5.3.3.2.5 CM-CONNECTED with RRC_INACTIVE state

RRC_INACTIVE state applies to NG-RAN. UE support for RRC_INACTIVE state is defined in TS 38.306 [69] for NR and TS 36.306 [70] for E-UTRA connected to 5GC. RRC_INACTIVE is not supported by NB-IoT connected to 5GC.

The AMF shall provide assistance information to the NG-RAN, to assist the NG-RAN's decision whether the UE can be sent to RRC_INACTIVE state except due to some exceptional cases such as:

• PLMN (or AMF set) does not support RRC_INACTIVE;
• The UE needs to be kept in CM-CONNECTED State (e.g. for tracking).

The "RRC Inactive Assistance Information" includes:

• UE specific DRX values;
• UE specific extended idle mode DRX values (cycle length and Paging Time Window length);
• The Registration Area provided to the UE;
• Periodic Registration Update timer;
• If the AMF has enabled MICO mode for the UE, an indication that the UE is in MICO mode;
• Information from the UE identifier, as defined in TS 38.304 [50] for NR and TS 36.304 [52] for E-UTRA connected to 5GC, that allows the RAN to calculate the UE's RAN paging occasions;
• An indication that Paging Cause Indication for Voice Service is supported;
• AMF PEIPS Assistance Information (see clause 5.4.12.2) for paging a UE in CM-CONNECTED with RRC_INACTIVE state over NR as defined in TS 38.300 [27];
• AMF LP-WUSPS Assistance Information (see clause 5.4.12A) for paging a UE in CM-CONNECTED with RRC_INACTIVE state over NR as defined in TS 38.300 [27];
• AMF provided "LP-WUS Disabled" indication (see clause 5.4.12a.1);
• CN based MT communication handling support indication for RRC_INACTIVE state (see clause 5.31.7.2.1).

The RRC Inactive Assistance Information mentioned above is provided by the AMF during N2 activation with the (new) serving NG-RAN node (i.e. during Registration, Service Request, Handover) to assist the NG RAN's decision whether the UE can be sent to RRC_INACTIVE state. If the AMF allocates a new Registration Area to the UE, the AMF should update the NG-RAN with the new Registration Area by sending the RRC Inactive Assistance Information accordingly. The Paging Cause Indication for Voice Service is used to assist NG RAN to perform RAN based paging.

RRC_INACTIVE state is part of RRC state machine and it is up to the RAN to determine the conditions to enter RRC_INACTIVE state. If any of the parameters included in the RRC Inactive Assistance Information changes as the result of NAS procedure, the AMF shall update the RRC Inactive Assistance Information to the NG-RAN node.

When the UE is in CM-CONNECTED state, if the AMF has provided RRC Inactive assistance information, the RAN node may decide to move a UE to CM-CONNECTED with RRC_INACTIVE state.

The state and "endpoints" (in the case of Dual Connectivity configuration) of the N2 and N3 reference points are not changed by the UE entering CM-CONNECTED with RRC_INACTIVE state. A UE in RRC_INACTIVE state is aware of the RAN Notification area and periodic RAN Notification Area Update timer.

The 5GC network is not aware of the UE transitions between CM-CONNECTED with RRC_CONNECTED and CM-CONNECTED with RRC_INACTIVE state, unless the 5GC network is notified via N2 notification procedure in clause 4.8.3 of TS 23.502 [3].

At transition into CM-CONNECTED with RRC_INACTIVE state, the NG-RAN configures the UE with a periodic RAN Notification Area Update timer taking into account the value of the Periodic Registration Update timer value indicated in the RRC Inactive Assistance Information and uses a guard timer with a value longer than the RAN Notification Area Update timer value provided to the UE.

If the periodic RAN Notification Area Update guard timer expires in NG-RAN, the NG-RAN shall initiate AN Release procedure as specified in clause 4.2.6 of TS 23.502 [3].

When the UE is in CM-CONNECTED with RRC_INACTIVE state, the UE performs PLMN selection procedures as defined in TS 23.122 [17] and TS 24.501 [47].

When the UE is CM-CONNECTED with RRC_INACTIVE state, the UE may resume the RRC Connection due to:

• Uplink data pending;
• Mobile initiated NAS signalling procedure;
• As a response to RAN paging;
• Notifying the network that it has left the RAN Notification Area;
• Upon periodic RAN Notification Area Update timer expiration.

If the UE resumes the connection in a different NG-RAN node within the same PLMN or equivalent PLMN or within the same SNPN or equivalent SNPN, the UE AS context is retrieved from the old NG-RAN node and a procedure is triggered towards the CN (see clause 4.8.2 of TS 23.502 [3]).

NOTE 1: With Dual Connectivity configuration if the UE resumes the RRC connection in the Master RAN node, the Secondary RAN node configuration is defined in TS 38.300 [27].

If the RAN paging procedure applying DRX or eDRX value no longer than 10.24s, as defined in TS 38.300 [27], is not successful in establishing contact with the UE the procedure shall be handled by the network as follows:

• If NG-RAN has at least one pending NAS PDU for transmission, the RAN node shall initiate the AN Release procedure (see clause 4.2.6 of TS 23.502 [3]) to move the UE CM state in the AMF to CM-IDLE state and indicate to the AMF the NAS non-delivery.
• If NG RAN has only pending user plane data for transmission, the NG-RAN node may keep the N2 connection active or initiate the AN Release procedure (see clause 4.2.6 of TS 23.502 [3]) based on local configuration in NG-RAN.

NOTE 2: The user plane data which triggers the RAN paging can be lost, e.g. in the case of RAN paging failure.

If the RAN paging procedure applying eDRX value longer than 10.24s, as defined in TS 38.300 [27], has not requested the CN based mobile terminated (MT) communication handling as described in clause 5.31.7.2.1 and is not successful in establishing contact with the UE after paging the UE, the procedure shall be handled by network as follows:

• If NG-RAN has at least one pending NAS PDU for transmission, the RAN node shall initiate the AN Release procedure (see clause 4.2.6 of TS 23.502 [3]) to move the UE CM state in the AMF to CM-IDLE state and indicate to the AMF the NAS non-delivery.
• If NG-RAN has only pending user plane data for transmission, the NG-RAN node may keep the N2 connection active and based on implementation send indication to the CN requesting the CN based mobile terminated (MT) communication handling as described in clause 5.31.7.2.1, or initiate the AN Release procedure (see clause 4.2.6 of TS 23.502 [3]) based on local configuration in NG-RAN.

NOTE 3: The user plane data which triggers the RAN paging can be lost, e.g. in the case of RAN paging failure.

If a UE in CM-CONNECTED with RRC_INACTIVE state performs cell selection to GERAN/UTRAN/E-UTRAN, it shall follow idle mode procedures of the selected RAT as specified in clause 5.17.

In addition, a UE in CM-CONNECTED state with RRC_INACTIVE state shall enter CM-IDLE state and initiates the NAS signalling recovery (see TS 24.501 [47]) in the following cases:

• If RRC resume procedure fails,

If the UE receives Core Network paging,

• If the periodic RAN Notification Area Update timer expires and the UE cannot successfully resume the RRC Connection,
• In any other failure scenario that cannot be resolved in RRC_INACTIVE state and requires the UE to move to CM-IDLE state.

When a UE is in CM-CONNECTED with RRC_INACTIVE state and a trigger to change the UE's NG-RAN or E-UTRAN UE Radio Capability information happens, the UE shall move to CM-IDLE state and initiate the procedure for updating UE Radio Capability defined in clause 5.4.4.1. (For specific requirements for a UE operating in dual-registration mode see clause 5.17.2.1)

When UE is in CM-CONNECTED with RRC_INACTIVE state, if RAN has received Location Reporting Control message from AMF with the Reporting Type indicating single stand-alone report or continuously reporting whenever the UE changes the cell, the RAN shall perform location reporting as specified in clause 4.10 of TS 23.502 [3].

When the UE is CM-CONNECTED with RRC_INACTIVE state. If the AMF receives Nudm_UECM_DeregistrationNotification from UDM, the AMF shall initiate AN Release procedure as specified in clause 4.2.6 of TS 23.502 [3].

When UE is in CM-CONNECTED with RRC_INACTIVE state, if RAN has received Location Reporting Control message from AMF with the Reporting Type of the Area Of Interest based reporting, the RAN shall send a Location Report message to AMF including UE presence in the Area Of Interest (i.e. IN, OUT, or UNKNOWN) and the UE's last known location with time stamp.

When the UE is in CM-CONNECTED with RRC_INACTIVE state, if the old NG-RAN node that sends the UE into RRC_INACTIVE state receives the downlink N2 signalling, it initiates the RAN paging as defined in TS 38.300 [27]. If the UE resumes the RRC Connection towards a different NG-RAN node, the old NG-RAN node includes the "UE Context Transfer" indication into a response container to the NF (e.g. AMF or SMF) that generates such N2 downlink signalling. Then the NF shall reattempt the same procedure when the path switch from the old NG-RAN node to the new NG-RAN node is complete.