Emergency Session Transfer Number for DRVCC

Description

The Emergency Session Transfer Number for DRVCC (E-STN-DR) is a critical routing identifier within the IMS (IP Multimedia Subsystem) architecture for emergency service continuity. It is used specifically in the context of Dual Radio Voice Call Continuity (DRVCC), a scenario where the User Equipment (UE) is capable of simultaneously maintaining connections on both a packet-switched (PS) access (like LTE or 5G NR) and a circuit-switched (CS) access (like GSM or UMTS). When such a UE with an active emergency IMS voice call needs to handover the call from the PS to the CS domain (or vice versa), the E-STN-DR is the key that initiates the session transfer at the IMS layer.

Mechanically, the E-STN-DR is a tel URI (e.g., tel:+1234567890) configured in the network and associated with the Emergency Access Transfer Control Function (E-ATCF). The E-ATCF is the IMS entity that anchors the emergency call for the purpose of service continuity. When a DRVCC handover for an emergency call is required, the UE or the network initiates an IMS session transfer procedure. This involves sending a SIP INVITE or REFER request containing the E-STN-DR in the Request-URI or Refer-To header. The IMS core network routes this request based on the E-STN-DR number directly to the specific E-ATCF that is anchoring the original emergency call.

Upon receiving the request, the E-ATCF correlates it with the existing emergency IMS session using identifiers stored during call setup. It then executes the Access Transfer procedures, which involve updating the remote end's media path (e.g., the Public Safety Answering Point or another UE) to point from the old access leg (PS) to the new access leg (CS). This ensures the media flow continues uninterrupted on the new network bearer. The E-STN-DR is distinct from the E-STN-SR used for Single Radio VCC, as DRVCC procedures can be initiated later in the call and involve different device capabilities. Its definition and usage are spread across specifications like TS 23.237 (VCC), 23.271 (location services), and 23.771 (enhanced VCC).

Purpose & Motivation

E-STN-DR was introduced in 3GPP Release 13 to extend robust emergency call continuity support to devices with dual radio capabilities. Prior standards had defined E-SR-VCC and its associated E-STN-SR for single-radio devices, which must break the PS connection before establishing the CS connection. However, devices with independent transceivers (common in early VoLTE-capable phones that also had 3G radios) could use DRVCC, which allows for a more seamless 'make-before-break' handover. The existing emergency continuity mechanisms were not fully defined for this dual-radio scenario.

The creation of E-STN-DR solved the problem of how to uniquely and reliably identify and route the session transfer request for an *emergency* call in a DRVCC context. Without a dedicated emergency number, the network could not distinguish between a normal DRVCC handover and a critical emergency call handover, potentially leading to incorrect routing or lack of priority handling. By defining E-STN-DR, 3GPP ensured that emergency DRVCC procedures would always be routed to the correct E-ATCF, which is provisioned to handle the specific regulatory and priority aspects of emergency sessions, thereby maintaining the same high reliability for emergency services on advanced dual-radio devices as on single-radio devices.