5.31.4.1 General

The Control Plane CIoT 5GS Optimisation is used to exchange user data between the UE and the SMF as payload of a NAS message in both uplink and downlink directions, avoiding the establishment of a user plane connection for the PDU Session. The UE and the AMF perform integrity protection and ciphering for the user data by using NAS PDU integrity protection and ciphering. For IP and Ethernet data, the UE and the SMF may negotiate and perform header compression.

NOTE: In the context of Control Plane CIoT 5GS Optimisation, established or activated user plane resources/connection refers to radio user plane resources/connection i.e Data Radio Bearer and N3 tunnel.

UE and AMF negotiate support and use of Control Plane CIoT 5GS Optimisation as defined in clause 5.31.2. When the Control Plane CIoT 5GS Optimisation feature is used and the PDU Session Type is unstructured, the SMF selects either NEF or UPF based on information in the UE's subscription.

If UE and network have negotiated support and use of Control Plane CIoT 5GS Optimisation then the following paragraphs of this clause apply.

During the PDU Session Establishment procedure the AMF indicates to the SMF that Control Plane CIoT 5GS Optimisation is available for data transmission.

During the PDU Session Establishment procedure the AMF also determines based on Preferred and Supported Network Behaviour (see clause 5.31.2), subscription data, other already established PDU Sessions and local policy whether a new PDU Session shall only use the Control Plane CIoT 5GS Optimisation (i.e. that a user-plane connection shall never be established for the new PDU Session). If a PDU Session shall only use Control Plane CIoT 5GS Optimisation, the AMF provides a Control Plane Only Indicator to the SMF during the PDU Session Establishment. The SMF provides the Control Plane Only Indicator in the Session Management Request to the UE. A UE and SMF receiving the Control Plane Only Indicator for a PDU Session shall always use the Control Plane CIoT 5GS Optimisation for this PDU Session.

The following rules apply for the use of the Control Plane Only Indicator during PDU Session Establishment:

• If N3 data transfer was not successfully negotiated, all PDU Sessions shall include Control Plane Only Indicator.
• If N3 data transfer was successfully negotiated then:
• For a new PDU Session for a DNN/S-NSSAI for which the subscription data for SMF Selection includes an Invoke NEF indication (i.e. for a PDU Session which will be anchored in NEF), the AMF shall always include the Control Plane Only Indicator.
• For a new PDU Session for a DNN/S-NSSAI for which the subscription data for SMF Selection does not include an Invoke NEF indication (i.e. for a PDU Session which will be anchored in UPF) and that supports interworking with EPS based on the subscription data defined in TS 23.502 [3]:
• for the first PDU Session the AMF determines based on local policy whether to include the Control Plane Only Indicator or not;
• if the AMF previously included a Control Plane Only Indicator for PDU Sessions that support interworking with EPS based on the subscription data defined in TS 23.502 [3] and that are anchored in UPF, the AMF shall include it also for the new PDU Session;
• if the AMF previously did not include a Control Plane Only Indicator for any of the PDU Sessions that support interworking with EPS based on the subscription data defined in TS 23.502 [3] and that are anchored in UPF, the AMF shall not include it for the new PDU Session.
• For a new PDU Session for a DNN/S-NSSAI for which the subscription data for SMF Selection does not include an Invoke NEF indication (i.e. for a PDU Session which will be anchored in UPF) and that does not support interworking with EPS based on the subscription data defined in TS 23.502 [3], AMF determines individually per PDU Session whether to include the Control Plane Only Indicator or not.

As described in clause 5.31.4.2, if UE and AMF successfully negotiate N3 data transfer in addition to Control Plane CIoT 5GS Optimisation, the UE or SMF may request to establish N3 data transfer for one or more PDU Sessions for which Control Plane Only Indicator was not received. In CM-CONNECTED, the UE and the network use N3 delivery for PDU Sessions for which user plane resources are established and uses NAS for data transmission for PDU Sessions for which user plane resources are not established.

If the AMF determines that Control Plane Only indication associated with PDU Session is not applicable any longer due to e.g. change of Preferred and Supported Network Behaviour, subscription data and local policy, the AMF should request the SMF to release the PDU Session as specified in clause 4.3.4.2 or clause 4.3.4.3 of TS 23.502 [3].

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

The QoS model as defined in clause 5.7 is not supported for PDU Sessions using Control Plane CIoT 5GS Optimisation as user plane resources are not established for those PDU Sessions.