Vehicular ad hoc network is one of the major subjects of intelligent transportation system.
With mobile devices and high technology equipments on the transportations, such as on-board unit (OBU), the vehicles can get information by using the wireless signal hop by hop. Each mobile device or standalone vehicle computer can transmit information to their neighbor nodes with a limited transmission range. The transmitted information can be safety message, road condition report, or entertainment multimedia data, etc. These useful messages can help drivers to take an easily and smoothly path to their destination. Besides, the drivers can prevent disaster happening if the situation on the road can be taken care properly.
1.1. Vehicular Ad Hoc Network and Communication
Vehicular Ad Hoc Network (VANET) is envisioning for intelligent transportation system (ITS) applications. IEEE 1609 is a family standard for Wireless Access in Vehicular Environment (WAVE) for providing a definition of entire vehicular information system.
1609.0 is an over view of WAVE, their components and operation included. In 2006.10, 1690.1 [1] about resource management of WAVE is proposed. 1690.2 [2] describes the security services, and makes messages protected against eavesdropping attacker. 1609.3 [3] is to support the communication between vehicle-to-vehicle and vehicle-to-roadside infrastructures, it also defends the connectivity and the flow rules for vehicle to interact with each other, furthermore, it contains the service accessing with travel-related information. The last, 1609.4 [4] presents the channel management with multi-channel operation.
Vehicular Ad Hoc Network (VANET) is a special class Mobile Ad Hoc Network (MANET). There are some different characteristics between VANET and MANET. First, the speed and mobility of vehicle is very fast, it’s the major cause of frequently disconnection between vehicles. Second, vehicles move on a fixed road map with many lines topology.
Third, vehicles need to transmit information rapidly and rely on broadcast transmission frequently to disseminate data information. Furthermore, vehicles would not be concerned with power consuming and storage problem.
In IEEE 1609.3 [3], Dedicated Short Range Communication (DSRC) capable device with 5.9 GHz can support vehicular communication. There are two mode of communication for
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vehicle to get information and data, one is vehicle to road site unit (RSU), and the other one is vehicle to vehicle (V2V), here our thesis focus on the V2V ad hoc mode which vehicle communicate with each other vehicle by vehicle. As shown in Figure 1, when vehicle bump into a critical situation, it will broadcast the corresponding message to inform the other cars.
The cars in the transmission range will be notified and take action to the incident immediately.
Figure 1 Vehicle to vehicle communication
1.2. Information dissemination and its challenges
To propagate information, as shown in Figure 2, multi-hop broadcast is an important and frequently used transmission method to disseminate information. Many applications depend on the mechanism, such as route discovery, information exchanging, alarm notification. All vehicles rely on the broadcast protocol mostly. Intuitively, the simple flooding method can notify nodes in the network as many as possible. Figure 4 shows an example, when one source vehicle (Originator) starts to send a broadcast packet, the one-hop neighbors of the sender will receive the broadcast packet and rebroadcast, and so the two-hop neighbor will keep rebroadcasting the packet epidemically. Because all neighbors within the transmission range of originator have received the message, more and more redundant messages will be received by the neighbors of originator. The darker nodes means more redundant message they have.
With simple flooding method, not only many redundant messages will be generated, but also cause many contention for neighbors to rebroadcast. After rebroadcast by all neighbors, more and more collision will degrade the performance of the network system, such an overhead is also called “broadcast storm problem” [5]. Basically, more vehicles to be in forward status, more vehicles will be notified with the information packet. However, more forwarder vehicles means more same packet forwarded in the network system, that is, more forwarders, there would be more network congestion, more collision opportunities, and more media contention times. In Figure 3, both two packet receiver (R1, R2) will rebroadcast, and node A, B, and C will get one redundant message. If A, B, and C are covered by more receivers, they get more redundant messages. Therefore, an efficient forwarding method should be applied to decrease the broadcast storm problem.
Figure 2 Multi-hop broadcast
Figure 3 Redundant messages with A, B, and C
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Figure 4 Broadcast storm problem
1.3. Forwarding decision and its problem
In order to reduce redundant messages, many rebroadcast protocol has been proposed. By making forwarding decision to decide whether to rebroadcast a received packet, broadcast storm problem can be mitigated. There are many solutions to make forwarding decision. In [6], four schemes of broadcast are specified, simple flooding, probability-based methods, area-based methods, and neighbor-knowledge method. These methods depend on additional information (speed, direction, neighbor relationship, etc.) to rebroadcast and achieve higher coverage (reliability or notified nodes) about the information and lower overhead of broadcast (less redundant messages).
By the nature of vehicles on the road, cars move along roads but still lack of principle with high mobility, the relation between the vehicles changes rapidly. It’s hard to maintain the relationship between vehicles, and the behaviors of the drivers are also unpredictable. There are so many unexpected things on the road, how to overcome all the situations in a hurry is the problem we need to solve. Most of the recent methods need collected information on the road to make forwarding decision. However, with high mobility, neighbor nodes will have higher probability to go out of transmission range, especially the border ones as shown in Figure 5. If the outer border nodes are not yet notified to forwarders, forwarder will get out-of-date information and make wrong forwarding decision with out-of-date neighbor information. Many protocols suffer from the frequently changed topology of VANET. The methods by making forwarding with neighbor relationship will be influenced mostly.
Figure 5 Border nodes
1.4. Motivation and Objectives
In high mobility vehicular ad hoc environment, an efficient solution to disseminate data or information with broadcast should be concerned about two things, reducing redundant messages and forwarding message with correct parameters. We proposed a nodes covered forward (NCF) rebroadcast protocol to achieve these requirements. Forwarders will take the mobility of border nodes into account. To take precautions before it is too late, with more covered nodes, the forwarders will have higher probability to rebroadcast to cover un-notified nodes. With probability to rebroadcast, NCF can notify neighbor nodes about incident information with less retransmission times and also solve redundant messages problem.
1.5. Organization
The rest of this thesis is organized as follow: chapter 2 discusses some related work on forwarding algorithm of broadcast that reduces redundant messages. The chapter 3 gives a detail description of our proposed algorithm. And chapter 4 shows the performance of simulation and evaluation. Last, the future work and conclusion will be in chapter 5.
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