# Distributed Routing and Channel Selection for Multi-Channel Wireless Sensor Networks +
## 6 Performance Evaluation
### Evaluation in an Experimental Testbed  
We implement our proposed scheme DRCS in TinyOS using MICAz motes that use LowPowerListening with wake-up intervals of 125 milliseconds. The beacon interval, DATA interval and \(\tau\) are chosen to be 30, 60 and 180 s respectively. The transmit power is chosen to be \(-28.5\) dBm to enable multi-hop communications in a small place. We place 18 motes that periodically sense and forward sensor data to the sink using our proposed multi-channel routing scheme DRCS. The position of the sink is varied to form three different network topologies as shown in Figure 9a-c. For ease of obtaining packet counts, we disable retransmissions. The results obtained over a duration of 15 min are shown in Figure 9d,e. It is observed that in all three topologies, the number of packets received at the sink drops only marginally with increasing number of channels, even with no retransmissions. This implies that the packet delivery performance is not affected by channel switching in these data-rates. However, there is a significant reduction in the total number of overhearing packets by using 2 and 4 channels. With just 2 channels, the overhearing is reduced to \(\frac{1}{3}\). This experiment demonstrates that DRCS can significantly reduce energy wastage due to overhearing without sacrificing the delivery performance.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Var** & **Values** & **Var** & **Values** & **Var** & **Values** & **Var** & **Values** \\ \hline \(I_{Bt}\) & 20 mA & \(T_{Bt}\) & 140 ms & \(I_{Br}\) & 20 mA & \(T_{Br}\) & 140 ms \\ \(I_{Dt}\) & 20 mA & \(T_{Dt}\) & 140 ms & \(I_{Dr}\) & 20 mA & \(T_{Dr}\) & 140 ms \\ \(I_{P}\) & 20 mA & \(T_{P}\) & 3 ms & \(I_{S}\) & 7.5 mA & \(T_{S}\) & 112 ms \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation environment.  
Figure 9: Different deployment scenarios for the experimental testbed and test results. The sink locations are marked by yellow circles: (**a**) Scenario-1; (**b**) Scenario-2; (**c**) Scenario-3; (**d**) results obtained on the total packets overheard; and (**e**) the number of packets delivered to the sink, with 1, 2, and 4 channels.  
To show the effectiveness of the dynamic channel selection scheme, we set up a small network as shown in Figure 10a, and monitor the variations of the number of packets overheard in a specific node when its battery voltage (and hence, its capacity) is changed manually. In this experiment, we use only 2 channels and a data interval of 15 s. Initially, the battery capacities of all nodes are made to be 100%. After 30 min, the battery voltage of node D (provided by a programmable power supply) is reduced to artificially represent a capacity of 50%, keeping all others unchanged. This is done to artificially introduce a temporal energy variation at node D, and demonstrate how DRCS adapts to such energy variations. Figure 10b shows the variation of the number of packets overheard by node D over time. Each bar on the x-axis shows the number of overheard packets by D over time blocks of 5 min duration. It can be observed that after 30 min the overhearing on node D starts reducing as all other nodes switch their transmit channels to avoid the receiver channel of D. This experiment demonstrates that our proposed scheme helps in reducing energy consumption at a node with bad health-metric, which can occur due to deteriorating battery health.