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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
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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).
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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
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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.
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- 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