## 3 Reclassification of Emergency Messages Dissemination Approaches  
We propose a new message dissemination classification from global and local message dissemination perspectives, as shown in Figure 2, based on the previous classification from works observed in Section 2. A first superclass to be discerned is whether the approaches have the objective of making messages reachable for the majority of the population in the network, or in a reduced area in which data have space-time constraints and are considered relevant.  
_Scope and Limitations of Current Solutions and Why the Presented Work Is Necessary_  
It is clear that emergency message dissemination, a special and high-priority case of the task of communication in vehicular networks, remains an open problem mainly due to mobility of the entities involved. Therefore, several approaches and solutions have been proposed in the literature, as observed in Section 2. However, the main objective of the aforementioned approaches is to reach as many entities as possible with single broadcasts. Model systems include previous knowledge of the neighbors and/or data recipients, as in the routing cases, as well as location services for geo-routing, or both in the case of clustering approaches. We include a qualitative comparison of the dissemination approaches in Table 3.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Approach** & **Highlights and Advantages** & \begin{tabular}{c} **Assumptions for Models** \\ **Systems** \\ \end{tabular} & **Disadvantages and Drawbacks** \\ \hline \hline Flooding & Simplest in dissemination & \
\begin{tabular}{c} Previous knowledge of \\ neighbors is not mandatory \\ \end{tabular} &
\begin{tabular}{c} If not controlled, broadcast \\ storms hoard communications \\ and messages overflow \\ incoming buffers; the highest \\ cost of message dissemination to \\ as many vehicles as possible \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: Qualitative characteristics comparison of message dissemination approaches for vehicular networks.  
Note that issues such as data integrity, encryption, authentication, and privacy are security matters that are beyond the scope of the current research work and are not considered as characteristics in Table 3 or in this paper as a whole.  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Approach** & **Highlights and Advantages** & **Assumptions for Models Systems** & **Disadvantages and Drawbacks** \\ \hline \multirow{6}{*}{Clustering} & \multirow{6}{*}{No broadcast storms are present; only heads disseminate EMs among clusters} & Full knowledge of neighbors due to control messages for cluster formation and maintenance; an objective is that EMs reach most vehicles, whether they are useful or not & Control messages might be larger than EMs; cluster members must have the same speed and heading, which results in multiple executions of the cluster formation phase for urban scenarios and is impractical for sparse vehicular networks  
Proposed Approach: Pheromone Emergency Message System for Vehicular Networks (PhEMS4 VN) and Experiments  
This section includes a proposed feasible bio-inspired space-time communication approach to deal with the task of dissemination of emergency messages for mobile entities in vehicular networks and a comparison with flooding mechanisms.