Wireless sensor networks (WSNs) boast numerous applications ranging from home appliance control to environmental monitoring and smart healthcare. More recently, they have also demonstrated benefits for intelligent transport systems: monitoring aircraft systems and parameters [1] , monitoring wheel bearings on trains [2] , and connecting sensors, switches, and actuators inside cars [3] [4] [5] . In these applications, the ability to reliably aggregate data in one or several processing centers is critical to the monitoring capabilities of the sensors, which are typically constrained in both energy and computational power. For transport systems, this aggregation is further complicated by the dynamic channel properties that vehicles may experience as they travel through areas with different radio interference patterns or road quality. To date, several single-hop communication models based on Zigbee, RFID, and ultra-wideband technologies, have been examined for intra-car wireless networking [3] [4] [5] ; in these networks, all sensor nodes communicate in a point-to-point fashion (within a star topology) with the central node. For large-scale deployments, especially over large physical distances, it may be advantageous to arrange sensors in a multihop network, with some sensors relaying information from other sensors onto the central node (or collection root). Within vehicles, however, it is not clear whether multihop networking is worthwhile, as the small distances involved typically allow sensors to reach the central node using low or medium transmission power. Indeed, multihop networking adds communication overhead to the system, requiring exchange of metadata (e.g., topology information) and utilizing available bandwidth for packet relaying (with some incident interference to other sensors). In this work, we show that, notwithstanding the caveats mentioned above, multihop networking may provide clear benefits within cars. Specifically, despite its greater overhead, it can enhance system reliability, provide robust performance, and reduce communication energy. We utilize the Collection Tree Protocol (CTP) [6] as a multihop network layer protocol, as it is widely deployed, well researched, and the basis of the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) standard [7] . Our results show, for example, that the packet delivery rate of a node using a single-hop topology protocol can be below 80 percent in practical scenarios, whereas CTP improves reliability performance beyond 95 percent across all nodes while simultaneously reducing radio energy consumption. We perform several experiments to explore the effects of environmental conditions on the performance of intra-car WSN; for instance, our results show that engine noise does not have much effect on performance, even though it adds 2-4 dB in noise power [3] . On the other hand, CTP achieves clearly better performance when a vehicle is parked in a covered area (i.e., garage) than in an open area (i.e., outdoor parking). To the best of our knowledge, this is the first demonstration of the significant benefits possible from multihop networking within vehicles. A preliminary version of our results was presented in [8] . The rest of this article is organized as follows. In the following section we provide background Modern vehicles incorporate dozens of sensors to provide vital sensor data to electronic control units, typically through physical wires, which increase the weight, maintenance, and cost of cars. Wireless sensor networks have been contemplated for replacing the current physical wires with wireless links, although existing networks are all single-hop, presumably because cars are small enough to be covered by lowpower communication, and multihop networking requires organizational overhead. In contradiction with previous works, we experimentally investigate the use of multihop wireless communication to support intra-car sensor networking. Extensive tests, run under various vehicular environments, indicate the potential for significant reliability, robustness, and energy usage improvements over existing single-hop approaches. Our implementation is based on the Collection Tree Protocol, a state-of-the-art multihop data collection protocol. on existing intra-car WSN models and on CTP. After that our experimental setup is described. We follow that with a comparison of multihop CTP performance to a star topology protocol under static conditions. Then we extend the experiments into dynamic scenarios. We conclude with overall thoughts.
The authors of REF show that using multihop networking for data convergecast in a vehicle improves the performance compared to single-hop convergecast.
Intra-car multihop wireless sensor networking: a case study
IEEE Communications Magazine
IEEE Communications Magazine
Computer Science