Following two states are connection management states in EPC.
ECM-IDLE
A UE is in ECM-IDLE state when no NAS signalling connection between UE and network exists. In ECM-IDLE state, a UE performs cell selection/reselection according to TS 36.304 and PLMN selection according to TS 23.122.
There exists no UE context in E-UTRAN for the UE in the ECM-IDLE state. There is no S1_MME and no S1_U connection for the UE in the ECM-IDLE state.
In the EMM-REGISTERED and ECM-IDLE state, the UE shall:
1. perform a tracking area update if the current TA is not in the list of TAs that the UE has received from the network in order to maintain the registration and enable the MME to page the UE;
2. perform the periodic tracking area updating procedure to notify the EPC that the UE is available;
3. perform a tracking area update if the RRC (Radio Resource Control) connection was released with release cause "load balancing TAU required";
4. perform a tracking area update when the UE reselects an E-UTRAN cell and the UE's TIN indicates "P-TMSI";
5. perform a tracking area update for a change of the UE's Core Network Capability information or the UE specific DRX parameter;
6. perform a tracking area update when the UE manually selects a CSG cell, and the CSG ID of that cell is absent from both the UE's Allowed CSG list and the UE's Operator CSG list;
7. answer to paging from the MME by performing a service request procedure;
8. perform the service request procedure in order to establish the radio bearers when uplink user data is to be sent.
The UE and the MME shall enter the ECM-CONNECTED state when the signalling connection is established between the UE and the MME. Initial NAS messages that initiate a transition from ECM-IDLE to ECM-CONNECTED state are Attach Request, Tracking Area Update Request, Service Request or Detach Request.
When the UE is in ECM IDLE state, the UE and the network may be unsynchronized, i.e. the UE and the network may have different sets of established EPS bearers. When the UE and the MME enter the ECM CONNECTED state, the set of EPS Bearers is synchronized between the UE and network.
ECM-CONNECTED
The UE location is known in the MME with an accuracy of a serving eNodeB ID. The mobility of UE is handled by the handover procedure.
The UE performs the tracking area update procedure when the TAI in the EMM system information is not in the list of TA's that the UE registered with the network, or when the UE handovers to an E UTRAN cell and the UE's TIN indicates "P-TMSI".
For a UE in the ECM-CONNECTED state, there exists a signalling connection between the UE and the MME. The signalling connection is made up of two parts: an RRC connection and an S1_MME connection.
The UE shall enter the ECM-IDLE state when its signalling connection to the MME has been released or broken. This release or failure is explicitly indicated by the eNodeB to the UE or detected by the UE.
The S1 release procedure changes the state at both UE and MME from ECM-CONNECTED to ECM-IDLE.
NOTE 1: The UE may not receive the indication for the S1 release, e.g. due to radio link error or out of coverage. In this case, there can be temporal mismatch between the ECM-state in the UE and the ECM-state in the MME.
After a signalling procedure, the MME may decide to release the signalling connection to the UE, after which the state at both the UE and the MME is changed to ECM-IDLE.
NOTE 2: There are some abnormal cases where the UE transitions to ECM-IDLE.
When a UE changes to ECM CONNECTED state and if a radio bearer cannot be established, or the UE cannot maintain a bearer in the ECM-CONNECTED state during handovers, the corresponding EPS bearer is deactivated.
Source: http://www.lteandbeyond.com/2012/12/EMM-ECM-eps-mobility-management-and-connection-management.html
