5.6.13 Always-on PDU Session

An always-on PDU Session is a PDU Session for which User Plane resources have to be activated during every transition from CM-IDLE mode to CM-CONNECTED state.

Based on an indication from upper layers, a UE may request to establish a PDU Session as an always-on PDU Session. The SMF decides whether the PDU Session can be established as an always-on PDU Session. In Home Routed roaming case, based on local policies, the V-SMF shall be involved to determine whether the PDU Session can be established as an always-on PDU Session. If the S-NSSAI of an always-on PDU Session is subject to area restrictions (as defined in clause 5.15.17 and clause 5.15.18), the SMF, upon detecting that the UE enters the NS-AoS of the S-NSSAI, shall reactivate the always-on PDU Session by executing the Network Triggered Service Request procedure for the case of UE in CM-CONNECTED mode (steps 3a to 4a only) in clause 4.2.3.3 of TS 23.502 [3].

NOTE: When I-SMF/V-SMF is inserted for the PDU Session it is the I-SMF/V-SMF that handles the area restrictions for the S-NSSAI. The I-SMF/V-SMF reactivates the always-on PDU Session and SMF/H-SMF is not involved in this procedure.

If the UE requests the 5GC to modify a PDU Session, which was established in EPS, to an always-on PDU Session after the first inter-system change from EPS to 5GS, the SMF decides whether the PDU Session can be established as an always-on PDU Session based on the procedure described above.

The UE shall request activation of User Plane resources for always-on PDU Sessions even if there are no pending uplink data for this PDU Session or when the Service Request is triggered for signalling only or when the Service Request is triggered for paging response only.

If the UE has one or more established PDU Sessions which are not accepted by the network as always-on PDU Sessions and the UE has no uplink user data pending to be sent for those PDU Sessions, the UE shall not request for activating User Plane resources for those PDU Sessions.