Public Safety Answering Point

Description

A Public Safety Answering Point (PSAP) is the physical or logical contact center responsible for answering calls to a national or regional emergency telephone number (e.g., 112 in Europe, 911 in North America). In the context of 3GPP standards, the focus is on defining the network architectures, interfaces, and procedures to successfully route emergency calls and associated data from a User Equipment (UE) through the mobile network to the appropriate PSAP. The PSAP itself is typically considered a node outside the 3GPP network, but its interface and requirements heavily influence 3GPP specifications.

The architecture for emergency services involves several key 3GPP network functions. When a UE initiates an emergency call, the Radio Access Network (RAN) and Core Network (CN) must recognize the call as an emergency session. The Mobility Management Entity (MME) in 4G or the Access and Mobility Management Function (AMF) in 5G plays a central role. They prioritize the session establishment and may invoke the Emergency Call Session Control Function (E-CSCF) in the IP Multimedia Subsystem (IMS) for emergency call routing. The network must also provide location information for the caller. This is facilitated by the Gateway Mobile Location Centre (GMLC) or the Location Management Function (LMF) and Secure User Plane Location (SUPL) platforms, which obtain the UE's position and deliver it to the PSAP.

How it works: The UE indicates an emergency request during session establishment. The network authenticates the UE for emergency services only (it may not have a valid SIM) and establishes a prioritized bearer or QoS flow with specific attributes for emergency voice. Concurrently, location retrieval procedures are triggered. The call signaling (SIP messages) is routed through the IMS core, where the E-CSCF queries a Location Retrieval Function (LRF) to determine the correct PSAP based on the caller's location. The E-CSCF then routes the call to that PSAP via the Interconnection Border Control Function (IBCF) or directly over the IP network (e.g., using ESInet). The PSAP receives the call and the associated location data, allowing the operator to dispatch police, fire, or medical services. 3GPP's role is to ensure this complex chain of events works reliably across different network generations and administrative domains.

Purpose & Motivation

The standardization of interfaces to PSAPs was motivated by the critical public safety requirement to reliably connect mobile callers to emergency services. Early cellular systems had limited capabilities for emergency calls, often lacking automatic location identification, which posed a significant risk to public safety. As mobile phones became the primary device for placing emergency calls, regulatory bodies worldwide mandated enhanced features like automatic location delivery.

3GPP's work on PSAP interfaces solves several key problems: It defines how a mobile network identifies an emergency call attempt, even from an unauthenticated device. It standardizes the mechanisms for retrieving and formatting the caller's location (cell-ID, assisted-GNSS) to meet regulatory accuracy requirements. It also defines the routing logic to connect the caller to the geographically appropriate PSAP, which is non-trivial with mobile users. This work evolved from basic circuit-switched emergency calls in 2G/3G to sophisticated IP-based emergency services in 4G and 5G, incorporating multimedia emergency services (e.g., real-time text, video). The purpose is to create a reliable, interoperable, and feature-rich emergency service ecosystem that leverages the capabilities of modern mobile networks to save lives.