## 6 IoT AP Performance Evaluation  
To verify the reliability and performance of the proposed IoT AP, this section focuses on the ZigBee response time under dissimilar Wi-Fi environments. The ZigBee response time is defined as the time needed for the IoT AP completes the task of sending the command to the ZigBee devices and receiving the command from the corresponding ZigBee devices. Table 1 lists the parameters used to verify the existing ZigBee network (IEEE 802.15.4) and Wi-Fi network (IEEE 802.11n). The verification of the response time uses Java verification program, whereas all commands are sent and received by using the TCP method to ensure that the commands are sent to the correct destination. Figure 12 depicts the band variation between various Wi-Fi throughputs (0 KBps, 2000 KBps, and 4000 KBps) and ZigBee response time. The Wi-Fi channel is fixed to CH1 (2.412 GHz) and the ZigBee channel used for testing is changed. The horizontal axis represents the band variation between ZigBee and Wi-Fi (0.002-0.068 GHz) whereas the vertical axis represents the ZigBee response time. As shown in Figure 12, when the Wi-Fi throughputs are 0 KBps and 2000 KBps, the band variation between ZigBee and Wi-Fi has minimal impact against the ZigBee response time, which is 21 ms. When the Wi-Fi throughput is 4000 KBps, the bands used by ZigBee are closer to the Wi-Fi band, which results in longer response time. Upon the increasing amount of Wi-Fi packets, the probability and number of ZigBee packet collision will be higher. Therefore, some ZigBee packets need to be re-sent, which increases the ZigBee response time. However, the commands can be successfully sent to the ZigBee devices and the average of the response time is still less than 45 ms. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Wireless Protocol & IEEE 802.11n; IEEE 802.15.4 \\ Network Protocol & TCP \\ Channel Variation & Wi-Fi: CH1–CH14; ZigBee: CH11–CH26 \\ Band Variation & Wi-Fi: 2.412–2.484 GHz; ZigBee: 2.405–2.480 GHz \\ SoC Chip & 384 MHz MIPS24 Ke \\ Flash Memory & 64 MB \\ Simulation repetitions & 200 \\ \hline \end{tabular}
\end{table} 
Table 1: Simulation Parameters. Figure 12: The impact of ZigBee/Wi-Fi band variation on the ZigBee response time. Figure 13 shows the ZigBee response time affected by various Wi-Fi throughputs against various ZigBee channels (CH 11-CH 15). The Wi-Fi channel is fixed on CH1 (2.412 GHz) and the throughput is changed for the test. The horizontal axis represents the variation of Wi-Fi throughputs whereas the vertical axis represents the ZigBee response time. As shown in Figure 13, when the Wi-Fi throughput ranges between 0 KBps and 3000 KBps, there is a minimal impact on ZigBee response time, which is less than 22 ms. When the Wi-Fi throughput exceeds 3000 KBps, the channels used by ZigBee are closer to the Wi-Fi channels, which results in longer ZigBee response time. The probability and number of ZigBee packet collisions increase with the increment of Wi-Fi packets. Therefore, some ZigBee packets need to be re-sent, which increases the ZigBee response time. From Figure 13, we can conclude that the response time of ZigBee channel 15 is maintained to be less than 22 ms on any Wi-Fi throughputs. Figure 14 investigates the event response time which is measured by the duration from the time point of event occurrence to the time point that the Internet server has received the event report. The proposed IoT AP is compared with the gateway proposed in [12]. In [12], a gateway is designed to collect the sensing data and subsequently forward them to an Internet server via the Wi-Fi AP. Instead, the proposed IoT AP collects data from sensors and subsequently directly forward the data to the Internet server. The two tasks can be accomplished by the internal modules of IoT AP. As shown in Figure 14, we assigned the IoT device (smart meter) to report its sensing data to the compared gateway and the proposed IoT AP every one hour.