## 6 Evaluations
### Experimental Setup  
In the experiments, _Raspberry Pi 4B_ is used for each access point (AP) by running _hostapd_[54]. The embedded wireless NIC is used for the 11n interface, and the _Archer T4U_ wireless NIC adapter is used for the 11 ac interface. The _Linux_ laptop PCs are used for the client hosts and server. For both interfaces, the 40 \(MHz\) bonded channel is used. The hardware and software presented in Table 5 are taken into account in the measurements. To set the bonded channel for both interfaces, we can modify the /etc/hostapd/hostapd.conf file using the following Linux commands:  
$ #40 MHz for 11n  
$ rsn_pairwise=CCMP  
$ ht_capab=[HT40+][SHORT-GI-20][SHORT-GI-40][DSSS_CCK-40]  
[MAX-ANDU-3839]  
$ #40 MHz for 11 ac  
$ rsn_pairwise=CCMP  
$ vht_capab=[MAX-MPDU-3895][SHORT-GI-80][SU-BEAMFORMEE]  
The _[HT40+]_ needs to set the 20 MHz primary channel in the /etc/hostapd/hostapd.conf file. Then, the primary channel will be bonded with the secondary channel to form the 40 MHz channel. _ipperf 2.0.5_[55] software is adopted for the throughput measurements as a popular tool for measuring the TCP throughput by sending TCP packets from the server to the host. The TCP traffic at a link automatically saturates by this software. The TCP traffic was generated for each measurement point in this paper using a 477 KB TCP window size and an 8 KB buffer size for four minutes at intervals of 30 s, which was conducted twice on different days. Then, the average value is calculated for each point and is shown in this paper. The packet size of the generated TCP traffic is 1500 bytes. For each point, the average of the measured throughput is computed over four minutes at 30 s intervals on two separate days in order to eliminate the fluctuations in measurements. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Values** \\ \hline packet size & 1500 bytes \\ max. transmission rate & 150 Mbit/s \\ propagation model & log distance path loss model \\ rate adaptation algorithm & link speed estimation model [24] \\ carrier sense threshold & \(-\)85 dBm \\ transmission power & 19 dBm \\ collision threshold & 10 \\ RTS/CTS & yes \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Simulation parameters for the WIMNET simulator. \begin{table}
\begin{tabular}{c c} \hline \hline simulator & WIMNET \\ CPU & Intel Core i7 \\ memory & 8 GB \\ OS & Ubuntu LTS 14.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: PC environment. The elastic WLAN testbed system using _Raspberry Pi_ APs with single interfaces was implemented and used in [13, 14, 22, 56, 57, 58]. We extend this testbed by adding _Archer T4U_ at the AP for the dual interface, as shown in Figure 9. The testbed included four Linux laptop PCs for the hosts and one Linux laptop PC for the server that is used to collect the necessary information for the active AP configuration algorithm and controls the APs and the hosts. This server controls the overall system through the following steps:  
1. The server obtains active all the APs in the network. 2. It collects the information of the connected devices in the network using _arp-scan-tool_ in Linux, which includes the IP and MAC addresses of the APs and the hosts. 3. It collects the _receiving signal strength (RSS)_ at each host from each AP using _nm-tool_. 4. It converts the RSS to the throughput using the _sigmoid function_ in [23]. 5. It executes the _active AP configuration algorithm_ using the inputs derived in the previous steps and obtains the algorithm output that consists of the number of active APs, the host associations, and the channels assigned to the active APs. 6. It activates or deactivates the APs in the network according to the algorithm output using the following command. $ sudo /etc/init.d/hostapd start  
$ sudo /etc/init.d/hostapd stop
7. It changes the associations of the hosts with the APs according to the algorithm output using _nmcli-tool_. 8. It assigns the channels to the APs according to the algorithm output using _sed-tool_.