Time Sensitive Communication and Time Synchronization Function

Description

The Time Sensitive Communication and Time Synchronization Function (TSCTSF) is a pivotal 5G Core Network (5GC) Network Function (NF) standardized from 3GPP Release 17 onwards. It is the central architectural entity designed to enable Time Sensitive Networking (TSN) capabilities and precise time distribution over 5G systems. The TSCTSF resides in the 5GC control plane and interacts with other core functions like the Policy Control Function (PCF), Network Exposure Function (NEF), and the User Plane Function (UPF) to orchestrate end-to-end deterministic communication services.

Architecturally, the TSCTSF has several key roles. First, it acts as a bridge between the 5G system and an external TSN network (or a TSN-aware application), often represented by a TSN Translator or a Centralized Network Controller (CNC). It exchanges TSN-specific configuration and requirements, such as gate control lists for scheduled traffic and time synchronization information, via the NEF or direct interfaces. Second, the TSCTSF is responsible for managing and configuring the 5G system as a virtual TSN bridge. This involves translating TSN requirements into 5G-specific QoS parameters and policies, which it provisions to the PCF and subsequently to the Session Management Function (SMF) and UPF. For time synchronization, the TSCTSF participates in distributing precise timing from a grandmaster clock (which could be inside or outside the 5G network) to User Equipment (UE) and UPFs, ensuring all nodes in the communication path are aligned to a common time reference, often using the IEEE 802.1AS (gPTP) profile.

From an operational perspective, the TSCTSF works in conjunction with the 5G system's QoS framework to create dedicated QoS Flows with guaranteed flow bit rate (GFBR) and maximum flow bit rate (MFBR), along with strict priority scheduling to meet latency and reliability bounds. It configures the UPF to act as a TSN talker/listener and bridge, implementing traffic shaping (e.g., per-queue gate control as per IEEE 802.1Qbv) on the N6 interface towards the data network. The TSCTSF also supports mechanisms for redundancy and seamless redundancy as defined in IEEE 802.1CB (Frame Replication and Elimination for Reliability) by coordinating multiple UPF paths. Its interfaces, such as Ntsctsf, are defined for service-based interaction within the 5GC, allowing other NFs to request time-sensitive communication services.

Purpose & Motivation

The creation of the TSCTSF was motivated by the industrial sector's demand to use 5G as a unified communication backbone for critical applications like factory automation, motion control, and process monitoring. These applications have stringent requirements for deterministic latency (often sub-millisecond), ultra-high reliability (up to 99.9999%), and precise synchronization (microsecond accuracy) that traditional best-effort mobile networks could not guarantee. Previous 3GPP releases focused on enhanced Mobile Broadband (eMBB) and ultra-reliable low-latency communication (URLLC) but lacked specific support for the scheduling and synchronization paradigms standardized in the IEEE TSN suite.

The TSCTSF specifically addresses the problem of integrating 5G systems into TSN-aware industrial networks. Before its introduction, there was no standardized way for a 5G network to appear as a virtual, deterministic bridge within a TSN domain, nor to participate in TSN's centralized configuration model. The TSCTSF bridges this gap, allowing 5G to be managed as a component within a larger TSN-controlled network. It solves the challenge of translating TSN's time-aware shaping and scheduled traffic concepts into actionable 5G QoS and user plane configurations, enabling end-to-end deterministic flows that traverse both wired TSN and wireless 5G segments seamlessly. This enables the convergence of Operational Technology (OT) and Information Technology (IT) networks over a single 5G infrastructure.