## 6 Evaluation of RPL and RPAL in a Real World Environment
### Evaluation Results
#### 6.2.4 Energy Usage  
In Figure 8b, the effects of two routing models on the energy consumption performance is illustrated. In comparison with the simulation results in Section 5.2, we have to admit that the behavior is different from the expectations. This outcome can be explained by the following reasons: the nodes in the simulation tests have the same link conditions but the field test nodes do not have the same link conditions. In other words, the repair mechanism for detecting routing loops is initiated more often in the real world tests, and this could be responsible for an increase in the power consumption. Furthermore, the current prototype system cannot provide low MCU power mode due to the lack of an appropriate solution using Contiki process management to deal with the correlation between the AVR MCU and the NANO module. Thus, keeping MCU in normal working mode is a safe and temporary method to evade these potential issues. Besides, in our tests, if the radio chip enters sleeping mode or the ulPV6 process is trying to send/receive a packet, and at the same time, the process of reading NANO's state has a certain level of communication delay, the wake-up interval in this node will become unstable and lose the synchronization with its neighbor nodes, and this leads to the failure of subsequent radio transmissions. Although this is an event with a very small possibility, the tests show that it will happen within the  
Figure 8: **(a)** Packets lost ratio in the field tests of 11 IWoCore nodes; **(b)** Average energy usage of the testbeds in the 1 st and 2 nd experiments.  
duration of 20 to 30 h. Thus, the RDC mechanism (_i.e._, fast sleeping and low power listening) in the ContikiMAC model and the transmission of collect-view application packets are designed to have a short intermission when they have conflicts with the NANO process. This could also cause the augmentation of radio duty (depicted in Figure 9).  
The RDC results in Figure 9 further prove that the RPAL model can prevent network churns and reduce energy usage due to misbehaving nodes, and improve the topology stability by increasing the effects of hysteresis. For example, node 3 has smaller RDC in the 2 nd test after the energy consuming task is carried out, it sinks to lower rank of DODAG and the other nodes will select optional routing path. In other words, standard RPL model will continue a relatively greedier PDR searching algorithm to treat node 3 as the key router. In the case of our designed experiments, the two tested routing strategies will impact the lifetime of these two exceptional nodes (3 and 6). According to the recorded data, nodes 3 and 6 can survive around 31 and 14 min longer respectively in the 2 nd experiment than in the 1 st one. However, the obtained results cannot ensure the RPAL model is energy efficient since the number of redundant routers and the duration of experiments are still limited, but at least, RPAL outperforms the standard RPL model in the aspect of energy balance, which can lead to longer network lifetime.  
As the only difference between the 2 nd test and 3 rd test is whether nodes 3 and 6 can recharge their batteries from the emulated solar panel, energy power mode becomes the single factor that most impacts the network structure. However, no improved network performance in the 3 rd experiment can be observed, except in the packet reception ratio. More network churns emerged compared with the results in the 2 nd test. We also notice that the network topology is not stable during the period when the battery-depleting nodes have up-and-down battery volume because they will become optional key routers along with the restoration of their batteries, and this enables them to be selected as preferred parents again.