Emergency Service Query Key

Description

The Emergency Service Query Key (ESQK) is a critical element within the 3GPP IMS emergency services architecture, defined primarily in TS 23.167. It functions as a temporary, unique key assigned by the network for the duration of an emergency session. When a user initiates an emergency call via an IMS network, the call is routed to an Emergency Call Session Control Function (E-CSCF). The E-CSCF, in conjunction with a Location Retrieval Function (LRF), is responsible for obtaining the caller's location. The ESQK is generated and associated with this specific emergency session and its corresponding location data.

The ESQK is then inserted into the Session Initiation Protocol (SIP) signaling of the emergency call, typically in the Request-URI or a dedicated header. When the call is routed to the Public Safety Answering Point (PSAP), the PSAP system uses the received ESQK as a query key. The PSAP sends this ESQK in a query to a designated emergency services network entity, such as an Emergency Services Message Entity (ESME) or via the LoST (Location-to-Service Translation) protocol framework, to retrieve the associated location information and other call data from the network's database where the E-CSCF/LRF stored it.

This mechanism decouples the real-time voice call path from the location data retrieval path, which is essential for efficiency and reliability. The PSAP does not need to parse complex location objects within the SIP signaling itself; it simply uses the lightweight ESQK to fetch the data. This architecture supports both initial location retrieval and subsequent location updates, as the network can push updated location information to the same database record keyed by the ESQK. The ESQK is temporary and valid only for the life of the emergency session, after which it can be recycled, ensuring privacy and efficient resource usage.

Key components involved with the ESQK include the User Equipment (UE), the Proxy-CSCF (P-CSCF) that detects the emergency request, the E-CSCF that manages the emergency session, the LRF that obtains and formats location information, and the PSAP infrastructure. The ESQK enables a standardized, interoperable interface between the mobile operator's IMS core and disparate PSAP systems, which may be operated by different authorities. Its role is foundational for Next Generation emergency services (NG-eCall, IMS emergency calls) where accurate, network-derived location is paramount.

Purpose & Motivation

The ESQK was introduced to solve critical problems in routing emergency calls and delivering location information within IP-based networks, specifically IMS. Traditional circuit-switched emergency calls often embedded location data directly in the call signaling or relied on the cell ID available to the PSAP, which could be inaccurate or insufficient for rapid response, especially for mobile users. As networks evolved to all-IP architectures, a new, standardized method was needed to correlate a voice call with dynamically obtained, precise location data.

The primary motivation was to separate the delivery of the emergency voice media from the delivery of associated data (like GPS coordinates or civic address). This separation allows each system—the voice routing network and the location database system—to operate optimally. It also accommodates the fact that PSAPs may need to query for location after the call is answered and may require updated locations if the caller is moving. The ESQK provides a simple, universal 'key' that bridges these two worlds, enabling PSAPs from various vendors and regions to interoperate with 3GPP-compliant networks without needing deep integration into the operator's internal location systems.

Furthermore, it addresses privacy and security concerns by using a temporary, non-personal identifier. The ESQK does not reveal the caller's phone number (MSISDN) or permanent subscriber identifier to the PSAP until explicitly retrieved, if allowed by policy. This architecture, centered on the ESQK, was a foundational step towards reliable, location-enabled emergency services in LTE and 5G, supporting regulatory requirements like E911 in the US and eCall in Europe.