Mobility Management states in LTE

  • EPS mobility management (EMM) states are maintained by the NAS layers within the UE and MME. These states determine whether or not a UE is reachable, and whether or not UE can receive services.
  • The main two EMM states are
    • EMM De-registered
    • EMM Registered
  • A UE is in the EMM-Deregistered state when it is first switched on. The MME does not have knowledge of the UE location so the UE cannot be paged. In addition, The UE cannot have any user plane bearers while in the EMM-Deregistered state.
  • UE attempt to move into the EMM-Registered state whenever possible. This is achieved by completing the attach procedure. This procedure registers the UE with the MME and establishes a default bearer for application data transfer. UE can also move into the EMM-Registered state by completing a Tracking Area update after an incoming inter-system transition.
  • The location of the UE in the EMM-Registered state is known by the MME to at least an accuracy of the tracking area list allocated to the UE., i.e. the set of tracking area with which the UE is registered. If the tracking area list includes only a single TA then the UE is known to be within that Tracking area. Depending upon the EPS connection Management (ECM) state, the location of a UE can also be known by the MME to the accuracy of the single eNodeB.
  • The UE always has at least one active packet data network (PDN) connection when in the EMM-registered state(the default bearer is established during the attach procedure). De- activating all PDN connections causes the UE to move back to the EMM-Deregistered state.
  • UE can also make the transition from the EMM-registered state to the EMM-Deregistered state by completing the detach procedure or by having a Tracking area update rejected.

Similar Threads: