# An Adaptive Back-Off Mechanism for Wireless Sensor Networks
## 4 Simulation and Analysis
### Experimental Results and Analysis  
In the network adopting a mesh topology, the throughput, energy efficiency, and end-to-end delay comparison are presented in Figures 5-7, respectively. By introducing an adaptive contention window mechanism, we can see that our proposed back-off mechanism has a better performance than S-MAC and BEB in all aspects. The throughput performance of proposed MAC is also better than that of CH-MAC. Since the nodes adopting our proposed MAC can update their contention window according to the traffic load in real time, there are fewer collisions than that in S-MAC, BEB, and CH-MAC. As a result, a higher throughput is acquired, especially when the traffic load is heavy, which is illustrated in Figure 5. We can see that, when the packet interval is less than 3 s, the throughput  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(CW_{\text{min}}\) & 16 & CBR Packet Size & 512 (bytes) \\ \(CW_{\text{max}}\) & 1024 & Routing Protocol & AODV \\ TH\({}_{1}\) & 5 & Initial Energy & 1000 (J) \\ TH\({}_{2}\) & 9 & Transmit Power & 0.386 (W) \\ Radio Bandwidth & 20 (kbps) & Receive Power & 0.368 (W) \\ Node Transmission Range & 250 (m) & Idle Power & 0.344 (W) \\ Simulation Time & 1000 (s) & Sleep Power & 5.0e-5 (W) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters and corresponding values. \(CW\): contention window; CBR: constant bit rate; AODV: ad hoc on-demand distance vector.  
Figure 4: Network topology: (**a**) mesh topology; (**b**) linear topology.  
of our presented MAC protocol compared with S-MAC, BEB, and CH-MAC increases by 165%, 65%, and 15% on average, respectively. In order to assess the energy efficiency of the proposed protocol, we collected the information of overall energy consumption in the simulation and calculated the energy consumed per packet. Obviously, the lower this value is, the better the performance of the network is. Energy efficiency is shown in Figure 6, in which we can see lower energy consumption of the proposed mechanism when compared with S-MAC and BEB. From the statistical result, the energy consumed per packet adopting the proposed protocol decreases by 65% and 40% compared with S-MAC and BEB mechanisms. Owing to the redundant handshake frame and an initiative sleeping mechanism, CH-MAC performs well in the aspect of energy efficiency, which is comparative with our proposed MAC. The end-to-end delay comparison is illustrated in Figure 7. The proposed MAC outperforms S-MAC and BEB mechanism because of its proper size of contention window. When traffic load is heavy, S-MAC and BEB are vulnerable to data collision, leading to a large transmission delay; nodes adopting our proposed MAC tend to have a large contention window size, which will increase delay to some extent. Considering the simplified handshake procedure in CH-MAC, the delay of CH-MAC is slightly superior to the proposed MAC.  
Figure 5: Throughput comparison in mesh topology.  
Figure 6: Energy comparison in mesh topology.  
To sum up, we can see that the proposed MAC protocol adopting adaptive contention window mechanism has a better performance with respect to throughput and energy efficiency, especially in the heavy load network. By automatically adjusting the contention window size according to the channel competition fierce degree, data collision can be effectively avoided and higher throughput and energy efficiency are obtained.