## 5 Data Acquisition via V2V Communications
### Supplementary Storage at Vehicular Nodes  
A principle is that the supplementary storage should not interfere with the regular content storage and transmissions. The assistant contents are stored in a particular space, namely, the assistant storage. The priority to occupy the storage space is given to the contents that the assistant vehicles themselves request. Beyond these, if they still have free storage cells, they could use them as the assistant storage. In addition, in V2U communications, the roadside units respond to the content requests first and then the supplementary storage requests.  
In order to improve the supplementary storage efficiency, three problems need to be solved: (1) When a vehicle requests assistant contents, which contents should be sent by the roadside unit? (2) How long will the assistant vehicles carry the assistant contents? Or when will the assistant vehicles drop the assistant contents? (3) How will the assistant vehicles forward the assistant contents via V2V communications? Discussions about these issues are as follows.  
Figure 3: An instance of multi-replica content preprocessing. (**a**) original relationships; (**b**) after multi-replica content preprocessing.  
1. The selection of assistant contents. To increase the access ratio, the content selection depends on the transmission failures at roadside units and the routing plans of vehicular nodes. On the one hand, each roadside unit records its failed contents. When the transmission of a content \(c_{k}\) from a roadside unit \(u_{j}\) to a request sender \(v_{i}\) fails due to bandwidth competition, \(u_{j}\) finds the next roadside unit \(v_{i}\) will meet, denoted by \(u_{next}\), from \(g(i)\), and then increases the number of failed requests of \(c_{k}\) related to the path from \(u_{j}\) to \(u_{next}\), namely, the failure frequency, denoted by \(Fr(c_{k},u_{next})\) (\(Fr(c_{k},u_{next})\) is initialized as 0 at the start of \(\tau+1\)). On the other hand, a roadside unit selects assistant contents according to the failure frequency. When \(v_{i}\) requests an assistant content from \(u_{j}\), \(u_{j}\) obtains \(u_{next}\) from \(g(i)\) and sends a copy of the content having the biggest \(Fr(c_{k},u_{next})\) to \(v_{i}\). The selection of assistant contents gives priority to those having more unsatisfied requests, since there is a higher probability to meet the request senders. In addition, considering that vehicles only send data along its moving direction, different failure frequencies are calculated for different directions.
2. The lifetime of assistant contents. After \(v_{i}\) receives an assistant content \(c_{k}\) from \(u_{j}\), \(v_{i}\) moves from \(u_{j}\) to \(u_{next}\) carrying \(c_{k}\) and sends it to other vehicles requesting \(c_{k}\). The supplementary storage process ends with dropping \(c_{k}\) when \(v_{i}\) reaches \(u_{next}\).
3. The forwarding of assistant contents. In order to accelerate the transmission of assistant contents, when an assistant vehicle encounters other vehicles, it selects the neighbor in the same moving direction and nearest to the next roadside unit and then forwards the assistant content to this neighbor. As Figure 4 shows, a vehicle \(v_{1}\) has an assistant content and its next roadside unit to meet is \(u\). In V2V communication, \(v_{1}\) has three neighbors, i.e., \(v_{2}\), \(v_{4}\) and \(v_{5}\). According to the above method, since \(v_{5}\) is the neighbor nearest to and moving for \(u\), \(v_{1}\) forwards the assistant content to \(v_{5}\). Furthermore, \(v_{5}\) has a neighbor \(v_{6}\) who requests this content, so \(v_{5}\) replies to \(v_{6}\). It is apparent that the data forwarding helps to shorten the access latency.