Interaction between the AS and the NAS is maintained as long as a UE is powered on. As previously described, a UE's communication with the RAN is performed over the AS, while communication with a network's core is performed through the NAS.
When powered on, a UE first searches for a Public Land Mobile Network (PLMN). If successful, the UE then attempts to camp on a cell in the PLMN. This is performed through a cell search, which depends on frequency and timing synchronization between the UE and the selected eNB. The standard differentiates between camping on a cell and camping on any cell, where in the latter the UE camps regardless of the PLMN identity. Once camped, a UE is ready to initiate connections, or to maintain connectivity while mobile. To be connected, the UE goes through a connection setup that relies on RRC for setup, (re)configuration and security. Once a connection terminates, it is released.
Note that a UE can camp on either an acceptable or a suitable cell. An acceptable cell is one that allows the UE to initiate emergency calls, and to receive ETWS or CMAS can be received. A suitable cell, on the other hand, is one where the UE can achieve full connectivity, depending on its capabilities. In case of a CSG HeNB, the cell would be on the UE's white list, that is, among the cells accessible by the UE.
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