## 6 Performance Evaluation  
### Network Configurations  
The performance evaluation of our data acquisition model DAS is conducted on a simulated vehicular network with the environment configurations shown in Table 4. According to IEEE 802.11p, the communication radius of vehicular nodes and roadside units is 250 m and the data rate is 5 Mbps. For the mobility model, after a vehicle sets a destination, it selects a random speed from a preset range and moves to its destination along the map-based shortest path [32] at this speed. Moreover, we will analyze the effects of several parameters, including the storage cycle, the size of assistant storage, the communication radius and the request model in Section 6.3.  
In our previous research [33], we found that the data acquisition using roadside units had much better performances than those only using vehicular nodes because of the infrastructure with rich resources. Therefore, in our experiments, we compare those models using roadside units. In detail, we use a greedy replication algorithm as comparison, namely, Greedy. In Greedy, the control center prefers to distribute content a vehicle \(v_{i}\) requests to the first roadside unit \(v_{i}\) will meet (the roadside  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Network area & Grid network 7000 \(\times\) 5000 m\({}^{2}\), each grid 700 \(\times\) 500 m\({}^{2}\) \\ Number of vehicles & 100 \\ Number of roadside units & 10 \(\times\) 10, deployed at the intersections \\ Number of contents & 300 \\ Simulation time & 12,000 s \\ Storage cycle & 600 s \\ Communication radius & 250 m \\ Data rate & 5 Mbps \\ Mobility model & Map-based shortest path model [32] \\ Mobility velocity & 5.5\(\sim\)16.5 m/s \\ Size of a content & 3 MB \\ Number of requests & A vehicle sends at most 5 requests in \(\tau\) \\ Contents requested & Randomly selected \\ \hline \hline \end{tabular}
\end{table}
Table 4: Simulation environment configurations.  
unit \(u_{j}\) having \(h_{ij}=1\)). If the storage cells of this unit are full, then the content is distributed to the next roadside unit (\(h_{ij}=2\)) and so forth. It is apparent that Greedy aims for a short access latency firstly and a high access ratio secondly. In addition, in order to see the influences of the preference model and the priority model, a variant of DAS named vDAS is implemented, which takes the encountering latency factor as the preference of content over storage cells, and takes the response factor as the priority of a storage cell over contents. In addition, another compared method only has the content distribution at roadside units, which is generated by removing the supplementary storage from DAS, denoted by DAS-RU, so as to show the specific benefits of V2U or V2V communications.  
The simulation experiments evaluate four criteria, i.e., the access ratio, the average access delay, the number of content replicas, and the transmission overhead. The access ratio is the ratio of those satisfied requests to all the generated requests. A higher access ratio implies a better performance. The average access delay is calculated for those completed requests. An access delay of a request is the duration from the time this request is sent out to that it is replied to. A method with a short average access delay works well in those vehicular applications having a strict delay requirement. The number of content replicas directly indicates the storage cost and hence the storage efficiency, while the transmission overhead, which is the number of transmissions in the entire network, shows the consumption of communication resources.