Description
The Access Transfer Update - Session Transfer Identifier (ATU-STI) is a critical component within the IP Multimedia Subsystem (IMS) architecture that facilitates service continuity during access network transfers. It functions as a correlation identifier that binds together various signaling messages related to the same session transfer procedure. When a User Equipment (UE) moves between different access technologies (such as from LTE to Wi-Fi or vice versa), the ATU-STI ensures that all network elements involved in the transfer can correctly identify and process the ongoing session context.
Architecturally, ATU-STI operates within the IMS Service Continuity framework defined in 3GPP specifications. During an access transfer procedure, the Serving-Call Session Control Function (S-CSCF) or the Service Centralization and Continuity Application Server (SCC AS) generates the ATU-STI and includes it in the Access Transfer Update request. This identifier is then propagated through the signaling path to all relevant network functions, including the Target Access Transfer Control Function (ATCF) and the Access Transfer Gateway (ATGW). The ATU-STI enables these elements to correlate the transfer request with the specific session being moved, ensuring proper routing of media and signaling paths.
From a protocol perspective, ATU-STI is carried within SIP messages as part of the IMS service continuity procedures. It typically appears in SIP headers or message bodies during the Access Transfer Update flow. The identifier follows a specific format defined by 3GPP, ensuring interoperability between different vendor implementations. When the target network receives the transfer request with the ATU-STI, it uses this identifier to retrieve the corresponding session context and execute the necessary modifications to maintain the multimedia session without interruption.
The ATU-STI plays a crucial role in preventing session transfer failures and misassociations. Without this identifier, multiple simultaneous session transfers could become confused, leading to incorrect session routing or complete transfer failures. By providing a unique correlation mechanism, ATU-STI enables reliable handover of IMS-based services like voice over LTE (VoLTE), video calls, and other real-time communication services across heterogeneous access networks. This capability is fundamental to delivering seamless user experiences in modern mobile networks where devices frequently switch between cellular and Wi-Fi connectivity.
Purpose & Motivation
ATU-STI was introduced to address the challenges of maintaining IMS service continuity during access network transfers in heterogeneous network environments. Prior to its introduction, IMS networks faced difficulties in reliably correlating session transfer requests with specific ongoing sessions when users moved between different access technologies. This limitation became particularly problematic with the proliferation of multi-access devices that could simultaneously maintain connections to both 3GPP networks (like LTE) and non-3GPP networks (like Wi-Fi).
The primary problem ATU-STI solves is the reliable identification and correlation of session transfer procedures. In earlier implementations without standardized correlation mechanisms, network elements could misassociate transfer requests, leading to session drops, media path disruptions, or incorrect session routing. This was especially critical for real-time services like voice and video calls, where even brief interruptions significantly degrade user experience. The ATU-STI provides a standardized way to uniquely identify each transfer procedure, ensuring that all network elements involved in the handover process can correctly associate signaling messages with the appropriate session context.
Historically, the need for ATU-STI emerged with the development of IMS Service Continuity in 3GPP Release 10, which aimed to provide seamless service experience across different access networks. As operators began deploying Voice over LTE (VoLTE) and other IMS-based services, they needed mechanisms to maintain these services when users moved between cellular and Wi-Fi networks. ATU-STI became an essential component of this architecture, enabling the reliable handover of multimedia sessions while maintaining quality of service and security parameters. Its creation was motivated by the industry's move toward all-IP networks and the requirement for consistent user experiences across heterogeneous access technologies.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (10 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-10, normative work from Rel-15.
In Release 15, the ATU-STI function was extended to support the session transfer of IMS emergency sessions, specifically introducing procedures for PS to CS SRVCC during the early dialogue phase of such an emergency call. This enhancement ensures service continuity for emergency sessions when a user equipment transfers from a Packet Switched to a Circuit Switched access network. The update integrates this emergency SRVCC capability within the existing framework for Access Transfer and Session Transfer Identifiers.
- PS to CS SRVCC for IMS emergency session in early dialogue phase TS 23.237CR0504
In Release 16, the ATU-STI function was enhanced to enable SRVCC for emergency calls transferred to EPS. Furthermore, the release introduced a mechanism for the UDM to update the Session Transfer Number for SRVCC (STN-SR) to the AMF, supporting these emergency session transfers.
In Release 17, the specification for the ATU-STI (Access Transfer Update - Session Transfer Identifier) function was updated to its Rel-17 version. This maintenance update ensured alignment within the broader framework of procedures like PS-CS Access Transfer and Inter-UE Transfer, which rely on the STI for the UE to request session transfer from the SCC AS. The changes involved a version update to reflect the current release, incorporating the established technical context for session transfer mechanisms.
In Release 18, the ATU-STI function was updated to its Rel-18 version, as reflected in the Change Requests for the MCC (Mobile Country Code) parameter. This maintenance update ensures alignment with the broader Release 18 specifications, which define the Session Transfer Identifier (STI) as the identifier used by the UE to request the SCC AS to perform a Session Transfer, encompassing procedures like PS-CS Access Transfer.
In Release 19, the ATU-STI function was updated to its Rel-19 version, as indicated by the Change Requests. This update maintains the core function of the Session Transfer Identifier (STI) as the identifier a UE uses to request the SCC AS to perform a Session Transfer, which incorporates procedures like PS-CS Access Transfer.
Explore further
Broader topics and technologies where ATU-STI plays a role.
Defining Specifications
3GPP specifications that define or reference ATU-STI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.237 vj00 | IMS Service Continuity (ISC) Stage 2 | Rel-19 |
| TS 24.802 vc10 | IMS II-NNI Traversal Scenario Determination Study | Rel-12 |
| TR 29.949 vj00 | VoLTE IMS Roaming Architecture & Procedures | Rel-19 |