## 4 Performance Evaluation
### Results and Analysis
#### 4.4.2 V2V Channel Propagation Model  
This subsection presents the metrics obtained when considering the radio propagation model explicitly developed for highway scenarios introduced in [37]. Remember that for this channel a transmitted packet may not reach the transceiver nominal radio range because of the path loss and the shadowing (see Section 3.1). Therefore, for this case a packet may not reach its intended destination because of collisions or adverse channel conditions. The Figure 10 presents the PDR obtained with FPBR and DDOR when increasing the maximum vehicle acceleration, \(\alpha\). Four different vehicle densities, \(\lambda\), were considered for this figure. Note in Figure 10 that DDOR provides a much lower PDR than that observed when considering freespace propagation. In contrast, the PDR obtained when using FPBR shows a similar performance to that observed in Figure 7(a). This performance drop for DDOR is caused by the underestimation of the V2V highway channel effects when selecting the next forwarder node. As explained in Section 3, FPBR considers the specific constraints imposed by the V2V highway channel when selecting the next forwarder node. Thus, when considering the V2V highway channel FPBR does not show a significant performance drop, compared to the performance achieved when considering the freespace channel. Additionally, note that the PDR is not significantly affected by the acceleration rate when using FPBR. This means that the backup mechanism implemented in FPBR is able to cope with packet collisions and erroneous predictions made by the LP mechanism. The Figure 11 shows the PDR obtained by FPBR and DDOR for different source-destination distances. The plots in Figure 11 were obtained by setting \(\alpha=4\) with \(\lambda=66\) and \(\lambda=133\) Importantly, note in this figure how the PDR obtained with DDOR exhibits a decreasing behavior as the source-destination distance increases. This is caused by erroneous next-hop selections made by DDOR (remember that for this protocol the channel effects are not considered when performing the next-hop selections). In contrast, FPBR provides a significantly higher PDR when compared to DDOR. The FPBR protocol exhibits this behavior because of its backup and next-hop selection mechanisms enable the delivery of packets traveling longer source-destination distances. Figure 9: Network and MAC overhead obtained with FPBR and DDOR when increasing the maximum acceleration, \(\alpha\). Two different vehicle densities were considered for these plots: \(\lambda=133\) and \(\lambda=33\). (**a**) Network overhead _vs.a._ (**b**) MAC overhead _vs.a._Figure 11: Packet Delivery Ratio (PDR) obtained with FPBR and DDOR when increasing the source-destination distance. The maximum vehicle acceleration was set to \(\alpha=4\). Two different vehicle densities, \(\lambda=66\) and \(\lambda=133\), were considered for these figures. Figure 10: Packet Delivery Ratio (PDR) obtained with FPBR and DDOR when increasing the maximum acceleration, \(\alpha\). Four different vehicle densities, \(\lambda\), were considered for these plots. Each plot shows the PDR _vs._\(\alpha\) for: (**a**) \(\lambda=133\); (**b**) \(\lambda=100\); (**c**) \(\lambda=66\) and (**d**) \(\lambda=33\). Figure 12 illustrates the effects of increasing the maximum acceleration, \(\alpha\), on the average number of hops per packet for different vehicle densities, \(\lambda\). It can be seen in this figure that DDOR has a lower hop count. Although at first this may appear to be an advantage, the reason for the lower hop count in DDOR is the increase in the packet drop probability observed when the source-destination distance is incremented (see Figure 11). On the other hand, the hop count of FPBR is larger because it enables the delivery of packets traveling longer source-destination distances (see Figure 11). Figure 13 shows the effects of increasing the maximum acceleration, \(\alpha\), on the average delay per hop metric for different vehicle densities, \(\lambda\).