Emergency Service Routing Number

Description

The Emergency Service Routing Number (ESRN) is a standardized identifier within the 3GPP IMS emergency call architecture. It functions as a routing key within the core network to direct an emergency session request to the correct Emergency Call Session Control Function (E-CSCF). The E-CSCF is the IMS node responsible for processing emergency calls. The ESRN is not dialed by the user; instead, it is derived by network elements based on the location of the caller. When a User Equipment (UE) initiates an emergency call, the Proxy-CSCF (P-CSCF) detects the emergency session request. The P-CSCF then queries a Location Retrieval Function (LRF) to obtain both routing information (the ESRN) and the caller's location information (the Emergency Service Query Key or ESQK). The LRF determines the appropriate PSAP jurisdiction based on the provided or estimated location of the UE.

Once obtained, the P-CSCF includes the ESRN in the SIP INVITE request's Request-URI when forwarding the call to an E-CSCF. The ESRN acts as a token that tells the E-CSCF which specific PSAP gateway or Border Control Function (BCF) the call should be routed towards. The E-CSCF uses the ESRN to select the correct outgoing trunk or interface toward the PSAP network. This mechanism decouples the complex location-based routing logic (handled by the LRF) from the basic session routing performed by the CSCFs. The architecture ensures that the core IMS network does not need to maintain detailed, real-time PSAP boundary maps.

Alongside the ESRN, the ESQK is also provided to the E-CSCF and is ultimately delivered to the PSAP. The PSAP can use the ESQK in a subsequent callback to the LRF to retrieve the precise location of the caller. This two-number system (ESRN for routing, ESQK for location retrieval) is fundamental to IMS emergency services. The ESRN is defined to be globally unique, typically following an E.164 number format, to prevent conflicts and ensure unambiguous routing across different network operators and countries. Its implementation is detailed in 3GPP TS 23.167, which specifies the overall architecture and procedures for IMS emergency sessions.

Purpose & Motivation

The ESRN was created to solve the critical problem of routing emergency calls in all-IP networks like IMS, where traditional circuit-switched methods based on dialed digits (e.g., 911, 112) are insufficient. In legacy networks, emergency calls were routed based on the cell site or exchange serving the caller. IMS introduces mobility, nomadic access, and Voice over IP (VoIP) services, where a user's IP address or network attachment point does not reliably indicate their physical location for emergency response. The primary problem is ensuring that an emergency call reaches the PSAP responsible for the caller's current geographical area, regardless of their home network or subscription status.

The motivation stemmed from regulatory requirements for providing reliable emergency services in Next Generation Networks (NGNs). Previous approaches lacked a standardized, scalable mechanism for dynamic location-based routing in a decomposed, service-based architecture. The ESRN, in conjunction with the LRF, provides this mechanism. It allows the routing decision to be based on the most accurate available location (civil address, geodetic coordinates) rather than network topology, which is essential for VoIP calls from laptops, fixed wireless terminals, or visiting users.

It addresses the limitation of requiring every CSCF to have full PSAP routing tables. By centralizing the routing decision in the LRF and passing a simple token (the ESRN), the system becomes more scalable, maintainable, and adaptable to changes in PSAP jurisdictions. It also facilitates interconnection between different operator networks and PSAP infrastructures by providing a standardized number for routing purposes.