## 6 Simulations and Testbed Experiments
### Simulation Results
#### 6.2.2 Energy Consumption  
To clarify the effects of the two CDSAs on reducing energy consumption, we summarize the energy consumed in each state of the three groups, which are present in Table 4 and Figure 10. Idle state normally consumes the most energy among the node's states in a contention network [1,5]. From Table 4 and Figure 10, we can see that the CDSAs reduce the energy consumption significantly in the Idle and Receive states. The energy reduction is caused by the increase in the Sleep state. Also, the energy consumption in Transmit state is reduced in CDSAs due to the collision avoidance, which helps to reduce total energy consumption. Therefore, the total energy consumption of nodes using our CDSAs is less than the non-scheduling approach. Table 4 and Figure 10 show further that the T-CDSA can reduce energy much more than O-CDSA in the states of Idle, Receive and Transmit.  
We compare the energy consumption of the three approaches in Figure 11. Figure 11(a) reflects the relationship between the average energy consumption and the nodes' hop. Figure 11(b) shows the relationship between the energy consumption and the number of descendants of nodes. Each data point in Figure 11a,b is the average value of 20 simulation experiments in each group. From Figure 11(a), we find that the energy consumption of a node comes down when it is located close to the sink node, which applies to the non-scheduling, O-CDSA and T-CDSA. For the nodes with the same hop, the energy  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**State**} & \multicolumn{3}{c}{**Energy consumption (W)**} \\ \cline{2-4}  & **Non-scheduling** & **O-CDSA** & **T-CDSA** \\ \hline Idle & 100.867504 (0.788271) & 61.526519 (3.212833) & 29.996916 (0.181777) \\ Receive & 35.164835 (1.914381) & 24.498266 (0.556466) & 23.112505 (0.845375) \\ Transmit & 0.929933 (0.042540) & 0.776610 (0.017748) & 0.728381 (0.056686) \\ Sleep & 0.000000 (0.000000) & 0.057428 (0.002256) & 0.066978 (0.000280) \\ \hline \end{tabular} where the statistic value out of parenthesis denotes the mean value of 20 experiments in each group, and the statistic value in parenthesis represents the standard deviation of the 20 experiments.  
\end{table}
Table 4: Energy consumption in each state.  
Figure 10: Energy consumption in each state.  
consumption of O-CDSA is less than that of non-scheduling, and T-CDSA consumes the least. The more descendants a node has, the more workload it has. Therefore Figure 11(b) reflects the relation between the workload of a node and the amount of energy consumption. As is seen from this figure, for all the three schemes, the workload affects the amount of energy consumption, i.e. when the workload increases, the amount of energy consumption increases accordingly. However, the energy dissipation of the CDSAs rises up quicker compared with the non-scheduling scheme. Furthermore, the energy consumption of O-CDSA increases much faster than that of T-CDSA when the workload increases. When the number of descendants is big enough in O-CDSA, its energy consumption will be similar to or a bit higher than that of non-scheduling under the same conditions. This may be caused by re-transmission of data packets in many times when its parent node is in Sleep state. On the contrary, the energy consumption of the T-CDSA is always lower than that of non-scheduling, i.e. the T-CDSA is much stable. From this figure, we can also conclude that the two CDSAs are well adapted to the workload of all the nodes. Figure 11(c) depicts the total energy consumption at each round. And all the data are from the fist experiment in each group. As is seen from the Figure 11(c), T-CDSA can save more energy than the O-CDSA.  
Figure 11: Energy consumption analysis: (a) Average energy consumption versus the hop of nodes. (b) Average energy consumption versus the number of descendants of nodes. (c) Total energy consumption in WSN at each round.