Evolved Mobility Management

Description

Evolved Mobility Management (EMM) is a core protocol and state management entity defined for the Evolved Packet System (EPS) and continued into the 5G System (5GS). It operates in the control plane between the User Equipment (UE) and the core network's Mobility Management Entity (MME) in 4G or the Access and Mobility Management Function (AMF) in 5G. EMM is responsible for establishing, maintaining, and releasing the mobility management context of a UE, which is essential for the network to know the UE's location (at the tracking/routing area level) and its connection state (EMM-REGISTERED or EMM-DEREGISTERED). This context is crucial for enabling the network to page the UE for incoming sessions and to authenticate the device for network access.

The protocol defines specific EMM procedures, which are initiated by either the network or the UE. Key UE-initiated procedures include the initial Attach, which registers the UE with the network and establishes a default bearer, and the Tracking Area Update (TAU), which updates the network when the UE moves into a new tracking area. Network-initiated procedures include Authentication and Security Mode Control to establish ciphering and integrity protection, and Detach to gracefully remove the UE's context. EMM works in conjunction with the ESM (EPS Session Management) protocol, which handles bearer management; EMM procedures often carry ESM messages as payload.

EMM states are central to its operation. The primary states are EMM-DEREGISTERED, where the UE is not known to the MME/AMF, and EMM-REGISTERED, where a context exists. Within the EMM-REGISTERED state, sub-states like EMM-IDLE and EMM-CONNECTED indicate whether a signaling connection to the radio access network is active. Transitions between these states are triggered by specific events, such as successful attach (deregistered to registered) or connection release (connected to idle). The protocol ensures that security keys are established during registration and are used to protect all subsequent NAS (Non-Access Stratum) signaling messages, providing a secure foundation for mobility management.

Purpose & Motivation

EMM was created as part of the System Architecture Evolution (SAE) in 3GPP Release 8 to provide a unified, IP-based mobility management framework for the new Evolved Packet Core (EPC). Previous 3GPP systems like GPRS and UMTS had mobility management (GMM) that was more tightly coupled with the circuit-switched domain and the legacy network architecture. The purpose of EMM was to design a cleaner, more efficient protocol optimized for all-IP networks, supporting seamless mobility between 3GPP and non-3GPP access networks (like Wi-Fi).

It solved the problem of managing mobility in a flatter network architecture where the control plane (handled by the MME) was separated from the user plane. EMM provides a standardized way for the UE and network to negotiate capabilities, establish security, and maintain location information without the overhead of the older circuit-switched paradigms. Its creation was motivated by the need for higher data rates, lower latency, and simplified network architecture required for LTE, and it laid the groundwork for the mobility management in 5G, where it evolved into the NAS protocol for Registration and Connection Management handled by the AMF.