# Time-Sensitive Networking Technologies for Industrial Automation in Wireless Communication Systems
## 5 Integration WLAN with 5G TSN
### Technical Challenges in Wireless Communications for TSN  
In this section, we discuss technical challenges in wireless communication for the TSN service. As discussed in Section 5.2, WiFi as well as 5G NR should guarantee deterministic transmission when 5 GS uses 5G NR and WiFi accesses for TSN service. Figure 11 shows technologies for guaranteeing deterministic QoS between the UPF/NW-TT and UE/DS-TT based on the proposed architecture in Figure 9. To support deterministic transmission within the 5 GS, DS-TT and NW-TT provide hold and forward buffering for de-jittering. 5G core network (CN) supports delay-critical guaranteed bit rate (GBR) and redundant transmission between the UPF and the gNB, and 5G RAN uses periodic transmission with TSCAI as well as delay-critical GBR and redundant transmission between gNB and UE. Eventually, the 5 GS via 5G NR guarantees deterministic QoS delivery. However, WLAN currently does not guarantee the deterministic transmission although it uses enhanced technologies such as OFDMA, TWT, and SR.  
Due to the characteristics of WLAN, unstable communication environment and collision avoidance mechanism, WLAN cannot provide deterministic QoS service. This is because the unstable communication environment causes unexpected packet loss and unnecessary retransmission, and the collision avoidance mechanism causes unnecessary waiting time and increased latency. Therefore, 802.11 ax has proposed OFDMA, SR, and TWT to reduce latency, improve reliability, and support power saving mode scheduling, respectively. Since OFDMA allocates a RU to each station, some RUs are not used when the number of active stations is less than the number of available RUs, so the delay is not significantly reduced. SR intends to improve reliability by reducing interference between  
Figure 11: Technologies for deterministic QoS in 5 GS.  
Figure 10: WiFi access within the 5 GS.  
adjacent APs. However, only one of the adjacent APs reduces transmission power when interference occurs between them, so there is a limit to improve reliability. In TWT, the AP does not provide QoS scheduling based on traffic priority although it schedules traffic transmission between the AP and the node.  
After 802.11 ax, 802.11 be has been proposed as next MAC protocol of WLAN for further improvement [40, 41]. For wireless TSN, several 802.11 be technologies has been addressed to achieve deterministic QoS [40]. The 802.11 be improved OFDMA performance by allocating multiple RUs for each station and reduced the latency by allocating RUs for direct links between the stations. For that, the description scheme for multiple RUs and allocation scheme of RU for direct transmission should be addressed. The 802.11 be improves transmission reliability by performing multi-AP cooperation-based coordinated spatial reuse (CSR) [42]. As shown in Figure 7, in CSR, both APs adjust the transmission power to reduce mutual interference. The 802.11 be proposed EDCA improvement and Multi-link operation to support TSN. New access categories (ACs) were added to the existing four ACs proposed, and transmit opportunities (TXOPs) are allocated to the corresponding ACs. EDCA reduces the latency and contention, and improves the transmission reliability. In a Multi-link operation, as shown in Figure 12, one MAC address is assigned to a wireless NIC; however, multiple physical interfaces are simultaneously activated, and data are transmitted through all physical interfaces. The joint mode in which packets are transmitted in multiple physical interfaces reduces the delay time, and the duplicate mode in which packets are redundantly transmitted to multiple physical interfaces improves transmission reliability. We expect that 802.11 be will provide periodic deterministic communication using newly developed technologies such as enhanced OFDMA, CSR, EDCA with TXOP, and Multi-link.