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LTE EPS Mobility Management States Explained

This document describes the LTE EPS (Evolved Packet System) Mobility Management States.

EPS mobility management (EMM) states are maintained by the NAS (Non-Access Stratum) layers within both the UE (User Equipment) and the MME (Mobility Management Entity). These states are crucial as they determine whether a UE is reachable and whether it can receive services.

As shown in the figure below, the two primary EMM states are:

EMM-Deregistered
EMM-Registered

LTE EPS Mobility Management States

A UE starts in the EMM-Deregistered state when it is first powered on. In this state, the MME has no knowledge of the UE’s existence, and therefore, the UE cannot be paged. The UE also cannot have any user plane bearers established.

The LTE UE will attempt to transition to the EMM-Registered state whenever possible. This transition is achieved by completing the Attach procedure. This procedure registers the UE with the MME and establishes a default bearer for application data transfer.

The UE can also enter the EMM-Registered state after an incoming inter-system transition by completing a Tracking Area Update procedure. When in the EMM-Registered state, the MME knows the UE’s location to an accuracy of at least the “tracking area list” allocated to the UE. The tracking area list is the set of tracking areas with which the UE is registered.

If the tracking area list includes only a single tracking area, the location of the UE is known to be within that specific tracking area. Depending on the ECM (EPS Connection Management) state, the MME can know the location of the UE with the accuracy of a single eNodeB.

Crucially, the LTE UE has at least one active PDN (Packet Data Network) connection when in the EMM-Registered state. Deactivating all PDN connections will cause the UE to move back to the EMM-Deregistered state.

Note that the default bearer is established during the Attach Procedure.

The LTE UE transitions from the EMM-Registered state to the EMM-Deregistered state by completing the Detach procedure or by having a Tracking Area Update rejected.

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