Static Replication Strategies for Content Availability in Vehicular Ad-hoc Networks

This study investigates replication strategies for reducing latency to desired content in a vehicular peer-to-peer network. We provide a general constrained optimization formulation for efficient replication and study it via analysis and simulations employing a discrete random walk mobility model for the vehicles. Our solution space comprises of a family of popularity based replication schemes each characterized by an exponent n. We find that the optimal replication exponent depends significantly on factors such as the total system storage, data item size, and vehicle trip duration. With small data items and long client trip durations, n ∼ 0.5 i.e., a square-root replication scheme provides the lowest aggregate latency across all data item requests. However, for short trip durations, n moves toward 1, making a linear replication scheme more favorable. For larger data items and long client trip durations, we find that the optimal replication exponent is below 0.5. Finally, for these larger data items, if the client trip duration is short, the optimal replication exponent is found to be a function of the total storage in the system. Subsequently, the above observations are validated with two real data sets: one based on a city map with freeway traffic information and the other employing encounter traces from a bus network. ∗A shortened version of this study, specifically, Section 7 was presented at ACM MDM in 2005. †Corresponding author 1 Section Name Section No. Small data items with long client trip duration 4 Analysis 4.1 Low density of replicas 4.1.1 High density of replicas 4.1.2 Simulation results 4.2 Small data items with short client trip duration 5 Analysis 5.1 Low density of replicas 5.1.1 High density of replicas 5.1.2 Simulation results 5.2 Large data items with long client trip duration 6 Large data items with short client trip duration 7 Evaluation employing a mobility model based on a map of San Francisco city 8 Evaluation employing real world bus traces 9 Table 1: Organization of this paper