Effectiveness of Using Periodic Messages for Disseminating Warning Information in Vehicular Ad

Hoc Networks

Zaydoun Yahya Rawashdeh and Syed Masud Mahmud Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202

Abstract- In Vehicular Ad Hoc Networks, vehicles use periodic messages to inform all nearby vehicles about their current state. Roadside hazardous information can be sent to the vehicles to prevent accidents. Hazardous roadside information can be sent either by using separate messages or by embedding the information into the regular periodic messages. This paper investigates the effectiveness of using only the periodic messages for disseminating warning information. Results show that the dissemination of warning information, by only using periodic messages, will be as effective as using two different types of messages: periodic messages and separate warning messages.

I. INTRODUCTION

Vehicular Ad hoc Networks (VANET) is one of the most active topics in Intelligent Transportation System (ITS). Automobile manufacturers started to equip communication interfaces and Global Positioning system (GPS) with the new vehicles. With the rapid advance of technology, it becomes easy to support VANET and then provide an ITS services to drivers. One example of these services is to forward safety information to vehicles in a certain range and within certain time latency.

Some VANET applications use the traditional way to broadcast messages. In this way, a node sends packets to all its neighbors, and the neighboring nodes retransmit these packets. Even though this method can provide us with a solution to deliver messages to all nodes within the network, it will increase the network congestions and the message latency. Therefore, a number of methods that attempt to overcome this problem have been developed. The idea behind these methods is to select a small subset of appropriate nodes to rebroadcast the packets such that the bandwidth is efficiently utilized and network congestion is minimized. Some methods rely on the periodic messages that are explicitly exchanged by all nodes [1]. Other methods try to avoid exchanging periodic messages. In these methods each message contains the sender’s position, and the receiving nodes insert defer time slots for each rebroadcasted message [2]. This time is inversely proportional to the distance from the sending vehicle.

The goal of this paper is to investigate whether periodic messages can be used to deliver warning information on time. For this purpose, we propose a method that uses periodic messages to carry event data, called Periodic Messages (PM). PM will be compared with the method that uses periodic and warning messages separately, called Periodic and Warning

Messages (PWM). The paper presents a brief description of these methods. Detailed performance analysis of these techniques will be presented in our future full paper.

This paper is organized as follows: Section II presents an overview; Section III contains assumptions and system design; Section IV presents the simulation model; Section V contains the performance evaluation; finally, Section VI presents the conclusion along with the future work.

II. OVERVIEW

In this paper, we assume that two types of messages will be sent by all vehicles: event driven and periodic messages. Event driven messages are the result of the detection of an unsafe situation; e.g., slippery road, a vehicle crash, etc. Periodic messages, called ‘hello’ messages, are generated to make vehicles aware of their environment. Periodic messages contain the state of the sending vehicle, i.e., position, velocity, direction, etc.

Consider the scenario in which vehicles are moving along a road. Vehicles send periodic messages to inform other vehicles in their vicinity of their current state. Assume that at time T, vehicle A detects an event (e.g., slippery road, or a vehicle crash) and then sends a message to inform vehicles in the back about the existence of the event and its location. This message will be propagated in a multi-hop manner until it reaches its destination.

III. ASSUMPTIONS AND SYSTEM DESIGN

We assume that every vehicle is equipped with a GPS that can update the location of the vehicle every 100 msec. Therefore, periodic messages are sent every 100 msec. A. PWM Method

The vehicle that detects the event generates a message, inserts its position and sends it. Vehicles that receive a message must determine their position with respect to the position of the event. Vehicles will consider the message, only if they are approaching the event area and the message is received for the first time. Among the receiving vehicles, the farthest vehicle retransmits the message to inform the rest. Vehicles in the front, including the relaying node, ignore the message; vehicles in the back receive the message, and repeat the same process. This process will be repeated until the message gets delivered or there is a network discontinuity. Vehicles should stop forwarding the message if it has traveled

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE CCNC 2008 proceedings.

1-4244-1457-1/08/$25.00 © IEEE707

N hops. In case of network discontinuity, the vehicle either holds the message for some time until it detects another vehicle in the back and then releases the message or forwards the message to a nearby base station, which periodically broadcasts the message to inform passing vehicles. B. PM Method

A vehicle that detects the event waits to send the message, until the periodic message time elapses. After that the vehicle changes the type of the message from periodic message only to periodic message carrying event data and then sends the message. Vehicles that receive the message must determine their position with respect to the position of the event. Vehicles will consider the message if they are approaching the event area and if the message is received for the first time. Among the receiving vehicles, the farthest vehicle waits to send the message until the periodic message time elapses and then retransmits it. Vehicles in the front ignore the message; vehicles in the back receive it and repeat the same process. This process will be repeated until the message reaches its destination or there is a network discontinuity. Vehicles should stop forwarding the message if it has traveled N hops. In case of network discontinuity, this method will use the same technique used in the PWM method.

IV. SIMULATION MODEL

We developed a simulator in C++. Table I shows some parameters that were used in the simulator. In the simulator, vehicles move in one direction and enter the road according to the poisson process. The simulator doesn’t take the MAC layer into consideration. In case of a data collision, vehicles must wait for a random period of time, generated by the simulator, before sending the message.

TABLE I SIMULATOR PARAMETERS

Parameter Value Road length 4000 meters with 3 lanes Wireless range 250 meters Vehicle’s length 5 meters Vehicle’s speed 80 to 96 kilometers/hour Vehicle’s acceleration -2 to +2 meters / Sec2 Min. distance between vehicles 10 meters Periodic messages length 8 bytes Event message length 20 bytes

V. PERFORMANCE EVALUATION

The simulation is used to evaluate the PM, PWM, and the simple broadcast methods. We used three metrics to evaluate the results. The first metric is the delivery latency. This is the time needed to deliver the message from the location of the event to the target area. We found that, PM method needs around 525 msec to deliver the warning messages to its destination. The second metric is the reachability. Reachability is the number of vehicles that received the message divided by the total number of vehicles on the road when the message was created. Fig. 1 shows the reachability. The figure shows that the reachability remains close to 100%

for all methods when the density of vehicles is very high. The third metric is the overhead. The overhead is the total number of bytes, periodic and event messages, which occupy the media for every density. Fig. 2 shows the overhead. The figure shows that the simple broadcast makes vehicles rebroadcast messages which results in unnecessary transmission, and this is the reason why simple broadcast perform poorly. However, PM and PWM show significant improvement, the figure shows that PM outperforms PWM because PM embeds the event message into periodic message. However, the figure shows that the difference is not significant.

0

0.2

0.4

0.6

0.8

1

1 0.5 0.33 0.25 Arrival rate (Cars/Sec)

Rea

chab

ility

PMPWMBroadcast

Figure 1. Reachability vs Vehicle’s density

0

0.2

0.4

0.6

0.8

1

1 0.5 0.33 0.25 Arrival rate (Cars/Sec)

Num

ber o

f byt

es(N

orm

aliz

ed)

PWMPMBroadcast

Figure 2. Number of bytes sent vs Vehicle’s density

VI. CONCLUSION AND FUTURE WORK

We proposed a new method that embeds warning information into periodic messages. We compared this method with the traditional approach that uses periodic and warning messages separately. Preliminary simulation results show that using this method would be good enough to deliver safety information. In our future work, we aim to consider the data link layer to evaluate collisions and packet loss. We also want to simulate scenarios in an urban environment using different transmission intervals and different message lengths.

REFERENCES [1] L.Briesemeister and G.Hommel. Overcoming Fragmentation in mobile

Ad Hoc Networks, Journal of Communications and Networks. Vol. N 3. pp. 182-187, September 2000

[2] S.Y. Ni et al.. The broadcast storm problem in Mobile Ad Hoc Network.Mobicom’99 Seattle Washington USA. PP. 151-62. 1999

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE CCNC 2008 proceedings.

708