5.31.11 Inter-RAT idle mode mobility to and from NB-IoT

Tracking Areas are configured so that they do not contain both NB-IoT and other RATs' cells, so when the UE is changing RAT type to or from NB-IoT while remaining registered with 5GC, the UE will perform the Mobility Registration Update procedure, see clause 5.3.2.3. When the UE is changing RAT type to or from NB-IoT and moving between 5GC and EPC, during the Registration, Attach or TAU procedure the RAT type change is determined.

The specification in this clause does not apply to RAT type corresponding to Non-3GPP Access type.

PDU session handling is controlled by "PDU Session continuity at inter RAT mobility" in the UE's subscription data, which indicates per DNN/S-NSSAI whether to;

• maintain the PDU session,
• disconnect the PDU session with a reactivation request,
• disconnect the PDU session without reactivation request, or
• leave it up to local VPLMN policy

when the UE moves between a "broadband" RAT (e.g. NR or WB-E-UTRA) and a "narrowband" RAT (NB-IoT).

During PDU session establishment the SMF retrieves the "PDU Session continuity at inter RAT mobility" subscription information (if available) from the UDM. Local SMF configuration is used if "PDU Session continuity at inter RAT mobility" is not available for a PDU Session.

The AMF informs the SMF at an inter-RAT idle mobility event, e.g. to or from NB-IoT connected to 5GC about the RAT type change in the Nsmf_PDUSession_UpdateSMContext message during the Registration procedure. Based on this (H-)SMF handles the PDU session according to "PDU session continuity at inter RAT mobility information" subscription data or based on local policy.

NOTE: The "PDU Session continuity at inter RAT mobility" and "PDN continuity at inter-RAT mobility" subscription should be the same so that the PDU sessions/PDN connections are handled the same by both CN types.

During inter-RAT idle mode mobility to NB-IoT, if a PDU session has more than one QoS rule, the SMF shall initiate a PDU session modification procedure as described in TS 23.502 [3] to remove any non-default QoS rule and maintain only the default QoS rule.