# Time-Sensitive Networking Technologies for Industrial Automation in Wireless Communication Systems
## 1 Introduction  
Industrial automation is a representative system attracting attention in the fourth industrial revolution. In the industrial automation system, data and controls are exchanged between manufacturing equipment within a fixed period of time [1]. The soft real-time manufacturing system that allows some deadline misses requires latency of 10-50 ms. In a hard real-time manufacturing system where operations must be completed within a deadline, the latency is less than 1 ms because of low cycle time [2]. In addition to low latency, a significant amount of data must be transmitted without data loss with low jitter for hard real-time networking. Therefore, to support deterministic transmission, researchers focused on time-sensitive networking (TSN). TSN, developed by the IEEE 802.1 Task Group, is a standard technology based on Ethernet (802.3) and can achieve low transmission delay, low jitter, and reliable packet transmission [3]. The Ethernet-based TSN technology can support services from soft real-time networking to hard real-time networking, and it requires wiring, network equipment, etc. to connect manufacturing equipment. However, in industrial networks, wired-based networks do not provide scalability and flexibility because the wiring cost increases as manufacturing equipment is added or relocated frequently. To construct scalable networks and offer mobility of manufacturing equipment, the wireless TSN, where the TSN technology is required to extend from wired networks to wireless networks (e.g., 5G and 802.11 be), has been proposed [4]. However, because communication environments and mechanisms differ between wired networks and wireless networks, the Ethernet-based TSN technology is not compatible with wireless technologies, and several researchers have addressed wireless TSN [2, 5, 6, 7, 8, 9, 10]. In wireless networks, the soft real-time manufacturing process is supported by LTE networks and 802.11 ax [2]. However, because the hard real-time manufacturing process requires sophisticated deterministic transmission, next generation of wireless communication technologies such as 5G and 802.11 be are required. For the 5G System (5 GS) tosupport TSN, a new function is required. To manage the periodically transmitted TSN traffic, time synchronization must be supported. Currently, because a 5 GS does not provide time synchronization between the user equipment (UE), radio access network (RAN), and user plane function (UPF), a new time synchronization mechanism is required. In addition, a new buffering and scheduling mechanism is required for processing TSN traffic because guaranteeing a deterministic delay as well as a low delay is necessary. The WLAN can be used for TSN besides 5 GS. The TSN over WLAN can be implemented without modification of TSN reference architecture because WLAN technology is one of the IEEE 802 family of MAC layer [2]. In wireless TSN over WLAN, time synchronization is required. However, while the Ethernet network interface card (NIC) supports precision time protocol (PTP) for accurate time synchronization [11; 12], less accurate time synchronization is achievable because the wireless NIC does not provide a PTP hardware clock (PHC). For time synchronization, timing measurement (TM) and fine timing measurement (FTM) have already been proposed in wireless TSN [5]. Both Ethernet and 802.11-based WLAN use contention-based packet transmission and determine whether packets are transmitted through channel-state monitoring. However, because WLAN cannot detect collisions, it uses a random backoff counter to avoid collisions. The random backoff mechanism increases the transmission delay and causes a high jitter. Thus, maintaining the delay within a particular range is challenging. In this paper, we have summarized TSN technologies in a wireless communication environment. As shown in Figure 1, TSN uses three technologies as underlying technology: Ethernet, WLAN, and 5 GS. For wireless TSN, we describe 5G technology and 802.11-based WLAN except for wired-based Ethernet.