# Efficient Dissemination of Safety Messages in Vehicle Ad Hoc Network Environments
## 3 The Proposed Safety Message Dissemination Scheme
### Cluster Management
#### 3.2.2 Cluster Head Candidates  
In urban environments, vehicles have high mobility and density. In V2V communication, this causes frequent cluster subscriptions and departures, which must be considered. The CH candidate algorithm proposed in this paper is a scheme to always maintain CMs that can perform the role of the next CH. The proposed scheme supports immediate CH replacement using the CH candidate algorithm if a problem arises with the CH, maintaining V2V communication. If the frequent reconfiguration of clusters is not considered, then when the CH suddenly departs, packet loss of safety messages will occur during the time it takes to reselect a CH. Therefore, a CH candidate algorithm considering the sudden departure of the CH from the cluster is needed.  
Figure 3 shows the proposed CH candidate selection process. Time 1 shows the process of CMs periodically sending scores to the CH. Time 2 shows the process of the CH receiving scores from the CMs and disseminating the Cluster Head Candidate List along with the cluster information. The CH aggregates and sorts the scores from the CMs, and then creates and disseminates the Cluster Head Candidate List. Each CM that receives this list learns who the next CH candidate is and prepares for the sudden departure of the CH. If the CH suddenly departs, the CMs wait for a certain time 't'. If time 't' passes, each CM assumes the CH has departed, and the next CM with the highest score declares itself as the CH. The proposed scheme minimizes packet loss for safety messages by having a CM immediately perform the role of the CH through the CH candidate algorithm.  
To select a cluster head candidate, each CM comprehensively calculates its reception quality, total number of messages, movement speed, and the number of nearby connected vehicles, and then sends its score to the CH. Reception quality refers to how well the vehicle can communicate. The most important role of the CH is to effectively disseminate messages. The proposed scheme considers vehicles with better reception quality to have a higher message dissemination success rate, as they have a lower signal attenuation rate [12]. The total number of messages represents how many messages a vehicle has. The proposed scheme considers that vehicles with more messages can respond to different requests from CMs. Movement speed refers to the vehicle's speed and how close it is to the average movement speed of the vehicles within the cluster. The proposed scheme considers that vehicles with speeds closer to the cluster's average speed are more likely to maintain the CH for a longer period without departing from the cluster. The number of connected vehicles refers to how many vehicles within the cluster the vehicle can communicate with. The proposed scheme considers that vehicles that can  
Equation (4) calculates the total number of connected vehicles (_TCV_). This score is calculated by dividing the number of connected vehicles (_CV_) within one hop by the total number of vehicles in the cluster (_TV_). A higher number of connected vehicles leads to a higher score.  
\[TCV_{(V_{i})}=\frac{CV(V_{i})^{\prime}s\ 1\ hop}{TV\in Cluster} \tag{4}\]  
Equation (5) calculates the final CH candidate score. This is the sum of the reception quality, total number of messages, movement speed, and number of connected vehicles scores. The CM with the highest score is the first candidate to become the next CH.  
\[Score_{(V_{i})}=RQ+TD+SS+TCV \tag{5}\]