8.7 RRC connection reestablishment

38.4013GPPArchitecture descriptionNG-RANRelease 17TS

This procedure is used for the case that UE tries to reestablish the RRC connection, as shown in Figure 8.7-1.

Figure 8.7-1: RRC connection reestablishment procedure

1. The UE sends a preamble to the gNB-DU.

2. The gNB-DU allocates new C-RNTI and responds with RAR.

3. The UE sends an RRCReestablishmentRequest message to the gNB-DU, which contains old C-RNTI and old PCI.

4. The gNB-DU includes the RRC message and, if the UE is admitted, the corresponding low layer configuration for the UE in the INITIAL UL RRC MESSAGE TRANSFER message and transfers to the gNB-CU. The INITIAL UL RRC MESSAGE TRANSFER message includes the new C-RNTI. If the gNB-DU identifies the UE as a Reduced Capability UE during the random access procedure, a NR RedCap UE Indication is provided in the INITIAL UL RRC MESSAGE TRANSFER message.

5. The gNB-CU includes an RRCReestablishment message and transfers to the gNB-DU. If the UE requests to re-establish RRC connection in the last serving gNB-DU, the DL RRC MESSAGE TRANSFER message shall include old gNB-DU UE F1AP ID.

6. The gNB-DU retrieves the UE context based on the old gNB-DU UE F1AP ID, and replaces old C-RNTI/PCI with new C-RNTI/PCI. It sends the RRCReestablishment message to UE.

7-8. The UE sends an RRCReestablishmentComplete message to the gNB-DU. The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and sends to the gNB-CU.

9-10. The gNB-CU triggers an UE Context Modification procedure by sending UE CONTEXT MODIFICATION REQUEST message, which may include DRBs to be modified and released list. The gNB-DU responses with the UE CONTEXT MODIFICATION RESPONSE message.

9′-10′. The gNB-DU triggers an UE Context Modification procedure by sending UE CONTEXT MODIFICIATION REQUIRED message, which may include DRBs to be modified and released list. The gNB-CU responses with UE CONTEXT MODIFICATION CONFIRM message.

NOTE: Here it is assumed that the UE accessed the original gNB-DU where the UE context is available for that UE, and either steps 9-10 or steps 9’-10’ may be executed or both could be skipped.

NOTE: If the UE accessed from a gNB-DU other than the original one, the gNB-CU should trigger the UE Context Setup procedure toward this new gNB-DU.

11-12. The gNB-CU includes an RRCReconfiguration message into the DL RRC MESSAGE TRANSFER message and transfers to the gNB-DU. The gNB-DU forwards it to the UE.

13-14. The UE sends an RRCReconfigurationComplete message to the gNB-DU, and the gNB-DU forwards it to the gNB-CU.