Emergency Access Transfer Function

Description

The Emergency Access Transfer Function (EATF) is a critical functional entity within the IP Multimedia Subsystem (IMS) for emergency services. Its primary role is to manage and facilitate the transfer of an ongoing emergency session (e.g., an IMS emergency call) from one access network to another, or between different domains within the core network, while maintaining the session's emergency context and connection to the appropriate Public Safety Answering Point (PSAP). The EATF acts as an anchor point for the emergency session. When a User Equipment (UE) with an active emergency call initiates a handover—for instance, moving from LTE to Wi-Fi, or between 3GPP and non-3GPP access—the EATF ensures the signaling and media paths are re-established without dropping the call.

Architecturally, the EATF interacts with several IMS core functions. It typically interfaces with the Proxy-Call Session Control Function (P-CSCF), which is the first contact point for the UE in IMS, and the Serving-Call Session Control Function (S-CSCF), which handles session control. During an emergency session establishment, the P-CSCF may identify the call as an emergency and involve the EATF. The EATF then inserts itself into the signaling path, often becoming a back-to-back user agent (B2BUA). This allows it to maintain control over both legs of the call: one leg towards the UE and the other towards the PSAP. It manages the session descriptors and can hold multiple alternative media addresses for the UE.

How it works involves continuous monitoring and proactive management. The EATF is aware of the UE's access capabilities and registration status. When a handover condition is detected or initiated (e.g., via network or UE indications), the EATF coordinates the transfer. It may use mechanisms like the Session Initiation Protocol (SIP) re-INVITE or REFER methods to update the session parameters on the PSAP side while the UE re-establishes connectivity via the new access. This process is designed to be seamless, minimizing interruption to the emergency communication. The EATF also plays a role in emergency call back; if a PSAP needs to call back a UE that initiated an emergency session, the EATF can assist in routing the callback to the UE's current location and access type.

Purpose & Motivation

The EATF was introduced to solve a critical problem in IMS-based emergency services: maintaining emergency call continuity during access mobility. Prior to its standardization, emergency calls established over one type of access (e.g., LTE) would likely drop if the user moved to another coverage area using a different technology (e.g., Wi-Fi or a legacy circuit-switched network). This was unacceptable for life-critical communications. The evolution towards all-IP networks and the increasing support for voice over non-3GPP access (like Wi-Fi) created a complex mobility landscape that traditional emergency service architectures could not handle.

Its creation in 3GPP Release 9 was motivated by regulatory requirements for reliable emergency services and the industry's move to IMS as the sole platform for voice and communication services. The EATF addresses the limitations of previous, access-specific emergency solutions by providing a centralized, access-agnostic function within the IMS core. It ensures that the emergency service context (like the PSAP connection and call priority) is preserved across network boundaries. This is essential not only for user mobility but also for network resilience scenarios, such as transferring emergency sessions during a core network node failure or when a UE's primary radio access deteriorates.