Emergency Call Session Transfer Number – Single Radio

Description

E-STN-SR is a critical functional element within the IP Multimedia Subsystem (IMS) emergency call architecture defined by 3GPP. It is a globally routable E.164 number, distinct from a regular telephone number, which is pre-configured in the network and associated with specific IMS application servers responsible for emergency sessions. When a User Equipment (UE) with single radio capability—meaning it cannot simultaneously transmit/receive on both LTE and legacy 2G/3G radios—undergoes a handover from a PS access like LTE to a CS access like GERAN or UTRAN for an ongoing emergency call, the network uses the E-STN-SR. The mechanism is part of the IMS Service Continuity procedures. During a handover, the MSC Server in the CS network receives an indication of the emergency session transfer. It initiates a session transfer by placing a new call to the E-STN-SR number. This call is routed through the PSTN/CS network to the IMS network, specifically to an IMS Application Server (e.g., an Emergency Call Session Control Function or a dedicated Service Centralization and Continuity Application Server). This server correlates the incoming CS call with the existing IMS emergency session using session identifiers, and bridges the media paths, thereby transferring the UE's leg of the call from the PS bearer to the CS bearer without dropping the call. The architecture ensures that the emergency service center (Public Safety Answering Point) remains connected to the user throughout the transition. Key components involved include the UE, the E-UTRAN, the MSC Server, the IMS Core (P-CSCF, I-CSCF, S-CSCF), and the relevant IMS Application Server. Its role is to provide a standardized, network-controlled anchor point for emergency session mobility, ensuring regulatory requirements for emergency call reliability are met even when the user moves out of LTE coverage.

Purpose & Motivation

E-STN-SR was created to solve the critical problem of maintaining active emergency calls when a single-radio device performs a handover from a Voice over LTE (VoLTE) or IMS-based emergency call to a legacy circuit-switched network. Prior to IMS-based emergency services, emergency calls were native to the access network (e.g., a CS call in 2G/3G). With the introduction of LTE as a packet-only access, emergency calls were defined to be routed over IMS. However, early LTE deployments often had coverage gaps, necessitating handovers to ubiquitous 2G/3G CS networks. Without a standardized transfer mechanism, an emergency call would drop during such a handover, creating a serious public safety risk. The E-STN-SR provides the necessary signaling address (the telephone number) that allows the CS network to 'call back' into the IMS ecosystem to retrieve the existing emergency session context. This addressed the limitation of earlier non-IMS emergency services which had no concept of session continuity between fundamentally different network domains (PS and CS). Its creation was motivated by regulatory mandates for reliable emergency services and the practical need for a smooth transition phase during the long-term migration from CS to all-IP networks.