Session Transfer Number - Single Radio

Description

The Session Transfer Number for Single Radio (STN-SR) is a critical data element stored in the user's subscription profile on the Home Subscriber Server (HSS). It is essentially a routable telephone number (E.164 format) that points to a specific network function: the MSC Server enhanced for SRVCC. When a UE capable of Voice over LTE (VoLTE) is engaged in a voice call and moves towards the edge of LTE coverage, the network initiates an SRVCC handover to a legacy circuit-switched (CS) network like GERAN or UTRAN. The STN-SR is the key that triggers this complex transfer.

The SRVCC procedure works as follows: The source LTE network's MME detects the need for an SRVCC handover based on measurement reports from the UE. The MME then retrieves the subscriber's STN-SR from the HSS (if not already cached). The MME uses this STN-SR to establish a signalling connection to the target MSC Server. This is done via the Sv interface, which is defined specifically for SRVCC. The STN-SR acts as the destination address for this Sv signalling. The MME sends an SRVCC PS-to-CS Request message to the MSC Server identified by the STN-SR. This message contains the necessary details of the ongoing IMS session. The MSC Server then uses its own capabilities and interfaces with the IMS network (via the IMS Service Centralization and Continuity Application Server, SCC AS) to perform a session transfer. It anchors the call in the CS domain and coordinates with the target radio network to complete the handover of the UE's radio connection, all while maintaining the voice call with minimal interruption to the user.

Architecturally, the STN-SR is a cornerstone of the SRVCC feature, which was designed for UEs with a single radio transceiver that cannot simultaneously communicate with LTE and 2G/3G networks. Without STN-SR, the MME would have no way of knowing which MSC Server in the potentially large network is responsible for handling the SRVCC procedure for that particular subscriber. It provides a subscriber-specific routing mechanism for the handover command. The STN-SR is provisioned per subscription, allowing for flexibility—for example, different subscribers could be homed to different, geographically appropriate MSC Servers for optimized handover performance.

Purpose & Motivation

STN-SR was created to solve a critical deployment challenge for early LTE networks: providing seamless voice service before LTE coverage was ubiquitous. LTE was designed as a packet-only network, with voice intended to be delivered as VoIP over IMS (VoLTE). However, LTE coverage initially had gaps. A UE on a VoLTE call moving out of LTE coverage would drop the call if no continuity mechanism existed. Dual-radio solutions were possible but increased device cost and battery consumption. SRVCC, enabled by STN-SR, provided an elegant solution for single-radio devices.

The problem it addressed was the lack of a native circuit-switched fallback in the LTE radio interface itself. Previous mechanisms like CS Fallback (CSFB) required the call to be set up in the CS domain from the start if the UE was camped on LTE. SRVCC, in contrast, allowed the call to originate optimally in the high-quality VoLTE/IMS domain and only fall back to CS when absolutely necessary due to radio conditions. The STN-SR was the essential identifier that made this dynamic, mid-call transfer possible. It connected the packet-core MME, which manages the LTE mobility event, with the correct circuit-switched entity (MSC Server) that could orchestrate the session transfer with the IMS core. This enabled operators to launch VoLTE services with the confidence that voice service continuity could be maintained, thereby improving the user experience and accelerating VoLTE adoption.