establishment cause is set to "MO exception data", as described in clause 5.31.14.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The AMF may immediately send the MO Exception Data Counter to the (H-)SMF. If no Downlink Data is expected based on the NAS RAI from the UE in step 1 and if the AMF is not aware of pending MT traffic, then AMF does not wait for step 7 and continues with step 12. 5. The (V-)SMF decompresses the header if header compression applies to the PDU session and forwards the data to the UPF. The UPF forwards the data to the DN based on data forwarding rule, e.g. in the case of unstructured data, tunnelling may be applied according to clause 5.6.10.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 6. [Conditional] In the non-roaming and LBO case, the UPF forwards available downlink data to the (V-)SMF, in the home-routed roaming case, the H-UPF forwards the data to the V-UPF then to the V-SMF. 7. [Conditional] The (V-)SMF compresses the header if header compression applies to the PDU session. The (V-)SMF forwards the downlink data and the PDU session ID to the AMF using the Namf_Communication_N1N2MessageTransfer service operation. 8. [Conditional] The AMF creates a DL NAS transport message with the PDU session ID and the downlink data. The AMF ciphers and integrity protects the NAS transport message 9. [Conditional] The AMF sends the DL NAS transport message to NG-RAN. If NAS RAI indicated for single uplink and single downlink packets (e.g. acknowledgment expected) and AMF has determined the data transmission is for single uplink and single downlink packets, the AMF includes an End Indication in the DL NAS transport message to indicate that no further data or signalling is expected with the UE. 10. [Conditional] NG-RAN delivers the NAS payload over RRC to the UE. 11. If no further data or signalling is pending and AMF received NAS RAI indicating single downlink data transmission, then AMF triggers the AN release procedure (clause 4.2.6) and the procedure stops after this step. 12. [Conditional] If no further activity is detected by NG-RAN, then NG-RAN triggers the AN release procedure. 13. [Conditional] The UE's logical NG-AP signalling connection and RRC signalling connection are released according to clause 4.2.6. NOTE: The details of the NGAP messages to be used for this procedure are specified in TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [10].