Broadcasting Protocols in Vehicular Ad-Hoc Networks (VANETs)

By

Mostafa M. I. Taha B.Sc. Electrical Engineering, Assiut University, 2004

A Thesis Submitted in partial fulfillment of the requirements

for the degree

MASTER OF SCIENCE

Department of Electrical Engineering Assiut University Assiut, EGYPT.

2008

Assiut University Faculty of Engineering

Broadcasting Protocols in Vehicular Ad-Hoc Networks (VANETs)

By

Mostafa M. I. Taha

B.Sc. Electrical Engineering, Assiut University, 2004

A Thesis Submitted in partial fulfillment of the requirements

for the degree

MASTER OF SCIENCE

Department of Electrical Engineering Assiut University Assiut, EGYPT.

2008

Supervised by: Prof. Abdel Karim El-Wardany

(Assiut University) Dr. Tarik K. Abdelhamid

(Assiut University) Dr. Yassin M. Yassin

(Assiut University)

Discussion committee: Prof. Ibrahim Elsayed Ziedan

(Zagazig University) Prof. Hosny M. Ibrahim

(Assiut University) Prof. Abdel Karim El-Wardany

(Assiut University) Dr. Yassin M. Yassin

(Assiut University)

Assiut University Faculty of Engineering

II

ABSTRACT

Wireless communications are becoming the dominant form of transferring information,

and the most active research field. In this dissertation, we will present one of the most

applicable forms of Ad-Hoc networks; the Vehicular Ad-Hoc Networks (VANETs). VANET

is the technology of building a robust Ad-Hoc network between mobile vehicles and each

other, besides, between mobile vehicles and roadside units.

The work begins with an introduction to VANET technology, its possible applications,

unique characteristics and promising challenges. It also demystifies some excerpts from the

IEEE 802.11 standard that are related to the operation in the Ad-Hoc mode and illustrates the

main points of its amendment in vehicular environments (IEEE 802.11p). Reliable

broadcasting of messages in self-organizing Ad-Hoc networks is a promising research field

with hundreds of published papers. This work presents a comprehensive study of the

significant broadcasting protocols in VANET environments.

The thesis contribution is a novel reliable broadcasting protocol that is especially designed

for an optimum performance of public-safety related applications. There are four novel ideas

presented in this thesis, namely choosing the nearest following node as the network probe

node, headway-based segmentation, non-uniform segmentation and application adaptive. The

integration of these ideas results in a protocol that possesses minimum latency, minimum

probability of collision in the acknowledgment messages and unique robustness at different

speeds and traffic volumes.

The performance of the proposed protocol has been studied using simulation programs and

it proved a superior performance over all previously published ones.

III

TABLE OF CONTENTS

Chapter Page

Chapter 1 Introduction ............................................................................................................... 1

1.1 What is VANET ............................................................................................................... 11.2 Why VANET .................................................................................................................... 21.3 What is Ad-Hoc ................................................................................................................ 41.4 Why Ad-Hoc .................................................................................................................... 61.5 Why Broadcasting ............................................................................................................ 61.6 Thesis Contributions ........................................................................................................ 71.7 Outline .............................................................................................................................. 7

Chapter 2 Background................................................................................................................ 8

2.1 VANET Applications ....................................................................................................... 82.2 VANET Characteristics .................................................................................................. 112.3 VANET Open-Research Challenges .............................................................................. 132.4 VANET Simulation ........................................................................................................ 142.5 IEEE 802.11 MAC ......................................................................................................... 16

2.5.1 Channel Access Functions ...................................................................................... 172.5.2 Interframe Spaces (IFS) .......................................................................................... 172.5.3 Random Backoff Time ............................................................................................ 202.5.4 RTS/CTS Handshaking ........................................................................................... 21

2.6 WAVE System Architecture .......................................................................................... 232.6.1 WAVE Physical Layer ............................................................................................ 252.6.2 WAVE Channel Coordination ................................................................................ 262.6.3 WAVE Basic Service Set ........................................................................................ 272.6.4 WAVE Communication Protocols .......................................................................... 28

2.6.4.1 Internet Protocol Version 6 (IPv6) ................................................................... 282.6.4.2 WAVE Short Message Protocol (WSMP) ....................................................... 28

2.6.5 WAVE Management Plane ..................................................................................... 292.6.6 WAVE Synchronization .......................................................................................... 29

Chapter 3 Previous Work ......................................................................................................... 31

3.1 Categories of Broadcasting Protocols ............................................................................ 313.2 Why not IEEE 802.11 .................................................................................................... 323.3 Reliable Protocols .......................................................................................................... 33

3.3.1 Rebroadcasting ........................................................................................................ 333.3.2 Selective Acknowledgment ..................................................................................... 353.3.3 Changing Parameters ............................................................................................... 35

3.4 Dissemination Protocols ................................................................................................. 363.4.1 Flooding .................................................................................................................. 37

IV

3.4.2 Single Relay ............................................................................................................ 38

Chapter 4 Theoretical Analysis ................................................................................................ 41

4.1 Introduction .................................................................................................................... 414.1.1 The Design Objective .............................................................................................. 414.1.2 Broadcasting Goals ................................................................................................. 424.1.3 Assumptions ............................................................................................................ 43

4.2 The Starting Block ......................................................................................................... 434.2.1 Frame Exchange Sequence ...................................................................................... 444.2.2 The Basic Algorithm ............................................................................................... 44

4.3 Step-1: Safety Related Applications .............................................................................. 454.3.1 Discussion ............................................................................................................... 46

4.4 Step-2: A Headway-Based Segmentation ...................................................................... 474.4.1 Discussion ............................................................................................................... 50

4.5 Step-3: Non-uniform Segmentation (Headway Model) ................................................. 524.5.1 Headway Model ...................................................................................................... 52

4.5.1.1 The Semi-Poisson Distribution ........................................................................ 544.5.2 Protocol Improvement ............................................................................................. 554.5.3 Analytical Results ................................................................................................... 59

4.6 Step-4: Application Adaptive (Modes of Operation) ..................................................... 604.6.1 Mode 0 Basic Broadcasting ................................................................................. 604.6.2 Mode 1 The Furthest Following Vehicle ............................................................. 614.6.3 Mode 2 The Nearest-in-time Following Vehicle ................................................. 614.6.4 Mode 3 The Furthest Leading Vehicle ................................................................ 62

4.7 The Proposed Algorithm ................................................................................................ 634.7.1 Algorithm of the Transmitting node ........................................................................ 634.7.2 Algorithm of Other Vehicles ................................................................................... 64

Chapter 5 Simulation Results ................................................................................................... 67

5.1 Performance Metrics ...................................................................................................... 675.2 Measurement Methodology ............................................................................................ 685.3 Simulation Parameters .................................................................................................... 695.4 Random Number Generator ........................................................................................... 695.5 Simulation Results .......................................................................................................... 695.6 Robustness at Different Traffic Volumes ....................................................................... 715.7 Protocol Comparison ...................................................................................................... 74

Chapter 6 Conclusion ............................................................................................................... 76

Appendix A - List of Co-authored Publications ....................................................................... 77Appendix B - Word-Wide VANET Projects ............................................................................ 78Appendix C - VANET Simulation Programs ........................................................................... 79Appendix D - MATLAB Scripts .............................................................................................. 80Appendix E - References .......................................................................................................... 94

V

LIST OF TABLES

Table Page Table 2-1 IEEE 802.11 channel access functions .............................................................................................. 17Table 2-2 QoS Access Categories ....................................................................................................................... 19Table 2-3 WAVE physical characteristics ........................................................................................................ 26Table 2-4 EDCA parameter set used in CCH ................................................................................................... 26Table 2-5 Default EDCA parameter set used in SCH ...................................................................................... 27Table 4-1 Best segmentation points for 330 vehicle /h (in headway sec) ........................................................ 59Table 5-1 Matlab parameters ............................................................................................................................. 69Table 5-2 Best segmentation points for 1300 vehicle /h (in headway sec) ...................................................... 72

VI

LIST OF FIGURES

Figure Page

Fig. 1-1. Node types in VANETs .......................................................................................................................... 2Fig. 1-2. Uses of Ad-Hoc networks in wars and emergencies ............................................................................ 4Fig. 1-3. Wireless Sensor Network and a sample tiny sensor ............................................................................ 4Fig. 1-4. Wireless Mesh Network ......................................................................................................................... 5Fig. 2-1. The GM's V2V system and a sample transceiver ................................................................................ 9Fig. 2-2. Interframe spaces in 802.11 ................................................................................................................. 18Fig. 2-3. Exponential increase of CW ................................................................................................................ 21Fig. 2-4. Hidden node problem ........................................................................................................................... 21Fig. 2-5. RTS/CTS/data/ACK timeline .............................................................................................................. 22Fig. 2-6. WAVE system components .................................................................................................................. 23Fig. 2-7. WAVE protocol stack .......................................................................................................................... 24Fig. 2-8. Spectrum of WAVE Channels ............................................................................................................. 25Fig. 2-9. WSM frame format .............................................................................................................................. 28Fig. 2-10. WAVE Synchronization .................................................................................................................... 30Fig. 3-1. Different categories of broadcasting protocols .................................................................................. 32Fig. 4-1. Arrangement of segments for the basic algorithm ............................................................................ 45Fig. 4-2. Arrangement of segments for step-1 modification ............................................................................ 45Fig. 4-3. Collisions at far range nodes ............................................................................................................... 46Fig. 4-4. Headway ................................................................................................................................................ 48Fig. 4-5. Distance-based segmentation ............................................................................................................... 49Fig. 4-6. Headway-based segmentation ............................................................................................................. 49Fig. 4-7. Assuming a single lane highway .......................................................................................................... 50Fig. 4-8. Sample Headway models ..................................................................................................................... 53Fig. 4-9. Headway at different traffic volumes ................................................................................................. 53Fig. 4-10. Semi-Poisson Headway Model .......................................................................................................... 54Fig. 4-11. Non-uniform headway-based segmentation ..................................................................................... 55Fig. 4-12. Study area of the analytical solution ................................................................................................. 55Fig. 4-13. Probabilities associated with an arbitrary segment ........................................................................ 57Fig. 4-14. Suggested Distribution of Collisions ................................................................................................. 58Fig. 4-15. Analytical calculation of Pc for best segmentation .......................................................................... 60Fig. 4-16. Mode 0 Basic Broadcasting ........................................................................................................... 61Fig. 4-17. Priority arrangement of mode 1 ........................................................................................................ 61Fig. 4-18. Priority arrangement of mode 2 ........................................................................................................ 62Fig. 4-19. Priority arrangement of mode 3 ........................................................................................................ 62Fig. 4-20. The suggested WSM frame format ................................................................................................... 63Fig. 4-21. Actions of the transmitting MAC ...................................................................................................... 64Fig. 4-22. Actions of other vehicles .................................................................................................................... 65Fig. 5-1. RTB/CTB/data/ACK timeline ............................................................................................................. 68Fig. 5-2. Histogram of one of the variables ....................................................................................................... 70Fig. 5-3. Simulated calculation of Pc for best segmentation ............................................................................ 70

VII

Fig. 5-4. Simulated calculation of latency at best segmentation ...................................................................... 71Fig. 5-5. Headway distribution at 330v/h and 1300v/h ..................................................................................... 72Fig. 5-6. PC for 6-seg at 1300v/h ......................................................................................................................... 73Fig. 5-7. Latency for 6-seg at 1300v/h ................................................................................................................ 73Fig. 5-8. Probability of Collision (protocol comparison) .................................................................................. 75Fig. 5-9. Latency (protocol comparison) ........................................................................................................... 75

VIII

LIST OF ABBREVIATIONS

AC Access Category ACK Acknowledgment AFR Asynchronous Fixed Repetition (Xu, et al. algorithm) AFR-CS Asynchronous Fixed Repetition with Carrier Sensing (Xu, et al. algorithm) AIFS Arbitration Interframe Space APR Asynchronous p-persistent Repetition (Xu, et al. algorithm) APR-CS Asynchronous p-persistent Repetition with Carrier Sensing (Xu, et al. algorithm) BMMM The Batch Mode Multicast MAC Protocol (Huang, et al. algorithm) BMW The Broadcast Medium Window (Tang, et al. algorithm) BPSK Binary Phase Shift Keying CCH WAVE Control Channel CEN European Committee for Standardization CSMA/CA Carrier Sense Multiple Access with Collision Avoidance CTB Clear to Broadcast CTS Clear to Send CW Contention Window DCF Distributed Coordination Function DDB The Dynamic Delayed Broadcasting (Heissenbttel, et al. algorithm) DHCP Dynamic Host Configuration Protocol DIFS Distributed Coordination Function Interframe Space DSRC Dedicated Short Range Communications EDCA Enhanced Distributed Channel Access Function edf empirical density function EDR Event Data Record EIFS Extended Interframe Space ETC Electronic Toll Collection GPS Global positioning systems HCCA Hybrid Controlled Channel Access IEEE Institute of Electrical and Electronics Engineers IFS Interframe Space IPv6 Internet Protocol Version 6 ITS Intelligent Transportation Systems LAN Local Area Networks LLC Logical Link Control MAC Media Access Control MANET Mobile Ad-Hoc Network MCDS Minimum Connected Dominating Set MLME MAC Layer Management Entity NAV Network Allocation Vector OBU On Board Unit PB Probability of success broadcast PC Probability of Collision

IX

PCF Point Coordination Function pdf probability density function PHY Physical Layer PIFS Point Coordination Function Interframe Space PLME Physical Layer Management Entity QAM Quadrature Amplitude Modulation QoS Quality of Service QPSK Quadrature Phase-Shift Keying RAK Request for Acknowledgment (Huang, et al. algorithm) RRAR The Round-Robin Acknowledge and Retransmit (Xie, et al. algorithm) RSU Road Side Unit RTB Ready to Broadcast RTS Ready to Send SB The Smart Broadcasting Protocol (Fasolo, et al. algorithm) SCH WAVE Service Channel SFR Synchronous Fixed Repetition (Xu, et al. algorithm) SIFS Short Interframe Space SPR Synchronous p-persistent Repetition (Xu, et al. algorithm) TCP Transmission Control Protocol TS Time-slot UDP User Datagram Protocol UMB The Urban Multihop Broadcast Protocol (Korkmaz, et al. algorithm) UMB Urban Multi-Hop UTC Coordinated Universal Time VANET Vehicular Ad-Hoc Network VCWC Vehicular Collision Warning Communication protocol (Yang, et al. algorithm) WAVE Wireless Access in Vehicular Environments WBSS WAVE Basic Service Set WME Wave Management Entity WSM Wave Short Message WSMP Wave Short Message Protocol

1

Chapter 1

Introduction

Everything is becoming wireless. The fascination of mobility, accessibility and flexibility

makes wireless technologies the dominant method of transferring all sorts of information.

Satellite televisions, cellular phones and wireless Internet are well-known applications of

wireless technologies. This work presents a promising wireless application and introduces a

tiny contribution to its research community.

Wireless research field is growing faster than any other one. It serves a wide range of

applications under different topologies every one of which comes with some new specialized

protocols. In this research, we will present an introduction to a wireless technology that is

expected to be adopted by both governments and manufacturers in the very near future. It

directly affects car accidents (which is the first cause of death in the age group 1 - 44 years

[35]) and the sales of one of the largest markets. It is the technology of building a robust

network between mobile vehicles; i.e. let vehicles talk to each other. This promising

technology is literally called Vehicular Ad-Hoc Networks (VANETs).

In this research, an introduction to the technology of VANETs will be presented as well as

a new contribution with a novel broadcasting protocol.

1.1 What is VANET

VANET is the technology of building a robust Ad-Hoc network between mobile vehicles

and each other, besides, between mobile vehicles and roadside units.

As shown in Fig. 1-1, there are two types of nodes in VANETs; mobile nodes as On Board

Units (OBUs) and static nodes as Road Side Units (RSUs). An OBU resembles the mobile

2

network module and a central processing unit for on-board sensors and warning devices. The

RSUs can be mounted in centralized locations such as intersections, parking lots or gas

stations. They can play a significant role in many applications such as a gate to the Internet.

Fig. 1-1. Node types in VANETs

VANET presents a new and promising field of research, development and standardization.

Throughout the world, there are many national and international projects in governments,

industry, and academia devoted to the development of VANET protocols (Appendix

B). These projects include consortiums like The Dedicated Short Range Communications

(DSRC) (USA) [8], the Car-to-Car Communication (Europe) [6] and the Intelligent

Transportation Systems (Japan) [27], and standardization efforts like the IEEE 802.11p

Wireless Access in Vehicular Environment (WAVE) [22]. An introduction to the WAVE

standard will be discussed in Sec 2.6.

1.2 Why VANET

The Bureau of Transportation Statistics [44] reported that, in 2004 within the USA only,

there were more than 6.4 million kilometers of highway, with more than 243 million

registered vehicles of different types running through them. During that year, there were more

than 6.18 million vehicle crashes causing approximately 2.79 million injuries and 42,000

fatalities. Car accidents are the leading cause of death in the age group of 1 to 44 years [35].

These accidents cost more than $150 billion per year [11]. With these terrific numbers,

Internet

RSU

OBU

3

considerable governmental and other related agencies' as well as investments of vehicles

manufacturers have been there trying to safety of roads.

Accordingly, vehicle manufacturers are competing in equipping their vehicles with devices

that collect data from the interior and exterior of vehicles and deliver it to a central processing

unit that can analyze this data to boost the road safety while increasing the on-board luxury.

Global positioning systems (GPS), Event Data Record (EDR) resembling the Black-Box used

in avionics, small range radars, night vision, light sensors, rain sensors and navigation

systems are well-known intelligent devices used in many newly produced vehicles, what is

rather referred to as "Computers-on-Wheels".

Communication researchers have been recently working on a prominent step; if each

vehicle has a device that can communicate with other vehicles, vehicles will have a gigantic

new source of information that extends beyond the capabilities of all previously mentioned

devices. For example, all of these devices cannot warn the driver of a stopping vehicle in the

next turn and of course cannot let travelers enjoy video chatting and file sharing at no charge.

Moreover, with this technology, vehicles can talk to each other and inform each other of any

probable danger and may even respond to that danger in a cooperative manner, i.e.,

introducing what may be rather referred to as "Computer Networks-on-Wheels".

Under heavy industrial pressure, it is obvious that VANETs are likely to become the most

relevant realization of mobile Ad-Hoc networks. Motivations of the promising VANET

technology include but are not limited to,

1. Increase traveler safety

2. Enhance traveler mobility

3. Decrease travelling time

4. Conserve energy and protect the environment

5. Magnify transportation system efficiency

6. Boost on-board luxury

Related governmental authorities (e.g. [10]) are expected to set a number of new rules and

regulations forcing all vehicle manufacturers to equip their vehicles with VANET transceivers

employing some of the required safety applications.

4

1.3 What is Ad-Hoc

Mobile Ad-Hoc Network (MANET) is a wireless technology where all nodes are one level

topology and can communicate directly with each other through a single hop or multi-hop

without the need of centralized nodes. The crucial usefulness of this technology arises when it

is required to build a network with a very fast deployment time and when is difficult to have

static centralized nodes such in cases of battlefields, forests or in natural catastrophes.

Fig. 1-2. Uses of Ad-Hoc networks in wars and emergencies

Before discussing why Ad-Hoc is the preferred topology for vehicular networks, it is

suitable to mention other respectful forms of MANET that took much research efforts with a

wide range of remarkable applications. These forms are Wireless Sensor Networks and

Wireless Mesh Networks. Distinguishable characteristics of VANETs will be highlighted

based on this brief introduction.

In wireless sensor networks [39] , a large set of sensors are thrown randomly in a large

area using an airplane or any other throwing sort. Each sensor is only of a coin size (Fig. 1-3

[31]) and equipped with a transceiver, small battery and any of temperature, vibration, light or

humidity sensors and even a microphone or camera.

Fig. 1-3. Wireless Sensor Network and a sample tiny sensor

Gateway Sensor Node

Sensor Node

5

These sensors coordinate between each other to scan the investigated area of any required

information such as conflagrations, earthquakes, animal activities or human activities. This

information could latterly be delivered to a single terminal node acting as a gateway to a

remote server. This information is of great usefulness in the prediction of natural catastrophes,

statistical studies and spying activities.

Wireless mesh networks [24] have better properties in terms of robustness, range

extendibility and density. It consists of multiple radio nodes, on condition that, there are at

least two communication links available at each node, hence redundancy and capability of

high density. The coverage area of these nodes forms a large mesh cloud. When any node can

no longer operate, all the rest nodes can still communicate with each other directly or through

one or more intermediate nodes, hence reliability. A new access to this cloud is dependent

only on being in a connection with any node in this cloud, hence extendibility. The figure

below shows a sample wireless mesh network (Fig. 1-4).

Fig. 1-4. Wireless Mesh Network

Both wireless sensor networks and wireless mesh networks received a considerable amount

of research in the past few years and resulted in new sets of standards. As for wireless sensor

networks, researchers suggest using the new ZigBee IEEE 802.15.4 [17] standard to cover

challenging problems such as low power at low data rates. As for wireless mesh networks, the

IEEE came up with IEEE 802.11s [24] as an amendment to the IEEE 802.11 Wireless

LAN Standard to cover challenging problems such as power consumption and security.

In Sec 2.2, we will present VANET distinguishable characteristics and how it is different

from other forms of MANET.

6

1.4 Why Ad-Hoc

Although positioning static centralized infrastructure nodes will even increase the

information offered to travelers and OBUs (as they may be used as gates to the Internet),

vehicular networks should make use of but not depend on these nodes. The elephantine size of

paved roads and high mobility of nodes limit the usefulness of any static infrastructure node.

Researchers recommend this network to be in the Ad-Hoc topology where RSUs act as

regular nodes. This topology will fasten the rate of deployment as the industry will not wait

for the infrastructure to be built. Besides, it will offer the service at no charge. Literally

speaking, VANET is a special case of the general MANET to provide communications among

nearby vehicles and between vehicles and nearby fixed roadside equipments.

1.5 Why Broadcasting

Duo to the high mobility of vehicles, the distribution of nodes within the network changes

very rapidly and unexpectedly that wireless links initialize and break down frequently and

unpredictably. Taking into consideration that VANET operates in the absence of servers,

force OBUs to organize network resources distributively. Thereupon, broadcasting of

messages in VANETs plays a crucial rule in almost every application and requires novel

solutions that are different from any other form of Ad-Hoc networks. Broadcasting of

messages in VANETs is still an open research challenge and needs some efforts to reach an

optimum solution.

So, what are the problems associated with broadcasting that we devoted a master level

study for its protocols? Although we let the entire Chapter 3 to answer this question, it is

convenient to summarize the answer. Broadcasting requirements are: high reliability and high

dissemination speed with minimum latency in single-hop as well as multi-hop

communications. Problems associated with regular broadcasting algorithms are: the high

probability of collision in the broadcasted messages, the lack of feedback and the hidden node

problem. In VANETs, there are two types of collisions, collisions of wireless messages in the

network domain and the physical collisions of running vehicles. Throughout this work, the

7

default type of collision is the collision between messages in the network domain except what

is explicitly said as a vehicular collision.

1.6 Thesis Contributions

The thesis contribution is a novel reliable broadcasting protocol that is especially designed

for an optimum performance of public-safety related applications. There are four novel ideas

presented in this thesis, namely choosing the nearest following node as the network probe

node, headway-based segmentation, non-uniform segmentation and application adaptive. The

integration of these ideas results in a protocol that possesses minimum latency, minimum

probability of collision in the acknowledgment messages and unique robustness at different

speeds and traffic volumes.

The performance of the proposed protocol has been studied using simulation programs and

it proved a superior performance over all previously published ones.

1.7 Outline

This dissertation is organized as follows;

- Chapter 2 is a background on the VANET technology. This chapter presents some of the

required applications of VANETs, introducing the outcomes of this new technology. It also

introduces the unique characteristics of VANETs, VANET challenging research areas,

simulation environments and the current state of standardization process. Although it is not

directly related to the new contribution, this background is mandatory to understand the area

of research.

- Chapter 3 provides a comprehensive study of the different objectives of broadcasting in

VANETs. Accordingly, this chapter provides a brief description of the currently published

broadcasting protocols formed in a new categorization.

- Chapter 4 provides the analytical analysis of the proposed protocol with excessive

description and analysis.

- Chapter 5 presents simulation results and protocol comparison.

- Chapter 6 presents the conclusion.

8

Chapter 2

Background

Since the first invention of mobile vehicles, governments and manufacturers have

researched accidents to reduce the number of vehicle crashes in order to reduce costs, injuries

and fatalities. The promising VANET technology complements this work with a research that

focuses on preventing crashes on the first place. Accordingly, related governmental

authorities initiated new projects to the study, research, development and standardization of

VANETs. The Dedicated Short Range Communications (DSRC) [8] is a pioneer ITS

(Intelligent Transportation Systems which is a branch of the U.S. Department of

Transportation [26]) project dedicated to VANET standardization. Then, the acronym

DSRC becomes a worldwide name of any set of standards that aim to put VANET

technology into life. The DSRC concerns with communication links between vehicle-to-

vehicle and vehicle-to/from-roadside units.

2.1 VANET Applications

According to the DSRC, there are over one hundred recommended applications of

VANETs. These applications are of two categories, safety and non-safety related. Moreover,

they can be categorized into OBU-to-OBU or OBU-to-RSU applications. Here we list some

of these applications

- Co-operative Collision Warning,

Co-operative collision warning is an OBU-to-OBU safety

application, that is, in case of any abrupt change in speed or driving

direction, the vehicle is considered abnormal and broadcasts a warning

9

message to warn all of the following vehicles of the probable danger. This application

requires an efficient broadcasting algorithm with a very small latency.

- Lane Change Warning,

Lane-change warning is an OBU-to-OBU safety application, that is,

a vehicle driver can warn other vehicles of his intention to change the

traveling lane and to book an empty room in the approaching lane.

Again, this application depends on broadcasting.

- Intersection Collision Warning,

Intersection collision warning is an OBU-to-RSU safety application.

At intersections, a centralized node warns approaching vehicles of

possible accidents and assists them determining the suitable

approaching speed. This application uses only broadcast messages.

In June 2007, General Motors GM addressed the previously mentioned applications and

announced for the first wireless automated collision avoidance system using vehicle-to-

vehicle communication (Fig. 2-1, [13]), as quoted from GM, If the driver doesnt respond to

the alerts, the vehicle can bring itself to a safe stop, avoiding a collision.

Fig. 2-1. The GM's V2V system and a sample transceiver

- Approaching Emergency vehicle,

Approaching emergency vehicle is an OBU-to-OBU public-safety

application, that is, high-speed emergency vehicles (ambulance or

police car) can warn other vehicles to clear their lane. Again, this

application depends on broadcasting.

10

- Rollover Warning,

Rollover warning is an OBU-to-RSU safety application. A RSU

localized at critical curves can broadcast information about curve angle

and road condition, so that, approaching vehicles can determine the

maximum possible approaching speed before rollover.

- Work Zone Warning,

Work zone warning is an OBU-to-RSU safety application. A RSU is

mounted in work zones to warn incoming vehicles of the probable danger

and warn them to decrease the speed and change the driving lane.

- Coupling/Decoupling,

Coupling/decoupling system is an OBU-to-OBU non-safety

application that is designed to link multiple buses or trucks into a train

to minimize the headway distance and traveling time and to decrease

rear-end crashes. In August 2003, California PATH project practically

tested this application on a three-bus platoon [5].

- Inter-Vehicle Communications,

Inter-vehicle communication is an OBU-to-OBU non-safety

application that enables travelers to communicate with each other using

instant file transfer, voice chatting or even video chatting.

- Electronic Toll Collection (ETC),

Electronic toll collection is an OBU-to-RSU non-safety application

that supports the collection of payment at toll plazas using automated

systems to increase the operational efficiency. Systems typically

consist of OBUs that are chargeable with prepaid smart cards. These

OBUs are identified by RSUs located in dedicated lanes at toll plazas.

ETC was the first widely accepted DSRC application and it is

practically implemented in many toll collection sites. As an example, it has

been used for the congestion charge region in London downtown since

2003 [43].

11

- Parking Lot Payment,

Parking lot payment is an OBU-to-RSU non-safety application that

provides benefits to parking lot operators, simplify payment for

customers, and reduce congestion at entrances and exits of parking lots.

- Traffic Management,

In-vehicle navigation is a non-safety application that is designed to

reduce driving time and fuel consumption by exchanging real-time

information about traffic conditions in the driving route.

2.2 VANET Characteristics

Although VANETs, Wireless Sensor Networks and Wireless Mesh Networks are special

cases of the general MANETs, VANETs possess some distinguishable characteristics that

make its nature a unique one. These properties present considerable challenges and require a

set of new especially designed protocols.

- Due to the high mobility of vehicles, that can be up to one hundred fifty kilometers per

hour, the topology of any VANET changes frequently and unexpectedly. Hence, the time that

a communication link exists between two vehicles is very short especially when the vehicles

are traveling in opposite directions. A one solution to increase the lifetime of links is to

increase the transmission power, but increasing a vehicles transmission range will increase

the collision probability and degrade the overall throughput of the system. The other solution

is to have a set of new protocols employing a very low latency.

- Yet another effect of the high mobility of nodes is that the usefulness of the broadcasted

messages is very critical to latency. Assuming for example that a vehicle is suddenly

stopping, it should send a broadcast message to warn other vehicles of the probable danger.

Considering that the driver needs at least 0.70 to 0.75 sec to initiate his response [14], the

warning message should be delivered at virtually zero sec latency.

- In VANETs, location of nodes changes very quickly and unpredictably, so that, building

an efficient routing table or a list of neighbor nodes will exhaust the wireless channel and

12

decrease the network efficiency. Protocols that rely on prior information about location of

nodes are likely to have a poor performance.

- Nevertheless, the topology of a VANET can be a benefit because vehicles are not

expected to leave the paved road, hence, the running direction of vehicles is predictable to

some extent.

- Although, the design challenge of protocols in wireless sensor networks is to minimize

the power consumption, this is not a problem in VANETs. Nodes in VANETs depend on a

good power supply (e.g. vehicle battery and the dynamo) and the required transmission power

is small compared with power consumption of on-board facilities (e.g. air-condition).

- It is expected that, as VANET is initially deployed, only a small percentage of vehicles

will be equipped with transceivers. Thus, the benefits of the new technology, especially OBU-

to-OBU applications, will not rise until many years. Moreover, the limited number of vehicles

with transceivers will lead to a frequent fragmentation of the network. Even when VANET is

fully deployed, fragmentation may still exist in rural areas, thereupon, any VANET protocol

should expect a fragmented network.

- Privacy and security are of crucial effect on the public acceptance of this technology. In

VANETs, every node represents a specific person and its location tells about his location.

Any lack of privacy can ease a third party monitoring persons daily activities. However, from

the other point of view, higher authorities should gain access to identity information to ensure

punishment of illegal actions, where, there is a fear of a possible misuse of this feature. The

tampering with messages could increase false alarms and accidents in some situations

defeating the whole purpose of this technology.

Finally, the key difference between VANET protocols and any other form of Ad-Hoc

networks is the design requirement. In VANETs, the key design requirement is to minimize

latency with no prior topology information. However, the key design requirement of Wireless

Sensor Network is to maintain network connectivity with the minimum power consumption

and the key design requirement of Wireless Mesh Network is reliability.

Concluding, the main characteristics of VANETs can be summarized as follows [28];

- High mobility of nodes

- No prior information about the exact location of neighbor nodes

13

- Predictable topology (to some extent)

- Critical latency requirement especially in cases of safety related applications

- No problem with power

- Slow migration rate

- High possibility to be fragmented

- Crucial effect of security and privacy

2.3 VANET Open-Research Challenges

VANET is still a virgin research area. This section walks through some of the currently

open-research challenging areas.

- Security

Authentication versus privacy [4] is considered the most intuitively confusing challenge in

the area of VANET security. Authentication of each message is a must to ensure that

messages are originated from actual vehicles suffering from actual situations. Consider what

may happen if a normal vehicle can transmit a warning beacon message of an ambulance just

to clear its travelling lane. Moreover, higher authorities (e.g. police officers) should be able to

determine causes of accidents by investigating the pre-accident transmitted messages.

However, a third party can use this information to track vehicles of important persons

remotely.

Vehicular networks, especially in cases of public-safety applications, have a very low

tolerance to errors, i.e. tampering with these messages can increase accidents.

The critical latency requirement of VANET messages prohibits the use of complicated

time-consuming cryptographic algorithms. The expected sheer scale of the network, assuming

full deployment, rules out protocols that require pre-stored information about participating

parities.

Concluding, VANET technology requires a completely new bundle of security protocols.

14

- Broadcasting

In a self-organizing Ad-Hoc network, the challenge is how we can design a protocol that is

capable of implementing a reliable broadcasting with a minimum probability of message

collision and minimum latency.

The deployed protocol should be highly distributed and does not need any prior control

messaging. Moreover, it should take into account that vehicles are expected to be travelling at

different speeds and different environments (urban and rural). Finally, as indicated in Sec 2.1,

broadcasting supports a vast range of applications that the implemented protocol should cope

with application differences efficiently.

2.4 VANET Simulation

The problem discussed in this section is how VANET researchers are going to evaluate

their proposed protocols? The ultimate evaluation tool is by doing outdoor experiments, but

this solution has many drawbacks:

- Neither easy nor cheap to have a high number of vehicles in real scenarios especially in

case of public safety related protocols.

- Difficult to analyze the performance in highly distributed environments like the case of

VANETs.

- Impossible to compare between two protocols in the exactly same situation.

Therefore, the only appropriate evaluation tool is by using simulation programs. Any

simulation program consists of two complementary parts; network model and mobility model.

The network model is responsible for identifying the communication stack; i.e. wireless

channel model, antenna model, MAC layer, network layer, application layer and similar

issues. The network model for VANET simulation programs is the same as that of MANET

programs.

The mobility model is responsible for identifying different aspects of vehicle movement. It

is the only new issue in VANET simulation programs. Vehicular mobility models are usually

classified as being either microscopic or macroscopic models. When focusing on the

macroscopic point of view, motion constraints such as roads, streets, crossroads and traffic

15

lights are considered and the generation of vehicular traffic such as traffic density, traffic

flows, and initial distribution of vehicles are defined. The microscopic point of view, instead,

focuses on the movement of each individual vehicle and on the vehicle behavior with respect

to neighbors such as lane changing and car following models. A realistic mobility model

should include [29]:

- Accurate and realistic topological maps: Such maps should include different types of

roads that consist of different number of lanes.

- Intersections with traffic lights: Maps should contain intersection where vehicles should

slow-down. Vehicles are expected to react with traffic lights appropriately.

- Lane changing models: Drivers are not expected to still in their lanes for the entire

journey. Hence, lane-changing maneuvers should be included in the simulation.

- Smooth deceleration and acceleration: Since vehicles do not breakdown and accelerate

abruptly, deceleration and acceleration models should be included.

- Obstacles: The simulation should include obstacles in the vehicular mobility and the

wireless channel.

- Intelligent driving patterns: Drivers interact with their environments, not only with

respect to static obstacles, but also to dynamic obstacles, such as neighboring cars and

pedestrians.

- Human behaviors: Drivers are humans not machines. All driving models should be

probabilistic with a tolerance of errors which results in simulated accidents.

- Non-random distribution of vehicles: As it can be observed in real life, initial positions of

vehicles are not uniformly distributed in the simulation area.

- Different types of vehicles: The VANET technology is not addressed to sedan cars only

buses, vans, trucks, trains and motorcycles are also involved. Each type should have its

own models.

- Effect of the implemented protocol: Almost all mobility models are used to generate a

predefined traffic prior to the simulation itself, without any effect of the implemented

protocol. If the researcher wants to measure the net improvement of his protocol on the

traffic flow, he must have a simulation program that allows changing of future

movements according to events from the network model.

16

All of these features are recommended for a mobility model to be as realistic as possible,

but the researcher may not use such very complicated models because this means many

variables and a lot of time. Such complicated models may be useful only in the final

evaluation of the protocol but not during the development cycle itself where the researcher

wants to study the effect of his protocol in specific situations. Note that, the network model

used in the simulation program should also be adequate to his needs with the possibility of

developing new protocols.

Although many simulation programs are available to VANET research community, it is

expected that choosing, and getting used to, an appropriate simulation tool is the most time-

consuming problem in the protocol development cycle.

Some of the popular network simulators are NS-2, GloMoSim, QualNet, OPNet, NCTUns

and MATLAB.

Some of the popular mobility generators are VanetMobiSim and CanuMobiSim.

Some of joint mobility and network simulators are TraNS and MOVE.

Web addresses for these simulators are listed in (Appendix C).

2.5 IEEE 802.11 MAC

This section provides an overview of some concepts from the IEEE 802.11 MAC standard

[23]. The IEEE 802.11 standard defines medium access control (MAC) and physical layer

(PHY) specifications for the wireless connectivity of fixed, portable, or moving stations

within a local area network. It defines a single set of MAC procedures to support packet

delivery services and several physical signaling techniques. The IEEE 802.11 includes a long

list of amendments [38] to make the standard more suitable for specific purposes. Each one of

these amendments shares the common MAC while defining some parameters of the physical

technique. Wireless Access in Vehicular Environments (WAVE) has got its own amendment

(802.11p). The first draft of which was just in Nov 2004 and it is still a draft [40]. In this

section, only general MAC concepts related to this work will be covered, based on the IEEE

802.11-REVma/D7.0 [23]; however, WAVE specific concepts will be discussed later in

Sec 2.6.

17

2.5.1 Channel Access Functions

The IEEE 802.11 MAC defines four access functions (as shown in Table 2-1)

- DCF The Distributed Coordination Function

- PCF The Point Coordination Function

- EDCA The Enhanced Distributed Channel Access Function

- HCCA The Hybrid Controlled Channel Access

Table 2-1 IEEE 802.11 channel access functions

Ad-Hoc Coordinator Point non-QoS DCF PCF

QoS EDCA HCCA

The DCF is the fundamental access function and the one that must be implemented by all

stations, whether the network was Ad-Hoc or server-based. The DCF is a distributed protocol

where all nodes, must first contend for access on the channel. The DCF access protocol

reduces collision probability by using carrier sense multiple access with collision avoidance

(CSMA/CA) and a random backoff time. The EDCA is similar to DCF but it is used when a

certain quality of service (QoS) is required. It provides four access priorities by assigning

each node one out of four access categories according to the running application.

Contrarily, the PCF is an optional access method, and is used in server-based networks

only. The PCF is a contention-free protocol where the coordinator point passes the channel

control to network nodes in a round robin fashion. Finally, the HCCA is just similar to PCF in

cases of QoS server-based networks.

The EDCA is the recommended access function in VANETs because the communications

in VANET environments does not depend on centralized infrastructure nodes and the

deployed applications should have different access priorities (from life-safety to file-sharing).

2.5.2 Interframe Spaces (IFS)

The Interframe space (IFS) is the time interval between transmission of two consecutive

frames from different nodes, whether it was a new session or just a handshaking packet in the

18

same session. Each station should wait for a different IFS according to its priority. There are

five different IFSs listed here from the shortest to the longest (Fig. 2-2)

- SIFS Short Interframe Space - PIFS Point Coordination Function (PCF) Interframe Space - DIFS Distributed Coordination Function (DCF) Interframe Space - AIFS Arbitration Interframe Space (used by the QoS facility) - EIFS Extended Interframe Space

Fig. 2-2. Interframe spaces in 802.11

The timing unit of the IEEE 802.11 is the Time-Slot, which is defined as the minimum

time that is required by nodes to sense the channel as idle and start a new transmission.

The SIFS should be used before transmission of frames that belong to the same session like

ACK frames, CTS frames, and the second or subsequent fragments of data. The SIFS is the

time interval from the end of a frame to the beginning of the next frame as seen at the air

interface assuming that the node responds directly without sensing the channel. the SIFS is

the shortest interframe space. It gives nodes involved in the current session the control over

the wireless medium until the end of the frame exchange sequence.

In case of server-based networks, the coordinator point should control access to the

wireless medium. Although all nodes in the network shall wait for DIFS before starting a new

session, the coordinator point gives a single node the permission to start after PIFS only. This

gives it a higher priority over other nodes. The PIFS is the tool used by the coordinator point

to maintain a contention-free medium.

PIFS = SIFS + Time-Slot

The DIFS is the default waiting time of nodes before starting a new session in both Ad-

Hoc and server-based networks. DIFS is longer than both SIFS and PIFS, which inhibits all

SIFSBusy media

PIFS

AIFS

DIFS

EIFS

time

19

nodes from interrupting a running session they are not involved in, or taking a time-slot that

they are not allowed to.

DIFS = SIFS + 2 Time-Slot

If all nodes start transmission after the same DIFS, an unavoidable collision will happen,

hence, the IEEE 802.11 utilizes a contention algorithm that depends on assigning a random

back-off time to each node (will be discussed in details in the next section). If a node wants to

start a new session, it must sense the channel as idle for the duration of DIFS and an extra

random time.

All nodes should use the AIFS instead of DIFS whenever it is required to employ a

quality of service (QoS). The AIFS is used by nodes deploying EDCA access function. The

EDCA provides differential access to the channel by assigning to each node one out of four

access categories. These access categories are labeled according to Table 2-2, where the

Voice gets the highest priority. The AIFS is a different value for each category with a

minimum value for the Voice (highest priority).

AIFS[AC] = SIFS + AIFSN[AC] Time-Slot

where AC is the access category and AIFSN[AC] is a number associated with AIFS[AC].

Table 2-2 QoS Access Categories

Priority AC Designation

Lowest

Highest

AC_BK Background

AC_BE Best Effort

AC_VI Video

AC_VO Voice

Unlike other IFSs, EIFS is not used to control access onto the radio link, but it is only used

when there has been an error in the last transmitted frame. If the present session ends

correctly, nodes wait for DIFS and a random backoff before starting a new transmission.

However, if the present session ends erroneously, all other nodes should use the EIFS waiting

time to provide enough time for session involved nodes to correct this error.

EIFS = SIFS + DIFS + ACK transmission time

20

2.5.3 Random Backoff Time

In contention-based access functions (DCF and EDCA), channel access protocol should be

efficient while being distributed, that network nodes should achieve low collision probability

without the help of coordinator points. Recalling that, if a node wants to start a new session, it

must sense the channel as idle for the duration of DIFS (or AIFS[AC]) and an extra random

backoff time. This section discusses specifications of the random backoff time. The pool of

random numbers that is used should be big enough for minimizing collision probability in

cases of high-density networks and small enough for shorter useless waiting time in cases of

low-density networks. The IEEE 802.11 employs an adaptive size of random pool by defining

the contention window size (CW) which increases in high-density cases and decreases in low-

density ones.

Backoff Time = Random Time-Slot

where Random is a uniformly distributed random integer in the interval (0, CW), and CW is

an integer of (CWmin CW CWmax).

The procedure is as follows,

1- The node must first sense the channel as idle for the DIFS (or AIFS[AC]) time.

2- Choose a random backoff counter in the interval (0) to (CWmin).

3- Sense the channel on every Time-Slot (TS).

4- If the channel was idle, decrement the backoff counter by one. If not (a busy medium),

hold the backoff counter.

5- If it reached zero, start the transmission.

If it received an ACK from the destination as an indication of a correct transmission, then it

should move on to the next fragment. However, if there was no ACK as an indication of a

collision in the transmitted message (there are two or more nodes got the same random

number and the network is denser than thought), it should increase the CW to a higher value

and redo the procedure from the beginning.

Summarizing, the CW should take a higher value if a collision happens until reaching

CWmax and it should be reset to CWmin after every successful transmission.

Note that, values of CW of nodes deploying DCF should be

CW = 2(i) - 1

21

where i equals 3 to 8 as shown in Fig. 2-3. In EDCA (VANET case), the CWmin and CWmax are different for each AC as will be shown in Sec 2.6.2.

Fig. 2-3. Exponential increase of CW

2.5.4 RTS/CTS Handshaking

So far, we have studied how the 802.11 minimizes collision probability by using carrier

sense mechanism and different channel-access waiting times (different IFSs and random

backoff times). However, there is still another source of collision that cannot be avoided by

the CSMA/CA, which is the hidden node problem.

Consider the case that there are four nodes arranged as shown in Fig. 2-4. N2 is in the

communication range of both N1 and N3, while N3 is out of range of N1. If there is a

concurrent transmission between N1 N2 and between N3 N4, there will be a collision at N2 because it can hear the transmission of both N1 and N3 simultaneously. Note that, the

CSMA/CA has nothing to do with this type of collision as when N3 is willing to initiate its

transmission, it cannot hear N1, hence it senses the channel as idle, and proceeds with the

transmission after the associated IFS.

Fig. 2-4. Hidden node problem

N1 N2 N3 N4

Busy mediumDIFS/AIFS

7 TS (CWmin)1st trial

Busy mediumDIFS/AIFS

15 TS2nd trial

Busy mediumDIFS/AIFS

255 TS / CWmax

6th and all following trials

A new session can start at any of these time-slots

22

The 802.11 standard addressed this problem and suggested that the transmitter should,

prior to any transmission, reserve his communication range as well as the receiver range (N1

and N2 in the example) by using ready to transmit / clear to transmit (RTS/CTS) handshaking.

In case that N1 (transmitter) has a long message to send to N2 (receiver), the procedure will be

as follows:

1- It sends an unencrypted broadcast with the RTS message indicating the transmitter address

(N1), intended receiver address (N2) and the expected time required.

2- The receiver (N2) should reply with an unencrypted broadcast with the CTS message

indicating the CTS-transmitter address (N2), CTS-receiver address (N1) and the expected

time required.

The RTS reserves the transmitter communication range, while the CTS reserves the receiver

communication range. The hidden node (N3) will hear the CTS message, know about the

medium reservation and wait for the time reservation before resuming contention for the

channel.

Each node should maintain a network allocation vector (NAV) as an indicator of time periods

when transmission is not allowed. Data in the NAV is updated by time requirements in the

RTS and CTS messages.

The timeline of the sequence [RTS/CTS/DATA/ACK] is shown in Fig. 2-5.

Fig. 2-5. RTS/CTS/data/ACK timeline

Note that, the RTS message itself may still suffer from unexpected collisions due to hidden

node problem and should only be used prior to long messages, however, for short messages,

the RTS/CTS handshaking will just increase the overhead.

SIFS

DIFS

SIFS

SIFS

RTS DATATransmitter

Receiver CTS ACK

23

2.6 WAVE System Architecture

Worldwide, hundreds of projects, laps, and consortiums are competing in developing a

robust set of standards for VANET environments (Appendix B). In USA, the Dedicated Short

Range Communication (DSRC) [8] Committee of the IEEE Transportation Technology

Council is preparing the new Wireless Access in Vehicular Environments (WAVE)

standard, which will be illustrated in this section. In Europe, the European Committee for

Standardization (CEN) [7] (CEN stands for Comit Europen de Normalisation) has got its

own standard namely General Specifications for Medium-Range Pre-Information Via

Dedicated Short-Range Communication (CEN ISO/TS 14822-1:2006). In Japan, the

Association of Radio Industries and Businesses [1] issued the standard Dedicated Short-

Range Communication System (ARIB STD-T75) in 2001 with an updated version in 2007.

This section presents a brief overview of the IEEE WAVE system architecture as an

indication of the current state of standardization process. WAVE system Architecture is a set

of standards that describes the communication stack of vehicular nodes and the physical

airlink between them (Fig. 2-6). Any RSU may have two interfaces, one for the wireless

WAVE stack and the other for external interfaces like wireline Ethernet that may be used to

enable connectivity to the Internet. Similarly, each OBU may have two interfaces, one for the

wireless WAVE stack and the other for sensor-connections and human interaction.

Fig. 2-6. WAVE system components

On-Board UnitRoad Side Unit

Applications Applications

WA

VE

stac

k

WA

VE

stac

k

Wir

elin

e st

ack

Wir

elin

e st

ack

Airlink Optional

External interface On-Board

Human Interfaces

Intra-Vehicle systems

External systems

Covered by WAVE standards

24

WAVE standard consists of five complementary parts,

- 802.11p Wireless Access in Vehicular Environments (WAVE) [22], which is an

amendment to the well-known IEEE 802.11 Wireless LAN Standard and covers the

physical layer of the system.

- 1609.1 Resource Manager [18] that covers optional recommendations for the

application layer.

- 1609.2 Security Services for Applications and Management Messages [19] that

covers security, secure message formatting, processing, and exchange.

- 1609.3 "Networking Services [20] that covers the WAVE communication stack.

- 1609.4 Multi-Channel Operation [21] that covers the arrangement of multiple

channels and how they should be used.

The WAVE communication stack and the coordination between standards are shown in

Fig. 2-7. Definition and operation of each layer of the stack will be demystified in the

following sections.

Fig. 2-7. WAVE protocol stack

Applications 1609.1,

et al.

1609.3

1609.4 802.11p

802.11p

LLC

Multi-ChannelOperation

IPv6UDP / TCP

Management Plane Data Plane

WME

MLME

WSMP

PLME

Air

link

WAVE PHY

WAVE MAC

25

2.6.1 WAVE Physical Layer

In October 1999, the Federal Communication Commission (FCC) allocated a 75 MHz of

bandwidth in the 5.9 GHz band (5.850 5.925 GHz) for applications of the DSRC [36]. The

WAVE spectrum is composed of seven channels of 10 MHz each, as shown in Fig. 2-8, with

an option of grouping two adjacent channels to have a spectrum of 20 MHz. Channel 178 is

the only control channel (CCH), and other channels are service channels (SCH). Channels

175 and 181 are the 20 MHz channels. Note that channel numbering are defined according to

the relation,

Channel center frequency = 5 GHz + (5 channel number) MHz

The modulation scheme used by WAVE is the Orthogonal Frequency Division

Multiplexing (OFDM) using 52 orthogonal subcarriers. The OFDM is a multi-carrier

modulation scheme where data is split into multiple lower rate streams. Each stream is used to

modulate one of the closely spaced orthogonal subcarriers. The primary advantage of OFDM

is its ability to cope with frequency-selective fading due to multipath channels without

complex equalization filters. This modulation scheme enables data rates of 3, 4.5, 6, 9, 12, 18,

24, and 27 Mbit/s in the 10 MHz channels and up to 54 Mbit/s in the 20 MHz channels. The

orthogonal subcarriers should be modulated using BPSK (Binary Phase Shift Keying), QPSK

(Quadrature Phase-Shift Keying), 16-QAM (Quadrature Amplitude Modulation), or 64-QAM

depending on the data rate required.

Fig. 2-8. Spectrum of WAVE Channels

Frequency 5.850 5.860 5.870 5.880 5.890 5.900 5.910 5.920 5.925 GHzChannel number 172 174 176 178 180 182 184

175 181

10 MHz 5 MHz

26

Before leaving the physical layer, Table 2-3 summarizes some of the physical-dependant

parameters related to 802.11 MAC [22].

Table 2-3 WAVE physical characteristics

Characteristic Value for WAVE Time-slot 16 s

SIFS 32 s DIFS 64 s

2.6.2 WAVE Channel Coordination

The WAVE spectrum is composed of only one control channel (CCH) and six service

channels (SCHs). The control channel is considered as the public room for all WAVE devices

and its critical resource. Efficient organization and minimization of traffic on the CCH is a

challenging problem. The CCH should only be used for service advertisement frames and

broadcast messages (i.e. when the transmitter has not negotiated with a specific receiver yet);

however, no active connections between two or more devices are allowed to exchange data

over the CCH (i.e. after handshaking, the transmitter and receiver must pursue talking in

another channel). The channel access function used to organize contention over the CCH (and

SCHs as well) is the EDCA. Table 2-4 summarizes CW and AIFSN parameters for different

access categories over the CCH. Note that, CWmin=15 and CWmax =1023

Table 2-4 EDCA parameter set used in CCH

ACI AC CWmin CWmax AIFSN 0 Background CWmin CWmax 9 1 Best Effort (CWmin +1)/2 1 CWmin 6 2 Video (CWmin +1)/4 1 (CWmin +1)/2 1 3 3 Voice (CWmin +1)/4 1 (CWmin +1)/2 1 2

The other six SCHs are considered as private rooms for any connection to exchange long

streams of data. Before initiating a connection over a SCH, a node must first join an active

logical private network (namely, the WAVE Basic Service Set WBSS). Advertisement of

new services should be transmitted over the CCH, however, actual data exchange of the

27

service is done over any SCH. Table 2-5 summarizes CW and AIFSN parameters for different

access categories over SCHs.

Table 2-5 Default EDCA parameter set used in SCH

ACI AC CWmin CWmax AIFSN 0 Background CWmin CWmax 7 1 Best Effort CWmin CWmax 3 2 Video (CWmin +1)/2 - 1 CWmin 2 3 Voice (CWmin +1)/4 - 1 (CWmin +1)/2 - 1 2

2.6.3 WAVE Basic Service Set

The WAVE Basic Service Set (WBSS) is a concept that should be clear before discussing

the deployed communication protocols. Duo to the distributed manner of WAVE protocols,

applications that want to establish a new connection with remote devices must first announce

for the new service on the CCH within a WBSS advertisement frame. The WBSS

advertisement frame contains the originating application, intended recipient devices (which

could be a broadcast), data rate and the intended SCH to be used.

On receiving of the WBSS advertisement frame, the provider node as well as user nodes

should switch to the indicated SCH to proceed with data exchange. Hence, the WBSS is a

logical private network of two or more WAVE devices having same active application(s) and

participating in data exchange over any of the SCHs (no WBSS is allowed on the CCH).

Any node can announce for a new WBSS while other nodes, on receiving of the

advertisement frame, have the right to join it according to their currently active applications.

A device can join only one WBSS at any time. A WBSS can support services for multiple

applications and can be joined by many users.

There are two types of WBSS, persistent WBSS and non-persistent WBSS. A persistent

WBSS is announced periodically in each CCH interval (the time interval when all WAVE

nodes listen to the CCH). This type could be used to support services of indefinite lifetime

(e.g. a RSU offering Internet access) so that they can be joined by nodes that newly come into

range. A non-persistent WBSS is announced only once on its initiation, and could be used to

support WBSS with limited lifetime.

28

2.6.4 WAVE Communication Protocols

WAVE supports two protocol stacks, the standard Internet Protocol Version 6 (IPv6) and a

new specially designed WAVE Short Message Protocol (WSMP).

2.6.4.1 Internet Protocol Version 6 (IPv6)

WAVE networking services support data exchange using the Internet Protocol version 6

(IPv6) [25] with both TCP and UDP at the transport layer. The existence of IPv6 protocol in

the wireless device within vehicles opens the Internet access with a tremendous variety of

possible applications. Connection using IPv6 is permitted only on SCHs after joining a

WBSS.

2.6.4.2 WAVE Short Message Protocol (WSMP)

The WAVE Short Message Protocol (WSMP) is a new protocol designed especially for an

optimized operation in WAVE environments. If any node prefers not to join a WBSS (for

example, a transmitter has a short data to broadcast) it will have to use only WSMP over the

CCH. WSMP is used for direct transmission of short messages without joining WBSS.

Messages of this protocol are designed to consume minimal channel capacity. Hence, it is the

only protocol allowed over the CCH (and may be used on any SCH as well). The suggested

frame format of a WAVE Short Message (WSM) is shown in Fig. 2-9 (lengths are in octets of

bits).

1 1 1 1 1 4 2 variable

WSM Version

Security Type

Channel Number

Data Rate

Tx Power Level

ProviderService

Identifier

WSM Length

WSM Data

Fig. 2-9. WSM frame format

The WSM Version is used version of WSMP (currently, its value is zero). The Security

Type indicates the security processing of the WSM Data i.e. the transmitter application can

sign or encrypt the message with an indication in security field. The Channel Number is

used to identify the radio channel used for the WSM. The Data Rate indicates the data rate

used for the WSM. The Tx Power Level indicates the transmit power used for the WSM.

29

The Provider Service Identifier identifies the application that originated the WSM (each

application will have a unique number). The WSM Length indicates the length in octets of

the following WSM Data field (limited to 1400 in its default value). The WSM Data

contains the application data being transferred.

2.6.5 WAVE Management Plane

The WAVE management plane is considered a logical low-level database of the system

and performs system configuration and maintenance functions. It consists of the WAVE

management entity (WME) with a special part to serve the MAC layer namely MAC layer

management entity (MLME) and another one to serve the physical layer namely Physical

layer management entity (PLME). Examples of its use include:

- Prior to the first operation of the transceiver (i.e. network configuration phase) different

system parameters are loaded into the devices WME. This field is known as Local

Information.

- Active applications register their parameters with the WME. Therefore, MAC layer can

determine whether a received WBSS advertisement is of interest to any of its applications or

not. This field is known as User Service Information.

- The WME is responsible for generating the WAVE service advertisement frame on an

application request. This field is known as Provider Service Information.

- On the initiation or joining of a WBSS, network parameters are registered in the WME.

2.6.6 WAVE Synchronization

During data exchange within a WBSS over a SCH, critical events (e.g. public safety

related messages) and new service advertisements with higher priorities may take place over

the CCH. Thereupon, the WAVE system requires that all participating devices should monitor

the CCH during a small common time interval (CCH interval) on a regular basis.

WAVE depends on GPS devices to acquire synchronization with reference to the

Coordinated Universal Time (UTC). Each UTC second is divided into ten sync intervals,

which in turn divided into a CCH interval followed by a SCH interval separated by a guard

interval, as shown in Fig. 2-10 .

30

CCH interval SCH interval CCH interval SCH interval CCH interval

Fig. 2-10. WAVE Synchronization

Devices without access to a precise timing signal (e.g. GPS) may acquire synchronization

from other WAVE devices upon receiving of WAVE advertisement frames, as the time will

be included within the frame.

This concludes the introduction of the field of study. Next chapter will briefly cover

published broadcasting protocols in VANET environments.

Guard interval

Sync Interval (0.1 sec) Start of every UTC second

31

Chapter 3

Previous Work

Although broadcasting has a limited usage in Ethernet and MANET (e.g. a DHCP

Dynamic Host Configuration Protocol request), it has got a wider range of implementation

in VANET applications. Almost all applications discussed in Sec 2.1 depend on sending

messages to intended vehicles without explicitly determining their identity, which is a

broadcast in its nature. Note that, all signaling techniques that are currently deployed in

vehicles (e.g. brake lights and turning right / left lights) are considered a broadcast. With

VANET technology, these signals will be exchanged directly between vehicles themselves.

This will increase the driver awareness of the road and the traveling luxury as well.

In this chapter, we will discuss previous promising contributions in broadcasting protocols

in VANET environments. Within the discussion of each protocol, we will clarify the work

objective, the new algorithm proposed and the key strengths / weaknesses regarding VANET

environments.

3.1 Categories of Broadcasting Protocols

All of these contributions try to solve just two questions; the first one is "How to deliver

the broadcast message to nodes within a single communication range with the highest

possible reliability?" which will be designated as reliable protocols. The second one is "How

to deliver the broadcast message to the entire network?" which will be designated as

dissemination protocols. Although both questions look similar to each other, the first one is

used with applications related to direct neighbors (e.g. collision avoidance) and the second is

used with applications related to the entire network (e.g. traffic management).

32

Fig. 3-1 shows different categories of broadcasting protocols along with some sample

protocols that will be discussed in this chapter.

Broadcasting Protocols

Reliable Protocols Dissemination Protocols

Rebroadcasting Selective ACK Changing Parameters Flooding Single relay

Xu [46] (2004)

Tang [42] (2001)

Balon [2] (2006)

Ni [37] (1999)

Zanella [48] (2004)

Yang [47] (2004)

Huang [16] (2002)

Heissenbttel [33] (2006)

Korkmaz [30] (2004)

Alshaer [15] (2005)

Xie [45] (2005)

Fasolo [12] (2006)

Fig. 3-1. Different categories of broadcasting protocols

Published reliable protocols use three methods: Rebroadcasting where the transmitter

node retransmits the same message for many times, Selective ACK where the transmitter

requires ACK from a small set of the neighbors, and Changing parameters where the

transmitter changes transmission parameters according to the expected state of the network.

Published dissemination protocols use two methods: Flooding where each node is

responsible for determining whether it will rebroadcast the message or not, and Single relay

where the transmitter is responsible for determining the next hop node.

3.2Why not IEEE 802.11

As quoted from the IEEE 802.11 standard [23], There is no MAC-level recovery on

broadcast or multicast frames. As a result, the reliability of this traffic is reduced, relative to

the reliability of directed traffic, due to the increased probability of lost frames from

interference, collisions, or time-varying channel properties.

Although the probability of collisions may be dropped down using the RTS/CTS

mechanism, the 802.11 standard says that, The RTS/CTS mechanism cannot be used for

33

messages with broadcast and multicast immediate destination because there are multiple

recipients for the RTS, and thus potentially multiple concurrent senders of the CTS in

response. As a result, the area of reliable broadcasting is still an open research challenge and

needs some new innovations.

3.3 Reliable Protocols

Broadcasting in wireless networks can serve numerous applications where reliability is not

necessary and time is not a critical requirement. The emergence of VANETs opened a new

research challenge of time-critical reliable broadcasting that intended to serve a bunch of

public safety related applications. The problem statement for reliable protocols is to design a

protocol that can deliver a message from a single source to every node in his transmission

range with the highest possible reliability and minimum latency.

The key performance metrics for reliable protocols are:

Success rate: the number of nodes that have successfully received the broadcast, divided by,

the number of nodes in the transmitter communication range.

Latency: the total time required in a single broadcast phase.

Researchers used three methods to increase the broadcast reliability: Rebroadcasting,

Selective Acknowledgment and Changing Parameters.

3.3.1 Rebroadcasting

The first method of increasing broadcast reliability is by retransmitting the same message

for many times. The problem discussed in this situation is, how many times are considered

practically enough?

Xu, et al. (2004) [46] explored the effect of retransmission on increasing the reliability and

developed six MAC protocols:

- Asynchronous Fixed Repetition (AFR): where the message is repeated in each time-slot for

a fixed number of times.

- Asynchronous p-persistent Repetition (APR): where the transmitter node transmits the

message in each time-slot with probability P, where P is a configurable parameter.

34

- Synchronous Fixed Repetition (SFR): is the same as AFR except that all nodes in the

network are synchronized to a global clock.

- Synchronous p-persistent Repetition (SPR): is the same as APR except that all nodes in the

network are synchronized to a global clock.

- Asynchronous Fixed Repetition with Carrier Sensing (AFR-CS): is the same as AFR except

sensing the channel before transmission.

- Asynchronous p-persistent Repetition with Carrier Sensing (APR-CS): is the same as APR

except sensing the channel before transmission.

Although both SFR and AFR-CS protocols gave the best success rate, the author suggests

using the AFR-CS as it does not require a global synchronization and it uses the minimum

overhead.

Key strengths: He was the first to address retransmission as a method of increasing

reliability.

Key weaknesses: He did not solve the hidden node problem, and the AFR-CS protocol

requires the same number of repetitions neglecting the effect of network condition and traffic

volume.

Vehicular Collision Warning Communication protocol (VCWC) (Yang, et al. 2004) [47]

proposed two new concepts. The first one shows that, the same degree of reliability can be

achieved by retransmitting with a decreasing rate, and hence the protocol saves some