This TS gives an overview of the tasks undertaken by the Core network protocols of a Mobile Station
(MS) when in idle mode, that is, switched on but typically not having a dedicated channel allocated. It also describes the
corresponding network functions. The idle mode functions are also performed by a GPRS MS as long as no dedicated
channel is allocated to the MS.
When an MS is switched on, it attempts to make contact with a public land mobile network (PLMN). The particular
PLMN to be contacted may be selected either automatically or manually.
The MS looks for a suitable cell of the chosen PLMN 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
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 and camps on that cell. If the new cell is in a different registration area, an LR request is performed.