Inactive state
RRC_INACTIVE: resume and RNA update
How an NR UE in RRC_INACTIVE resumes a connection or updates its RAN-based notification area, with UE context retrieval over Xn.
Overview
In RRC_INACTIVE the UE remains in CM-CONNECTED and can move within an area configured by NG-RAN, the RAN-based notification area (RNA), without notifying NG-RAN. The last serving gNB keeps the UE context and the UE-associated NG connection with the serving AMF and UPF (TS 38.300 9.2.2.1).
The RNA can cover one or several cells and is contained within the CN registration area. It is configured either as a list of cells or as a list of RAN areas, and a UE must support all options. A RAN-based notification area update (RNAU) is sent periodically and when cell reselection picks a cell outside the configured RNA (TS 38.300 9.2.2.3).
If the UE accesses a gNB other than the last serving gNB, the receiving gNB triggers the XnAP Retrieve UE Context procedure to fetch the UE context. If no valid context is found, the receiving gNB can set up a new RRC connection instead of resuming, and a change of serving AMF also makes a new connection necessary.
Call flow
- 1UEtogNB
RRCResumeRequest
Carries the I-RNTI allocated by the last serving gNB.
- 2gNBtoLast serving gNB
RETRIEVE UE CONTEXT REQUEST (XnAP)
Sent if the gNB can resolve the gNB identity in the I-RNTI.
- 3Last serving gNBtogNB
RETRIEVE UE CONTEXT RESPONSE
Provides the UE context data.
- 4gNBtoUE
RRCResume, then RRCResumeComplete
Completes resumption of the RRC connection.
- 5gNBtoAMF
Path switch
The gNB performs the path switch toward the core.
- 6gNBtoLast serving gNB
UE context release
Resources at the last serving gNB are released.
Timers and parameters
| Name | Where | Role |
|---|---|---|
| T380 | UE | Periodic RNA update timer, configured in suspendConfig. Values: 5, 10, 20, 30, 60, 120, 360 or 720 minutes. |
| T319 | UE | Started when the RRCResumeRequest is submitted. Failure handling is in TS 38.331 5.3.13.5. |
Specifications
| Spec | What it covers |
|---|---|
| TS 38.300 | Mobility in RRC_INACTIVE. RNA is 9.2.2.3, state transitions 9.2.2.4 and RNA update 9.2.2.5.(see clause 9.2.2) |
| TS 38.331 | RRC connection resume. RNA update is 5.3.13.8 and failure to resume is 5.3.13.5.(see clause 5.3.13) |
| TS 38.423 | XnAP Retrieve UE Context. RAN Paging is 8.2.5.(see clause 8.2.4) |
Failure behaviour in the specs
- If context retrieval fails, the last serving gNB indicates failure and the gNB falls back to RRC establishment by sending RRCSetup (TS 38.300 9.2.2.4.1).
- A gNB that cannot handle the resume request, for example due to congestion, can send RRCReject with a wait time to keep the UE in RRC_INACTIVE.
- RNA update: the last serving gNB may provide the context, may keep the UE in RRC_INACTIVE by answering with a RETRIEVE UE CONTEXT FAILURE that encapsulates an RRCRelease with suspend indication, or may move the UE to RRC_IDLE with an RRCRelease (TS 38.300 9.2.2.5).
- Network-triggered resume: RAN paging is sent in the last serving gNB's cells and, with XnAP RAN Paging, in neighbouring gNBs' cells in the RNA. The UE is paged with its I-RNTI (TS 38.300 9.2.2.4.2).
Related procedures
Idle mode
Cell selection and reselection
How an idle or inactive UE selects a cell by the S-criterion and keeps the best one by priority and ranking.
Failure handling
Radio link failure and RRC re-establishment
Out-of-sync handling with N310, N311 and T310, then re-establishment supervised by T311 and T301.
Registration
5G registration and 4G tracking area update
NAS mobility management: registration in 5GS, tracking area update in EPS, and the reject causes.