In this paper we conduct a feasibility study of delay-critical safety applications over vehicular ad hoc networks based on the emerging dedicated short range communications (DSRC) standard. In particular, we quantify the bit error rate, throughput and latency associated with vehicle collision avoidance applications running on top of mobile ad hoc networks employing the physical and MAC layers of DSRC. Towards this objective, the study goes through two phases. First, we conduct a detailed simulation study of the DSRC physical layer in order to judge the link bit error rate performance under a wide variety of vehicles speeds and multi-path delay spreads. We observe that the physical layer is highly immune to large delay spreads that might arise in the highway environment whereas performance degrades considerably at high speeds in a multi-path environment. Second, we develop a simulation testbed for a DSRC vehicular ad hoc network executing vehicle collision avoidance applications in an attempt to gauge the level of support the DSRC standard provides for this type of applications. Initial results reveal that DSRC achieves promising latency performance, yet, the throughput performance needs further improvement.

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The authors introduce hybrid freespace optical and RF wireless links os potential technology for designing next-generation broadband wireless networks. AB s T R A CTIn this article, we introduce hybrid free-space optical and R F wircless links as potential technology for designing next-gencration broadband wireless nctworks. We present various design challenges and potential solutions for real-time link performance characterization and adaptation for enhanced performancc during adverse wcather conditions. First, we introduce the hybrid wireless architecture and emphasize its significant role in achieving ubiquitous carrier-grade wireless conncctivity. Second, we propose a link monitoring scheme that accurately reflccts the performance of optical wireless links under various weather conditions. In addition, we examine the role of known link performance rcstoration schcmcs -powcr and data rate control. Third, we propose two novel link restoration schemes that cfficiently utilize the hybrid architccture: dynamic load switching and multihop routing. Finally, the article dcscribes an elaborate field testbed based on thc hybrid architecture and various link restoration techniques. The dynamic load switching schemc is shown to h a w a profound impact on the ovcrall hybrid link availability. The results, recorded from the experiments during extreme weather conditions, validate thc impact of thc hybrid architecture concept and conclusively prove the availability and rcliability of the architecture in achieving sustained highspeed wireless connectivity.