Overview of a UE's State Transitions

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.

Resources for this chapter can be sought as follows.

• Overall description of RRC services, states and transitions can be found in 36.300.
• Detailed descriptions for IDLE processes are described in 36.304 (AS) and 23.122 (NAS).
• PHY layer aspects of cell search are described in 36.213.
• Descriptions for RRC processes in both IDLE and CONNECTED states are described in 36.331.