LTE EPS Mobility Management States
This page describes LTE EPS Mobility Management States. EPS mobility management i.e. EMM states are maintained by NAS layers within UE and MME. These states determine whether or not UE is reachable and whether or not a UE can receive services.
As shown in the figure, the two main EMM states are :
A UE will be in the EMM-deregistered state when it is first powered ON. This time MME does not have knowledge of UE and hence it can not be paged. In this state UE cannot have any user plane bearers.
LTE UE will attempt to change the state to EMM-registered whenever posssible.
This is done by completing attach procedure.
This procedure registers UE with MME and establishes a default bearer for application data transfer. UE can also enters into EMM-registered state by completing Tracking Area Update after an incoming inter-system transition.
The location of UE in the EMM-registered state is known by MME to atleast an accuracy of 'tracking area list' allocated to UE. Tracking area list is the set of tracking area with which UE is registered. If the tracking area list includes only a single tracking area then the location of the UE is known to be within that tracking area. Depending upon ECM state (EPS Connection Management state) , location of UE can be known by the MME to the accuracy of a single eNodeB.
The LTE UE has atleast one active PDN connection when in the EMM-registered state. Deactivating all PDN connections cause UE to move back to EMM-deregistered state. Pls. note that default bearer is esyablished during the 'Attach Procedure'.
LTE UE make transition from EMM-registered state to EMM-deregistered state by completing the 'detach procedure' or by having a tracking area update rejected.
REFERENCES: 3GPP TS 23.401, 3GPP TS 24.301