EPS Mobility Management CONNECTED Mode

Description

EMM-CONNECTED mode is one of the two EPS Mobility Management (EMM) states defined for a User Equipment (UE) in an LTE/EPS network, the other being EMM-IDLE mode. A UE enters EMM-CONNECTED mode when a NAS signalling connection is established between the UE and the Mobility Management Entity (MME). This connection is typically initiated by the UE during procedures such as Attach, Tracking Area Update (TAU), or Service Request, but it can also be triggered by the network via paging. In EMM-CONNECTED mode, the UE can exchange NAS messages with the MME for signaling purposes, and user data can be transferred over the established EPS bearers.

From Release 17 onwards, the definition was expanded to include the case where the UE resumes an RRC connection from a suspended state (EMM-IDLE mode with suspend indication). This enhancement, introduced with the Control Plane CIoT EPS Optimization and later refined for NR interworking, allows the UE to transition to EMM-CONNECTED mode more quickly without a full NAS connection establishment. This is particularly beneficial for IoT devices and for reducing latency in mobile broadband services.

In EMM-CONNECTED mode, the MME maintains a context for the UE and knows the exact cell or base station (eNodeB) serving the UE. This enables the network to deliver downlink data and signaling messages directly to the UE without paging. The UE also performs radio measurements and can report them to the network for handover decisions. The EMM-CONNECTED mode is closely tied to the RRC_CONNECTED state at the radio access layer; however, the EMM state is managed by the NAS protocol and is independent of the RRC state transitions, although they are coordinated.

The transition from EMM-IDLE to EMM-CONNECTED involves the establishment of both an RRC connection and a NAS signalling connection. This process includes security procedures such as authentication and NAS security mode control if not already established. The UE remains in EMM-CONNECTED mode as long as there is active signaling or data exchange, or until the network releases the connection due to inactivity, moving the UE back to EMM-IDLE mode.

The EMM-CONNECTED mode is defined in 3GPP TS 24.301 for the NAS protocol and is referred to in TS 24.801. It corresponds to the ECM-CONNECTED state in TS 23.401, which describes the overall EPS architecture. The mode is fundamental for ensuring efficient mobility management, timely paging, and seamless service delivery in LTE networks.

Purpose & Motivation

EMM-CONNECTED mode exists to provide a state where the UE and the network can actively exchange information with minimal latency and full context awareness. In earlier GPRS networks, the equivalent state was known as GMM-CONNECTED or READY state, but it had limitations in terms of always-on connectivity and support for high-speed data. The EMM-CONNECTED mode was designed to support the LTE requirement for always-on connectivity, where UEs frequently transition between idle and connected states to balance battery consumption and responsiveness.

The mode solves the problem of UE reachability: when in EMM-CONNECTED, the network knows the exact location of the UE and can deliver downlink data without paging, enabling real-time services like voice over LTE (VoLTE) and video streaming. It also provides a secure channel for signaling and data, with ciphering and integrity protection applied at the NAS level. The introduction of resume from suspend in Release 17 further optimizes this by reducing the signaling overhead and latency associated with state transitions, which is crucial for machine-type communications and 5G interworking.

Historically, the EMM-CONNECTED mode was introduced in Release 8 as part of the EPS specifications. Its definition has been stable, with a notable enhancement in Release 17 to accommodate suspend/resume procedures. This evolution reflects the ongoing need to support diverse service requirements, from high-speed mobile broadband to low-power IoT, within the same mobility management framework.