The idle mode tasks can be subdivided into four processes:

• PLMN selection;
• Cell selection and reselection;
• Location registration;
• Support for manual CSG selection.

The relationship between these processes is illustrated in Figure 4.1-1.

When a UE is switched on, a public land mobile network (PLMN) is selected by NAS. For the selected PLMN, associated RAT(s) may be set TS 23.122. The NAS shall provide a list of equivalent PLMNs, if available, that the AS shall use for cell selection and cell reselection. With the cell selection, the UE searches for a suitable cell of the selected PLMN and chooses that cell to provide available services, further the UE shall tune to its control channel. This choosing is known as "camping on the cell". For E-UTRA a cell may be associated with more than one CN type (EPC and/or 5GC) and hence the selected cell can be suitable for more than one CN type. The CN type(s) for which the selected cell is suitable are reported to NAS which selects a CN type to be used for camping and for the NAS registration procedure (see below). Note that CN type selection is only applicabe for UE supporting E-UTRA connected to 5GC. The UE shall, if necessary, then register its presence, by means of a NAS registration procedure, in the tracking area of the chosen cell and as outcome of a successful Location Registration the selected PLMN becomes the registered PLMN TS 23.122. If the UE finds a more suitable cell, according to the cell reselection criteria, it reselects onto that cell and camps on it. Similar to cell selection procedure, if the reselected cell is an E-UTRA cell and the UE supports E-UTRA connected to 5GC, the CN type(s) for which the cell is suitable are reported to NAS which selects one of them. If the new cell does not belong to at least one tracking area to which the UE is registered, location registration is performed. In RRC_INACTIVE state, if the new cell does not belong to the configured RNA, a RNA update procedure is performed. If necessary, the UE shall search for higher priority PLMNs at regular time intervals as described in TS 22.011 and search for a suitable cell if another PLMN has been selected by NAS. Search of available CSGs may be triggered by NAS to support manual CSG selection. If the UE loses coverage of the registered PLMN, either a new PLMN is selected automatically (automatic mode), or an indication of which PLMNs are available is given to the user, so that a manual selection can be made (manual mode). Registration is not performed by UEs only capable of services that need no registration. The UE may perform sidelink communication or V2X sidelink communication or sidelink discovery or NR sidelink communication while in-coverage or out-of-coverage for sidelink, as specified in clause 11. The purpose of camping on a cell in idle mode is fivefold:

1. It enables the UE to receive system information from the PLMN.
2. When registered and if the UE wishes to establish an RRC connection, it can do this by initially accessing the network on the control channel of the cell on which it is camped.
3. If the PLMN receives a call for the registered UE, it knows (in most cases) the set of tracking areas (in RRC_IDLE state) or RNAs (in RRC_INACTIVE state) in which the UE is camped. It can then send a "paging" message for the UE on the control channels of all the cells in this set of tracking areas. The UE will then receive the paging message because it is tuned to the control channel of a cell in one of the registered tracking areas and the UE can respond on that control channel.
4. It enables the UE to receive ETWS and CMAS notifications.
5. It enables the UE to receive MBMS services.

If the UE is unable to find a suitable cell to camp on or if the location registration failed (except for LR rejected with cause #12, cause #14, cause #15 or cause #25, see TS 23.122 and TS 24.301), it attempts to camp on a cell irrespective of the PLMN identity, and enters a "limited service" state. When NAS indicates that PSM starts, the AS configuration (e.g. priorities provided by dedicated signalling and logged measurements) is kept, all running timers continue to run but the UE need not perform any idle mode tasks. If a timer expires while the UE is in PSM it is up to UE implementation whether it performs the corresponding action immediately or the latest when PSM ends. When NAS indicates that PSM ends, the UE shall perform all idle mode tasks.

Table 4.2-1 presents the functional division between UE non-access stratum (NAS) and UE access stratum (AS) in idle mode. The NAS part is specified in TS 23.122 and the AS part in the present document.

This clause defines the level of service that may be provided by the network to a UE in Idle mode. The action of camping on a cell is necessary to get access to some services. Three levels of services are defined for UE:

• Limited service (emergency calls, ETWS and CMAS on an acceptable cell). It is not applicable to RRC_INACTIVE state.
• Normal service (for public use on a suitable cell)
• Operator service (for operators only on a reserved cell)

Furthermore, the cells are categorised according to which services they offer:   acceptable cell: An "acceptable cell" is a cell on which the UE may camp to obtain limited service (originate emergency calls and receive ETWS and CMAS notifications), and it is not applicable to RRC_INACTIVE state. Such a cell shall fulfil the following requirements, which is the minimum set of requirements to initiate an emergency call and to receive ETWS and CMAS notification in a E-UTRAN network:

• The cell is not barred, see clause 5.3.1;
• The cell selection criteria are fulfilled, see clause 5.2.3.2;

  suitable cell: A "suitable cell" is a cell on which the UE may camp on to obtain normal service. The UE shall have a valid USIM and such a cell shall fulfil all the following requirements.

• The cell is part of either:
  • the selected PLMN, or:
  • the registered PLMN, or:
• a PLMN of the Equivalent PLMN list
• For a CSG cell, the cell is a CSG member cell for the UE;

According to the latest information provided by NAS:

• The cell is not barred, see clause 5.3.1;
• The cell is part of at least one TA that is not part of the list of "forbidden tracking areas for roaming" TS 22.011, which belongs to a PLMN that fulfils the first bullet above;
• The cell selection criteria are fulfilled, see clause 5.2.3.2;
• Except for NB-IoT, if the UE supports authorization of coverage enhancements and upper layers indicated that use of coverage enhancements is not authorized for the selected PLMN:
  • the cell selection criterion S in normal coverage shall be fulfilled;
• If the UE supports CE mode B and upper layers indicated that CE mode B is restricted:
  • the cell selection criterion S in normal coverage based on values Q_rxlevmin and Q_qualmin or in enhanced coverage based on values Q_{rxlevmin_CE} and Q_{qualmin_CE} shall be fulfilled.

If more than one PLMN identity is broadcast in the cell, the cell is considered to be part of all TAs with TAIs constructed from the PLMN identities and the TAC broadcast in the cell.   barred cell: A cell is barred if it is so indicated in the system information TS 36.331.   reserved cell: A cell is reserved if it is so indicated in system information TS 36.331. Following exceptions to these definitions are applicable for UEs:

• camped on a cell that belongs to a tracking area that is forbidden for regional provision of service; a cell that belongs to a tracking area that is forbidden for regional provision service (TS 23.122, TS 24.301) is suitable but provides only limited service.
• as an outcome of the manual CSG selection procedure the UE is allowed to access an acceptable cell which fulfils the cell selection criteria and is not barred or reserved for operator use for UEs not belonging to AC 11 or 15 and inform NAS that access is possible (for location registration procedure).
• if a UE has an ongoing emergency call, all acceptable cells of that PLMN are treated as suitable for the duration of the emergency call.
• if the UE in RRC_IDLE fulfils the conditions to support sidelink communication or PS related sidelink discovery in limited service state as specified in TS 23.303, clause 4.5.6, the UE may perform sidelink communication or PS-related sidelink discovery.
• if the UE in RRC_IDLE fulfils the conditions to support V2X sidelink communication or NR sidelink communication in limited service state as specified in TS 23.285, clause 4.4.8 and TS 23.287, clause, 5.7, the UE may perform V2X sidelink communication or NR sidelink communication.

For E-UTRA the cell categorization defined above is per CN type. In this specification, when the term suitable/acceptable cell is used without specifying the CN type, it means the cell is suitable/acceptable for any of the CN type(s) supported by the UE.

This specification is applicable to NB-IoT, except for the following functionality which is not applicable to NB-IoT:

• Acceptable cell
• Accessibility measurements
• Access Control based on ACDC categories
• Camped on Any cell state
• CSG, including support for manual CSG selection and CSG or Hybrid cell related functionality in PLMN selection, or HNB name (SIB9), Cell selection and Cell reselection.
• Emergency call
• E-UTRAN Inter-frequency Redistribution procedure
• Inter-RAT Cell Selection and Reselection including measurements in other RATs
• Logged measurements
• Mobility History Information
• Mobility states of a UE
• Priority based reselection
• Public warning system including CMAS, ETWS, PWS.
• RAN-assisted WLAN interworking
• RRC_INACTIVE state
• Sidelink operation