# Distributed Routing and Channel Selection for Multi-Channel Wireless Sensor Networks +
## 6 Performance Evaluation
### Simulation Results  
We conduct simulations to evaluate the performance of our proposed scheme in a larger network and to also evaluate the lifetime improvement achieved by DRCS. A deployment area of \(200\times 200\) m is assumed, where the nodes are deployed uniformly. The transmission power is assumed to be 0 dBm. The initial battery capacities of the nodes are assumed to be uniformly (randomly) distributed between 75% to 100%. The capacity of a fresh battery (100% capacity) is assumed to be 5000 mAh. The random variations in battery capacities allow us to evaluate how DRCS performs energy management and optimize the network lifetime under arbitrary energy variations. The beacon interval is set to 30 s and the maximum retransmission count is set to 30. We assume a log-normal shadowing model with path-loss exponent \(n=2.4\), and channel variance \(\sigma=4\) dBm. The path loss at a reference distance \(d_{0}=1\) m is assumed to be of 55 dBm.  
_Comparison with different datarate:_ Figure 11 shows the variation of the packet delivery ratios, overhearing counts and the _worst case network lifetime_ with different number of channels and transmission rates. The worst case network lifetime is defined as the time when the first node of the network dies. We distribute 200 nodes for these set of figures. It is observed that the packet delivery ratio is above 80% for all cases. This is consistent with the findings from the experimental testbed, indicating that at these data rates, the packet delivery ratio is not significantly affected by the channel switching scheme employed in DRCS. However, overhearing is reduced by nearly 60% with 2 channels and by almost 80% with 4 channels. This significantly reduces the average current consumption in the nodes and improves the network lifetime as seen from Figure 11b.  
Figure 10: Illustration of experimental tests used to evaluate the effectiveness of dynamic transmit channel selection. (**a**) Network layout; (**b**) variations of packets overheard by node D over time.  
_Comparison with TMCP [35]_: Figure 12 shows the comparison of DRCS with another well-known channel assignment scheme TMCP with different number of channels. Here, we assume a communication range of 40 m and an interference range that is 1.5 times of the communication range. Several observation can be made from the comparative results presented in Figure 12. It must be noted that DRCS generates a higher packet delivery ratio in comparison to TMCP. At the same time the overhearing is reduced by almost 50% which doubles the network lifetime as obtained from Figure 12a,b. This is due to several reasons. Firstly, TMCP uses a distance-based communication and interference model that does not effectively capture the link qualities, especially with a high channel variance. Secondly, DRCS uses channels more efficiently than TMCP. In TMCP, nodes select the same channels as that of their parents. Hence, if the sink has \(n\) immediate neighbors and there are \(k\) channels where \(k>n\), then at least \(k-n\) channels will be unused, since there will be at most \(n\) sub-trees in the network. On the other hand, nodes on the same sub-tree in DRCS may use multiple channels, thereby improving channel utilization. Also in case of TMCP, the parent and channel assignments are static. These do not change even with variations of congestion and link quality. These result in poor route quality that leads to higher packet loss, retransmissions, and overhearing. Moreover, the channel quality may vary over time, which requires a dynamic protocol. It is also observed that the benefits of multiple channels drops with increasing number of channels and is not significant beyond 6 channels.