Priyantha Mudalige

Broadcast storm mitigation techniques in vehicular ad hoc networks

Several multihop applications developed for vehicular ad hoc networks use broadcast as a means to either discover nearby neighbors or propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem, a scenario in which there is a high level of contention and collisions at the link layer due to an excessive number of broadcast packets. While this is a well-known problem in mobile ad hoc wireless networks, only a few studies have addressed this issue in the VANET context, where mobile hosts move along the roads in a certain limited set of directions as opposed to randomly moving in arbitrary directions within a bounded area. Unlike other existing works, we quantify the impact of broadcast storms in VANETs in terms of message delay and packet loss rate in addition to conventional metrics such as message reachability and overhead. Given that VANET applications are currently confined to using the DSRC protocol at the data link layer, we propose three probabilistic and timer-based broadcast suppression techniques: weighted p-persistence, slotted 1-persistence, and slotted p-persistence schemes, to be used at the network layer. Our simulation results show that the proposed schemes can significantly reduce contention at the MAC layer by achieving up to 70 percent reduction in packet loss rate while keeping end-to-end delay at acceptable levels for most VANET applications.

Mobile Vehicle-to-Vehicle Narrow-Band Channel Measurement and Characterization of the 5.9 GHz Dedicated Short Range Communication (DSRC) Frequency Band

This study presents narrow-band measurements of the mobile vehicle-to-vehicle propagation channel at 5.9 GHz, under realistic suburban driving conditions in Pittsburgh, Pennsylvania. Our system includes differential Global Positioning System (DGPS) receivers, thereby enabling dynamic measurements of how large-scale path loss, Doppler spectrum, and coherence time depend on vehicle location and separation. A Nakagami distribution is used for describing the fading statistics. The speed-separation diagram is introduced as a new tool for analyzing and understanding the vehicle-to-vehicle propagation environment. We show that this diagram can be used to model and predict channel Doppler spread and coherence time using vehicle speed and separation.

Routing in Sparse Vehicular Ad Hoc Wireless Networks

A vehicular ad hoc network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on 1-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time). Our results show that, depending on the sparsity of vehicles or the market penetration rate of cars using Dedicated Short Range Communication (DSRC) technology, the network re-healing time can vary from a few seconds to several minutes. This suggests that, for vehicular safety applications, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. In addition, the developed analytical framework and its predictions provide valuable insights into the VANET routing performance in the disconnected network regime.

Broadcasting in VANET

In this paper, we report the first complete version of a multi-hop broadcast protocol for vehicular ad hoc networks (VANET). Our results clearly show that broadcasting in VANET is very different from routing in mobile ad hoc networks (MANET) due to several reasons such as network topology, mobility patterns, demographics, traffic patterns at different times of the day, etc. These differences imply that conventional ad hoc routing protocols such as DSR and AODV will not be appropriate in VANETs for most vehicular broadcast applications. We identify three very different regimes that a vehicular broadcast protocol needs to work in: i) dense traffic regime; ii) sparse traffic regime; and iii) regular traffic regime. We build upon our previously proposed routing solutions for each regime and we show that the broadcast message can be disseminate efficiently. The proposed design of the distributed vehicular broadcast (DV-CAST) protocol integrates the use of various routing solutions we have previously proposed.

On the Broadcast Storm Problem in Ad hoc Wireless Networks

Routing protocols developed for ad hoc wireless networks use broadcast transmission to either discover a route or disseminate information. More specifically, reactive routing protocols has to flood the network with a route request (RREQ) message in order to find an optimal route to the destination. Several applications developed for vehicular ad hoc wireless networks (VANET), which is a subset of MANET, rely on broadcast to propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem. In this paper, we explore how serious the broadcast storm problem is in both MANET and VANET by examining how broadcast packets propagate in a 2-dimensional open area and on a straight road or highway scenarios. In addition, we propose three novel distributed broadcast suppression techniques; i.e., weighted p-persistence, slotted 1-persistence, and slotted p- persistence schemes. Our simulation results show that the proposed schemes can achieve up to 90% reduction in packet loss rate while keeping the end-to-end delay at acceptable levels for most VANET applications. They can also be used together with the route discovery process to guide the routing protocols to select routes with fewer hop counts.

On the Routing Problem in Disconnected Vehicular Ad-hoc Networks

Vehicular ad hoc wireless network (VANET) exhibits a bipolar behavior in terms of network topology: fully connected topology with high traffic volume or sparsely connected topology when traffic volume is low. In this work, we develop a statistical traffic model based on the data collected on 1-80 freeway in California in order to study key performance metrics of interest in disconnected VANETs, such as average re-healing time (or the network restoration time). Our results show that, depending on the sparsity of vehicles, the network re-healing time can vary from a few seconds to several minutes. This suggests that, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as dynamic source routing (DSR) and ad hoc on-demand distance vector routing (AODV) will not work with such long re-healing times.

GrooveNet: A Hybrid Simulator for Vehicle-to-Vehicle Networks

Vehicular networks are being developed for efficient broadcast of safety alerts, real-time traffic congestion probing and for distribution of on-road multimedia content. In order to investigate vehicular networking protocols and evaluate the effects of incremental deployment it is essential to have a topology-aware simulation and test-bed infrastructure. While several traffic simulators have been developed under the intelligent transport system initiative, their primary motivation has been to model and forecast vehicle traffic flow and congestion from a queuing perspective. GrooveNet is a hybrid simulator which enables communication between simulated vehicles, real vehicles and between real and simulated vehicles. By modeling inter-vehicular communication within a real street map-based topography it facilitates protocol design and also in-vehicle deployment. GrooveNets modular architecture incorporates mobility, trip and message broadcast models over a variety of link and physical layer communication models. It is easy to run simulations of thousands of vehicles in any US city and to add new models for networking, security, applications and vehicle interaction. GrooveNet supports multiple network interfaces, GPS and events triggered from the vehicles on-board computer. Through simulation, we are able to study the message latency, and coverage under various traffic conditions. On-road tests over 400 miles lend insight to required market penetration

Bounded-Latency Alerts in Vehicular Networks

Vehicle-to-vehicle communication protocols may be broadly classified into in three categories; bounded-delay safety alerts, persistent traffic warnings and streaming media for telematics applications. We focus on the first category of time-critical messaging as is it of greatest value to the driver and passengers. Safety alerts are transmitted from a vehicle during events such as loss of traction, sudden braking and airbag deployment. The objective for a safety protocol is to relay messages across multiple vehicles within a 1.5-2 km distance to alert approaching vehicles within a bounded end-to-end delay (e.g. 1.5 sec). Due to high mobility and ephemeral connectivity we must employ broadcast protocols, as well as mitigation strategies to curtail inherent issues associated with broadcast protocols, such as broadcast storm problem. In this paper, we present a location division multiple access (LDMA) scheme to suppress the broadcast storm problem and ensure bounded end-to-end delay across multiple hops. This scheme requires participating vehicles to time synchronize with the GPS time and receive the regional map definitions consisting of spatial cell resolutions and temporal slot schedules via an out-of-band FM/RDBS control channel. We use the GrooveNet vehicular network virtualization platform with realistic mobility, car-following and congestion models to evaluate the performance of LDMA in simulation and on the road.

Vehicular Networks for Collision Avoidance at Intersections

A substantial fraction of automotive collisions occur at intersections. Statistics collected by the Federal Highway Administration (FHWA) show that more than 2.8 million intersection-related crashes occur in the United States each year, with such crashes constituting more than 44 percent of all reported crashes [12]. In addition, there is a desire to increase throughput at intersections by reducing the delay introduced by stop signs and traffic signals. In the future, when dealing with autonomous vehicles, some form of co-operative driving is also necessary at intersections to address safety and throughput concerns. In this paper, we investigate the use of vehicle-to-vehicle (V2V) communications to enable the navigation of traffic intersections, to mitigate collision risks, and to increase intersection throughput significantly. Specifically, we design a vehicular network protocol that integrates with mobile wireless radio communication standards such as Dedicated Short Range Communications (DSRC) and Wireless Access in a Vehicular Environment (WAVE). This protocol relies primarily on using V2V communications, GPS and other automotive sensors to safely navigate intersections and also to enable autonomous vehicle control. Vehicles use DSRC/WAVE wireless media to periodically broadcast their position information along with the driving intentions as they approach intersections. We used the hybrid simulator called GrooveNet [1, 2] in order to study different driving scenarios at intersections using simulated vehicles interacting with each other. Our simulation results indicate that very reasonable improvements in safe throughput are possible across many practical traffic scenarios.

Reliable intersection protocols using vehicular networks

Autonomous driving will play an important role in the future of transportation. Various autonomous vehicles have been demonstrated at the DARPA Urban Challenge [3]. General Motors has recently unveiled their Electrical-Networked Vehicles (EN-V) in Shanghai, China [5]. One of the main challenges of autonomous driving in urban areas is transition through cross-roads and intersections. In addition to safety concerns, current intersection management technologies such as stop signs and traffic lights can introduce significant traffic delays even under light traffic conditions. Our goal is to design and develop efficient and reliable intersection protocols to avoid vehicle collisions at intersections and increase the traffic throughput. The focus of this paper is investigating vehicle-to-vehicle (V2V) communications as a part of co-operative driving in the context of autonomous vehicles. We study how our proposed V2V intersection protocols can be beneficial for autonomous driving, and show significant improvements in throughput. We also prove that our protocols avoid deadlock situations inside the intersection area. The simulation results show that our new proposed V2V intersection protocols provide both safe passage through the intersection and significantly decrease the delay at the intersection and our latest V2V intersection protocol yields over 85 % overall performance improvement over the common traffic light models.