# Smart Preliminary Channel Access to Support Real-Time Traffic in Wi-Fi Networks
## 5 Results  
To study the methods described in Section 4, we develop a discrete-event simulator similar to ns-3 [48]. We consider a Wi-Fi network with an AP, \(N\) non-RTA STAs, and \(M\) RTA STAs. All the STAs sense transmissions of each other perfectly, so there are no hidden STAs. Non-RTA STAs support the 802.11n standard, but without significant changes in  
Figure 2: Transmission scheme of Smart PCA. results, we can consider 802.11 ac STAs. Non-RTA STAs operate in saturation mode and serve the traffic of one AC with parameters \(W_{min}\), \(W_{max}\), AIFSN and TXOP Limit. They always occupy the channel for the maximum time defined by TXOP Limit and use the RTS/CTS mechanism to increase throughput. RTA STAs support the 802.11 be standard and generate quasiperiodic traffic (Figure 3). The expected frame arrival time has a period \(T_{period}\), and the real arrival time is normally distributed around the expected time with a standard deviation \(\sigma\). This model of traffic can describe RTA that periodically generates data, but the real frame arrival time is random due to the implementation issues, such as clock inaccuracy or unpredictable operation system delays. Let us define the interval around the expected frame arrival time as \(T_{exp}=10\sigma\). This value of \(T_{exp}\) guarantees that more than 99.999% of data frames are generated inside \(T_{exp}\). All the RTA STAs have the same periods \(T_{period}\), but the cycles of different RTA STAs are shifted relative to each other by a random value uniformly distributed in \([0,T_{period}]\). The AC for RTA traffic has the following parameters: \(W_{min}[\text{RTA}]=W_{max}[\text{RTA}]\triangleq W[\text{RTA}]\) and AIFSN[RTA]. We assume that all the RTA STAs use the same modulation and coding scheme, which is reliable enough to neglect frame losses because of random noise in the channel. The size of RTA MAC Protocol Data Units (MPDUs) is fixed and equal for all RTA STAs. Note that we consider a short duration of RTA frames; therefore, in the Tuned EDCA, RTA STAs do not use the RTS/CTS mechanism in order not to increase the delay because of the additional frames transmissions. For the methods described in Section 4, we consider that all the STAs operate in a 40 MHz channel. For the multi-link method [44] described in Section 3, we assume that AP and RTA STA simultaneously operate in two 20 MHz channels, while \(N\) non-RTA STAs can operate only in one channel and are uniformly distributed among two 20 MHz channels. Note that as receiver sensitivity is higher in the narrower channel, RTA STAs in 20 MHz can use faster MCS than in 40 MHz without losing reliability. The modeling parameters are listed in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(T_{\epsilon}\) & 9 μs & RTA MPDU size & 2500 bytes \\ \(T_{CTS}\) & 24 μs & \(N\) & 8 \\ \(SIFS\) & 16 μs & & \\ \hline
**Parameter** & **Value for RTA STAs** & **Value for non-RTA STAs** \\ \hline AIFSN & 2 & 10 \\ \(W_{min}\) & 8 & 16 \\ \(W_{max}\) & 8 & 1024 \\ Standard & 802.11 be & 802.11n \\ OFDM Symbol & 13.6 μs & 4 μs \\ Guard Interval & 0.8 μs & 0.8 μs \\ Preamble & 48 μs & 40 μs \\ MCS for 40 MHz & MCS 2 & MCS 6 \\ MCS for 20 MHz & MCS 3 & MCS 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Modeling parameters. Figure 3: Quasiperiodic traffic of RTA STAs. To compare the RTA service methods, we consider the following QoS metrics. For RTA traffic, we use the _0.99999-quantile_ of delay, which shows the minimum delay that can be guaranteed with a probability of 0.99999. The QoS metric for non-RTA traffic is the efficiency of channel usage by non-RTA STAs. The further description is organized as follows. In Section 5.1, we consider how the described metrics depend on the number of RTA STAs. Here and further, we consider a fixed number \(N\) of non-RTA STAs.