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full Contents for  TS 23.501  Word version:   16.4.0

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5.3.3  Connection ManagementWord-p. 77
5.3.3.1  General
Connection management comprises the functions of establishing and releasing a NAS signalling connection between a UE and the AMF over N1. This NAS signalling connection is used to enable NAS signalling exchange between the UE and the core network. It comprises both the AN signalling connection between the UE and the AN (RRC Connection over 3GPP access or UE-N3IWF connection over untrusted N3GPP access or UE-TNGF connection over trusted N3GPP access) and the N2 connection for this UE between the AN and the AMF.
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5.3.3.2  5GS Connection Management statesWord-p. 78
5.3.3.2.1  General
Two CM states are used to reflect the NAS signalling Connection of the UE with the AMF:
  • CM-IDLE
  • CM-CONNECTED
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.
5.3.3.2.2  CM-IDLE 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 5.3.4.1:
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.
NOTE:
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 4.2.3.3), 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 5.4.1.3).
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5.3.3.2.3  CM-CONNECTED state
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 5.21.1.2 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.
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5.3.3.2.4  5GS Connection Management State modelsWord-p. 79
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When a UE enters CM-IDLE state, the UP connection of the PDU Sessions that were active on this access are deactivated.
NOTE:
The activation of UP connection of PDU Sessions is documented in clause 5.6.8.
5.3.3.2.5  CM-CONNECTED with RRC Inactive state
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).
NOTE 1:
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.
NOTE 2:
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 5.4.4.1.
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.
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5.3.3.3  NAS signalling connection managementWord-p. 81
5.3.3.3.1  General
NAS signalling connection management includes the functions of establishing and releasing a NAS signalling connection.
5.3.3.3.2  NAS signalling connection establishmentWord-p. 82
NAS signalling connection establishment function is provided by the UE and the AMF to establish a NAS signalling connection for a UE in CM-IDLE state. The AMF shall provide the list of recommended cells/ TAs / NG-RAN node identifiers for paging, if the NG-RAN had provided that information in an earlier AN Release Procedure in the AN (see clause 4.2.6 of TS 23.502).
When the UE in CM-IDLE state needs to transmit an NAS message, the UE shall initiate a Service Request, a Registration or a Deregistration procedure to establish a NAS signalling connection to the AMF as specified in TS 23.502, clauses 4.2.2 and 4.2.3. If the NAS signalling connection is to be established via an NG-RAN node, but the AMF detects that this UE has already established a NAS signalling connection via old NG-RAN node, the AMF shall release the old established NAS signalling connection by triggering AN Release Procedure.
Based on UE preferences, UE subscription, Mobility Pattern and network configuration, the AMF may keep the NAS signalling connection until the UE de-registers from the network.
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5.3.3.3.3  NAS signalling connection Release
The procedure of releasing a NAS signalling connection is initiated by the AN node (either 5G (R)AN node or N3IWF) or the AMF. The NG-RAN node may include the list of recommended cells/ TAs / NG-RAN node identifiers for paging, during the AN Release Procedure in the AN (see clause 4.2.6 of TS 23.502). The AMF stores this information, if provided by the NG-RAN.
The UE considers the NAS signalling connection is released if it detects the AN signalling connection is released. The AMF considers the NAS signalling connection is released if it detects the N2 context is released.
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5.3.3.4  Support of a UE connected over both 3GPP and Non-3GPP access
The AMF manages two CM states for an UE: a CM state for 3GPP access and a CM state for Non-3GPP access. An N2 interface can serve the UE for either 3GPP access or for Non 3GPP access. UE connected over both 3GPP and Non-3GPP has got two N2 interfaces, one for each access. A UE may be in any combination of the CM states between 3GPP and Non-3GPP access, e.g. a UE may be CM-IDLE for one access and CM-CONNECTED for the other access, CM-IDLE for both accesses or CM-CONNECTED for both accesses.
When the UE CM state in the AMF is CM-IDLE for 3GPP access and CM-CONNECTED for Non-3GPP access, the AMF shall perform a network triggered Service Request procedure, when it has downlink data to be sent to this UE for 3GPP access, by sending either the Paging Request via 3GPP access or the NAS notification via Non-3GPP access to this UE (see TS 23.502, clause 4.2.3.3).
Connection Management over Non-3GPP access is further defined in clause 5.5.2.
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5.3.4  UE Mobility
5.3.4.1  Mobility Restrictions
5.3.4.1.1  General
Mobility Restrictions restrict mobility handling or service access of a UE. The Mobility Restriction functionality is provided by the UE (only for mobility restriction categories provided to the UE), the radio access network and the core network.
Unless otherwise stated, Mobility Restrictions only apply to 3GPP access and wireline access, they do not apply to other non-3GPP accesses.
Service Area restrictions and handling of Forbidden Areas for CM-IDLE state and, for CM-CONNECTED state when in RRC Inactive state are executed by the UE based on information received from the core network. Mobility Restrictions for CM-CONNECTED state when in RRC-Connected state are executed by the radio access network and the core network.
In CM-CONNECTED state, the core network provides Mobility Restrictions to the radio access network within Mobility Restriction List.
Mobility Restrictions consists of RAT restriction, Forbidden Area, Service Area Restrictions, Core Network type restriction and Closed Access Group information as follows:
  • RAT restriction:
  • Defines the 3GPP Radio Access Technology(ies), a UE is not allowed to access in a PLMN. In a restricted RAT a UE based on subscription is not permitted access to the network for this PLMN. For CM-CONNECTED state, when radio access network determines target RAT and target PLMN during Handover procedure, it should take per PLMN RAT restriction into consideration. The RAT restriction is enforced in the network, and not provided to the UE.
  • Forbidden Area:
  • In a Forbidden Area, the UE, based on subscription, is not permitted to initiate any communication with the network for this PLMN. The UE behaviour in terms of cell selection, RAT selection and PLMN selection depends on the network response that informs the UE of Forbidden Area. A Forbidden Area applies either to 3GPP access or to non-3GPP access.
    Further description on Forbidden Area when using wireline access is available in TS 23.316.
    NOTE 1:
    If the N3GPP TAI (see clause 5.3.2.3) is forbidden in a PLMN, non-3GPP Access is forbidden altogether in this PLMN.
    NOTE 2:
    The UE reactions to specific network responses are described in TS 24.501.
  • Service Area Restriction:
  • Defines areas in which the UE may or may not initiate communication with the network as follows:
    • Allowed Area:
    • In an Allowed Area, the UE is permitted to initiate communication with the network as allowed by the subscription.
    • Non-Allowed Area:
    • In a Non-Allowed Area a UE is service area restricted based on subscription. The UE and the network are not allowed to initiate Service Request or SM signalling (except for PS Data Off status change reporting) to obtain user services (both in CM-IDLE and in CM-CONNECTED states). The UE shall not use the entering of a Non-Allowed Area as a criterion for Cell Reselection, a trigger for PLMN Selection or Domain selection for UE originating sessions or calls. The RRC procedures while the UE is in CM-CONNECTED with RRC Inactive state are unchanged compared to when the UE is in an Allowed Area. The RM procedures are unchanged compared to when the UE is in an Allowed Area. The UE in a Non-Allowed Area shall respond to core network paging or NAS Notification message from non-3GPP access with Service Request and RAN paging.
    NOTE 3:
    When the services are restricted in 5GS due to Service Area Restriction, then it is assumed that the services will be also restricted in all RATs/Systems at the same location(s) using appropriate mechanisms available in the other RATs/Systems.
  • Core Network type restriction:
  • Defines whether UE is allowed to connect to 5GC only, EPC only, both 5GC and EPC for this PLMN. The Core Network type restriction when received applies in the PLMN either to both 3GPP and non-3GPP Access Types or to non-3GPP Access Type only.
    NOTE 4:
    The Core Network type restriction can be used e.g. in network deployments where the E-UTRAN connects to both EPC and 5GC as described in clause 5.17.
  • Closed Access Group information:
  • As defined in clause 5.30.3.
For a given UE, the core network determines the Mobility Restrictions based on UE subscription information, UE location and/or local policy (e.g. if the HPLMN has not deployed 5GC, HPLMN ID of the UE and the operator's policy are used in the VPLMN for determining the Core Network type restriction). The Mobility Restriction may change due to e.g. UE's subscription, location change and local policy. Optionally the Service Area Restrictions or the Non-Allowed Area may in addition be fine-tuned by the PCF e.g. based on UE location, PEI and network policies. Service Area Restrictions may be updated during a Registration procedure or UE Configuration Update procedure.
NOTE 5:
The subscription management ensure that for MPS service subscriber the Mobility Restrictions is not included.
If the network sends Service Area Restrictions to the UE, the network sends only either an Allowed Area, or a Non-Allowed Area, but not both at the same time, to the UE. If the UE has received an Allowed Area from the network, any TA not part of the Allowed Area is considered by the UE as non-allowed. If the UE has received a Non-Allowed Area from the network, any TA not part of the Non-Allowed Area is considered by the UE as allowed. If the UE has not received any Service Area Restrictions, any TA in the PLMN is considered as allowed.
If the UE has overlapping areas between Forbidden Areas, Service Area Restrictions, or any combination of them, the UE shall proceed in the following precedence order:
  • The evaluation of Forbidden Areas shall take precedence over the evaluation of Service Area Restrictions.
The UE and the network shall override any Forbidden Area, Non-Allowed area restrictions and Core Network type restriction whenever access to the network for regulatory prioritized services like Emergency services and MPS.
The UDM shall provide to the AMF the information defined in TS 23.008 about the subscriber's NR or E-UTRA access restriction set by the operator determined e.g. by subscription scenario and roaming scenario:
  • For NR:
    • NR not allowed as primary access.
    • NR not allowed as secondary access.
    • NR in unlicensed bands not allowed as primary access.
    • NR in unlicensed bands not allowed as secondary access.
  • For E-UTRA:
    • E-UTRA not allowed as primary access.
    • E-UTRA not allowed as secondary access.
    • E-UTRA in unlicensed bands not allowed as secondary access.
    • NB-IoT not allowed as primary access.
    • LTE-M not allowed as primary access.
In order to enforce all primary access restrictions, the related access has to be deployed in different Tracking Area Codes and the subscriber shall not be allowed to access the network in TAs using the particular access.
With all secondary access restrictions, the subscriber shall not be allowed to use this access as secondary access.
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5.3.4.1.2  Management of Service Area RestrictionsWord-p. 84
This clause describes Service Area Restrictions for 3GPP access. For Service Area Restrictions when using wireline access, see TS 23.316.
A Service Area Restriction may contain one or more (e.g. up to 16) entire Tracking Areas each or the Service Area Restriction may be set as unlimited (i.e. contain all Tracking Areas of the PLMN). The UE's subscription data in the UDM includes a Service Area Restriction which may contain either Allowed or Non-Allowed Areas-specified by using explicit Tracking Area identities and/or other geographical information (e.g., longitude/latitude, zip code, etc). The geographical information used to specify Allowed or Non-Allowed Area is only managed in the network, and the network will map it to a list of TAs before sending Service Area Restriction information to the PCF, NG-RAN and UE.
When the AMF assigns a limited allowed area to the UE, the AMF shall provide the UE with Service Area Restrictions which consist of either Allowed Areas or Non-Allowed Areas. The Allowed Areas included in the Service Area Restrictions can be pre-configured and/or dynamically assigned by the AMF.
The Allowed Area may alternatively be configured as unlimited i.e. it may contain all Tracking Areas of the PLMN. The Registration Area of a UE in the Non-Allowed Area should consist of a set of TAs which belongs to a Non-Allowed Area of the UE. The Registration Area of a UE in the Allowed Area should consist of a set of TAs which belongs to an Allowed Area of the UE. The AMF provides the Service Area Restriction in the form of TA(s), which may be a subset of full list stored in UE's subscription data or provided by the PCF, to the UE during the Registration procedure.
NOTE:
As the finest granularity for Service Area Restrictions are at TA level, subscriptions with limited geographical extent, like subscriptions for Fixed Wireless Access, will be allocated one or a few TAs and will consequently be allowed to access services in a larger area than in e.g. a FWA system.
The limited allowed area may also be limited by the AMF by a maximum allowed number of Tracking Areas, even though this limitation is not sent to the UE. If maximum allowed number of Tracking Areas is used in combination with Allowed Area, the maximum allowed number of Tracking Areas indicates (to the AMF) the maximum number of TAs allowed in limited allowed area inside the Allowed Area. If maximum allowed number of Tracking Areas is used in combination with Non-Allowed Area, the maximum allowed number of Tracking Areas indicates (to the AMF) the maximum number of TAs allowed in limited allowed area outside of the Non-Allowed Area.
The UDM stores the Service Area Restrictions of a UE as part of the UE's subscription data. The PCF in the serving network may (e.g. due to varying conditions such as UE's location, application in use, time and date) further adjust Service Area Restrictions of a UE, either by expanding an Allowed Area or by reducing a Non-Allowed Area or by increasing the maximum allowed number of Tracking Areas. If NWDAF is deployed, the PCF may use analytics (i.e. statistics or predictions) on UE mobility from NWDAF (see TS 23.288) to adjust Service Area Restrictions. The UDM and the PCF may update the Service Area Restrictions of a UE at any time. For the UE in CM-CONNECTED state the AMF updates the UE and RAN immediately. For UE in CM-IDLE state the AMF may page the UE immediately or store the updated service area restriction and update the UE upon next signalling interaction with the UE, as defined in TS 24.501.
During registration, if the Service Area Restrictions of the UE is not present in the AMF, the AMF fetches from the UDM the Service Area Restrictions of the UE that may be further adjusted by the PCF. The serving AMF shall enforce the Service Area Restrictions of a UE. A limited allowed area given by a maximum allowed number of Tracking Areas, may be dynamically assigned by the AMF adding any not yet visited (by the UE) Tracking Areas to the limited allowed area until the maximum allowed number of Tracking Areas is reached (i.e. the AMF adds new TAs to the limited allowed area until the number of TAs is equal to the maximum allowed number of Tracking Areas). The AMF deletes the list of TAs that have been used up under the maximum allowed number of Tracking Areas quota at every Initial Registration.
For a UE in CM-CONNECTED state the AMF shall indicate the Service Area Restrictions of this UE to the RAN, using a Mobility Restriction List.
The UE shall store the received Service Area Restrictions and, if there is previously stored Service Area Restrictions, replace them with the newly received information. If the Service Area Restrictions include a limited allowed area, the Service Area Restrictions are applicable for the Tracking areas indicated in Service Area Restrictions. If the Service Area Restrictions included an unlimited allowed area, the received Service Area Restrictions are applicable for the registered PLMN and its equivalent PLMN(s) that are available in the Registration Area. The RAN uses the Service Area Restrictions for target cell selection in Xn and N2 based handover.
Upon change of serving AMF due to mobility, the old AMF may provide the new AMF with the Service Area Restrictions of the UE that may be further adjusted by the PCF.
The network may perform paging for a UE to update Service Area Restrictions with Generic UE Configuration Update procedure (see in TS 23.502, clause 4.2.4).
In the case of roaming, the Service Area Restrictions are transferred from the UDM via the serving AMF to the serving PCF in the visited network. The serving PCF in the visited network may further adjust the Service Area Restrictions.
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5.3.4.2  Mobility PatternWord-p. 85
The Mobility Pattern is a concept that may be used by the AMF to characterise and optimise the UE mobility. The AMF determines and updates Mobility Pattern of the UE based on subscription of the UE, statistics of the UE mobility, network local policy, and the UE assisted information, or any combination of them. The statistics of the UE mobility can be historical or expected UE moving trajectory. If NWDAF is deployed, the statistics of the UE mobility can also be analytics (i.e. statistics or predictions) provided by the NWDAF (see TS 23.288).
The Mobility Pattern can be used by the AMF to optimize mobility support provided to the UE, for example, Registration area allocation.
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5.3.4.3  Radio Resource Management functionsWord-p. 86
To support radio resource management in RAN the AMF provides the parameter 'Index to RAT/Frequency Selection Priority' (RFSP Index) to RAN across N2. The RFSP Index is mapped by the RAN to locally defined configuration in order to apply specific RRM strategies, taking into account any available information in RAN. The RFSP Index is UE specific and applies to all the Radio Bearers. Examples of how this parameter may be used by the RAN:
  • to derive UE specific cell reselection priorities to control idle mode camping.
  • to decide on redirecting active mode UEs to different frequency layers or RATs.
The HPLMN may set the RFSP Index taking into account the Subscribed S-NSSAIs. The AMF receives the subscribed RFSP Index from the UDM (e.g., during the Registration procedure). For non-roaming subscribers, the AMF chooses the RFSP Index in use according to one of the following procedures, depending on operator's configuration:
  • the RFSP Index in use is identical to the subscribed RFSP Index, or
  • the AMF chooses the RFSP Index in use based on the subscribed RFSP Index, the locally configured operator's policies, the Allowed NSSAI and the UE related context information available at the AMF, including UE's usage setting, if received during Registration procedures (see TS 23.502).
NOTE:
One example of how the AMF can use the "UE's usage setting," is to select an RFSP value that enforces idle mode camping on E-UTRA for a UE acting in a "Voice centric" way, in the case voice over NR is not supported in the specific Registration Area and it contains NR cells.
The AMF may report to the PCF the subscribed RFSP Index received from the UDM for further evaluation as described in clause 6.1.2.1 in TS 23.503. When receiving the authorized RFSP Index from the PCF, the AMF shall replace the subscribed RFSP Index with the authorized RFSP Index.
For roaming subscribers the AMF may choose the RFSP Index in use based on the visited network policy, but can take input from the HPLMN into account (e.g., an RFSP Index value pre-configured per HPLMN, or a single RFSP Index value to be used for all roamers independent of the HPLMN).
The RFSP Index in use is also forwarded from source to target RAN node when Xn or N2 is used for intra-NG-RAN handover.
The AMF stores the subscribed RFSP Index value received and the RFSP Index value in use. During the Registration procedure, the AMF may update the RFSP Index value in use (e.g. the AMF may need to update the RFSP Index value in use if the UE related context information in the AMF has changed). When the RFSP Index value in use is changed, the AMF immediately provides the updated RFSP Index value in use to NG-RAN node by modifying an existing UE context or by establishing a new UE context in RAN or by being configured to include the updated RFSP Index value in use in the NGAP DOWNLINK NAS TRANSPORT message if the user plane establishment is not needed. During inter-AMF mobility procedures, the source AMF forwards both RFSP Index values to the target AMF. The target AMF may replace the received RFSP Index value in use with a new RFSP Index value in use that is based on the operator's policies and the UE related context information available at the target AMF.
In order to enable UE idle mode mobility control and priority-based reselection mechanism considering availability of Network Slices at the network and the Network Slices allowed for a UE, an RFSP is derived as described in clause 5.3.4.3, considering also the Allowed NSSAI for the UE.
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5.3.4.4  UE mobility event notification
5G System supports the functionality of tracking and reporting UE mobility events.
The AMF provides the UE mobility related event reporting to NF that has been authorized to subscribe to the UE mobility event reporting service. Any NF service consumer such as SMF, NEF or NWDAF that wants to be reported on the UE location is able to subscribe to the UE mobility event notification service to the AMF with the following parameters:
  • Event reporting type that specifies what to be reported on UE mobility (e.g. UE location, UE mobility on Area of Interest).
  • Event filters indicating the:
    • Area Of Interest that specifies a geographical area within 3GPP system. The Area Of Interest is represented by a list of Tracking Areas, list of cells or list of (R)AN node identifiers. In the case of LADN, the event consumer (e.g. SMF) provides the LADN DNN to refer the LADN service area as the Area Of Interest. In the case of PRA, the event consumer (e.g. SMF or PCF) may provide an identifier for Area Of Interest to refer predefined area as the Area Of Interest.
    • S-NSSAI and optionally the NSI ID(s).
  • Event Reporting Information: event reporting mode, number of reports, maximum duration of reporting, event reporting condition (e.g. when the target UE moved into a specified Area Of Interest, immediate reporting flag).
  • Notification Endpoint of NF service consumer to be notified.
  • The target of event reporting that indicates a specific UE, a group of UE(s) or any UE (i.e. all UEs). Further details on the information provided by the NF service consumer are provided in clause 4.15 of TS 23.502.
If an NF service consumer subscribes to the UE mobility event notification service provided by AMF for reporting of UE presence in Area Of Interest, the AMF tracks UE's location considering UE's CM state and using NG-RAN procedures (if RRC Inactive state applies to NG-RAN) in order to determine the UE presence in the Area Of Interest, as described in clause 4.15.4.2 of TS 23.502. Upon detecting the change of the UE presence in the Area Of Interest, the AMF notifies the UE presence in the Area Of Interest and the new UE location to the subscribed NF consumer.
When the AMF is changed, the subscription of mobility event is transferred from the old AMF. The new AMF may decide not to notify the SMF with the current status related to the subscription of mobility event if the new AMF determines that, based on MM Context of the UE, the event is reported by the old AMF.
In the network deployment where a UE may leave or enter the Area Of Interest without any notification to the 5GC in CM-CONNECTED state (i.e. in the case that RRC Inactive state applies to the NG-RAN), the AMF may initiate the NG-RAN location reporting as described in clause 5.4.7 or N2 Notification as described in TS 23.502, clause 4.8.3 to track the UE presence in the Area Of Interest.
The AMF may provide UE mobility event reporting to PCF, using Policy Control Report Triggers defined in TS 23.503.
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