Traffic Steering Support Function

Description

The Traffic Steering Support Function (TSSF) is a core network function introduced in 3GPP Release 13, primarily for the Service Capability Exposure Function (SCEF) architecture, and later adapted for the 5G Core (5GC). Its primary role is to interpret and enforce traffic steering policies, which dictate how user data flows should be routed. These policies are typically provided by the Policy Control Function (PCF) and are based on factors such as subscription data, application requirements, network conditions, and slice selection policies. The TSSF acts as a policy enforcement point, translating high-level steering rules into actionable instructions for other network functions, such as the Session Management Function (SMF) or the Access and Mobility Management Function (AMF).

Architecturally, the TSSF can be implemented as a standalone function or integrated within other control plane functions. It interfaces with key 5GC elements via standardized reference points, such as N7 to the PCF for policy reception and N11 to the AMF for mobility and session context. When a Protocol Data Unit (PDU) session is established or modified, the SMF may consult the TSSF to determine the appropriate Data Network Name (DNN), Network Slice Selection Assistance Information (NSSAI), or access type (e.g., 3GPP, non-3GPP). The TSSF evaluates the policy rules against the current session context, subscriber profile, and local configuration to make a steering decision.

Internally, the TSSF comprises policy rule storage, a rule matching engine, and interfaces for communication with other network functions. It processes Policy and Charging Control (PCC) rules that contain traffic steering information, such as route selection descriptors. These descriptors may specify priorities for different slice instances or access technologies. The TSSF's decision-making process is dynamic, allowing for real-time adjustments based on network load, user mobility, or changes in service requirements. This capability is fundamental to realizing the 5G vision of network slicing, where multiple logical networks with distinct characteristics are created on a shared physical infrastructure.

In operation, the TSSF plays a pivotal role in end-to-end service assurance. For example, for an Ultra-Reliable Low-Latency Communication (URLLC) service, the TSSF might steer traffic to a slice optimized for low latency, potentially using a specific radio access network (RAN) configuration. Conversely, for a massive Internet of Things (mIoT) application, it might direct traffic to a slice designed for high connection density and energy efficiency. By centralizing traffic steering logic, the TSSF simplifies network management, enhances policy consistency, and enables automated, intelligent traffic distribution across heterogeneous network environments.

Purpose & Motivation

The TSSF was created to address the growing complexity of traffic management in modern mobile networks, particularly with the advent of 5G and network slicing. Prior to its introduction, traffic steering was often handled in an ad-hoc manner through static configurations in network elements like the Gateway GPRS Support Node (GGSN) or Packet Data Network Gateway (PGW). This approach lacked the dynamism and granularity needed to support diverse 5G services with stringent and varied requirements for latency, bandwidth, and reliability. The limitations of previous systems became apparent as operators sought to offer differentiated services, such as enhanced mobile broadband (eMBB), critical communications, and IoT, over a common infrastructure.

The primary motivation for developing the TSSF was to provide a standardized, policy-driven mechanism for intelligent traffic steering. It solves the problem of how to automatically and efficiently direct user data flows to the most appropriate network resources based on real-time conditions and service-level agreements (SLAs). This is essential for network slicing, a cornerstone of 5G, which allows the creation of multiple virtual networks on a single physical platform. Without a dedicated function like the TSSF, implementing and managing slice-specific traffic routing would be cumbersome, error-prone, and difficult to scale.

Historically, the concept evolved from earlier policy control frameworks defined in 3GPP, such as the Policy and Charging Control (PCC) architecture. Release 13 initially specified the TSSF within the context of the SCEF for Cellular IoT (CIoT) optimizations, focusing on steering non-IP data. With the transition to the 5G Service-Based Architecture (SBA) in Release 15, its role was expanded and integrated into the 5GC to manage steering for all types of PDU sessions across slices and access types. The TSSF thus represents a maturation of policy control, evolving from simple charging and quality of service (QoS) enforcement to encompass sophisticated, context-aware traffic orchestration that is vital for delivering the promised flexibility and efficiency of 5G networks.