Transparent Single-connection mode

Description

Transparent Single-connection mode (TSCM) is a specific operational mode defined within the 3GPP System Architecture Evolution (SAE) for non-3GPP access integration, as specified in TS 23.402. It applies when a User Equipment (UE) connects to the Evolved Packet Core (EPC) via a trusted non-3GPP access network, such as a trusted WLAN. In this mode, the UE establishes and maintains a single IP connection (a single point of attachment) to the EPC through the non-3GPP access. This connection is anchored at the Packet Data Network Gateway (PGW) in the EPC. The key architectural component enabling TSCM is the Trusted Non-3GPP Access Gateway (TNGF or ePDG in untrusted scenarios, but for TSCM it's specifically for trusted access), which acts as the termination point in the access network and interfaces with the 3GPP AAA server for authentication and with the PGW for the user plane via the S2a interface based on PMIPv6 or GTP.

From a procedural standpoint, when a UE attaches via a trusted non-3GPP access network opting for TSCM, it performs access authentication and authorization with the 3GPP EPC. Upon successful authentication, the TNGF establishes a PMIPv6 or GTP tunnel with the PGW, creating a single IP connection for the UE. All of the UE's IP traffic is routed through this single tunnel. The UE is assigned an IP address from the PGW's pool, and this address remains stable as long as the connection is maintained. The mode is termed 'transparent' because the non-3GPP access network does not perform any IP address allocation or Network Address Translation (NAT) for the UE's traffic towards the EPC; the UE's IP address is fully visible to the PGW.

The primary role of TSCM in the network is to provide a simplified and efficient model for integrating trusted non-3GPP accesses. It supports IP flow mobility (IFOM) and service continuity as defined by 3GPP, allowing IP flows to be moved between 3GPP and non-3GPP accesses without breaking the application session. Since the UE has a single IP connection anchored at the PGW, mobility events (like adding a 3GPP access path) can be managed by the network (the PGW) using dual-stack Mobile IPv6 (DSMIPv6) or by the UE if it supports multiple accesses. TSCM contrasts with the Multiple-connection mode, where a UE could have multiple simultaneous independent connections to the EPC. TSCM's single-connection model reduces signaling complexity and is suitable for scenarios where the UE requires a simple, stable IP attachment point through alternative access technologies.

Purpose & Motivation

TSCM was introduced to address the growing need for seamless integration of non-3GPP wireless access technologies, like Wi-Fi, into the 3GPP mobile packet core. Prior to its standardization, non-3GPP access was often treated as a completely separate network, leading to service discontinuity, complex roaming scenarios, and an inability to leverage the policy and charging controls of the mobile operator's core network. The motivation was to create a unified core network (the EPC) that could provide consistent services regardless of the underlying radio access technology.

The specific problem TSCM solves is providing a standardized, efficient method for a UE to attach to the EPC via a *trusted* non-3GPP access network using a single, persistent IP connection. This model simplifies the network architecture and UE behavior compared to supporting multiple simultaneous attachments. It enables operators to offload data traffic to Wi-Fi networks while maintaining core network control over authentication, policy enforcement (via the PCRF), and charging. Furthermore, by anchoring the connection at the PGW, TSCM is a foundational enabler for 3GPP-defined IP flow mobility, allowing operators to dynamically steer specific data flows between 3GPP and non-3GPP accesses based on network conditions, policies, or user preferences, all while maintaining session continuity for the end user.