## 4 Performance Evaluations
### Simulation Results
In contrast, both DM-MAC  
Figure 5: Performance comparisons for randomly selected 50 sources with different data rates when end-to-end reliability is considered (a) network throughput, (b) average end-to-end delay. Figure 6: Performance with varying number of sources from a randomly selected location: (a) network throughput, (b) delivery ratio. and LPMC add channels when the network becomes overloaded and, thus, achieve a higher packet delivery ratio. Figure 7(a) shows the average end-to-end delays of the packets. Because TMCP does not utilize all of the channels, the network becomes overloaded when the number of nodes increases. This increases the packet loss rate and more packets tend to be retransmitted, which increases the average end-to-end delay. At higher traffic loads, DM-MAC adds new channels to share the traffic of an overloaded region. However, unlike LPMC, the nodes in DM-MAC need to frequently switch channels in a path. The channel switching increases the medium access time in DM-MAC, so the delay is higher than that of LPMC. Finally, Figure 7(b) shows the energy required for each mechanism to successfully deliver a data byte. Because of the increased packet delivery ratio, the energy consumption in LPMC is lower than that of DM-MAC and TMCP as we increase the number of nodes. Figures 8 and 9 demonstrate the impact of the channel quality and external interference. We vary the packet loss rate randomly from 10 to 80 percent. All of the nodes send their data to the sink. Both LPMC and DM-MAC assign a new channel when the channel quality degrades, whereas the nodes using the interfered (or low quality) channel cannot deliver their data in TMCP. More specifically, when the traffic load is low, LPMC and DM-MAC can find an unused or new channel to replace a low-quality channel. However, when the network becomes overloaded, these mechanisms cannot find an unused channel that can be added to the network. Therefore, LPMC and DM-MAC achieve the required delivery ratio, as shown in Figure 8(b) (and therefore, a higher throughput as shown in Figure 8(a)), as long as the offered load is less than or equal to the aggregate channel capacity. However, LPMC achieves higher throughput and reliability than DM-MAC, because it does not require channel switching. In contrast, the throughput in TMCP increases for higher source rates, because of the static channel assignment. Figure 9(a) shows the average end-to-end delays when the offered loads are increased. As the traffic load is increased, all of the mechanisms tend to experience more delays. However, DM-MAC has the largest delay, due to the channel switching required by the nodes to forward or receive packets. Because all of the channels are active in TMCP, the delay is lower than that of LPMC for low data rates. However, as the traffic load increases, the nodes that are using low-quality channels experience a higher delay in  
Figure 7: Performance with varying number of sources from a randomly selected location: (a) average end-to-end delay, (b) energy consumption per delivered data byte. TMCP. Furthermore, TMCP has a higher delay than LPMC for higher source rates, because the queuing and medium access delay is very high for low-quality channels. The vertical lines show the maximum and minimum average delays among the channels. Finally, Figure 9 represents the energy consumption per byte as we increase the load of the network. Since the packet delivery ratio of LPMC is more tolerant of an increasing network load, it also proves to be more energy-efficient than DM-MAC and TMCP. As shown in the figure, energy consumption for each successfully delivered data byte in LPMC increases from \(2.23\times 10^{-7}\) mWr to only \(2.36\times 10^{-7}\) mWr as we increase the network load. Figure 8: Impact of channel quality and external interference on the performance: (a) network throughput, (b) delivery ratio.