## 4 Equipment and Methods  
### Equipment  
_IEEE 802.11 client_. An Apple iPhone SE (manufactured in 2016) was used as the IEEE 802.11 client. This model supports IEEE 802.11a/b/g/n/ac standards, as well as IEEE 802.11 tethering. It also supports HSPA, GSM, CDMA, EVDO and LTE. The device belongs to Category 4 according to the LTE-A standard. _Non-IEEE 802.11 interferer_. A Line6 Relay G30 wireless audio transmitter was used. It was set up to operate in RF2 mode with two channels (2428 and 2453 MHz). The wireless audio transmitter created two spikes in the 2.4 GHz spectrum (Figure 4), which were present on IEEE 802.11 channels 3, 4, 5, 6, 8, 9, 10, and 11. Manufacturer specifications indicate a range up to 30 m, depending on the surroundings. Figure 3: Illustration of CSMA/CA involving two devices and one interfering device on one channel of an IEEE 802.11 network in the presence of severe interference. ACK, Acknowledgment Control Frame. Figure 2: Illustration of CSMA/CA involving two devices and one interfering device on one channel of an IEEE 802.11 network in the presence of sporadic interference. ACK, Acknowledgment Control Frame; Dest, Destination. _IEEE 802.11 access point_. The D-Link DIR-615 access point was used (Table 2), which supports the IEEE 802.11 b, g and n standards. We opted for these standards because they are still widely used in various industry environments [2; 3; 4; 5]. It has two fixed omni-directional antennas with a gain of 2 dBi. The access point was secured with WPA/WPA2 authentication to prevent any other unwanted clients to connect to our network. We adopted the approach from [22] and opted for channel 6 in order to avoid potential interference from IEEE 802.11 networks near our laboratory set-up. Continuous monitoring using a spectrum analyzer detected only one additional IEEE 802.11 access point on channel 6, whose signal was weaker than \(-80\) dBm-much weaker than the \(>-40\) dBm signal of our network set-up. This monitoring confirmed minimal use of channel 6 during measurements. _Spectrum analyzer_. A spectrum analyzer allowed us to examine the channel utilization and recognize various radiofrequency signal patterns in the PHY layer (Figure 4). An Ekahau Spectrum Analyzer Model III (Ekahau, Helsinki, Finland) was used to collect radiofrequency spectral data. This device has an external RP-SMA antenna, and it detects the range between \(-100\) and \(-6.5\) dBm at a resolution of \(0.5\) dBm. It supports both 2.4 and 5 GHz bands. The radiofrequency spectral data were represented in real time using Ekahau Pro 10 software, which allowed us to monitor the radiofrequency spectrum before and during measurements. \begin{table}
\begin{tabular}{c c} \hline \hline
**Feature** & **Setting** \\ \hline BSSID & d\_8 \\
802.11 mode & b/g/n mixed mode \\ Band & 2.4 GHz \\ Channel no. & 6 \\ Channel width & 22 MHz \\ \hline \hline \end{tabular}
\end{table} 
Table 2: General settings used for the D-Link DIR-615. Figure 4: Spectrum analyzer comprising an Ekahau Spectrum Analyzer Model III and Ekahau Pro 10 software. The image shows data collected during a preliminary network run in the presence of interference. Measurement of throughput_. The Speedcheck Internet Speed Test ([[https://apps.apple.com/us/app/speedcheck-internet-speed-test/id616145031](https://apps.apple.com/us/app/speedcheck-internet-speed-test/id616145031)]([https://apps.apple.com/us/app/speedcheck-internet-speed-test/id616145031](https://apps.apple.com/us/app/speedcheck-internet-speed-test/id616145031)), accessed on 26 March 2021) was used to measure the throughput of the wireless network. This test is ranked number 1 among applications for measuring network throughput on the Apple App Store.