When UE is in Idle mode It has to do certain tasks with the Network, It can be divided in to four Parts:
- PLMN selection
- Cell selection and re-selection
- Location registration
- Support for manual CSG ID selection
All Processes are tried to be mentioned in below diagram and explained further:
When UE is switched on, a PLMN is selected by NAS, For the selected PLMN, associated RAT(s) may be set. The NAS shall provide a list of equivalent PLMNs, if available, that the AS shall use for cell selection and cell re-selection.
Now if you have selected the cell, UE starts a search 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 whole process is known as "camping on the cell".
The UE shall register its presence if it is necessary by NAS registration procedure in tracking area of the chosen cell and as outcome of a successful Location Registration the selected PLMN becomes the registered PLMN.
By doing the cell re-selection if UE finds a more suitable cell then it will camp on to it. If the new cell is from other tracking area where UE did not register then UE has to perform Location registration procedure.
UE can also search for higher priority PLMNs as per his mentioned priority lists at regular intervals of time. Search of available CSG IDs may be triggered by NAS to support manual CSG ID selection.
As we know there are two modes of network selection in our mobile phones. In Automatic Mode if the UE looses coverage of the registered PLMN it will start searching for the other PLMN where as in Manual mode we have to do it manually and have to start searching for another PLMN.
Below is the reasons for camping on a cell:
a) It enables the UE to receive system information from the PLMN.
b) 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.
c) If the PLMN receives a call for the registered UE, it knows (in most cases) the set of tracking areas 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.
d) It enables the UE to receive ETWS and CMAS notifications.
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). It attempts to camp on a cell irrespective of the PLMN identity, and enters a "limited service" state.