Emergency – Call Session Control Function

Description

The Emergency – Call Session Control Function (E-CSCF) is a critical component within the IP Multimedia Subsystem (IMS) architecture, specifically designed to manage emergency sessions. It operates as a specialized SIP proxy server. When an emergency session initiation request (e.g., an INVITE message with an emergency service URN) is received by the IMS, typically via a Proxy-CSCF (P-CSCF), it is forwarded to the E-CSCF. The E-CSCF's primary role is to ascertain the caller's location, either from information embedded in the SIP signaling (e.g., PANI, CAI, or civic address) or by querying other network entities like a Location Retrieval Function (LRF).

Using the obtained location information, the E-CSCF interacts with a Location-to-Service Translation (LoST) server or a similar functional entity. This interaction translates the geographic coordinates or civic address into a uniform resource identifier (URI) for the appropriate Public Safety Answering Point (PSAP). The PSAP is the contact point for emergency services like police, fire, or ambulance. The E-CSCF then routes the emergency session request (SIP INVITE) to this PSAP URI, ensuring the call reaches the correct local emergency authority.

Furthermore, the E-CSCF plays a vital role in scenarios where normal IMS registration or subscription checks might fail. For emergency sessions, the E-CSCF may bypass certain authentication and authorization procedures to ensure the call is established even for unregistered users or users without a valid subscription, adhering to regulatory requirements for universal access to emergency services. It also coordinates with the LRF for possible subsequent location requests from the PSAP, facilitating emergency call back and accurate dispatch of first responders.

Purpose & Motivation

The E-CSCF was introduced to address the critical need for reliable, location-aware emergency call handling in IP-based networks, specifically within the 3GPP IMS framework. Traditional circuit-switched mobile networks had established mechanisms for emergency calls, but the migration to all-IP core networks like IMS presented new challenges. In an IP environment, routing is not inherently tied to a physical network location, making it difficult to determine which local emergency service center should receive a call.

Its creation was motivated by regulatory and safety requirements mandating that emergency calls be routed to the correct local PSAP based on the caller's current location, regardless of their home network or registration status. The E-CSCF solves the problem of translating a user's device location (which could be dynamic in a mobile network) into a routable address for the appropriate emergency service endpoint. It ensures that the advanced session control capabilities of IMS are leveraged for emergency services, providing a standardized, robust, and future-proof architecture for emergency communications in LTE, 5G, and subsequent generations of mobile networks.

Key Features

• Receives and processes SIP-based emergency session initiation requests within IMS.
• Acquires caller location information from SIP signaling or by querying the Location Retrieval Function (LRF).
• Translates geographic location data into a URI for the correct Public Safety Answering Point (PSAT) using LoST or similar protocol.
• Routes the emergency session (SIP INVITE) to the determined PSAP.
• May bypass normal IMS authentication/authorization for unregistered users to ensure universal emergency access.
• Coordinates with the LRF for possible post-establishment location requests from emergency services.

Evolution Across Releases

Defining Specifications