## 6 Evaluation and Analysis
### Simulation Results  
#### 6.3.1 MAC Layer Results  
This section evaluates the performance of three MAC protocols, DR-MAC, context-aware DR-MAC and IEEE 802.15.4 unslotted CSMA/CA, with AODV as the common network routing protocol. The results are shown for two traffic settings (normal and high traffic) as summarized in Table 2.  
Figure 8 shows the throughput results of the three MAC protocols. It is observed that the context-aware DR-MAC can achieve 22% improvements to the original DR-MAC and 36% improvements to CSMA/CA under the normal traffic. For high traffic scenario, the improvements are 22%, and 64%, respectively. The multiple backoff settings of the DR-MAC based protocols improve the frames' delivery success rate, which in turn improves their overall throughput over that of CSMA/CA. This is evidenced from the fewer number of retransmissions per frame delivered by DR-MAC-based protocols than CSMA/CA as shown in Table 3. In addition, by utilizing the AmI context information, the sensor nodes can control the frame transmissions based on their priority, and thus can further improve the overall throughput.  
Figure 9 compares the frame delay among the three protocols. The context-aware DR-MAC exhibits the lowest frame delay in both normal and high traffic scenarios. The frame delay is reduced by 38%, and 30%, over CSMA/CA, and original DR-MAC, respectively, under normal traffic scenario, while it is reduced by 45%, and 28%, respectively, under high traffic scenario. The DR-MAC based protocols may increase frame delay at the MAC layer for a pair of neighbor nodes. However, it could reduce the overall end-to-end frame delay. By improving the ratio of the frames being delivered between a pair of neighbor nodes, fewer route error packets containing the link breakage messages are issued by the routing protocol of the relay nodes. Therefore, it can reduce the need of a source node to rediscover a path and retransmit the data packet to the destination node. In turn, this can reduce the overall frame delay.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Scenario/Parameter** & **Description/Value** \\ \hline Scenario 1 & Normal traffic \\ \(\mathrm{D_{freq}}\) & 2 frames per second \\ \(\mathrm{D_{size}}\) & 512 bits \\ Scenario 2 & High traffic \\ \(\mathrm{D_{freq}}\) & 10 frames per second \\ \(\mathrm{D_{size}}\) & 1024 bits \\ \hline \hline \end{tabular}
\end{table}
Table 2: Traffic parameter settings.  
Figure 10 shows the average energy cost to successfully deliver a frame to the VB. Under normal traffic scenario, the context-aware DR-MAC can achieve 25%, and 10%, improvement over CSMA/CA, and the original DR-MAC, respectively. In high traffic scenario, the improvement is 35%, and 18%, respectively. By using context information to further enhance the delivery success of the data frames, the context-aware DR-MAC incurred less energy for transmissions associated with path rediscovery and packet retransmission. Therefore, it is more energy-efficient as compared to the original DR-MAC.  
\begin{table}
\begin{tabular}{c c c c} \hline  & **CSMA/CA** & **DR-MAC** & **DR-MAC (Context-Aware)** \\ \hline Normal traffic & 1.924 & 1.522 & 1.302 \\ High traffic & 2.727 & 2.465 & 2.273 \\ \hline \end{tabular}
\end{table}
Table 3: Average number of retransmissions for a successful frame delivery.  
Figure 8: Frame throughput of MAC protocols.  
Figure 9: Frame delay of MAC protocols.