5.31.18 User Plane CIoT 5GS Optimisation

User Plane CIoT 5GS Optimisation enables transfer of user plane data from CM-IDLE without the need for using the Service Request procedure to establish Access Stratum (AS) context in NG-RAN and UE.

If the following preconditions are met:

• UE and AMF negotiated support User Plane CIoT 5GS Optimisation (see clause 5.31.2) over NAS,
• the UE has indicated support of User Plane CIoT 5GS Optimisation in the UE radio capabilities as defined in TS 36.331 [51],
• AMF has indicated User Plane CIoT 5GS Optimisation support for the UE to NG-RAN,
• the UE has established at least one PDU session with active UP connection, i.e. AS context is established in NG-RAN and the UE,

then the RRC connection can be suspended by means of the Connection Suspend Procedure (see clause 4.8.1.2 of TS 23.502 [3]).

Based on a trigger from the NAS layer when a UE is in CM-IDLE with Suspend, the UE should attempt the Connection Resume in CM-IDLE with Suspend procedure (clause 4.8.2.3 of TS 23.502 [3]). If the Connection Resume in CM-IDLE with Suspend procedure fails, the UE initiates the pending NAS procedure. To maintain support for User Plane CIoT 5GS Optimisation for UE mobility across different NG-RAN nodes, the AS Context should be transferred between the NG-RAN nodes, see TS 38.300 [27] and TS 38.423 [99].

For MT data or signalling when the UE is in CM-IDLE with Suspend, Network Triggered Service Request procedure (clause 4.2.3.3 of TS 23.502 [3]) applies.

By using the Connection Suspend Procedure:

• the UE at transition into CM-IDLE stores the AS information;
• NG-RAN stores the AS information, the NGAP UE association and the PDU session context for that UE;
• AMF stores the NGAP UE association and other information necessary to later resume the UE, interacts with the SMF(s) to deactivate the user plane resources for the UE's PDU Sessions and enters CM-IDLE.

NG-RAN may decide based on implementation to delete the stored UE context and NGAP association. In that case, the RAN shall initiate the AN Release procedure as described in clause 4.2.6 of TS 23.502 [3]. NG-RAN does not initiate any RRC procedure to notify the UE of the UE context release.

By using the Connection Resume in CM-IDLE with Suspend procedure:

• the UE resumes the connection from CM-IDLE with the network using the AS information stored during the Connection Suspend procedure;
• NG-RAN notifies the AMF that the connection with the UE has been resumed;
• AMF enters CM-CONNECTED and interacts with the SMF to activate the user plane resources for the UE's PDU Sessions.

Early Data Transmission may be initiated by the UE for mobile originated User Plane CIoT 5GS Optimisation during Connection Resume.

If the AMF establishes an NGAP UE association with a new NG-RAN node different from the stored NGAP UE association, e.g. the UE initiates service request or registration procedure from a different NG-RAN node, the AMF initiates UE N2 release command towards the old NG-RAN node.

NG-RAN maintains the N3 tunnel endpoint information while a UE is in CM-IDLE with Suspend. UPF is instructed to remove DL N3 Tunnel Info of AN during Connection Suspend procedure, while UPF keeps UL N3 Tunnel Info (i.e. UPF accepts and forwards UL data). If a UE sends MO data with resume procedure, the NG-RAN can send the MO data to the UPF which is addressed by the N3 tunnel endpoint information. In the case of change of serving NG-RAN node due to UE mobility, if NG-RAN determines that it is not able to connect to the UPF which is addressed by the N3 tunnel endpoint information, NG-RAN performs Path Switch procedure before sending the MO data received from the UE.

Early Data Transmission may be initiated by the UE for mobile originated User Plane CIoT 5GS Optimisation when the RAT Type is E-UTRA.