Understanding LTE EPS Connection Management States

This page describes LTE EPS Connection Management States.

EPS connection management (ECM) states are maintained by NAS layers within the UE and MME. These states determine the signalling connections between the UE and the EPC.

There are two LTE ECM states: ECM IDLE and ECM CONNECTED.

An LTE UE in ECM IDLE state does not have a NAS signalling connection to the MME entity. The UE completes cell selection and cell reselection when it is in ECM IDLE state.

The location of the UE in the ECM IDLE state is known to the MME to the accuracy of the Tracking Area List allocated.

The following procedures are executed when the LTE UE is in EMM-registered and ECM-IDLE states:

• TAU triggered by mobility and periodic TAU (Tracking Area Update) timer T3412
• TAU triggered for MME load balancing (meaning relocating UE’s signalling connection to a different MME).
• TAU triggered for the UE to signal a change of its core network capability or its UE specific DRX cycle.
• Responding to paging messages by performing a service request procedure
• Performing the service request procedure when the uplink user plane data is to be sent

The UE and MME enter ECM-CONNECTED state after a NAS signalling connection has been established.

From the UE perspective, this corresponds to establishing an RRC connection between the UE and eNodeB.

From the MME perspective, this corresponds to establishing an S1 connection between the eNodeB and MME.

Initial NAS messages which are used to initiate a transition from ECM-IDLE to ECM-CONNECTED are as follows. All these procedures require the UE to be in RRC connected mode.

• ATTACH REQUEST
• Tracking Area Update REQUEST
• Service Request
• Detach request

The location of the UE in ECM-CONNECTED state is known by the MME to the accuracy of the serving eNodeB. LTE UE mobility is handled using handovers rather than cell re-selection when UEs are in ECM-CONNECTED states.

UE continue to complete TAU due to mobility when in ECM CONNECTED state, i.e. when UE moves into a cell which does not belong to a Tracking Area within the UE’s list of registered tracking areas.

LTE UE make the transition from ECM-CONNECTED to ECM IDLE when their signalling connection is released or when their signalling connection fails.

REFERENCES: 3GPP TS 23.401 , 3GPP TS 24.301