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Dec 8, 2012

EPS Mobility Management (EMM) and Connection Management (ECM) states

The EPS Mobility Management (EMM) states describe the Mobility Management states that result from the mobility management procedures e.g. Attach and Tracking Area Update procedures.
There are two EMM states described in Specs:
  • EMM-DEREGISTERED
  • EMM-REGISTERED

The EPS Connection Management (ECM) states describe the signalling connectivity between the UE and the EPC.
There are two ECM states described in Specs:
  • ECM-IDLE
  • ECM-CONNECTED

In general, the ECM and EMM states are independent of each other. Transition from EMM-REGISTERED to EMM-DEREGISTERED can occur regardless of the ECM state, e.g. by explicit detach signalling in ECM-CONNECTED or by implicit detach locally in the MME during ECM-IDLE. However there are some relations, e.g. to transition from EMM-DEREGISTERED to EMM-REGISTERED the UE has to be in the ECM-CONNECTED state.


Definition of main EPS Mobility Management states

EMM-DEREGISTERED

In the EMM DEREGISTERED state, the EMM context in MME holds no valid location or routing information for the UE. The UE is not reachable by a MME, as the UE location is not known.
In the EMM-DEREGISTERED state, some UE context can still be stored in the UE and MME, e.g. to avoid running an AKA procedure during every Attach procedure.
During the successful Inter-RAT TAU/RAU/handover procedure and ISR activated is not indicated to the UE, the old S4 SGSN/old MME changes the EMM state of the UE to GPRS-IDLE/PMM-DETACHED/EMM-DEREGISTERED.

EMM-REGISTERED

The UE enters the EMM-REGISTERED state by a successful registration with an Attach procedure to either E-UTRAN or GERAN/UTRAN. The MME enters the EMM-REGISTERED state by a successful Tracking Area Update procedure for a UE selecting an E-UTRAN cell from GERAN/UTRAN or by an Attach procedure via E-UTRAN. In the EMM-REGISTERED state, the UE can receive services that require registration in the EPS.
The UE location is known in the MME to at least an accuracy of the tracking area list allocated to that UE (excluding some abnormal cases).
In the EMM-REGISTERED state, the UE shall:
  • always have at least one active PDN connection;
  • setup the EPS security context.
After performing the Detach procedure, the state is changed to EMM-DEREGISTERED in the UE and in the MME. Upon receiving the TAU Reject and Attach Reject messages the actions of the UE and MME depend upon the 'cause value' in the reject message, but, in many cases the state is changed to EMM-DEREGISTERED in the UE and in the MME.
If all the bearers belonging to a UE are released (e.g., after handover from E UTRAN to non-3GPP access), the MME shall change the MM state of the UE to EMM-DEREGISTERED. If the UE camps on E-UTRAN and the UE detects that all of its bearers are released, the UE shall change the MM state to EMM-DEREGISTERED. If all the bearers (PDP contexts) belonging to a UE are released, while the UE camps on GERAN/UTRAN, the UE shall deactivate ISR (Idle mode Signalling Reduction) by setting its TIN (Temporary Identity used in Next update) to "P-TMSI". This ensures that the UE performs Tracking Area Update when it re-selects E-UTRAN. If the UE switches off its E UTRAN interface when performing handover to non-3GPP access, the UE shall automatically change its MM state to EMM-DEREGISTERED.
The MME may perform an implicit detach any time after the Implicit Detach timer expires. The state is changed to EMM-DEREGISTERED in the MME after performing the implicit detach.

Definition of EPS Connection Management states

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:
  • 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;
  • perform the periodic tracking area updating procedure to notify the EPC that the UE is available;
  • perform a tracking area update if the RRC (Radio Resource Control) connection was released with release cause "load balancing TAU required";
  • perform a tracking area update when the UE reselects an E-UTRAN cell and the UE's TIN indicates "P-TMSI";
  • perform a tracking area update for a change of the UE's Core Network Capability information or the UE specific DRX parameter;
  • 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;
  • answer to paging from the MME by performing a service request procedure;
  • 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.

State transition and functions

 
Fig. 1. EMM state model in UE
Fig. 1. EMM state model in UE


Fig. 2. EMM state model in MME
Fig. 2. EMM state model in MME


Fig. 3. ECM state model in UE
Fig. 3. ECM state model in UE


Fig. 4. ECM state model in MME
Fig. 4. ECM state model in MME





Source(s):
3GPP TS 23.401