With today's ubiquity of thin computing devices, mobile users are accustomed to having rich location-aware information at their fingertips, such as restaurant menus, shopping mall maps, movie showtimes, and trailers. However, delivering rich content is challenging, particularly for highly mobile users in vehicles. Technologies such as cellular-3G provide limited bandwidth at significant costs. In contrast, providers can cheaply and easily deploy a small number of WiFi infostations that quickly deliver large content to vehicles passing by for future offline browsing. While several projects have proposed systems for disseminating content via roadside infostations, most use simplified models and simulations to guide their design for scalability. Many suspect that scalability with increasing vehicle density is the major challenge for infostations, but few if any have studied the performance of these systems via real measurements. Intuitively, per-vehicle throughput for unicast infostations degrades with the number of vehicles near the infostation, while broadcast infostations are unreliable, and lack rate adaptation. In this work, we collect over 200 h of detailed highway measurements with a fleet of WiFi-enabled vehicles. We use analysis of these results to explore the design space of WiFi infostations, in order to determine whether unicast or broadcast should be used to build high-throughput infostations that scale with device density. Our measurement results demonstrate the limitations of both approaches. Our insights lead to Starfish, a high-bandwidth and scalable infostation system that incorporates device-to-device data scavenging, where nearby vehicles share data received from the infostation. Data scavenging increases dissemination throughput by a factor of 2-6, allowing both broadcast and unicast throughput to scale with device density.

Dynamic spectrum access networks are designed to allow today's bandwidth-hungry “secondary devices” to share spectrum allocated to legacy devices, or “primary users.” The success of this wireless communication model relies on the availability of unused spectrum and the ability of secondary devices to utilize spectrum without disrupting transmissions of primary users. While recent measurement studies have shown that there is sufficient underutilized spectrum available, little is known about whether secondary devices can efficiently make use of available spectrum while minimizing disruptions to primary users. In this paper, we present the first comprehensive study on the presence of “usable” spectrum in opportunistic spectrum access systems, and whether sufficient spectrum can be extracted by secondary devices to support traditional networking applications. We use for our study fine-grain usage traces of a wide spectrum range (20 MHz–6 GHz) taken at four locations in Germany, the Netherlands, and Santa Barbara, CA. Our study shows that on average, 54% of spectrum is never used and 26% is only partially used. Surprisingly, in this 26% of partially used spectrum, secondary devices can utilize very little spectrum using conservative access policies to minimize interference with primary users. Even assuming an optimal access scheme and extensive statistical knowledge of primary-user access patterns, a user can only extract between 20%–30% of the total available spectrum. To provide better spectrum availability, we propose frequency bundling, where secondary devices build reliable channels by combining multiple unreliable frequencies into virtual frequency bundles. Analyzing our traces, we find that there is little correlation of spectrum availability across channels, and that bundling random channels together can provide sustained periods of reliable transmission with only short interruptions.

Vehicle-to-Vehicle and Vehicle-to-Roadside communications are going to become an indispensable part of the modern day automotive experience. For people on the move, vehicular networks can provide critical network connectivity and access to real-time information. Infostations play a vital role in these networks by acting as gateways to the Internet and by extending network connectivity. In this context, an important question is “What is the minimum number of infostations that need to be deployed in an area in order to support vehicular applications?” Optimizing infostation density is vital to understanding and reducing the cost of deployment and management. In this paper, we examine the required infostation density in a highway scenario using different data dissemination models. We start from a simple analysis that captures the required density under idealized assumptions. These models are validated by an event-driven simulator. We then run detailed QualNet simulations on both controlled and realistic vehicular traces to observe the information density trends in practical environments, and consequently propose techniques to improve dissemination performance and reduce the required infostation density.

Dynamic spectrum access networks are designed to allow today's bandwidth hungry "secondary devices" to share spectrum allocated to legacy devices, or "primary users." The success of this wireless communication model relies on the availability of unused spectrum, and the ability of secondary devices to utilize spectrum without disrupting transmissions of primary users. While recent measurement studies have shown that there is sufficient underutilized spectrum available, little is known about whether secondary devices can efficiently make use of available spectrum while minimizing disruptions to primary users.

Today's mobile devices support many wireless technologies to achieve ubiquitous connectivity. Economic and energy constraints, however, are driving the industry to implement multiple technologies into a single radio. This system-on-a-chip architecture leads to competition among networks when devices toggle across different technologies to communicate with multiple networks. In this paper, we study the impact of such network competition using a representative scenario where devices split their time between WiMAX and WiFi connections. We show that competition with WiMAX significantly lowers WiFi's throughput, but this performance degradation is largely unnecessary, and can be attributed to the fact that WiMAX's transmission scheduling does not consider competing networks. We propose PACT, a new coexistence-aware WiMAX scheduling policy that cooperates with WiFi links hosted by its users without compromising its own transmission requirements. We derive PACT's design using an analytical model of network competition, and apply it to design practical WiMAX scheduling algorithms for various traffic classes. We evaluate PACT using OPNET's realistic models for WiFi and WiMAX. Using real network topologies, our experiment results show that PACT significantly improves WiFi performance by up to 17 fold without affecting the WiMAX user experience.

Wireless Mesh Networks (WMNs) can provide seamless broadband connectivity to network users, with the advantage of low setup and maintenance costs. To support next-generation applications with real-time requirements, however, these networks must provide improved Quality of Service guarantees. Most current mesh network routing protocols are adapted from MANET protocols, and do not optimize for mesh network properties. In this paper, we propose QUORUM (QUality Of service RoUting in wireless Mesh networks ), a routing protocol optimized for WMNs that addresses these drawbacks. QUORUM integrates a novel end-to-end packet delay estimation mechanism with stability-aware routing policies, allowing it to more accurately follow QoS requirements while minimizing misbehavior of selfish nodes.