Adding Reliability of Broadcast Methods in Vehicular Ad hoc Networks

Samaneh Khakbaz, Mahmood Fathy Iran University of Science and Technology

Samaneh.khakbaz@gmail.com, Mahfathy@iust.ac.ir

Abstract

Many of applications in Vehicular Ad hoc Networks

(VANET) are based on dissemination of information, so broadcasting is one of the fundamental services in these networks. Most of applications in VANET, especially safety applications, require broadcasting of messages with high reliability in different node densities. In this paper we propose a new broadcast method, which assures broadcasting of message to back and forward directions .To analyzes its performance, it has been evaluated by network Simulator (NS2).Simulation results show ,this method has high reliability to deliver message in both directions, especially in low node densities.

1. Introduction

Recently, mobile computing has become a hot topic

in research. Although computer and communication devices are becoming smaller and more powerful, mobility still challenges applications of mobile computing especially in the area of ad hoc networking. A mobile ad hoc network consists of mobile hosts that communicate via wireless links. Due to mobility, the topology of the network changes continuously and wireless links break down and reestablish frequently. Moreover, an ad hoc network operates in the absence of fixed infrastructure forcing the hosts to organize the exchange of information decentrally. A prominent type of mobile ad hoc networks is direct wireless communication between vehicles in road traffic. In this network, the vehicles are equipped with a computer controlled radio modem allowing them to contact other equipped vehicles in their vicinity. This type of network is named Vehicular Ad hoc Network (VANET).We believe that the best applications of inter-vehicle communication are to provide improved comfort and additional safety in driving. Most of safety applications require dissemination of

information among participating vehicles, so broadcasting is one of fundamental services in these networks.Because of high importance of exchanged messages especially in safety applications, broadcasting requires high reliability for delivering messages. [1]

As we will describe in this paper,according to current broadcast methods, there is probability of broadcasting stop in forward or back directions.This probability is high especially in low node densities. In this paper we have proposed a broadcast method, which assures broadcasting of message to back and forward directions. Our method obtains this reliability through using location information more precisely.

The remainder of this paper is organized as follows: Section 2 presents related work about broadcasting in VANET, section 3 describes the problem and proposed method; section 4 presents the simulation model, results and analysis of proposed approach, the last section concludes this paper.

2. Related Works

Early research on inter vehicle communication began in the 1990s, inspired by research in the area of intelligent transportation systems (ITS) initiated by the Department of Transportation (DOT) in the U.S. and by the PROMETHEUS project of the EUREKA program. With the decreasing cost of components for communication and positioning [e.g., global positioning systems (GPS)] in the recent past, Inter Vehicle Communication(IVC) became more attractive [2]. Various research projects were initiated [3] [4] [5], some explicitly focused on IVC, others considering IVC as one of many possibilities for data distribution.

Among IVC systems ,Vehicular Ad hoc Network (VANET) that relay on direct communication of vehicles with no need to any infrastructure has attracted a lot of interests. Data dissemination is one of the frequently used services in VANET ,so several research groups have explored the idea of data dissemination for it. Flooding is suggested as the most

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DOI 10.1109/NGMAST.2008.94

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common approach for broadcasting without need to explicit neighbour information [6] .Simulation shows that flooding results in severe performance degradation, especially with high node densities, as a result of the broadcast storm problem [7]. Following the high interest and potential of inter-vehicle communications, several strategies have been suggested that improve performance of simple flooding. For example, in [8] and [11], proposed methods are based on selection of only furthest receiver nodes as the next forwarders of message. These selections reduce broadcast overhead in comparison with simple flooding .In [9] the proposed method assigns the duty of forwarding the broadcast packet to only one vehicle by dividing the road portion inside the transmission range into segments and choosing the vehicle in the furthest empty segment. In [10], a selective message forwarding method is presented. In this method nodes that have least common neighbour with sender are chosen as forwarders.

Above approaches try to broadcast messages with low overhead, but as we will describe, these methods suffer from unreliability that occur in broadcasting to forward and back directions, especially in low node densities.

3. Proposed Method

When a vehicle broadcasts a message, all vehicles in its communication range receive it, to deliver messages to nodes out of sender’s communication range, some intermediate vehicles must be selected as forwarder to inform vehicles which haven’t not received the message yet,figure1.

In figure1, after broadcasting of main sender all

vehicles in its communication range receive the message. Among all of receivers only furthest vehicles are selected as forwarder of message. Other vehicles (white vehicles) do not broadcast the message, because broadcasting of forwarders covers their broadcasting

range. By this mechanism that selects furthest nodes as forwarders message is propagated in road.

In our proposed method we assume all vehicles are equipped with Global Positioning Systems (GPS).When a vehicle broadcast the message, it includes its position in the broadcast message.

To select furthest vehicles, when each node in transmission range of sender receives the message, by knowing its own position and position of the sender that is included in broadcast message determines a waiting time. This waiting time is determined based on the distance d to the sender, such that the waiting time is shorter for more distant receivers, equation 1.

Where d: distance to the sender, Max WT: maximum waiting time, Range: transmission range

By this method small waiting time of furthest nodes expires first and they are selected as the forwarder of message. The furthest nodes broadcast message.

While furthest nodes broadcast the message , waiting time of other vehicles haven’t expired yet. These nodes must cancel their broadcasting because the retransmission range that they cover by their broadcasting is covered by transmission range of forwarders. These vehicles upon receiving of duplicate message from a forwarder understand that must cancel their broadcast timer.

This mechanism reduces redundant retransmissions of other vehicles. There are some troubles in broadcasting especially in low node densities. Consider figure 1.

In this figure, vehicle A broadcast a message, the goal is broadcasting message to forward and back vehicles. After broadcasting by A Vehicles B, C and D receive the message, and compute a waiting time based on their distance to A. Vehicle C is the furthest receiver, so its defer time expires first and is selected as the forwarder of message to ahead vehicles. By retransmission of vehicle C, Vehicles D and B receive

Fig 1.Furthest node are selected as forwarder

(1)MaxWTd*Range

MaxWTWT(d) +−=

Fig 1 .Adding between condition increases reliability

D

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(x1,y1)

(x2,y2)

(x,y) Main Sender

Forwarder

A

duplicates and to avoid redundant retransmission they must cancel their broadcasting. But as you see in this case, no forwarder node is selected to broadcasting backward so message could not be delivered to back vehicles.

To overcome this problem we suggest using location information more precisely by adding between conditions. By using this between condition, After selecting furthest node as a forwarder, among all non forwarder vehicles only vehicles which are between the main sender and forwarder that they have received duplicate message from cancel their retransmission. This condition omits effect of broadcasting of forward and backward vehicles on each other.

By applying this between condition in figure 1, upon receiving duplicates by vehicle B and D, only vehicle D, that is between B and A, cancels its retransmission time .Vehicle B after expiration of its defer time is selected as a forwarder and delivers message to back vehicles.

For specifying that a receiver vehicle is between the main sender and last relay it calculates following equation.

In this equation (x1, y1) is the coordination of main sender and (x2, y2) is the coordination of forwarder. By putting every node coordination in above equation, negative results mean the receiver node is between the main sender and forwarder. By computing above equation, every node makes decision to cancel broadcasting or broadcast message.

By adding between conditions we have assured broadcasting by forward and backward nodes.

4. Simulation 4.1. Scenario

The simulation has been implemented in NS2 (version 2.30) .As an example we model a straight road 10 km long. An accident happens in the middle of simulated road. The goal is broadcasting the alarm message to all vehicles on the road. We have used the Freeway model for distribution of vehicles .In this scenario all vehicles depart at different position, and change speed as time goes periodically. Speed of vehicles is chosen between 80km/hr to 120km/hr. Vehicles do not change moving direction during simulation. The number of lanes is four. We assume that transmission range of all vehicles is 200m. 4.2. Performance Analysis

The proposed method has been analyzed for its

performance with a flooding method that doesn’t attempt to reduce the redundant retransmission and every receiver node retransmits the message. In flooding method retransmission of every node causes a lot of redundant packets while in our method rebroadcast of relays makes a little overhead.

We have compared both methods on different node density (50 ~ 500). Diagram 1 compares the overhead of these methods.

As the diagram shows, our method has much fewer

redundant retransmissions in comparison with flooding .Flooding method especially in high node densities causes a lot of retransmissions. But in our method every vehicle receives at most two copies of each message that one of them is redundant, so it has a little overhead. We either have compared success of our proposed method with UMB [7] that doesn’t consider between condition that we mentioned in this paper .In UMB method every vehicle when receives a duplicate

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−=−−−

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Fig 2.Vehicle A is between main sender and lastForwarder

Diagram 1.Comparison of our Method with Flooding in number of redundant retransmission (overhead)

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packet, concludes that its retransmission is redundant so cancels its retransmission. But this mechanism causes unreliability in low node densities that our mechanism overcomes it.

Diagram 2 shows performance of our proposed method in comparison with UMB. As the results show our method by using between condition has assured delivering message to both directions.

As you see in this diagram , our method delivers

message to more vehicles than UMB method in low node densities .This is because our method delivers message to vehicles in both directions . But UMB suffers unreliability and there is the probability of broadcasting stop in one direction. 5. References [1] T. Nadeem, P. Shankar, L. Iftode, “A Comparative Study of Data Dissemination Models for VANETs”, In Proc of the International Conference on Mobile and Ubiquitous Systems (MOBIQUITOUS), San Jose, CA, USA, July 2006. [2] H. Sabbineni, K. Chakrabarty, ”Location-aided flooding: an energy-efficient data dissemination protocol for wireless-sensor networks”, IEEE Transactions on Computers , Volume 54, Issue 1, Jan. 2005, pp. 36 – 46.

[3] S. Yu, G. Cho, “A Selective Flooding Method For Propagating Emergency Messages in Vehicle Safety Communications”, International Conference On Hybrid Information Technology (ICHIT06). [4] L. Campelli, M. Cesana, R. Fracchia, “Directional broadcast forwarding of alarm messages in VANETs “, Wireless on Demand Network Systems and Services, Jan. 2007, pp. 72 – 79. [5] M. Sun, W. Feng, T. Lai, K. Yamada, H. Okada, “GPS-Based Message Broadcasting for Inter-vehicle Communication”, In Proc. of International Conference on Parallel Processing (ICPP), Toronto, Canada, August 2000. [6] M. Nekovee, “Efficient and reliable information dissemination in intermittently connected vehicular adhoc networks”, IEEE Vehicular Technology Conference (VTC2007), Dublin, Ireland, April 2007. [7] Korkmaz, G. Ekici, E. Ozguner, F. And Ozguner, U. ”Urban Multi-Hop Broadcast Protocol for Inter-Vehicle Communication Systems,” First ACM VANET, Oct. 2004. [8] E. Chiasserini, R. Fasolo, R. Furiato, M. Gaeta, M. Garetto, M. Gribaudo, A. Sereno, A. Zanella, ” Smart Broadcast of Warning Messages in Vehicular Ad-Hoc Networks”, Workshop Interno Progetto NEWCOM (NoE), Turin, Italy, November 2005. [9] T. Fukuhara, T. Warabino, T. Ohseki, K. Saito, K. Sugiyama, T. Nishida, K. Eguchi, ” Broadcast methods for inter-vehicle communications system”, March 2005, Volume. 4, pp. 2252- 2257. [10] L. Briesemeister, G. Hommel, “Role-based multicast in highly mobile but sparsely connected ad hoc networks,” In First Annual Workshop on Mobile Ad Hoc Networking and Computing, Aug. 2000, pp. 45–50. [11] H. Alshaer, E. Horlait,” An Optimized Adaptive Broadcast Scheme for Inter-vehicle Communication” , In Proceedings of the IEEE Vehicular Technology Conference (IEEE VTC2005-Spring), Stockholm, Sweden, May 2005,pp. 2840– 2844.

Diagram 2.Comparison of our Method with UMB innumber of informed vehicles

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