# A Reliable Handoff Mechanism for Mobile Industrial Wireless Sensor Networks
## 6 Evaluation and Experiments
### Testbed Experiments  
Our testbed positions 20 nodes in a 50 \(\times\) 50 m\({}^{2}\) square at random. Nodes are not too close to each other for practical reasons, and there is no case where _MN_ cannot detect any of the infrastructure nodes. The model of mobility is random waypoint [38], it imitates a man catching a node. Each round of the experiment lasts for 30 min. It contains several pause and movement, which sum 10 and 20 min respectively. The pause time is randomly selected within one minute. Considering the length of the superframe, less than four nodes are served by the same parent. Another assumption is that every node takes probability to be a temporary parent of _MN_. The mobility imitates a person walking or running, which is why the speed and direction of movement are not precisely controlled but rather practical. Interference is added by putting Wi-Fi devices around the testbed. The interference is determined by adjusting the Wi-Fi, including frequency, signal strength and quantity. Two levels of interference with packet error rate 2% and 6% are used in the experiments to demonstrate a normal and a bad wireless channel respectively. To facilitate counting the packet delivery and to measure the energy consumption of _MN_, two nodes are considered to be the mobile nodes. The results of _MN_ shown in the figures are the average of two _MN_s. To evaluate the performance of our proposed mechanism, comparisons of RLP-\(V\), REP-\(V\), RLP-_MN_ and REP-_MN_ are made using WirelessHART and RSSI-based handoff. The results of the packet delivery with a 2% packet error rate are shown in Figures 12a-d. Although the RLP-\(V\) of the two solutions are not much lower than our mechanism, three other metrics show a big gap between WirelessCAN and two other solutions. Firstly, it roughly indicates that the mobility has a great effect on IWSNs without mobility support. Secondly, the common solution of RSSI-based handoff cannot solve this problem particularly well. Compared with WirelessHART, our mechanism decreases 2.34% RLP-_MN_ and 2.61% REP-_MN_, even though there are two mobile nodes and the results represent their average value. Meanwhile, the decreases of 1.16% RLP-_MN_ and 0.83% REP-_MN_ are only contributed by the solution of RSSI-based handoff. Most expired packets in WirelessHART and RSSI-based solutions are caused by the packets of _MN_ not being transmitted timely during movement. The reason why the RSSI-based solution causes 1.34% REP-\(V\) is that the ping-pong effect can occur on any of the nodes due to imprecise triggering. Figure 11: RLP-\(V\), REP-\(V\), REP-\(MN\) and energy consumption of mobile node with variation of \(\mu_{T}\). As the interference deteriorates to 6% packet error rate, the performance of the WirelessHART and RSSI-based solution is worse still that the performance under 2% packet error rate as shown in Figures 13a-d. The packet error rate is approximately 6%, or even higher in most industrial environments. However, RLP-\(V\), REP-\(V\), RLP-\(MN\) and REP-\(MN\) just respectively decrease 0.07%, 0.34%, 0.12% and 0.22% using WirelessCAN system with interference of 6% packet error rate. The results of comparison between the two levels of interference show that our mechanism is largely unaffected by increasing the interference. However, the more interferences there are, the more instability is generated during triggering when using RSSI-based handoff. Moreover, the ping-pong effect problem in the RSSI-based solution causes more energy consumption. Figure 14 shows the energy consumption of \(MN\) in each solution, denoted as \(E_{MN}\). As demonstrated, \(E_{MN}\) of our mechanism is close to WirelessHART, but much lower than the RSSI-based solution. Consecutive registration in WirelessCAN does not increase energy consumption, but in the remaining two solutions it does. The extra \(E_{MN}\) in our solution is consumed for the transmission during the movement. Thus our WirelessCAN system improves the reliability under frequent mobility in IWSNs.