Understanding MME States in the EPC: EMM vs. ECM

In the LTE Evolved Packet Core (EPC), the Mobility Management Entity (MME) is the brain of the control plane. To manage a device (UE) efficiently, the MME uses two parallel state machines: EPS Mobility Management (EMM) and EPS Connection Management (ECM).

Think of it this way: EMM determines if you are “checked in” to the network, while ECM determines if you are “actively talking” to it.

1. EPS Mobility Management (EMM)

EMM states describe whether a UE is registered with the MME and authenticated to use the network.

EMM-DEREGISTERED

  • The Scenario: The phone is powered off, in airplane mode, or failed its initial attach.

  • MME Knowledge: The MME has no valid location or routing information for the UE.

  • Status: No IP address (EPS Bearer) is assigned.

EMM-REGISTERED

  • The Scenario: The UE has successfully performed an Attach or Tracking Area Update (TAU).

  • MME Knowledge: The MME knows which Tracking Area (TA) the UE is currently in.

  • Status: The UE has an IP address and a “Default Bearer” assigned, even if it isn’t sending data right now.

2. EPS Connection Management (ECM)

ECM states describe the connectivity between the UE and the EPC. You can only have an ECM state if you are already in the EMM-REGISTERED state.

ECM-IDLE

  • The Scenario: The phone is on and registered, but it’s in your pocket. No apps are actively using the data connection.

  • Resources: To save battery, the Radio Resource Control (RRC) connection and S1-MME signaling connections are released.

  • Mobility: The UE moves between cells using Cell Reselection (the phone decides where to go).

  • MME Action: If data arrives for the UE, the MME must Page the device across the entire Tracking Area to “wake it up.”

ECM-CONNECTED

  • The Scenario: You are browsing the web, streaming a video, or the phone just responded to a page.

  • Resources: An active signaling connection exists (RRC + S1-MME), and the data path (S1-U) is established.

  • Mobility: The network manages movement via Handover.

  • MME Knowledge: The MME knows the specific eNodeB and Cell ID the UE is using.