# Smart Preliminary Channel Access to Support Real-Time Traffic in Wi-Fi Networks
## 3 Related Works  
Solutions for RTA traffic service in Wi-Fi networks are studied in the IEEE 802.11 working group and in the literature. Some RTA-aware solutions rely on mechanisms that appeared in the 802.11 ax standard amendment [16; 17], e.g., Orthogonal Frequency Division Multiple Access (OFDMA) [18; 19; 20; 21; 22]. OFDMA allows the AP to allocate different subcarriers of the same channel to multiple STAs for simultaneous reception or transmission of data. An OFDMA-based approach to RTA traffic service was explored in [8] and presented in RTA TIG. Note that to allocate frequency resources in OFDMA, the AP needs to know when and which STAs have data. As a result, the transmission of information about the appearance of frames in the queue causes additional delays. To avoid this problem, the AP can leave a part of the channel for random access [23; 24; 25; 26; 27; 28; 29] so that an STA can attempt to transmit as soon as it has a frame. However, with random access, STAs transmit data according to the access method that is similar to the ALOHA [30; 31], the efficiency of which significantly reduces with a large number of contending STAs. The 802.11 ax standard also defines the Quiet Time Period (QTP) mechanism [32], which allows STAs to request time intervals during which other STAs are not allowed to access the channel. QTP is important for RTA because transmissions in such time intervals avoid contention-related delays. For example, the authors of [33] propose an RTA traffic service approach based on periodic reservations implemented with QTP. With this approach, the AP periodically allocates time intervals for transmissions of only one of the RTA STAs. However, the QTP mechanism is optional, and the presence of STAs not supporting it may violate the restrictions on low delays. In addition, if several Wi-Fi networks overlap, this approach requires an additional central coordinator, which distributes time among the networks. As RTA traffic service is an important use case for Wi-Fi 7 networks, many works are devoted to the study of the mechanisms introduced in the 802.11 be amendment. Overviews of Wi-Fi 7 innovations are included in [34; 35; 36; 37; 38], and their application for serving delay \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**AC** & **Wmin** & **Wmax** & **AIFSN** & **TXOP Limit** \\ \hline Voice & 4 & 8 & 2 & 2.08 ms \\ \hline Best Effort & 16 & 1024 & 3 & 2.528 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Default EDCA parameters for client STAs. sensitive traffic is discussed in [39]. The standard defines the restricted target wake time (r-TWT) mechanism to reserve channel resources and provide low delays for predictable and delay-sensitive traffic. However, the mechanism becomes unreliable in the presence of legacy devices that do not support r-TWT and have poor clock accuracy. Moreover, r-TWT is an optional mechanism, and the standard states [14] (Section 35.9.4.2) that the Wi-Fi 7 STAs that do not support this method can ignore the r-TWT service periods, which violates the prioritization of RTA packets. This behavior is favorable for non-RTA STAs and implicitly makes the STAs not support this method if they do not have RTA traffic. The 802.11 be standard also introduces a multi-link operation (MLO) technology that can be used to serve RTA. Notably, some RTA-aware ideas based on simultaneous work in several channels had been studied even before MLO became a part of Wi-Fi. For instance, the paper [40] suggests using an additional radio interface to maintain a control channel. In this proposal, a STA sends a busy tone in the control channel when an RTA frame is enqueued. All STAs listen to the control channel and will immediately stop non-RTA transmissions once they detect the busy tone. However, low delays cannot be guaranteed in the presence of STAs that do not support such a mechanism. Another example of RTA traffic service using several channels is introduced in [41; 42; 43].