In EUTRAN, a UE transitions between RRC_IDLE and RRC_CONNECTED. The UE's state model in view of the NAS is two dimensional. The first dimension defines the EPS Mobility Management (EMM) states that dictate whether a UE is attached or detached, and the tracking area to which the UE belongs. The second dimension details the EPS Connection Management (ECM) states that essentially describe the connectivity between the UE and the network's core.
The two relevant EMM states are EMM-DERGSTERED and EMM-REGISTERED. The two relevant ECM states are ECM-IDLE and ECM-CONNECTED. While the EMM and ECM states are independent, for example, a UE can become (EMM)-deregistered regardless of the ECM state, there are certain relations between the two, and between both NAS states and the AS states.
An EMM-DERIGSTERED UE is one with valid location or routing information and is hence unreachable by the MME. An EMM-REGISTERED UE, on the other hand, is one that has been attached to either a EUTRAN or GERAN/UTRAN network. In such a state, the UE's location in the MME is known to at least an accuracy of the tracking area. A UE returns to the DERIGESTERED state upon a detach procedure or upon being handed over to a non-3GPP network.
An ECM-IDLE state indicates that no connection for NAS signaling has been setup between the UE and the core. An UE in ECM-IDLE needs to perform PLMN and cell selection and reselection in order to become ECM-CONNECTED. Once ECM-CONNECTED, a UE location ...