8.6 RRC state transition

38.4013GPPArchitecture descriptionNG-RANRelease 17TS

8.6.1 RRC connected to RRC inactive

This clause gives the RRC connected to RRC inactive state transition given that gNB consists of gNB-CU and gNB-DU(s), as shown in Figure 8.6.1-1.

Figure 8.6.1-1: RRC connected to RRC inactive state transition procedure

0. At first, the gNB-CU determines the UE to enter into RRC inactive mode from connected mode.

1. The gNB-CU generates RRCRelease message which includes suspend configuration towards UE. The RRC message is encapsulated in UE CONTEXT RELEASE COMMAND message to the gNB-DU.

2. The gNB-DU forwards RRCRelease message to UE.

3. The gNB-DU responds with UE CONTEXT RELEASE COMPLETE message.

8.6.2 RRC inactive to other states

This clause gives the RRC inactive to other RRC states transition given that gNB consists of gNB-CU and gNB-DU(s), as shown in Figure 8.6.2-1.

Figure 8.6.2-1: RRC inactive to other RRC states transition procedure

1. If data is received from 5GC, the gNB-CU sends PAGING message to the gNB-DU.

2. The gNB-DU sends Paging message to UE.

NOTE: Step 1 and 2 only exist in case of DL data arrival.

3. The UE sends RRCResumeRequest message either upon RAN-based paging, UL data arrival or RNA update.

4. The gNB-DU includes RRCResumeRequest in a non-UE associated INITIAL UL RRC MESSAGE TRANSFER message and transfer to the gNB-CU.

5. For UE Inactive to UE Active transitions, excluding transitions due to signalling exchange only, the gNB-CU allocates gNB-CU UE F1AP ID and sends UE CONTEXT SETUP REQUEST message to gNB-DU, which may include SRB ID(s) and DRB ID(s) to be setup, CellGroupConfig stored in gNB-CU or retrieved from the old NG-RAN node may also be included. In case of NG-RAN sharing, the gNB-CU includes the serving PLMN ID (in case of SNPNs the serving NID).

6. The gNB-DU responds with UE CONTEXT SETUP RESPONSE message, which contains RLC/MAC/PHY configuration of SRB and DRBs provided by the gNB-DU.

NOTE: Step 5 and step 6 exist for inactive to active transitions, excluding transitions due to signalling exchange only. When gNB-CU successfully retrieves and verifies the UE context, it may decide to let the UE enter into RRC active mode. gNB-CU shall trigger UE context setup procedure between gNB-CU and gNB-DU, during which both SRB1, SRB2 and DRB(s) can be setup. For signalling exchange only transitions, gNB-CU does not trigger UE Context Setup procedure. For inactive to Idle transitions the gNB-CU does not trigger the UE Context Setup procedure.

7. The gNB-CU generates RRCResume/RRCSetup/RRCReject/RRCRelease message or receives RRCRelease message from the old NG-RAN node towards the UE. The RRC message is encapsulated in DL RRC MESSAGE TRANSFER message together with SRB ID.

8. The gNB-DU forwards RRC message to the UE either over SRB0 or SRB1 as indicated by the SRB ID.

NOTE: In step 7, it is expected that gNB-CU takes appropriate action, e.g. generates RRC resume message for inactive to active state transition(for both cases of signaling exchange only, and UP data exchange), generates RRCSetup message for fallback to establish a new RRC connection, and generates or receives from the old NG-RAN node either RRCRelease message without suspend configuration for inactive to idle state transition, or RRCRelease message with suspend configuration to remain in inactive state.
If step 5 and 6 are not performed, the gNB-DU deduces the SRB on which to deliver the RRC message in step 7 from the SRB ID, i.e. SRB ID “0” corresponds to SRB0, SRB ID “1” corresponds to SRB1.

9. The UE sends RRCResumeComplete/RRCSetupComplete message to the gNB-DU.

10. The gNB-DU encapsulates RRC in UL RRC MESSAGE TRANSFER message and send to the gNB-CU.

NOTE: Step 9 and step 10 exist for inactive to active state transition (for both cases of signaling exchange only, and UP data exchange). UE generates RRCResumeComplete/RRCSetupComplete message for resume the existing RRC connection or fallback to a new RRC connection respectively.