The aim of the idle mode processes is to achieve the following conditions ("normal service state"):

1. The selected PLMN is one of the equivalent PLMNs;
2. The MS is camped on a suitable cell of the registration area on which the last successful LR was performed, and that cell has the highest value of C2 for any unbarred cell in that registration area;
3. The value of C2 for the MS is not lower than the value of C2 for any suitable cell in another registration area of an equivalent PLMN by more than the hysteresis value CRH.

Instead of the parameter C2, a GPRS MS applies the corresponding GPRS parameter if provided. An MS supporting SoLSA with SoLSA subscription shall use the SoLSA cell re-selection parameters. Registration is not performed by MSs only capable of services which need no registration. There may be temporary conditions under which not all these are fully satisfied, e.g., during cell selection on a PLMN, while in the process of reselecting another cell or, in case EC operation is enabled, when none of the criteria triggering neighbour cell measurements are fulfilled (see TS 45.008). However, if the above cannot be satisfied for a certain period of time, the MS will normally enter a "limited service state" in which it will try to camp on an acceptable cell. In this state, only emergency calls and warning notifications are possible.

Each of the processes of PLMN selection, cell selection and location registration can be described by a set of states. The overall state of the mobile is thus a composite of the states of the three processes. In some cases, an event which causes a change of state in one process may trigger a change of state in another process, e.g., camping on a cell in a new registration area triggers an LR request. The relationship between the processes is illustrated in figure 1 in TS 23.122. In the event of any conflict between the diagrams and the text in this ETS, the text takes precedence.

Whenever a PLMN is selected, the MS attempts to find a suitable cell of that PLMN to camp on. Two methods of searching for a suitable cell are possible:

1. Normal cell selection ("Normal cell selection" state) Here the MS has no prior knowledge of which RF channels are BCCH or CPBCCH carriers. It searches at least the number, given in subclause 3.2.1, of the strongest RF channels in descending order of received signal level to see which are BCCH or CPBCCH carriers. (If no BCCH or CPBCCH carriers have yet been found, searching will continue until at least one BCCH or CPBCCH carrier is found.). The first BCCH or CPBCCH carrier found which is from a suitable cell and on which there is a normal priority indication is taken and that cell is camped on. If at least the number, given in subclause 3.2.1, of the strongest RF channels have been tried and the only suitable cells found have low priority indication the MS shall camp on the strongest of these cells.
2. Stored list cell selection (optional) ("Stored list cell selection" state) Here the MS has a list of the BCCH and CPBCCH carriers used by the PLMN. (This list may be derived by the MS from information gathered during previous selections of the PLMN). The BCCH and CPBCCH carriers in the list are searched in descending order of received signal level, and the first BCCH or CPBCCH carrier found which is from a suitable cell and on which there is a normal priority indication is taken, and that cell is camped on. If an MS decodes system information from a cell of the selected PLMN but is unable to camp on that cell, the BA of that cell shall be examined and any BCCH and CPBCCH carriers in the BA which are not in the list of BCCH or CPBCCHcarriers to be searched shall be added to the list. If all these BCCH and CPBCCH carriers have been tried and the only suitable cells found have low priority indication, the MS shall camp on the strongest of these cells. When required to search for CPBCCH, the stored list cell selection is mandatory for CPBCCH carrier from the HPLMN. A MS supporting EC-GSM-IoT may store a separate list of BCCH carriers of cells supporting EC-GSM-IoT.

If no suitable cell is found using method b), method a) is then tried. An MS with GSM voice capability shall not search for CPBCCH carriers. An MS supporting GSM circuit-switched data without supporting GSM voice may search for CPBCCH carriers. While camped on a cell of the registered PLMN ("camped normally"), the MS may need to select a different cell ("normal cell reselection" state). The following events trigger a cell reselection: In addition, for PEO and EC operation the following events also trigger a cell reselection: A GPRS MS in Ready state applies the READY_STATE CELL RESELECTION HYSTERESIS together with the path loss criterion when reselecting the cell within the registration area. The GPRS MS in Ready state shall inform the network about cell reselection within the registration area by the cell update procedure. The network may control cell reselection of a GPRS MS in Ready state (see TS 43.064). Instead of the parameter C2, a GPRS MS applies the corresponding GPRS parameter if provided. If this GPRS parameter is provided the cell shall also broadcast a list BA(GPRS) indicating BCCH or CPBCCH carriers to be monitored by GPRS MSs for cell re-selection purpose. A MS that supports EC-GSM-IoT applies EC-specific C1 and C2 parameters. An MS supporting SoLSA with SoLSA subscription shall use the SoLSA cell re-selection parameters. The MS will then reselect a new cell in order to fulfil the process goal (see subclause 4.1). Before camping on the cell after re selection, the MS shall attempt to decode the full set of system information. The MS shall check that the parameters affecting cell re selection are unchanged. If a change is detected the MS shall check if the cell re selection criterion is still valid using the changed parameters. If the cell selection criteria are still valid, the MS shall camp on the cell. If they are not still valid, the MS shall repeat this process for the cell with the next highest value of C2 or corresponding GPRS parameter or SoLSA parameters, respectively. Once the MS has re tuned to the chosen cell, it shall monitor its paging subgroup (if known) for that cell. If the MS and the cell support group or broadcast calls, it shall also monitor the notification channel. A GPRS MS shall monitor the relevant GPRS control channel(s). If the MS receives a page before having decoded the relevant system information for the new cell, the MS shall store the page and respond, if permitted, once the relevant system information has been decoded. If not permitted, no page response shall be made. When the MS leaves "connected mode" (e.g., after a call), and returns to idle mode, a cell must be chosen ("choose cell" state) as soon as possible (e.g., to allow a new call to be made). The chosen cell will not necessarily be the same cell as the MS was camped on when the call started e.g., if the MS moved a significant distance while the call was in progress. Two cases are distinguished in TS 45.008, for which the algorithm is slightly different:

1. Normal case The call is terminated, either because the user (or network) has terminated the call, or because a radio link failure has occurred and call re establishment is not being attempted.
2. Call re establishment A radio link failure has occurred and a call re establishment is being attempted. (The MS returns to idle mode to start the call re establishment attempt and needs to select a new cell on which to make the attempt as soon as possible in order to minimize the interruption to the call).

If no suitable cell is found by either the stored list cell selection, normal cell reselection or choose cell tasks (all of which use a limited search), the normal cell selection task (which searches all RF channels for a suitable cell) is entered. If no suitable cell is found by the normal cell selection task, or if there is no valid SIM in the MS, the MS attempts to find an "acceptable cell" on which it may camp so that emergency calls may be made and warning notifications may be received ("any cell selection" state). The MS ignores the PLMN information when selecting an acceptable cell, and no attempt at location registration is made. This task is also entered if an LR reject is received with one of the following cause values:

• "IMSI unknown in HLR" (only for non-GPRS MS as this impacts only the non-GPRS update state, MSs capable of GPRS and non-GPRS services take a cell selection state according to the outcome of the routing area update);
• "Illegal MS";
• "illegal ME";
• "PLMN not allowed" (However, this will trigger a new PLMN selection if the MS is in automatic mode);
• "GPRS not allowed" (only for MS capable of GPRS only, where this will trigger a new PLMN selection if the MS is in automatic mode; MSs capable of GPRS and non-GPRS services take a cell selection state according to the outcome of location updating as "GPRS not allowed" has only impact on the GPRS update status).

If an acceptable cell is found, the MS camps on it ("Camped on any cell" state). If one of the cell reselection trigger events (i) to (v) above occurs, the MS attempts to find a new acceptable cell to camp on ("any cell reselection" state) but using a zero dB value of CRH. If no acceptable cell can be found, the "any cell selection" task is re entered. If while camped on an acceptable cell:

1. an emergency call origination is made, the MS enters a connected mode and at the end of the call, chooses an acceptable cell ("choose any cell" state) so that it is ready for the next emergency call origination;
2. a warning notification is received, the MS continues to perform cell reselection to an acceptable cell.

In all cases, if a new PLMN is selected and there is a valid SIM in the MS, the MS searches for a suitable cell of that PLMN. However, if the MS has recently searched the strongest RF channels while a previous PLMN was selected, it may already have information about other PLMNs. The user may request a search of RF channels to determine which PLMNs are available. This search shall be done in such a way as to minimize interruptions to the MS's monitoring of its paging subchannel.

See TS 23.122.

See TS 23.122.

The BCCH allocation is the list of BCCH or CPBCCH carriers in use by a given PLMN (or co-operating PLMNs) in a given geographical area. (It indicates the RF channels that the MS is required to monitor while camped on a cell of that PLMN.) It is broadcast in the system information messages, and is referred to as the BA(BCCH). When the MS is first switched on in a PLMN, and has no knowledge of which RF channels are BCCH or CPBCCH carriers of that PLMN, it has to search the strongest RF channels in order to find which are BCCH or CPBCCH carriers. It then has to determine which BCCH or CPBCCH carriers belong to the selected PLMN. Once it has found a BCCH or CPBCCH carrier of the selected PLMN, it can read the BA(BCCH). It then knows which RF channels to monitor for cell selection and reselection purposes. Cells supporting GPRS may broadcast a list BA(GPRS) as system information. It indicates the BCCH or CPBCCH carriers to be monitored by a GPRS MS for cell re-selection. Whilst switched on in the PLMN, the MS can gain knowledge of the preferable carriers to use in subsequent cell selections in that PLMN. This knowledge may come from previously decoded BA(BCCH)s and from transmitted information from the network. When the MS switches off, it has the option of storing the last known BA(BCCH) in a non volatile store so that on a subsequent switch on in that PLMN, it does not need to search to find the BA(BCCH), and so can camp on a cell more quickly. If the BA(BCCH) is stored in the (U)SIM, it shall be stored in the format specified either in TS 51.011 (if the mobile station supports the SIM) or TS 31.102 (if the mobile station supports the USIM). Any other information used by the MS in cell selection shall be stored in a non volatile store on the ME. The BA(BCCH) may or may not be the total list of BCCH or CPBCCH carriers in use throughout the PLMN coverage area. For example there may be differences in different geographical areas, and there may be "umbrella cells" which are only used for hand over traffic and hence are not to be camped on. A stored BA(BCCH) is not necessarily valid when the MS is switched on again e.g., if the system operator has made a change in BA(BCCH) or if the MS is switched on in a different geographical area. In these cases, the MS may not find a suitable cell for camping on using the stored BA(BCCH). However, the MS may use other stored information to find a suitable cell before searching all the strongest RF channels. It should also be noted that the BA(BCCH) only applies to the PLMN on which it was received. If a new PLMN is selected, only a BA(BCCH) previously obtained from that PLMN should be used. There is another BCCH or CPBCCH carrier list, called BA(SACCH), which is sent on the SACCH when in connected mode. This contains the list of BCCH or CPBCCH carriers to be monitored by the MS for hand over purposes. The BA(BCCH) and BA(SACCH) need not necessarily be the same. For example, the BA(SACCH) might contain umbrella cells, or the BCCH or CPBCCH carrier of the serving cell might be omitted. If the MS stores a BA list in the SIM, and there is a valid stored LAI, the BA list must be of the PLMN indicated by the stored LAI.

See TS 23.122.

See TS 23.122.