Ai Hua Ho

Routing protocols for inter-vehicular networks: A comparative study in high-mobility and large obstacles environments

An ad hoc network is composed of mobile nodes without the presence of a fixed infrastructure. Communications among nodes are accomplished by forwarding data packets for each other, on a hop-by-hop basis along the current connection to the destination node. In particular, vehicle-to-vehicle communications have been studied, in recent years, to improve driver safety. As more of such applications of high-mobility ad hoc networks emerge, it is critical that the routing protocol employed is capable of efficiently coping with the high-frequency of broken links (i.e., robust with respect to high-mobility). This paper presents a comprehensive comparative study in a city environment of eight representative routing protocols for wireless mobile ad hoc networks and inter-vehicular networks developed in recent years. In a city environment, communication protocols need adapt fast moving nodes (e.g., vehicles on streets) and large obstacles (e.g., office buildings). In this paper, we elaborate upon extensive simulation results based on various network scenarios, and discuss the strengths and weaknesses of these techniques with regard to their support for highly mobile nodes.

A novel broadcast technique for high-density ad hoc networks

Broadcasting in ad hoc networks is required for many routing and other network-layer protocols to request information like routes or locations about destination nodes. Most of these routing protocols use a simple flooding mechanism that can cause broadcast storms, particularly in high density environments. Although many techniques have been proposed to address the problem of broadcast storms, they require additional periodic location beacons or do not satisfactorily reduce transmission redundancy in high density environments.We propose Cell Broadcast, a broadcast protocol that significantly reduces redundancy without the use of beaconing and while maintaining complete reachability in a high density environment. The proposed technique divides a terrain into cells. These cells help a node to determine its geographic relationship with a broadcasting node. This geographic relationship can eliminate rebroadcasts not only from nodes close to a broadcasting node but also from a majority of nodes near the transmission edge of the broadcasting node. The effect is that, in a high density environment, only a few nodes located near the 4 diagonal corners of a transmission range need to rebroadcast to maintain 100% reachability. To the best of our knowledge, this technique is not present in any existing techniques that do not use location beaconing.

An Efficient Broadcast Technique for Vehicular Networks

Vehicular networks are a promising application of mobile ad hoc networks. In this paper, we introduce an efficient broadcast technique, called CB-S (Cell Broadcast for Streets), for vehicular networks with occlusions such as skyscrapers. In this environment, the road network is fragmented into cells such that nodes in a cell can communicate with any node within a two cell distance. Each mobile node is equipped with a GPS (Global Positioning System) unit and a map of the cells. The cell map has information about the cells including their identifier and the coordinates of the upper-right and lower-left corner of each cell. CB-S has the following desirable property. Broadcast of a message is performed by rebroadcasting the message from every other cell in the terrain. This characteristic allows CB-S to achieve an efficient performance. Our simulation results indicate that messages always reach all nodes in the wireless network. This perfect coverage is achieved with minimal overhead. That is, CB-S uses a low number of nodes to disseminate the data packets as quickly as probabilistically possible. This efficiency gives it the advantage of low delay. To show these benefits, we give simulations results to compare CB-S with four other broadcast techniques. In practice, CB-S can be used for information dissemination, or to reduce the high cost of destination discovery in routing protocols. By also specify the radius of affected zone, CB-S is also more efficient when broadcast to a subset of the nodes is desirable.

Dynamic route diversion in vehicular networks

In vehicular ad hoc networks in a city environment, communication connections need to adapt to fast moving nodes (e.g., vehicles on streets) and large obstacles (e.g., office buildings). Fast moving nodes results in frequent topology changes. Large buildings surrounded by relative narrow streets allow only a short window of communication among nodes. Early solutions address this fundamental requirement by employing techniques that can reconnect a broken link quickly with low overhead. This strategy, however, cannot cope with a high frequency of broken links in a vehicular environment. To address this problem, a few connectionless-oriented techniques, e.g., connectionless approach for vehicular networks, have emerged. These schemes rely on any mobile host along the general direction towards the destination node to help forward the data packets. Extensive simulation results have shown that these methods are more robust, and perform significantly better than connection-oriented techniques. The current connectionless method, however, may suffer from packet drops since traffic congestion is not considered in the packet forwarding policy. We address this weakness, in this paper, by adapt the connectionless approach for vehicular networks with collision avoidance routing technique; and give simulation results, based on GloMoSim, to illustrate their performance advantage.

A Mobile Computing Approach to Automatic Traffic Evacuation Management

In this paper we first introduce our smart traffic evacuation management system (STEMS), an ITS system in development to quickly and efficiently generate dynamic evacuation plans facilitating the rapid evacuation of a city street network in response to a human-induced disaster. We then broaden STEMS to a 2-tier architecture designed to interact with mobile devices. When incidents occur, STEMS automatically generates an evacuation plan, and prioritizes the deployment of personnel to direct traffic at street intersections by utilizing a voting algorithm and heuristics. Through the use of mobile communication devices the system subsequently coordinates these personnel in directing the evacuation traffic in real time. Our simulation results indicate that STEMS, with the new mobile computing capability, is very effective in mitigating the impacts of the disaster. The benefit can be observed immediately even during an initial response when only a small subset of the intersections can be policed and coordinated before more personnel can arrive at the scene

A near-optimal broadcast technique for vehicular networks

A vehicular network is a promising application of mobile ad hoc networks. In this paper, we introduce a near-optimal broadcast technique, called CB-S (Cell Broadcast for Streets), for vehicular networks. In this environment, the road network is fragmented into cells such that nodes in a cell can communicate with any node within a two cell distance. Each mobile node is equipped with a GPS (Global Positioning System) unit and a map of the cells. The cell map has information about the cells including their identifier and the coordinates of the upper-right and lower-left corner of each cell. CB-S has the following desirable property. Broadcast of a message is performed by rebroadcasting the message from every other cell in the terrain. This characteristic allows CB-S to achieve near optimal performance. Our simulation results indicate that data can always reach all nodes in the wireless network. This perfect coverage is achieved with minimal overhead. That is, CB-S uses a near optimal (minimum) number of nodes to disseminate the data packets. This optimality gives it the advantage of minimum delay. To show these benefits, we give simulations results to compare CB-S with four other broadcast techniques. In practice, CB-S can be used for information dissemination, or to reduce the high cost of destination discovery in routing protocols. We also present in this paper a CB-SD (Cell Broadcast for Street Dissemination) variant that has all the advantages of CB-S, with the additional benefit of supporting partial broadcast. This scheme is more efficient when broadcast to a subset of the nodes is desirable.

Mercury-like routing for high mobility wireless ad hoc networks

Supporting high mobility is essential to mobile ad hoc networks in a wide range of emerging applications such as vehicular networks. Communication links of an established communication path that extends between source and destination nodes are often broken under a high mobility environment. Although a new communication route can be established when a break in the communication path occurs, repeatedly reestablishing new routes incurs delay and substantial overhead. To address this limitation, we introduce the Communication Path abstraction in this paper. A communication path is a dynamically-created geographical area that connects the source and destination nodes. The routing functionality of a communication path is provided by the physical nodes (i.e., mobile devices) currently within the geographical region served by the path. These physical nodes take turns in forwarding data packets for the path. Since a path can be supported by many alternative nodes, this scheme is much less susceptible to node mobility. Our simulation results show the Communication Path approach can achieve several times better performance than traditional approach based on a fixed sequence of physical links.

A Real-Time Route Diversion Management System

We present our work on the real-time route diversion system RTRDS, a module developed for the State of Floridas SunGuide Automated Transportation Management System ATMS. Our RTRDS is an extensible, adaptor-based framework designed to create optimal route diversions based upon available real-time and historical traffic information, and disseminate these plans in real time. Given the need for a route diversion, operators can choose to have the system generate a new route diversion, or select from an archive of historical route diversions. The user interface permits operators to modify generated and historical route diversion plans on-the-fly and then implement them with existing hardware (dynamic message signs, etc.). Furthermore, due to the tight integration with Dynasmart-P, operators can save the current route diversion plan and traffic conditions to files that can be used for later performance analysis.