4.8 RAN-CN interactions

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

4.8.1 Connection Inactive and Suspend procedure

4.8.1.1 Connection Inactive procedure

This procedure may be initiated by the serving NG-RAN node when the UE is in CM-CONNECTED with RRC Connected state and has received the "RRC Inactive Assistance Information" from the AMF as defined in clause 5.3.3.2.5 of TS 23.501 [2]. NG-RAN initiates the transition to RRC Inactive state as defined in TS 38.300 [9].

4.8.1.1a Connection Inactive procedure with CN based MT communication handling

This procedure may be initiated by the serving NG-RAN node when CN based mobile terminating (MT) communication handling is requested for a UE that is configured with eDRX cycle value longer than 10.24 seconds for RRC_INACTIVE state and has at least one PDU session with active user plane as defined in clause 5.31.7 of TS 23.501 [2].

Figure 4.8.1.1a-1: NG-RAN initiated Connection Inactive procedure with CN based MT communication handling

0. The UE is registered in the network with negotiated eDRX parameters for CM-IDLE state and is in CM-CONNECTED with RRC_CONNECTED state. The AMF provides the eDRX cycle value for CM-IDLE state to NG-RAN as part of the RRC Inactive Assistance Information as defined in clause 5.3.3.2.5 of TS 23.501 [2]. The AMF also includes support indication of CN based MT communication handling for UE in CM-CONNECTED with RRC_INACTIVE state.

1. NG-RAN determines eDRX cycle value for UE in RRC_INACTIVE state and decides to transit the UE to RRC_INACTIVE state with CN based MT communication handling.

2. The NG-RAN sends N2 message to AMF indicating the UE is transitioning to RRC_INACTIVE state and the CN handles MT communication. The NG-RAN also provides the eDRX cycle value for RRC_INACTIVE to AMF.

Editor’s note: Whether the N2 message to be used is a new message or an existing message, e.g. N2 notification, needs coordination with RAN WG considering also if it’s NGAP class 1 or class 2 procedure.

3. For each of the PDU sessions with user plane resources have been activated, the AMF invokes Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, Cause, Operation type, User Location Information, Age of Location Information, N2 SM Information (Secondary RAT usage data)) towards SMF. The Operation Type is set to a value that indicates to stop user plane DL data transmissions towards the UE and enable data buffering. The SMF starts data buffering for MT data if the data buffering is handled in SMF.

Editor’s note: Whether an existing Operation Type is used, e.g. "UP Suspend", or another value is used needs coordination with CT WGs.

4. If data buffering is handled in the UPF, the SMF updates the UPF with proper rules for MT data handling.

5. The SMF sends the Nsmf_PDUSession_UpdateSMContext response.

6. The AMF sends N2 response to NG-RAN acknowledging the indication and the AMF considers the UE is in CM-CONNECTED with RRC_INACTIVE state.

7. NG-RAN initiates state transition from RRC_CONNECTED to RRC_INACTIVE with RRC configuring eDRX cycle value as specified in TS 38.300 [9].

Editor’s note: It is FFS on the handling of NG-RAN indication after entering RRC_INACTIVE.

4.8.1.2 Connection Suspend procedure

This procedure may be initiated by the serving NG-RAN node when the UE is in CM-CONNECTED and has at least one PDU session with active user plane connection and NG-eNB has received indication from the AMF that User Plane CIoT 5GS Optimisation, as defined in clause 5.31.18 of TS 23.501 [2], is supported for the UE.

Figure 4.8.1.2-1: NG-RAN initiated Connection Suspend procedure

1. NG-RAN to AMF: The NG-RAN sends the N2 Suspend Request message to the AMF, see TS 38.413 [10]. The AMF enters CM-IDLE with Suspend indicator. Context information related to the NGAP UE association, UE Context and PDU session context, necessary to resume the connection is stored in the UE, NG-RAN node and in the AMF. The NG-RAN may include the Suspend cause and the N2 SM information.

If the UE is served by an NG-eNB that supports WUS, then the NG-eNB should include the Information On Recommended Cells And RAN nodes For Paging in the N2 Suspend Request message; otherwise NG-RAN may include the Information On Recommended Cells And NG-RAN For Paging in the N2 Suspend Request message. If available, the AMF shall store this information to be used when paging the UE.

The NG-RAN includes Information for Enhanced Coverage, if available, in the N2 Suspend Request message.

If Service Gap Control is being applied to the UE (see clause 4.3.17.9) and the Service Gap timer is not already running, the Service Gap timer shall be started in the AMF when entering CM-IDLE, unless the connection was initiated after a paging of an MT event, or after a mobility registration procedure without Follow-on Request indication or after a mobility registration procedure for regulatory prioritized services like Emergency services or exception reporting.

2. AMF to SMF: For each of the PDU Sessions in the N2 Suspend Request, the AMF invokes Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, Cause, Operation type, User Location Information, Age of Location Information, N2 SM Information (Secondary RAT usage data)). The Operation Type is set to "UP Suspend" to indicate suspend of user plane resources for the PDU Session.

3. SMF to UPF: N4 Session Modification Request (AN Tunnel Info to be suspended, Buffering on/off).

The SMF initiates an N4 Session Modification procedure indicating the need to release the tunnel info of AN terminating N3 between AN and UPF. Buffering on/off indicates whether the UPF shall buffer incoming DL PDU or not.

The UPF sends N4 Session Modification Response to acknowledge the SMF request.

The SMF shall maintain the N3 tunnel info (including both AN Tunnel Info and the CN Tunnel Info).

NOTE: The UPF maintains the CN tunnel info as it may receive uplink packets from the AN.

4. SMF to AMF: The SMF sends Nsmf_PDUSession_UpdateSMContext response to the AMF.

5. AMF to NG-RAN: After response for each PDU session in step 4, the AMF sends N2 Suspend Response to NG-RAN to successfully terminate the Connection Suspend procedure initiated by the NG-RAN, see TS 38.413 [10].

6. The NG-RAN sends RRC message to suspend the RRC Connection towards the UE including UE Resume ID, see TS 36.300 [46]).

If Service Gap Control is applied for the UE (see clause 5.31.16 of TS 23.501 [2]) and the Service Gap timer is not already running, the Service Gap timer shall be started in the UE when entering CM-IDLE, unless the connection was initiated as a response to paging of an MT event, or after a mobility registration procedure without Follow-on Request Indication set or after a mobility registration procedure for regulatory prioritized services like Emergency services or exception reporting.

4.8.2 Connection Resume procedure

4.8.2.1 General

The Connection Resume procedure is used by a UE to request the establishment of a secure connection between a UE and the network when the UE is in CM-CONNECTED with RRC Inactive state, or in CM-IDLE with Suspend indicator for the UE supporting User Plane CIoT 5GS Optimisation. The UE initiates the procedure when upper layers or the AS (when responding to RAN paging or upon triggering RNA updates) requests the resumption of a suspended RRC connection. NG-RAN details are specified in TS 38.300 [9] and TS 38.331 [12] for NR and in TS 36.300 [46] and TS 36.331 [16] for E-UTRA.

4.8.2.2 UE Triggered Connection Resume in RRC Inactive procedure

The Connection Resume procedure is used by the UE to perform RRC Inactive to RRC Connected state transition. Triggers for the UE to initiate this procedure are defined in clause 5.3.3.2.5 of TS 23.501 [2].

Figure 4.8.2.2-1: Connection Resume in RRC Inactive

1. UE to NG-RAN: RRC message (Resume ID).

The UE initiates the transition from RRC Inactive state to RRC Connected state, see TS 38.300 [9]. The UE provides its Resume ID needed by the NG-RAN to access the UE’s stored Context.

2. [Conditional] NG-RAN performs UE Context Retrieval.

UE Context Retrieval is performed when the UE Context associated with the UE attempting to resume its connection is not locally available at the accessed NG-RAN. The UE Context Retrieval procedure via NG-RAN is specified in TS 38.300 [9].

3a. [Conditional] N2 Path switch procedure.

If the accessed NG-RAN is able to retrieve the UE Context, the accessed NG-RAN node initiates N2 Path Switch procedure, i.e. steps 1 to 8 of clause 4.9.1.2.2 and including Xn data forwarding.

If the Connection Resume procedure is a response to RAN paging which is triggered by 5GC due to an N2 interface procedure, NG-RAN and 5GC handle the N2 interface procedure as a collision described in clause 4.9.1.2.

3b. [Conditional] N2 Notification,

3b.1 If the accessed NG-RAN is the same as the NG-RAN that configured RRC Inactive and still has the UE context, NG-RAN send an N2 Notification to the AMF indicating the UE is in RRC_CONNECTED, if an AMF requested N2 Notification (see clause 4.8.3) or Connection Inactive procedure with CN based MT communication handling (see clause 4.8.1.1a) has been performed previously.

3b.2 The AMF invokes Nsmf_PDUSession_UpdateSMContext Request towards SMF indicating the Downlink data delivery for each PDU session with active user plane, if the AMF has requested data buffering as described in clause 4.8.1.1a.

3b.3 N4 session modification procedure is triggered by the SMF. If data buffering is handled in the UPF, the SMF updates the UPF with appropriate rules to trigger data delivery.

3b.4 The SMF sends the Nsmf_PDUSession_UpdateSMContext response.

3b.5 The AMF sends the N2 response to NG-RAN.

Editor’s note: Whether the N2 message to be used is a new message or an existing message, e.g. N2 notification, needs coordination with RAN WGs considering also if it’s NGAP class 1 or class 2 procedure.

4. NG-RAN to UE: RRC message.

The NG-RAN confirms to the UE that the UE has entered RRC_CONNECTED state.

NOTE: Steps 3 and 4 can be executed in parallel.

4.8.2.2a Network Triggered Connection Resume in RRC Inactive procedure

The Network Triggered Connection Resume in RRC Inactive procedure is used when the NG-RAN needs to send data (e.g. the N1 NAS PDU and/or downlink user plane PDU) to a UE in the RRC Inactive state. During the procedure, the NG-RAN sends a RAN Paging to the UE in order to trigger the Connection Resume in RRC Inactive procedure in clause 4.8.2.2.

Figure 4.8.2.2a-1: Network Triggered Connection Resume for UE in RRC Inactive

1. The NG-RAN receives downlink data (e.g. the N1 NAS PDU and/or the user plane PDU) for a UE in RRC Inactive State, the NG-RAN buffers the downlink data and triggers RAN Paging message.

If the NG RAN supports the Paging Cause Indication for Voice Service feature and if the UE context in NG-RAN indicates that the UE supports the Paging Cause Indication for Voice Service feature, the NG RAN shall provide the Voice Service Indication in the RAN Paging message for the UE when it detects that the downlink data which triggers the RAN Paging message is related to voice service, as specified in clause 5.38.3 in TS 23.501 [2].

2. The NG-RAN sends the RAN Paging message to the UE.

3. If the UE is in RRC Inactive State, based on the RAN paging and the UE decides to accept the paging, the UE initiates the UE Triggered Connection Resume in RRC Inactive procedure (see clause 4.8.2.2).

4.8.2.2b Network Triggered Connection Resume in RRC Inactive with CN based MT communication handling

When the UE is in CM-CONNECTED with RRC_INACTIVE state with CN based mobile terminating (MT) communication handling, high latency communication as described in clause 5.31.8 of TS 23.501 [2] is applied.

This procedure may be triggered by MT data, or a N1 procedure from SMF and UPF as shown in Figure 4.8.2.2b-1. When the procedure is triggered by other NFs (e.g. SMSF, LMF, GMLC), the UPF (or SMF) in the following figure should be replaced by the respective NF (the corresponding service operations used by other NFs when they communicate with AMF may also be different from the service operations used by SMF/UPF).

During the procedure, the NG-RAN (i.e. gNB) performs RAN paging towards the UE based on the N2 message from the AMF in order to trigger the UE triggered Connection Resume procedure in clause 4.8.2.2.

Figure 4.8.2.2b-1: Network Triggered Connection Resume for UE in RRC Inactive with CN based MT communication handling

1a. When downlink data is received and the SMF/UPF is requested to perform buffering as specified in clause 4.8.1.1a, the UPF/SMF checks with AMF for the possibility of data delivery, similar to step 2 of clause 4.24.2 with the following differences:

– The AMF determines if the UE is reachable based on the stored eDRX cycle value for RRC_INACTIVE state provided by NG-RAN in clause 4.8.1.1a. If the UE is unreachable, the AMF provides the Estimated Maximum Wait time in the response message based on the eDRX cycle value for RRC INACTIVE in AMF (steps 2-5 are skipped). If the UE is considered reachable, step 2 is triggered.

NOTE: This handling is similar to CM-IDLE with eDRX. When the AMF provides the Estimated Maximum Wait time, it can consider the time needed for RRC level procedures (e.g. RRC RNA update procedure) when UE wakes up from the eDRX cycle.

2. The AMF sends an N2 message to NG-RAN with the request for the UE to be transitioned to RRC_CONNECTED.

Editor’s note: Whether the N2 message to be used is a new message or an existing message, e.g. N2 notification, needs coordination with RAN WGs.

3. NG-RAN performs RAN paging towards the UE.

4. When the UE receives RAN paging, it initiates the UE triggered Connection Resume procedure and NG-RAN notifies CN as specified in clause 4.8.2.2 including the N2 Notification in step 3b.

5. The UPF triggers downlink data delivery if there is any. The AMF sends downlink NAS messages if there is any.

4.8.2.3 Connection Resume in CM-IDLE with Suspend procedure

For the UE supporting User Plane CIoT 5GS Optimisation, the Connection Resume procedure is used by the UE to perform RRC_IDLE with Suspend to RRC_CONNECTED state transition. Triggers for the UE to initiate this procedure are defined in clause 5.31.18 of TS 23.501 [2].

Figure 4.8.2.3-1: Connection Resume in CM-IDLE with Suspend

1. UE to NG-RAN: RRC message (Resume ID).

The UE initiates the transition from CM-IDLE and RRC IDLE state with Suspend to CM-CONNECTED and RRC Connected state, see TS 36.300 [46]. The UE provides its Resume ID needed by the NG-RAN to access the UE’s stored Context.

2. [Conditional] NG-RAN performs UE Context Retrieval.

UE Context Retrieval may be performed when the UE Context associated with the UE attempting to resume its connection is not locally available at the accessed NG-RAN. The UE Context Retrieval procedure via NG-RAN is specified in TS 38.300 [9].

3. NG-RAN and UE continues the resume procedure and access stratum configuration synchronization is performed between the UE and the network. UE enters CM-CONNECTED and RRC_CONNECTED.

4. NG-RAN to AMF:

a) If the NG-RAN is the same as the NG-RAN when UE is suspended, the NG-RAN sends N2 Resume Request to AMF including Resume cause and N2 SM information which indicates the PDU sessions successfully resumed and PDU sessions failed or partially failed to resume the user plane resource.

b) If the NG-RAN is different from the NG-RAN when UE is suspended and the new NG-RAN is able to retrieve the UE Context from the old NG-RAN, the new NG-RAN node initiates N2 Path Switch Request towards AMF, i.e. Steps 1b of clause 4.9.1.2.2.

5. AMF to SMF: For each of the PDU Sessions indicated in step 4, the AMF invokes Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, Cause, Operation type, User Location Information, Age of Location Information, N2 SM Information). The Operation Type is set to "UP Resume" to indicate resume of user plane resources for the PDU Session.

For PDU Session(s) to be switched to the new NG-RAN, upon receipt of the Nsmf_PDUSession_UpdateSMContext request, the SMF determines whether the existing UPF can continue to serve the UE. If the existing UPF cannot continue to serve the UE, steps 3 to 7 of clause 4.9.1.2.3 or clause 4.9.1.2.4 are performed depending on whether the existing UPF is a PDU Session Anchor and flow continues in step 7. Otherwise, step 6 is performed if the existing UPF can continue to serve the PDU Session.

If the RRC connection is resumed and the UE is accessing via the NB-IoT RAT with the RRC resume cause set to "MO exception data" the AMF updates all (H-)SMFs. If AMF indicates "MO exception data" then it includes the MO Exception Data Counter. The AMF maintains the MO Exception Data Counter for Small Data Rate Control purposes as described in clause 5.31.14.3 of TS 23.501 [2]. Each (H-)SMF should be updated for every RRC Connection which is triggered for "MO exception data".

In the home-routed roaming case, if Small Data Rate Control applies, the V-SMF waits for H-SMF response before proceeding with the N3 establishment.

6. SMF to UPF: N4 Session Modification Request (AN Tunnel Info to be resumed, Buffering on/off).

The SMF initiates an N4 Session Modification procedure indicating the resume of AN tunnel. Buffering on/off indicates whether the UPF shall buffer incoming DL PDU or not.

If step 4a) is performed, the AN tunnel info is the one maintained by the SMF during Connection Suspend procedure, step 3 of clause 4.8.1.2. If step 4b) is performed, the AN tunnel info is part of the N2 SM information received by SMF in step 5.

The UPF sends N4 Session Modification Response to acknowledge the SMF request.

7. SMF to AMF: The SMF sends Nsmf_PDUSession_UpdateSMContext response to the AMF.

If new CN tunnel information is allocated for the PDU session, i.e. in the case of new AN tunnel is received in step 6, the SMF includes the new CN tunnel information as part of the N2 SM information.

If the resume for PDU session is unsuccessful, the SMF shall include the resume failure as part of the N2 SM information.

8. AMF to NG-RAN: After response for each PDU session in step 7, the AMF sends N2 Resume Response to NG-RAN and indicates success, including N2 SM information for PDU session received in step 7, if at least one PDU session is resumed successfully. If none of the PDU sessions is resumed successfully, AMF indicates failure to NG-RAN.

The AMF sends N2 Path Switch Acknowledge with PDU session resume information, if Path Switch Request is received in step 4.

The AMF may provide Extended Connected Time value to the NG-RAN. If the NG-RAN receives the Extended Connected Time value, the NG-RAN may take this information into account when determining user inactivity.

9. [Conditional] NG-RAN to UE: RRC message.

The NG-RAN may reconfigure the RRC connection based on resume result received from AMF.

10. If the AMF has paged the UE to trigger the Connection Resume procedure, the AMF shall initiate the UE configuration update procedure as defined in clause 4.2.4.2 to assign a new 5G-GUTI.

4.8.2.4 Connection Resume in CM-IDLE with Suspend and MO EDT procedure

The Connection Resume with Early Data Transmission procedure is used by the UE to optimise sending user data in a single uplink packet and single uplink followed by single downlink packet cases for a UE in CM-IDLE with Suspend.

Figure 4.8.2.4-1: Connection Resume in RRC Idle with Suspend with EDT

1. UE to NG-eNB: RRC message (Resume ID, AS RAI) with UL EDT.

The UE initiates the transition from RRC IDLE with Suspend state to RRC Connected state, see TS 36.300 [46].

The UE may include AS Release Assistance information indicating:

– No further Uplink and Downlink Data transmission, or

– Only a single Downlink Data transmission subsequent to the Uplink transmission.

2. The NG-eNB deciphers the EDT UL data received from the UE and forwards it to the UPF using the N3 UL TEID in the AS context.

3. The NG-eNB sends N2 Resume Request to AMF including Resume cause and N2 SM.

If the UE included AS Release Assistance information indicating No further Uplink and Downlink Data transmission in step 1, NG-eNB may request for immediate transition to RRC IDLE with Suspend.

If the NG-eNB requests for immediate transition to RRC IDLE with Suspend, the NG-eNB should include Paging Assistance Data for CE capable UE in the N2 Resume Request message. If the NG-eNB supports WUS and requests for immediate transition to RRC IDLE with Suspend, the NG-eNB should include Information on Recommended Cells and RAN Nodes for Paging in the N2 Resume Request message. The AMF stores this information in the UE context for subsequent Paging procedure.

4. [Conditional] The AMF interacts with SMF to establish the N3 tunnel, except for the case:

– The AMF receives a request for immediate transition to RRC IDLE with Suspend in step 3; and

– the AMF is not aware of any downlink data or signalling pending.

5. The AMF sends an N2 Resume Response to NG-eNB.

If the AMF received a request for immediate transition to RRC IDLE with Suspend in step 3 and there is no downlink data or signalling pending, the AMF includes a Suspend indication, keeps the UE in CM-IDLE with Suspend and restarts the Periodic Registration Timer unless the Strictly Periodic Registration Timer Indication has been provided to the UE during the previous registration procedure.

Otherwise the AMF sends an N2 Resume Response to NG-eNB after the N3 Connectivity has been established and moves the UE to CM-CONNECTED. If the AMF knows of mobile terminating data or signalling pending, the AMF may include the Extended Connected Time value to the RAN

6. [Conditional] RRC procedure:

6a. If the AMF included the Suspend indication, the NG-eNB releases the RRC Connection with Suspend. The procedure is complete and following steps are skipped.

6b. If the AMF did not include the Suspend indication, and:

– The UE did not include AS Release Assistance Indication; or

– The AMF included the Extended Connected Time value.

the NG-eNB sends an RRC Resume message to the UE and the UE moves to CM-CONNECTED and RRC Connected. The procedure is complete and the following steps are skipped.

6c. Otherwise, if the AMF did not include Extended Connected Time value and the UE included AS Release Assistance information with only a single Downlink Data transmission subsequent to the Uplink transmission, the NG-eNB waits for the DL data to arrive and proceeds to steps 7-10.

7-9. The NG-eNB releases the N3 connectivity by sending an N2 Suspend Request to AMF including Suspend cause and N2 SM. Steps 2-4 of Connection Suspend procedure in clause 4.8.1.2 is executed.

10. [Conditional] NG-eNB to UE: RRC message (with DL data).

The NG-eNB ciphers received DL data.

The NG-eNB releases the RRC Connection with Suspend including the DL EDT data.

4.8.3 N2 Notification procedure

This procedure is used by an AMF to request the NG-RAN to report RRC state information, when the target UE is in CM-CONNECTED state. When AMF has requested reporting of subsequent state changes, the need for the NG-RAN to continue reporting ceases when the UE transitions to CM-IDLE or the AMF sends a cancel indication. This procedure may be used for services that require RRC state information (e.g. 5GC MT control and paging assistance, O&M and collection of statistics), or for subscription to the service by other NFs. See TS 38.413 [10] for details of the procedure.

Reporting of RRC state transitions can be requested per UE by AMF. Continuous reporting of all RRC state transitions can be enabled by operator local configuration.

Figure 4.8.3-1: RRC state transition notification

1. The AMF sends a UE State Transition Notification Request to the NG-RAN as described in TS 38.413 [10]. The UE State Transition Notification Request message shall identify the UE for which notification(s) are requested and may contain a reporting type. The reporting type either indicates subsequent state transitions shall be notified at every RRC state transition (i.e. from RRC Connected state to RRC Inactive state, or from RRC Inactive to RRC Connected state), or it indicates Single RRC-Connected state notification.

2. The NG-RAN sends the UE Notification message to report the current RRC state for the UE (i.e. RRC Inactive state or RRC Connected state). The current UE location information (i.e. TAI + Cell Identity) is always included when RRC state information is reported.

2b. When the AMF has requested reporting about subsequent state transitions, the NG-RAN sends subsequent UE Notification messages to the AMF at every RRC state transition until the UE transitions to CM-IDLE or NG-RAN receives a Cancel UE State Notification message from the AMF.

When the AMF has requested reporting for Single RRC-Connected state notification and UE is in RRC-Connected state, the NG-RAN sends one UE Notification message but no subsequent messages. If UE is in RRC-Inactive state, the NG-RAN sends one UE Notification message plus one subsequent UE Notification message when RRC state transits to RRC-Connected.

3. The AMF can send a Cancel UE State Notification message to inform the NG-RAN that it should terminate notifications for a given UE. This message should only be used when notification(s) about subsequent state transitions was requested at every RRC state transition.