Non-EPS Alert Flag

Description

The Non-EPS Alert Flag (NEAF) is a parameter used within the 4G Evolved Packet Core (EPC), specifically in the context of the Mobility Management Entity (MME). It is a boolean indicator stored in the MME for a user. The flag is set when the MME receives an indication that the User Equipment (UE) is not reachable for EPS services (e.g., due to being out of LTE coverage or in a power-saving state like Extended Idle Mode DRX), but there is a possibility the UE is attached to a non-EPS service. Non-EPS services primarily refer to Circuit-Switched (CS) services provided by 2G (GERAN) or 3G (UTRAN) radio access networks. The flag is used in conjunction with the SGs interface, which connects the MME (EPS) to the Mobile Switching Center (MSC) server (CS domain). When a mobile-terminated event (like an SMS or a CS voice call) arrives for the UE at the MSC, and the MME indicates the UE is EPS-detached but the NEAF is set, the MSC is informed that an alerting mechanism should be invoked. This typically triggers the MSC to page the UE in the CS domain. The primary application of the NEAF is for the delivery of SMS over SGs (SMS in MME) and for CS Fallback (CSFB) procedures. When the MME receives a downlink CS service notification from the MSC via the SGs interface, it checks the UE's mobility management context. If the UE is marked as not reachable for EPS but the NEAF is TRUE, the MME will accept the service request and initiate procedures (like paging in the CS domain) to alert the UE. This mechanism ensures that services like SMS and voice calls can still be delivered to a UE that is temporarily outside LTE coverage but is registered in a 2G/3G network.

Purpose & Motivation

The NEAF was created to bridge the service continuity gap between the all-IP 4G EPS and the legacy circuit-switched networks during the transition phase of mobile networks. It solves the problem of a UE being unreachable for EPS services (like IMS-based SMS or VoLTE) but potentially reachable via a legacy CS network. Without this flag, the network might simply treat the UE as completely unreachable, causing missed SMS messages or failed call setup attempts when the UE is camped on 2G/3G. Its introduction, particularly with the SGs interface in Release 8, was motivated by the need to support SMS and CS Fallback for voice calls in early LTE deployments, which often lacked full IMS-based voice (VoLTE) coverage. It addressed the limitation of pure EPS, which initially had no native circuit-switched functionality, by providing a signaling mechanism to leverage the ubiquitous CS core for essential services until IMS services became widespread. This allowed operators to launch LTE data services while maintaining reliable support for ubiquitous services like SMS and circuit-switched voice.

Key Features

• Boolean flag stored in the MME's mobility management context for a subscriber
• Indicates potential UE reachability in non-EPS (2G/3G CS) networks when EPS reachability fails
• Key enabler for SMS delivery over the SGs interface (SMS in MME)
• Used in CS Fallback (CSFB) procedures for mobile-terminated voice calls
• Works in conjunction with the SGs interface between MME and MSC Server
• Helps maintain service continuity between EPS and legacy CS domains

Evolution Across Releases

Defining Specifications