AF Time Sensitive Networking Application Function

Description

The TSN Application Function (TSN AF) is a critical component defined by 3GPP for integrating 5G systems into IEEE 802.1 Time-Sensitive Networking (TSN) ecosystems, which are central to industrial Ethernet and deterministic communication. It resides in the 5G Core network as a specialized Application Function, interacting with other core network functions like the Policy Control Function (PCF) and Network Exposure Function (NEF) via service-based interfaces. The TSN AF's primary role is to represent the TSN network (or the TSN System) to the 5G system, acting as a gateway for TSN-specific configuration and requirements.

Architecturally, the TSN AF interfaces with a TSN Network Controller (or Centralized Network Controller - CNC), which is the entity in the TSN domain responsible for overall schedule and resource management. The TSN AF receives TSN requirements from the CNC, which include deterministic communication parameters such as periodicity, maximum latency, reliability (packet error rate), and time synchronization accuracy for data flows that will traverse the 5G system. The 5G system, in this context, is modeled as a virtual TSN bridge (or a set of bridges) from the TSN network's perspective. The TSN AF is responsible for making the 5G system's capabilities and resources visible to the TSN CNC and for mapping the TSN flow requirements into 5G-specific QoS parameters and policies.

How it works involves a multi-step process. First, during capability exposure, the TSN AF informs the TSN CNC about the 5G system's characteristics, such as supported latency bounds, time synchronization support (via 5G system as a timing slave or master), and available bandwidth. When the CNC computes a global schedule for TSN traffic, it includes the 5G virtual bridge. The CNC sends this schedule, including gate control lists for the 5G bridge ports, to the TSN AF. The TSN AF then translates these TSN constructs into 5G policy rules. It interacts with the PCF to create or modify PCC (Policy and Charging Control) rules that enforce the required QoS—for example, by allocating a dedicated 5G QoS Flow with guaranteed bit rate and packet delay budget for a specific TSN stream. It may also interact with the SMF (Session Management Function) and UPF (User Plane Function) to configure the user plane for deterministic forwarding.

Key components it interacts with include the TSN Translator in the UE and/or in the UPF, which handle the actual adaptation of Ethernet frames to 5G packets and vice versa, including timestamping for synchronization. The TSN AF's role is purely in the control plane, managing the configuration. It enables end-to-end deterministic connectivity where a 5G wireless link can be seamlessly integrated into a wired TSN network, supporting critical Industry 4.0 applications like motion control, machine vision, and closed-loop control systems that require ultra-reliable, low-latency, and time-synchronized communication.

Purpose & Motivation

The TSN AF was created to bridge two historically separate worlds: deterministic industrial networking (TSN) and cellular mobile networks (5G). Industrial automation has long relied on wired fieldbus and industrial Ethernet technologies (like PROFINET, EtherCAT) that provide hard guarantees on latency, jitter, and synchronization. These are essential for coordinating machines on a production line. Wireless solutions were traditionally unsuitable due to lack of determinism, reliability, and precise timing.

The advent of 5G, with its URLLC (Ultra-Reliable Low-Latency Communication) capabilities, promised to break this barrier, enabling flexible wireless connectivity for moving parts like AGVs (Automated Guided Vehicles) and robotic arms. However, simply providing a low-latency pipe was not enough. For true integration, the 5G network needed to appear as a standard, manageable component within the TSN ecosystem, which is controlled by a central CNC. The TSN AF solves this problem by acting as the 5G system's agent to the TSN control plane.

It addresses the key limitation of previous wireless solutions—their opacity and lack of deterministic scheduling integration. Without the TSN AF, a TSN CNC could not see or control the 5G link, making end-to-end deterministic scheduling impossible. The TSN AF provides the necessary translation layer, allowing the CNC to treat the 5G radio link as just another TSN bridge with known characteristics. This motivated its creation in 3GPP Release 16 as part of the 5G system's support for vertical industries, specifically factory automation. It enables the convergence of OT (Operational Technology) and IT networks, allowing 5G to become a viable replacement for cables in the most demanding industrial control applications, thereby enabling new levels of flexibility and reconfigurability in smart manufacturing.