Two CM states are used to reflect the NAS signalling Connection of the UE with the AMF:
The CM state for 3GPP access and Non-3GPP access are independent of each other, i.e. one can be in CM-IDLE state at the same time when the other is in CM-CONNECTED state.
A UE in CM-IDLE state has no NAS signalling connection established with the AMF over N1. The UE performs cell selection/cell reselection according to TS 38.304
and PLMN selection according to TS 23.122
There are no AN signalling connection, N2 connection and N3 connections for the UE in the CM-IDLE state.
If the UE is both in CM-IDLE state and in RM-REGISTERED state, the UE shall, unless otherwise specified in clause 18.104.22.168
When the UE state in the AMF is RM-REGISTERED, UE information required for initiating communication with the UE shall be stored. The AMF shall be able to retrieve stored information required for initiating communication with the UE using the 5G-GUTI.
In 5GS there is no need for paging using the SUPI/SUCI of the UE.
The UE provides 5G-S-TMSI as part of AN parameters during AN signalling connection establishment as specified in TS 38.331
and TS 36.331
. The UE shall enter CM-CONNECTED state whenever an AN signalling connection is established between the UE and the AN (entering RRC Connected state over 3GPP access, or at the establishment of the UE-N3IWF connectivity over untrusted non-3GPP access or the UE-TNGF connectivity over trusted non-3GPP access). The transmission of an Initial NAS message (Registration Request, Service Request or Deregistration Request) initiates the transition from CM-IDLE to CM-CONNECTED state.
When the UE states in the AMF are CM-IDLE and RM-REGISTERED, the AMF shall:
perform a network triggered Service Request procedure when it has signalling or mobile-terminated data to be sent to this UE, by sending a Paging Request to this UE (see TS 23.502, clause 22.214.171.124), if a UE is not prevented from responding e.g. due to MICO mode or Mobility Restrictions.
The AMF shall enter CM-CONNECTED state for the UE whenever an N2 connection is established for this UE between the AN and the AMF. The reception of initial N2 message (e.g., N2 INITIAL UE MESSAGE) initiates the transition of AMF from CM-IDLE to CM-CONNECTED state.
The UE and the AMF may optimize the power efficiency and signalling efficiency of the UE when in CM-IDLE state e.g. by activating MICO mode (see clause 126.96.36.199
A UE in CM-CONNECTED state has a NAS signalling connection with the AMF over N1. A NAS signalling connection uses an RRC Connection between the UE and the NG-RAN and an NGAP UE association between the AN and the AMF for 3GPP access. A UE can be in CM-CONNECTED state with an NGAP UE association that is not bound to any TNLA between the AN and the AMF. See clause 188.8.131.52
for details on the state of NGAP UE association for an UE in CM-CONNECTED state. Upon completion of a NAS signalling procedure, the AMF may decide to release the NAS signalling connection with the UE.
In the CM-CONNECTED state, the UE shall:
enter CM-IDLE state whenever the AN signalling connection is released (entering RRC Idle state over 3GPP access or when the release of the UE-N3IWF connectivity over untrusted non-3GPP access or the UE-TNGF connectivity over trusted non-3GPP access is detected by the UE), see TS 38.331 for 3GPP access.
When the UE CM state in the AMF is CM-CONNECTED, the AMF shall:
enter CM-IDLE state for the UE whenever the logical NGAP signalling connection and the N3 user plane connection for this UE are released upon completion of the AN Release procedure as specified in TS 23.502.
The AMF may keep a UE CM state in the AMF in CM-CONNECTED state until the UE de-registers from the core network.
A UE in CM-CONNECTED state can be in RRC Inactive state, see TS 38.300
. When the UE is in RRC Inactive state the following applies:
UE reachability is managed by the RAN, with assistance information from core network;
UE paging is managed by the RAN.
UE monitors for paging with UE's CN (5G S-TMSI) and RAN identifier.
When a UE enters CM-IDLE state, the UP connection of the PDU Sessions that were active on this access are deactivated.
The activation of UP connection of PDU Sessions is documented in clause 5.6.8
RRC Inactive state applies to NG-RAN. UE support for RRC Inactive state is defined in TS 38.306
for NR and TS 36.306
for E-UTRA connected to 5GC. RRC Inactive is not supported by NB-IoT connected to 5GC.
The AMF shall provide assistance information to the NG-RAN, to assist the NG-RAN's decision whether the UE can be sent to RRC Inactive state except due to some exceptional cases such as:
PLMN (or AMF set) does not support RRC Inactive;
The UE needs to be kept in CM-CONNECTED State (e.g. for tracking).
The "RRC Inactive Assistance Information" includes:
UE specific DRX values;
UE specific extended idle mode DRX values (cycle length and Paging Time Window length);
The Registration Area provided to the UE;
Periodic Registration Update timer;
If the AMF has enabled MICO mode for the UE, an indication that the UE is in MICO mode;
Information from the UE identifier, as defined in TS 38.304 for NR and TS 36.304 for E-UTRA connected to 5GC, that allows the RAN to calculate the UE's RAN paging occasions.
The RRC Inactive Assistance Information mentioned above is provided by the AMF during N2 activation with the (new) serving NG-RAN node (i.e. during Registration, Service Request, Handover) to assist the NG RAN's decision whether the UE can be sent to RRC Inactive state. If the AMF allocates a new Registration Area to the UE, the AMF should update the NG-RAN with the new Registration Area by sending the RRC Inactive Assistance Information accordingly.
RRC Inactive state is part of RRC state machine, and it is up to the RAN to determine the conditions to enter RRC Inactive state. If any of the parameters included in the RRC Inactive Assistance Information changes as the result of NAS procedure, the AMF shall update the RRC Inactive Assistance Information to the NG-RAN node.
When the UE is in CM-CONNECTED state, if the AMF has provided RRC Inactive assistance information, the RAN node may decide to move a UE to CM-CONNECTED with RRC Inactive state.
The state and "endpoints" (in the case of Dual Connectivity configuration) of the N2 and N3 reference points are not changed by the UE entering CM-CONNECTED with RRC Inactive state. A UE in RRC inactive state is aware of the RAN Notification area and periodic RAN Notification Area Update timer.
The 5GC network is not aware of the UE transitions between CM-CONNECTED with RRC Connected and CM-CONNECTED with RRC Inactive state, unless the 5GC network is notified via N2 notification procedure in TS 23.502, clause 4.8.3
At transition into CM-CONNECTED with RRC Inactive state, the NG-RAN configures the UE with a periodic RAN Notification Area Update timer taking into account the value of the Periodic Registration Update timer value indicated in the RRC Inactive Assistance Information, and uses a guard timer with a value longer than the RAN Notification Area Update timer value provided to the UE.
If the periodic RAN Notification Area Update guard timer expires in NG-RAN, the NG-RAN shall initiate AN Release procedure as specified in TS 23.502, clause 4.2.6
When the UE is in CM-CONNECTED with RRC Inactive state, the UE performs PLMN selection procedures as defined in TS 23.122
and TS 24.501
When the UE is CM-CONNECTED with RRC Inactive state, the UE may resume the RRC Connection due to:
Uplink data pending;
Mobile initiated NAS signalling procedure;
As a response to RAN paging;
Notifying the network that it has left the RAN Notification Area;
Upon periodic RAN Notification Area Update timer expiration.
If the UE resumes the connection in a different NG-RAN node within the same PLMN or equivalent PLMN, the UE AS context is retrieved from the old NG-RAN node and a procedure is triggered towards the CN (see TS 23.502, clause 4.8.2
With Dual Connectivity configuration if the UE resumes the RRC connection in the Master RAN node, the Secondary RAN node configuration is defined in TS 38.300
If the RAN paging procedure, as defined in TS 38.300
, is not successful in establishing contact with the UE the procedure shall be handled by the network as follows:
If NG-RAN has at least one pending NAS PDU for transmission, the RAN node shall initiate the AN Release procedure (see TS 23.502, clause 4.2.6,) to move the UE CM state in the AMF to CM-IDLE state and indicate to the AMF the NAS non-delivery.
If NG RAN has only pending user plane data for transmission, the NG-RAN node may keep the N2 connection active or initiate the AN Release procedure (see TS 23.502, clause 4.2.6) based on local configuration in NG-RAN.
The user plane data which triggers the RAN paging can be lost, e.g. in the case of RAN paging failure.
If a UE in CM-CONNECTED with RRC Inactive state performs cell selection to GERAN/UTRAN/E-UTRAN, it shall follow idle mode procedures of the selected RAT as specified in clause 5.17
In addition, a UE in CM-CONNECTED state with RRC Inactive state shall enter CM-IDLE state and initiates the NAS signalling recovery (see TS 24.501
) in the following cases:
If RRC resume procedure fails,
If the UE receives Core Network paging,
If the periodic RAN Notification Area Update timer expires and the UE cannot successfully resume the RRC Connection,
In any other failure scenario that cannot be resolved in RRC Inactive state and requires the UE to move to CM-IDLE state.
When a UE is in CM-CONNECTED with RRC Inactive state, and a trigger to change the UE's NG-RAN UE Radio Capability information happens, the UE shall move to CM-IDLE state and initiate the procedure for updating UE Radio Capability defined in clause 184.108.40.206
When UE is in CM-CONNECTED with RRC Inactive state, if RAN has received Location Reporting Control message from AMF with the Reporting Type indicating single stand-alone report, the RAN shall perform RAN paging before reporting the location to AMF.
When UE is in CM-CONNECTED with RRC Inactive state, if RAN has received Location Reporting Control message from AMF with the Reporting Type indicating continuously reporting whenever the UE changes cell, the RAN shall send a Location Report message to AMF including UE's last known location with time stamp.
When the UE is CM-CONNECTED with RRC Inactive state. If the AMF receives Nudm_UEContextManagement_DeregistrationNotification
from UDM, the AMF shall initiate AN Release procedure as specified in TS 23.502, clause 4.2.6
When UE is in CM-CONNECTED with RRC Inactive state, if RAN has received Location Reporting Control message from AMF with the Reporting Type of the Area Of Interest based reporting, the RAN shall send a Location Report message to AMF including UE presence in the Area Of Interest (i.e., IN, OUT, or UNKNOWN) and the UE's last known location with time stamp.
When the UE is in CM-CONNECTED with RRC Inactive state, if the old NG-RAN node that sents the UE into RRC Inactive state receives the downlink N2 signalling, it initiates the RAN paging as defined in TS 38.300
. If the UE resumes the RRC Connection towards a different NG-RAN node, the old NG-RAN node includes the "UE Context Transfer" indication into a response container to the NF (e.g. AMF or SMF) that generates such N2 downlink signalling. Then the NF shall reattempt the same procedure when the path switch from the old NG-RAN node to the new NG-RAN node is complete.