# Coordination of Congestion and Awareness Control in Vehicular Networks
## 5 Discussion  
The integrated and coordinated approaches have been analyzed under scenarios where the protocols' CS were not disjoint. It can be possible that certain operating conditions produce an empty ICS (i.e., a set with no elements). In this case, it is not possible to find a configuration of the communication parameters that simultaneously satisfies the requirements of all protocols. Considering awareness and congestion control, this situation could occur when the channel load and application's requirements are so high that the maximum communication parameters allowed by the congestion control process are lower than the minimum ones demanded by the awareness control process. In other words, this situation can occur when the channel capacity is not sufficient to simultaneously satisfy the application's requirements of all vehicles. It can be considered a general problem of vehicular wireless networks that has not been fully solved yet. In fact, this is not a specific problem of COMPASS, since it is also produced with other protocols dealing with awareness control (e.g., INTERN and MINT in this study). Mechanisms exist to increase the channel capacity and try avoiding the described situation. Vehicles could use higher-order modulation and coding schemes to increase the data rate and therefore the channel capacity [31]. In addition, the information to be transmitted could be compressed by the higher layers of the protocol stack to reduce the packet size before each transmission [32]. Another interesting approach would be the combination of networking coding and multi-hop beaconing. Network coding can exploit the characteristics of the broadcast communication channel to increase the network's capacity or throughput [33]. When using network coding, a forwarding node combines multiple packets into a single coded one. An adequate selection of nodes to forward broadcast messages using network coding could enable the reduction of the transmission power levels needed by awareness control, and thereby decrease the overall channel load. Certain studies claim that multi-hop beaconing can be inefficient compared to single-hop beaconing schemes employing higher transmission power levels [34], although recent studies  
Figure 11: Upper bound of the CPU time needed to execute COMPASS for 3 CPU speeds, \(C=3\) communication parameters and \(P=3\) interacting protocols. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Algorithm Line** & **CPU Cycles** & **Repetitions** \\ \hline
1 & 1 & C + 1 \\
2 & 1 & C \\
3 & 1 & C\(\cdot N\) \\
4 & 3 & C\(\cdot N\) \\
6 & 39 & C \\
8 & 1 & C \\
11 & 1 & C \\
12 & 1 & C \\
13 & 9 & C \\
14 & 7 & C \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Computational cost of Algorithm 2.  
have shown its potential to improve the communications range if the relays are properly selected [35; 36]. The combined use of multi-hop beaconing with network coding could offer room to improve the channel capacity [37; 38]. Reducing the number of messages transmitted on the channel could also be an alternative approach to reduce the probability of an empty ICS due to insufficient channel capacity. In this case, the packet transmission frequency could be decreased, or lower priority messages (taking into account the vehicles' context) could be discarded, postponed or transmitted on different channels [39]. Despite these alternatives, it might still happen that an empty ICS cannot be avoided, even with the integrated approach since it can be considered a general problem of vehicular wireless networks. In this case, achieving a configuration of parameters that satisfies all protocols will not be possible, and it will be necessary to define certain priority levels among protocols. In the scenario discussed in this paper, this is equivalent to decide whether it is more relevant to guarantee a load below \(CBR_{max}\), provide the awareness levels required by the application's requirements, or maintain a certain number of neighbor vehicles.