School of Specs 38.300v19.3.0

3GPP 38.300 v19.3.0 — the document's own text

9.2.2.1 Overview

Taught in 6. The three states a device sits in (TSG RAN — the group that specifies the radio, in depth), 13. Where RAN meets the groups that write the rest of 3GPP (TSG RAN — the group that specifies the radio, in depth), 5. The layers a packet falls through, and the three states a device sits in (TSG RAN — the group that specifies the radio, overview), 3. What NR is, read off the document itself (TSG RAN — the group that specifies the radio, quick start).

RRC_INACTIVE is a state where a UE remains in CM-CONNECTED and can move within an area configured by NG-RAN (the RNA) without notifying NG-RAN. In RRC_INACTIVE, the last serving gNB node keeps the UE context and the UE-associated NG connection with the serving AMF and UPF.

For a UE in RRC_INACTIVE with eDRX cycle longer than 10.24 seconds, the NG-RAN node may, based on implementation, send a request to the AMF to perform MT Communication Handling as described in TS 23.501 [3]. If the UE accesses a gNB other than the last serving gNB and upon successful UE context retrieval, the AMF considers that the UE is reachable (i.e., the MT Communicating Handling is deactivated) and triggers the MT DL data/signalling delivery upon receiving the NGAP Path Switch Request message.

If the last serving gNB receives DL data from the UPF or DL UE-associated signalling from the AMF (except the UE Context Release Command message) while the UE is in RRC_INACTIVE, it pages in the cells corresponding to the RNA and may send XnAP RAN Paging to neighbour gNB(s) if the RNA includes cells of neighbour gNB(s).

Upon receiving the RAN Paging Request message from the AMF while the UE is in RRC_INACTIVE with eDRX beyond 10.24 seconds, the last serving gNB may page in its cells comprised in the RNA and may send XnAP RAN Paging to neighbour gNB(s) if the RNA includes cells of neighbour gNB(s), in order to trigger the UE to resume connection in RRC_CONNECTED state, or in RRC_INACTIVE state for DL SDT, based on the DL data size if included in the NGAP RAN Paging Request message.

Upon receiving the UE Context Release Command message while the UE is in RRC_INACTIVE, the last serving gNB may page in the cells corresponding to the RNA and may send XnAP RAN Paging to neighbour gNB(s) if the RNA includes cells of neighbour gNB(s), in order to release UE explicitly.

Upon receiving the NG RESET message while the UE is in RRC_INACTIVE, the last serving gNB may page involved UEs in the cells corresponding to the RNA and may send XnAP RAN Paging to neighbour gNB(s) if the RNA includes cells of neighbour gNB(s) in order to explicitly release involved UEs.

Upon RAN paging failure, the gNB behaves according to TS 23.501 [3].

The AMF provides to the NG-RAN node the Core Network Assistance Information to assist the NG-RAN node's decision whether the UE can be sent to RRC_INACTIVE, and to assist UE configuration and paging in RRC_INACTIVE. The Core Network Assistance Information can include:

  • The UE registration area, which is taken into account by the NG-RAN node when configuring the RNA.
  • The UE specific DRX and UE Identity Index value, which are used by the NG-RAN node for RAN paging.
  • The Periodic Registration Update timer, which is taken into account by the NG-RAN node to configure Periodic RNA Update timer.
  • An indication if the UE is configured with Mobile Initiated Connection Only (MICO) mode by the AMF.
  • The Expected UE Behaviour, which is taken into account by the NG-RAN node to assist the UE RRC state transition decision.
  • The UE Radio Capability for Paging, which may be used by the NG-RAN node during RAN Paging.
  • The PEI with Paging Subgrouping assistance information, which may be taken into account by the NG-RAN node for subgroup paging in RRC_INACTIVE, except when the UE context contains an emergency PDU session in which case the PEI with Paging Subgrouping assistance information shall not be used according to TS 24.501 [28].
  • The Extended UE Identity Index Value, which is taken into account by the NG-RAN node for RAN paging.
  • The LP-WUS with Paging Subgrouping assistance information, which is taken into account by the NG-RAN node for subgroup paging in RRC_INACTIVE.
  • The LP-WUS disable indication, which is taken into account by the NG-RAN node to disable LP-WUS to UE, including both CN based subgrouping and UE ID based subgrouping.
  • The Further Extended UE Identity Index value, if LP-WUS is not disabled, which is taken into account by the NG-RAN node for LP-WUS subgroup paging in RRC_INACTIVE.
  • The NR Paging eDRX Information, which is taken into account by the NG-RAN node to configure the RAN Paging when the NR UE is in RRC_INACTIVE.
  • The Paging Cause Indication for Voice Service, which is taken into account by the NG-RAN node to include the Paging Cause in RAN Paging for a UE in RRC_INACTIVE state.
  • The Hashed UE Identity Index Value, which is used by the NG-RAN node to determine the start point of PTW.
  • The CN support indication for MT Communication Handling, which is taken into account by the NG-RAN node when deciding to request the AMF for MT Communication Handling for a UE in RRC_INACTIVE state with long eDRX beyond 10.24 seconds as described in TS 23.501 [3].

When sending the XnAP RAN Paging to neighbour NG-RAN node(s), the PEI with Paging Subgrouping assistance information, the Extended UE Identity Index Value, the LP-WUS with Paging Subgrouping assistance information, the LP-WUS disable indication, the NR Paging eDRX Information for RRC_IDLE and for RRC_INACTIVE, the Paging Cause, the Hashed UE Identity Index Value and the Further Extended UE Identity Index Value may be included.

At transition to RRC_INACTIVE the NG-RAN node may configure the UE with a periodic RNA Update timer value. At periodic RNA Update timer expiry without notification from the UE, the gNB behaves as specified in TS 23.501 [3].

If the UE accesses a gNB other than the last serving gNB, the receiving gNB triggers the XnAP Retrieve UE Context procedure to get the UE context from the last serving gNB and may also trigger an Xn-U Address Indication procedure including tunnel information for potential recovery of data from the last serving gNB. Upon successful UE context retrieval, the receiving gNB shall perform the slice-aware admission control in case of receiving slice information and becomes the serving gNB and it further triggers the NGAP Path Switch Request and applicable RRC procedures. After the path switch procedure, the serving gNB triggers release of the UE context at the last serving gNB by means of the XnAP UE Context Release procedure.

In case the UE is not reachable at the last serving gNB, the gNB shall fail any AMF initiated UE-associated class 1 procedure which allows the signalling of unsuccessful operation in the respective response message. It may trigger the NAS Non Delivery Indication procedure to report the non-delivery of any non PDU Session related NAS PDU received from the AMF as specified in TS 38.413 [26].

If the UE accesses a gNB other than the last serving gNB and the receiving gNB does not find a valid UE Context, the receiving gNB can perform establishment of a new RRC connection instead of resumption of the previous RRC connection. UE context retrieval will also fail and hence a new RRC connection needs to be established if the serving AMF changes.

A UE in the RRC_INACTIVE state is required to initiate RNA update procedure when it moves out of the configured RNA. When receiving RNA update request from the UE, the receiving gNB triggers the XnAP Retrieve UE Context procedure to get the UE context from the last serving gNB and may decide to send the UE back to RRC_INACTIVE state, move the UE into RRC_CONNECTED state, or send the UE to RRC_IDLE. In case of periodic RNA update, if the last serving gNB decides not to relocate the UE context, it fails the Retrieve UE Context procedure and sends the UE back to RRC_INACTIVE, or to RRC_IDLE directly by an encapsulated RRCRelease message.