When an MS is switched on, it attempts to make contact with a public land mobile network (PLMN) or stand-alone non-public network (SNPN). The particular PLMN or SNPN to be contacted may be selected either automatically or manually. The MS looks for a suitable cell of the chosen PLMN or SNPN and chooses that cell to provide available services, and tunes to its control channel. This choosing is known as "camping on the cell". The MS will then register its presence in the registration area of the chosen cell if necessary, by means of a location registration (LR), GPRS attach, IMSI attach or registration procedure. If the MS loses coverage of a cell, or find a more suitable cell, it reselects onto the most suitable cell of the selected PLMN or SNPN and camps on that cell. If the new cell is in a different registration area, an LR request is performed. If the MS loses coverage of a PLMN or SNPN, either a new PLMN or SNPN is selected automatically, or an indication of which PLMNs or SNPNs are available is given to the user, so that a manual selection can be made. Registration is not performed by MSs only capable of services that need no registration. The purpose of camping on a cell in idle mode is fourfold:

1. It enables the MS to receive system information from the PLMN or SNPN.
2. If the MS wishes to initiate a call, 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 or SNPN receives a call for the MS, it knows (in most cases) the registration area of the cell in which the MS is camped. It can then send a "paging" message for the MS on control channels of all the cells in the registration area. The MS will then receive the paging message because it is tuned to the control channel of a cell in that registration area, and the MS can respond on that control channel.
4. It enables the MS to receive cell broadcast messages.

If the MS is unable to find a suitable cell to camp on, or the SIM is not inserted, or there is no valid entry in "list of subscriber data" in case the MS is operating in SNPN access operation mode over 3GPP access, or if it receives certain responses to an LR request (e.g., "illegal MS"), it attempts to camp on a cell irrespective of the PLMN identity or the SNPN identity, and enters a "limited service" state in which it can only attempt to make emergency calls or to access RLOS. An MS operating in NB-S1 mode, never attempts to make emergency calls or to access RLOS. An MS operating in N1 mode never attempts to access RLOS. If the MS is in eCall only mode, it attempts to camp on a suitable cell and enters an "eCall inactive" state in which it can only attempt an eCall over IMS, or a call to a non-emergency MSISDN or URI for test or terminal reconfiguration services as specified in TS 31.102. If the MS is in eCall only mode and is unable to find a suitable cell to camp on, it attempts to camp on an acceptable cell in limited service state, and enters an "eCall inactive" state in which it can only attempt an eCall over IMS. While in eCall inactive state, the MS does not perform LR with the PLMN of the cell on which the MS is camped. In A/Gb mode, if the CTS MS is in CTS mode only or in automatic mode with CTS preferred, it will start by attempting to find a CTS fixed part on which it is enrolled. The idle mode tasks can be subdivided into the following processes:

• PLMN selection;
• SNPN selection (N1 mode only);
• CSG selection (Iu mode and S1 mode only);
• Cell selection and reselection;
• Location registration;
• CTS fixed part selection (A/Gb mode only); and
• CAG selection (N1 mode only).

In A/Gb mode, to make this initial CTS fixed part selection, the MS shall be enrolled on at least one fixed part. Except for SNPN selection, the relationship between these processes is illustrated in Figure 1 in clause 5. The states and state transitions within each process are shown in Figure 2a, Figure 2b, and Figure 3 in clause 5. In the present document, EMM-IDLE mode with suspend indication (see TS 24.301) and 5GMM-IDLE mode with suspend indication (see TS 24.501) shall be considered the same as idle mode.