## 7 Performance Evaluation
### Discussion  
In this subsection, we discuss the results and clarify the limitations of our analysis. The goal of our performance evaluation is to show that our proposed schemes are more energy-efficient than  
Figure 16: Total communication overhead.  
Figure 17: Total energy consumption.  
existing schemes when integrating with adaptive code dissemination protocols which change the packet size depending on the link quality. To take into account the effect of the various link quality, we defined a simple 3-hop topology where each hop is assigned a static BER to represent different link quality. We additionally assumed that no collisions due to channel contention occur in order to focus on the channel error which is the dominant factor for the adaptive code dissemination. Finally, we adopted TelosB as a representative platform and the operation of the CC2420 wireless transceiver is simplified by ignoring the transition time among TX, RX, and off state. Under these assumptions and simplifications, we showed that our proposed schemes outperform Secure Deluge which is a well-known secure code dissemination scheme using fixed-size packets. As shown in Section 7.3, our proposed schemes work well in all kinds of link quality by adapting the packet size to the link quality while Secure Deluge uses small fixed-size packets of 22 bytes which is only good for the bad link quality. It is very important to note that our performance evaluation is conducted under the following assumptions, which will be considered carefully when applying to the real-world environment.  
* the use of a simple 3-hop topology with static link quality
* the assumption of no collisions due to channel contention
* the use of TelosB
* the simplification about the behavior of the CC2420 wireless transceiver
* the use of Deluge  
We used a simple 3-hop topology where each hop is assigned a static BER for the sake of simplicity. To achieve broader applicability of our work, we must further consider the wide range of network configurations and the effect of more realistic link quality by applying our schemes to the real-world testbeds with a variety of network configurations, which is one of our future works. We also assumed that no collisions due to channel contention happen. Even if the system performance such as energy consumption will be degraded by collisions due to channel contention, the proposed schemes will be influenced by the channel error more than collisions since they focus on the design about dynamic packet size depending on the channel conditions. However, the impact of collisions must be taken into account in order to obtain more accurate results in the real-world environment. We used TelosB as a representative platform because TelosB is slightly out-of-date [28] but it is still commonly used in WSNs [15; 16; 17]. Since the energy consumption can be different according to the platforms, the new performance evaluation is required before applying our schemes to the new platform. For simplicity, we ignored the transition time of the CC2420 wireless transceiver, which will have impact on the energy consumption of the real sensor nodes but it is difficult to reflect the operation of transitions of the CC2420 without experiments. Thus, we must perform the real-world experiment to obtain the exact energy consumption of each sensor node, which is included in our future works. Finally, our proposed schemes are based on Deluge which is the _de facto_ standard code dissemination protocol in WSNs. Even though our work can provide source authentication for other code disseminations with slight modifications, we do not consider other code dissemination protocols in this paper since our works are originally targeted at WSNs where Deluge is the _de facto_ standard.