# Efficient Dissemination of Safety Messages in Vehicle Ad Hoc Network Environments
## 4 Performance Evaluation
### Performance Evaluation Results
#### 4.2.2 Comparison of the Number of RSU Processing Requests  
We compared the number of RSU processing requests with the existing scheme to demonstrate the superiority of the proposed scheme. PBE and other existing schemes do not consider either the cluster or the V2I environments using RSUs, so all vehicles send duplicate safety message requests to the RSUs. In contrast, in the proposed cluster environment scheme, vehicles only request messages from the RSU that are not held within the cluster. In the experimental evaluation, we measured the number of RSU processing requests in two cases: when vehicles within the RSU communication range request only safety messages (CRD) not in the cluster according to the proposed operation algorithm, and when all vehicles within the communication range send safety messages to the RSU.  
Figure 8 shows the number of RSU processing requests according to the probability of holding a message within the cluster. This experiment assumed 1 cluster, with 20 vehicles  
Figure 6: Packet loss rate as a function of CH departure cycle.  
Figure 7: Packet loss rate as a function of number of CM vehicles.  
and 20 safety messages needed per vehicle, which were randomly selected. The probability of holding a safety message within the cluster was set so that vehicles within the same cluster could hold the message with the corresponding probability by randomly selecting messages for each vehicle. When the holding probability within the cluster was high, the proportion of CRDs containing messages not held within the cluster decreased. A decrease in the proportion of CRDs means that messages can be shared within the cluster, thus reducing the number of requests made to the RSU.  
Figure 9 shows the reduction in duplicate messages according to the probability of holding a message within the cluster. As the probability of processing the message within the cluster increased, there was no need to request the message from the RSU. Hence, the proposed scheme, where vehicles request only necessary messages, reduced the number of duplicate messages, making it much more efficient than the existing scheme, where all vehicles directly request all messages from the RSU. Therefore, the proposed scheme produced only 12.5% of nonduplicate safety message requests compared to the existing scheme. This means that the proposed scheme also generated only 12.5% of RSU requests compared to the existing scheme.  
Figure 10 shows the number of RSU processing requests according to the density. This experiment assumed 20 vehicles forming the cluster, with 20 safety messages needed per vehicle that were randomly selected. The vehicle's probability of holding the message within the cluster was set to 20%. The number of vehicles varied from 20 to 200 in the experimental evaluation. As the number of vehicles increased, the number of requests the RSU had to process also increased. At a low density, there were few messages shared between vehicles, so the number of RSU processing requests barely differed between the proposed scheme and the existing scheme. However, as the density increased, the number  
Figure 8: Number of RSU requests as a function of probability of message existing in cluster.  
Figure 9: Duplicate messages removed as a function of probability of message existing in cluster.  
of messages that could be shared within the cluster also increased due to the messages held by the vehicles.  
Figure 11 shows the reduction in duplicate messages according to density. The proposed scheme produced only 20% of nonduplicate safety message requests compared to the existing scheme, where all vehicles directly request messages from the RSU. This means that the proposed scheme also generated only 20% of RSU requests compared to the existing scheme. Particularly, as the density increased, the proposed scheme became more efficient than the existing scheme in terms of the number of message requests that the RSU had to process, reducing duplicate messages.