# Time-Sensitive Networking Technologies for Industrial Automation in Wireless Communication Systems
## 2 Background  
TSN is an extension of the audio video bridging (AVB) technology, which is based on Ethernet. The TSN technology aims to provide reliable packet delivery with deterministic latency. To achieve this, the IEEE 802.1 TSN Task Group developed a TSN standard encompassing time synchronization, bounded low latency, reliability, and resource management technologies [3].  
TSN technologies for bounded low latency, reliability, and resource management depend on the time synchronization between TSN nodes. IEEE 802.1 AS defined the gPTP to achieve a time synchronization in the millisecond range for all nodes in an Ethernet network [11; 12]. The gPTP performs time synchronization between network devices by delivering synchronization event messages (sync, delay, etc.) between the clock master and the clock slave. Because the Ethernet-based NIC provides the PHC, the clock time of the slave device is synchronized with that of the master device.  
Bounded low latency is a technology for transmitting packets within a limited transmission delay. TSN guarantees bounded low latency in a bridged network by adding delay-determined forwarding and queuing functions to the existing IEEE 802.1 bridge. Accordingly, each bridge provides a determined end-to-end delivery latency by processing the TSN stream in a cycle corresponding to the synchronized time. To achieve bounded low latency, time-aware scheduling and frame pre-emption have been proposed. First, time-aware scheduling divides Ethernet traffic into eight priorities and ensures that the allocated transmission time for each priority is within a communication cycle. Because high-priority traffic avoids congestion and channel contention with low-priority traffic, it achieves low latency [13]. Second, when more urgent or crucial packet transmission is required, frame pre-emption stops the transmission of the outgoing low-priority packet and transmits a packet with high priority. Then, the stopped packet transmission is resumed [14; 15].  
The TSN standard improves the reliability of packet transmission by redundantly transmitting packets through multiple paths [16]. This redundant transmission not only improves transmission reliability but also prevents retransmission delay due to packet loss. To minimize the increase in network traffic due to redundant transmission, when a packet arrives at the destination, the other redundant packets are removed from other transmission paths.  
Resource management for TSN uses three models: fully distributed model, centralized network/distributed user mode, and fully centralized model. For stream reservation in the fully distributed model, the talker sends network configuration information of the stream (i.e., service traffic) to the listener using the stream reservation protocol (SRP) [17]. Then the listener returns the stream ID. The centralized network/distributed user model allows talkers and listeners to reserve resources using SRP, and the user/network configuration information is delivered to centralized network configuration (CNC) for resources reservation. As shown in Figure 2, the fully centralized model reserves and manages resources through centralized user configuration (CUC) and CNC. The centralized network/distributed user model or fully centralized model using CNC/CUC manages resources using the YANG model [18; 19].  
As wireless communication technologies such as 5G and 802.11 ax emerges, TSN has been extended to wireless networks [4]. Figure 2 describes the example of 802.11-based wireless TSN for industrial network [20]. Since wireless TSN provides some benefits compared to wired TSN, several groups have addressed industrial use cases such as closed loop system and mobile robot system [21; 22; 23]. The wireless TSN enables deployment flexibility, mobility, low maintenance cost for closed loop system and mobile robot system.