Optimal content downloading in vehicular networks

Optimal content downloading in vehicular networks

Consider a system where users aboard communication-enabled vehicles are interested in downloading different contents from Internet-based servers. This scenario captures many of the infotainment services that vehicular communication is envisioned to enable, including news reporting, navigation maps and software updating, or multimedia file downloading. In this paper, we outline the performance limits of such a vehicular content downloading system by modeling the downloading process as an optimization problem, and maximizing the overall system throughput. Our approach allows us to investigate the impact of different factors, such as the roadside infrastructure deployment, the vehicle-to-vehicle relaying, and the penetration rate of the communication technology, even in presence of large instances of the problem. Results highlight the existence of two operational regimes at different penetration rates and the importance of an efficient, yet 2-hop constrained, vehicle to- vehicle relaying

Infrastructure deployment
Earlier studies focus on the feasibility of using IEEE 802.11 APs to inject data into vehicular networks, as well as on the connectivity challenges posed by such an environment. A random distribution of APs over the street layout can help routing data within urban vehicular ad hoc networks. The impact of several AP deployments studied on delay tolerant routing among vehicles is studied. More precisely, each AP is employed as a static cache for content items that have to be transferred between vehicles visiting the AP at different times. AP deployment is formulated as an optimization problem, where, however, the objective is not content downloading but the dissemination of information to vehicles in the shortest possible time. One of the studies estimated the minimum number of infrastructure nodes to be deployed along a straight road segment so as to provide delay guarantees to the data traffic that vehicles have to deliver to the infrastructure, possibly with the help of relays. Infrastructure placement strategies were studied that maximize the amount of time a vehicle is within radio range of an AP. Although longer periods of time under coverage can undoubtedly favor the download of contents by vehicular users, important differences with our work exist. First, our analysis is not limited to direct transfers from APs to vehicles, but includes traffic relaying. Second, while the problem formulation guarantees a minimum coverage requirement and the one maximizes the minimum-contact opportunity, we optimize the actual throughput, accounting for the airtime conflicts deriving from the contemporary presence of an arbitrary number of vehicles. Third, instead of studying a predefined set of paths over a given topology we process complete mobility traces.
An AP deployment strategy designed to favor content download through relaying in vehicular networks is introduced. The proposed optimization problem, however, aims at maximizing a metric reflecting the amount of vehicular traffic that enables V2V communication, and not the actual throughput. Moreover, such a formulation cannot capture the mutual interference among concurrent traffic transfers.

Content downloading and dissemination
With regard to content downloading in vehicular networks, focuses on the access to Web search and presents a system that makes such a service highly efficient by exploiting prefetching. Experimental and analytical results show the contribution of V2V and I2V communications to the system performance. The amount of data downloaded is maximized by vehicles through APs that form a wireless mesh network, given the AP deployment and an (imprecise) knowledge of the vehicles trajectory and of their connectivity with the APs.
Both I2V and V2V communications are considered and the performance evaluation is carried out through simulation and a testbed on a circular campus bus route. Furthermore, a comparison against the solution to a max-flow problem is presented, but (i) it is limited to a simplified, highway-like scenario featuring one AP and one downloader and (ii) it assumes atomic contacts between nodes, hence neglecting interference and channel contention. A vehicular peer-to-peer file sharing protocol, which allows vehicles to share a content of common interest.
Delay tolerant networks (DTNs)
The vehicular cooperation paradigm that we consider relates our work to DTNs. The benefit to content dissemination of adding varying numbers of base stations, mesh nodes and relay nodes to a DTN, through both a real testbed and an asymptotic analysis. We do not assume the contacts between mobile nodes to be atomic but to have arbitrary duration, and we build the network graph so as to account for the presence of roadside infrastructure and channel contention.

No multihop data transfer are investigated.


The aim of the Dynamic Network Topology Graph (DNTG) is to model all possible opportunities through which data can flow from the APs to the downloaders, possibly via relays. Given the mobility trace, we therefore identify the contact events between any pair of nodes (i.e., two vehicles, or an AP and a vehicle). Each contact event is characterized by:
(i) the quality level of the link between the two nodes. Several metrics could be considered; here, we specifically take as link quality metric the data rate achievable at the network layer;
(ii) the contact starting time, i.e., the time instant at which the link between the two nodes is established or the quality level of an already established link takes on a new value;
(iii) the contact ending time, i.e., the time instant at which the link is removed, or its quality level has changed. We stress that, by associating a time duration to the contact events, instead of considering them as atomic, we can model critical aspects such as channel contention.

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