## 4 TSN for Industrial Communications  
Real-time communication technologies that can provide guaranteed delivery of critical messages in a timely manner are of utmost importance in many manufacturing industries. Today, there exists a wide variety of wired and wireless communication technologies. Industrial use cases that demand ultra-high reliability and low latency, such as control systems in automation applications, robotic motion control, and safety-critical systems, rely on several dedicated open (e.g., PROFINET) and proprietary Ethernet protocols (e.g., EtherCAT, Ethernet/IP) [19]. These protocols, collectively known as Real-Time Ethernet, are built upon Ethernet's legacy features, which were mainly designed to handle best-effort traffic. OPC-UA (Open-Platform Communications--Unified Architecture) is another crucial communication standard. It is a platform-independent, service-oriented machine-to-machine communication interface and is considered the de facto communication standard for Industry 4.0 [50]. Recently, there has been an emerging trend in integrating OPC-UA with time-sensitive networking (TSN) to deliver real-time, deterministic data exchange and ensure interoperability [51; 52; 53]. TSN is a set of standards introduced to provide communication networks with real-time capabilities, such as zero/extreme low packet loss and bounded end-to-end latency. In this section, we aim to provide a brief overview of the key elements of TSN that support the vision of smart manufacturing. Figure 2: Key features of WiFi 7 and their applications in smart manufacturing. TSN aims to achieve deterministic real-time communication through time-triggered message delivery in the network, where all the network elements are globally time-synchronized. TSN accommodates traffic flows that demand ultra-high reliability (zero loss), low jitter, and low latency on a bridged network, along with best-effort traffic. This is achieved by combining various mechanisms, such as traffic shaping, defining various traffic queues, and implementing appropriate scheduling schemes, as well as reliability measures such as redundant/parallel message transmissions. Figure 3 outlines the main features/mechanisms associated with TSN, along with the respective standards. They can be broadly categorized into four pillars: (1) time synchronization, (2) bounded latency, (3) ultra-high reliability, and (4) network/resource management [54; 55]. **Time Synchronization**: To enable strictly bounded end-to-end latency and several other features, TSN requires precise time synchronization to sub-microsecond levels of accuracy among all network elements such as switches, bridges, and end devices (manufacturing robots, PLCs, etc.). TSN (wired) achieves this through the Precision Time Protocol (PTP--version 2) (IEEE 1588), using Ethernet frames in a distributed fashion to synchronize the timing of all network elements to that of a master device (known as the grandmaster). The TSN time synchronization standard, IEEE 802.1 AS (a subset of IEEE 1588), defines a generalized PTP (gPTP) profile to extend the time synchronization approaches beyond wired networks to wireless TSN. The currently active standard IEEE802.1 AS-2020 [57] covers the protocols and procedures involved in selecting the timing source (best master) in the network, the transport mechanisms for synchronized time, and indications of the occurrence and magnitude of impairments (e.g., phase and frequency discontinuities). The time synchronization mechanism is based on a master-slave approach, where all devices in the network are synchronized to the grandmaster (the device with the most accurate time source in the network) through periodic exchange of sync and follow-up messages over a spanning tree. IEEE 802.1 AS is an evolving standard, and details of active amendments (P802.1 ASdm, P802.1 ASdn, P802.1 ASdr, P802.1 ASds) can be found in [57].