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Now a day, everything is moving away from wired technology and leading towards 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.
Research in wireless communication field is growing faster, day by day, then any other field. It serves a very broad range or series of different kind of applications using different topologies. Every one of these comes with some new and specialized protocols. In this research, we will present an introduction to a wireless technology. This wireless technology directly affects car accidents and the sales of one of the largest markets. It is the technology of building a strong network between mobile vehicles; i.e. let vehicles communicate to each other. This promising technology is literally called Vehicular Ad-Hoc Networks (VANETs).
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. First of all, VANET technology is going to reduce crashes by doing research in this field. Accordingly, related governmental authorities initiated new projects to the learning institute for study, research, development in the field of wireless technology and VANETs also paying attention in its standards. The ‘Dedicated Short Range Communications (DSRC)'  is a pioneer ITS (Intelligent Transportation Systems which is a branch of the U.S. Department of Transportation ) project dedicated to VANET standardization. Then, the acronym or short form ‘DSRC' becomes a global or familiar name of kind of standards that aim to put VANET technology into life. The DSRC mainly concerns with the communication that is how to make different communication links between vehicle-to-vehicle and vehicle-to/from-roadside units.
In the last few years, vehicular network has gained great attention in industry. Federal communication commission (FCC) has assigned 5.850-5.925GHZ frequency band to promote safe and efficient road trips, which is planned for vehicle-to-vehicle and vehicle-to-infrastructure communication. Car manufacturers, e.g., Audi, BMW and DaimlerChrysler, also formed a Car2Car communication consortium , in which the prototype development for inter-vehicle communications is underway.
In near past, IEEE 802.11-based solutions for VANETs are also studied by IEEE 802.11p. IEEE 802.11p Wireless Access in the Vehicular Environment (WAVE) that defines changes to IEEE 802.11 to help Intelligent Transportation Systems (ITS) applications. IEEE 802.11p helps data exchanges between fast moving vehicles with each other and also exchanges data from vehicles to road side unit or from road side unit to vehicles in the licensed ITS band of 5.9 GHz. The Dedicated Short-Range Communications (DSRC) at 5.9 GHz is here today to provide safety that is increasing safety in case of road accidents, reducing highway or road maintenance cost and also improving mobility. Intersection and road departure collisions report for round about 50 percent of all crashes and victims on our roads. On an average day in the United States, vehicular collisions kill 116 and injure 7900. More health care dollars are consumed in the United States treating crash victims than any other cause of illness or injury , ; the situation in the European Union is similar, with over 100 deaths and 4600 injuries daily, and the annual cost of €160 billion . By getting rid from road victims and crashes, DSRC can provide or play important role in reducing road accidents, deaths, injuries, heavy traffic and increasing road safety by improving communication between vehicles and between vehicles and road side infrastructure. DSRC emerged from a partnership among automobile manufacturers, state and federal transportation officials, toll transponder equipment suppliers and the Federal Communications Commission. There is a recognized need for on-the-go communication with motor vehicles and reliable communication between vehicles to increase highway safety by providing warnings and alerts that enable drivers to take corrective and/or evasive actions. At the same time, it can be able to provide information i.e. real time information to drivers so that to improve mobility and motorist convenience, such as information on congestion or traffic incidents. The car manufacturing industry's determination to roll out vehicle-to-vehicle communication in the near future and, on the other hand, to the increasing disillusionment concerning the need for the vast number of protocols developed for general Mobile Ad-Hoc Networks (MANETs) in the past few years while on the other side that is for VANETs, industry pressure has created a situation in which an overwhelming interest in solutions to problems leads to a preference for real-world research as opposed to fancy theory. As the concept came from MANETs which totally depend on the subscribers motion as the motion is random it is difficult to cater it but this problem was very negligible when researchers observed it in VANETS. At highways vehicle move in an organized pattern with different speeds so initially it seemed that VANET will easily be implemented. Another major reason for VANET can be Traffic deaths and injuries which is a major health and social issue. While industrialized nations (e.g., the United States) have continuously reduced annual traffic deaths since 1970, annual traffic-related fatalities and injuries remain high (in the United States alone there were over 41,000 deaths and 5 million injuries in 2000, according to the NHTSA) . The economic impact of vehicle crashes in the United States exceeded US$230 billion or 2.3 percent of the U.S. GDP in 2000 . We want to remain connected with the world through net whether at home, airport, at work or even on the roads.
Stopped/Skidding/Slowing down vehicle warning, road obstacle/object-on-road warning
Lane Merge/Lane Change Assistance
Merging/Lane changing vehicles communicates with vehicles in lane to safely and smoothly merge.
Adaptive Cruise/Cooperative Driving
Automatically stop and go smoothly, when vehicles are in heavy roadway traffic; cooperates driving by exchanging cruising data among vehicles.
Intersection/Hidden Driveway Collision Warning
\vehicles communicates to avoid collisions at intersections without traffic lights or hidden driveway.
Roadway Condition Awareness
Vehicles communicates to extend vision beyond line of sight (e.g. beyond a big turn or over a hill)
Table-1.1: Example of Vehicle Safety Communication 
1.3 Scope of Project
Some of the industries and universities working on VANETs are as follow
Ø DaimlerChrysler AG
Ø Fraunhofer FOKUS
Ø NEC Europe Ltd.
Ø Robert Bosch GmbH
Ø Siemens AG
Ø TEMIC Telefunken
Ø Microelectronic GmbH
Ø Universities of Mannheim, Hamburg-Harburg, Karlsruhe, and Hannover.
1.4 Organization of Project
Thisthesisis mainly divided into four chapters. In the first two chapters (1-2) introduction and an overview over the topic and used technologies is given. In the following chapter (3), we have discussed the standards of IEEE and also discussed the MAC Layer and PHY Layer of IEEE 802.11 in detail. In chapter 4, simulation analysis of our work is shown along with the results. In the last chapter, we have summarized this whole thesis, what we have concluded from this project and future work needs to be done are discussed. Finally, in appendix some additional information can be found. In chapter two, VANET's characteristics, some of its applications and the research challenges faced by governments and car manufactures are discussed, continued by MAC Layer and PHY in chapter three. We have also discussed the WAVE architecture in chapter 2. From chapter three on, we have a look at some protocol improvements and extensions. Some thoughts, tests and their results on VANETs, those are related to our work, can be found in chapter 4.
VANETs (Vehicle ad-hoc Networks) is a form of Mobile Ad-hoc Networks (MANETs), which provide a communication between the vehicles and also fixed equipments, usually defined as road side equipments.
2.1 What is VANET
Vehicle ad hoc network comprises of three words.
“A machine such as a bus or car for transporting people or goods”. 
A lot of progress is happening in the field of vehicles since the invention of wheel. Development is due to provide services to the people and make their task easier.
It refers to dealing with special situations as they occur rather than functions that are repeated on a regular basis. For example you just meet someone outside your office and you exchange some words. On the other hand infrastructure system is a system which is fully installed and deployed than it works according to some predefined rules and regulations.
“A system, as in a business or university, consisting of a computer or computers and connected terminals, printers etc. specific, a local area network”.
The concept of networking is introduced because resources are limited and we have to utilize them efficiently. As it is not possible for firms to provide printer, faxes and other machines to everyone so they just inter linked all the devices so that each one can utilize it keeping the cost at minimum.
Vehicular connectivity can be fairly considered a future killer application, adding extra value to the car industry and operator's services. Taking into account the constant growth of automotive market and the increasing demand for the car safety, also driven by regulatory (governmental) domain, the potential of car-to-car connectivity is immense. Such system should be suitable for a wide spectrum of applications, including safety related, traffic and fleet control and entertainment. First, issues concerning architecture, security, routing, performance or QoS need to be investigated. Standardization of interfaces and protocols should be carefully planned to ensure interoperability, as vehicles coming from different vendors must communicate seamlessly. Having different competing systems would result in decreased market penetration and poor overall system efficiency, thus only one common system can be deployed. And finally, wise deployment strategy has to be proposed, as most application would become functional only after certain market penetration is reached. The first milestone of standardization process was the allocation of 75 MHz of DSRC (Dedicated Short Range Communications) spectrum to accommodate Vehicle-to-Vehicle (V2V) and Vehicle-to- Infrastructure (V2I) communication for safety-related applications by US Federal Communications Commission (1999). Commercial applications are also allowed to operate in this spectrum.
2.2 VANETs 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 application. Moreover, they can be categorized into OBU-to-OBU or OBU-to-RSU applications. Some of these applications are as followed:
2.2.1 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 message to warn all of the following vehicles of the probable danger. This application requires an efficient broadcasting algorithm with a very small latency.
2.2.2 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.
2.2.3 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 (Figure-2.1), as quoted from GM, If the driver doesn't respond to the alerts, the vehicle can bring itself to a safe stop by avoiding a collision.
2.2.4 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.
2.2.5 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.
2.2.6 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.
2.2.7 Near Term 
Ø Traffic Signal Violation Warning
Ø Curve Speed Warning
Ø Emergency Electronic Brake Lights
2.2.8 Mid Term 
Ø Pre-Crash Warning
Ø Cooperative Forward Collision Warning
Ø Left Turn Assistant
Ø Lane Change Warning
Ø Stop Sign Movement Assistance
Traffic Signal Violation
12V One-way, P2M
Signal Status, Timing, Surface Heading, Light Position, Weather
Curve Speed Warning
12V One-way, P2M
Curve Location, Curvature, Speed Limit, Bank, Surface
Emergency Brake Light
Vehicle to Vehicle Two-way, P2M
Position, Deceleration Heading, Velocity
Vehicle to Vehicle Two-way, P2P
Vehicle type, Yaw Rate, Position Heading, Acceleration,
Vehicle to Vehicle One-way, P2M
Vehicle type, Position, Heading Velocity, Acceleration, Yaw Rate
Left Turn Assist
12V and V21 One-way, P2M
Signal Status, Timing, Position, Direction, Road Geom., Vehicle Heading
Lane Change Warning
Vehicle to Vehicle One-way, P2M
Position, Heading, Velocity, Acceleration, Turn Signal Status
Stop Sign Assist
12V and V21 One-way
Position, Velocity, Heading, Warning
Table-2.1: Eight high-priority vehicular safety applications as chosen by NHTSA and VSCC. Note that communication freq. ranges from 1-50 Hz and Max. Communication range spam 50-300 meters. P2M represents “Point-to-Multipoint”, 12V represents “infrastructure to vehicle” and V21 represents “Vehicle-to-Infrastructure”. 
2.2.9 Comfort related applications
Ø Traffic efficiency
Ø Better navigation
Ø Internet access
The whole theme of these applications is improving passenger's comfort and traffic efficiency. That includes nearest POI (Points of Interest) localization, current traffic or weather information and interactive communication. All kinds of applications might be applied here. Another application is reception of data from commercial vehicles and roadside infrastructure about their businesses ('wireless advertising'). Enterprises (shopping malls, fast foods, gas stations, hotels) can set up stationary gateways to transmit marketing data to potential customers passing by.
The important feature of comfort/commercial applications is that they should not interfere with safety applications. In this context traffic prioritizing and use of separate physical channels is a viable solution.
2.2.10 Safety related applications
Ø Accidence avoiding
Ø Danger warnings
Ø Intersection coordination
Ø Cooperative driving
Safety-related applications may be grouped in three main classes: assistance (navigation, cooperative collision avoidance, and lane-changing), information (speed limit or work zone info) and warning (post crash, obstacle or road condition warnings). They usually demand direct communication due to their delay-critical nature. One such application would be emergency notifications, e.g. emergency braking alarms. In case of an accident or sudden hard breaking, a warning is sent to the subsequent cars. That information could also be propagated by cars driving in the opposite direction and, thereby, conveyed to the vehicles that might run into the accident. Another, more advanced example is cooperative driver assistance system, which exploits the exchange of sensor data or other status information among cars. The basic idea is to broaden the range of perception of the driver beyond his field of vision and further on to assist the driver with assistance applications. Transmitting this data to cars following on the same road, drivers get information about hazards, obstacles or traffic flow ahead; hence driving is more efficient and safer. Some applications of this kind are operating only when certain penetration of VANET enabled cars is reached. 
2.3 VANETs Characteristics
Although VANETs, Wireless Sensor Networks and Wireless Mesh Networks are special cases of the general MANETs, VANETs possess some noticeable characteristics that make its nature a unique one. These properties present considerable challenges and require a set of new especially designed protocols.
Ø Due to the high mobility of vehicles, that can be up to one hundred fifty kilometers per hour, the topology of several 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 vehicle's transmission range will increase the collision probability and mortify the overall throughput of the system. The other solution having a set of new protocols is employing a very low latency.
Ø Another effect of these high speed nodes is that the usefulness of the broadcasted messages is very critical to latency. For example, if we assume that a vehicle is unexpectedly stopping or suddenly stops, it should broadcast a 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 , the warning message should be delivered at virtually zero sec latency.
Ø In VANETs, location of nodes changes very quickly and unpredictably, so that, building an efficient routing table or a list of neighbor nodes will tire out the wireless channel and reduce the network efficiency. Protocols that rely on prior information about location of nodes are likely to have a poor performance.
Ø However, the topologies of a VANET can be a benefit because vehicles are not expected to leave the covered road; therefore, the running direction of vehicles is predictable to some extent.
Ø 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).
Ø It is predicted that, as VANET is deployed in the beginning, only a small percentage of vehicles will be outfitted with transceivers. Thus, the benefits of the new technology, especially OBU-to-OBU applications, will not go up until many years. Furthermore, the limited number of vehicles with transceivers will lead to a numerous 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.
Ø Privacy, safety and security are of fundamental effect on the public receiving of this technology. In VANETs, every node represents a specific person and its location tells about his location.
Any requirement of privacy can ease a third party monitoring person's 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 proposed design requirement of Wireless Mesh Network is reliability.
WE can summarize the main characteristics of VANETs as follows;
Ø High mobility of nodes
Ø No prior information about the exact location of neighbor nodes
Ø Predictable topology (to some extent)
Ø Significant latency requirement especially in cases of safety related applications
Ø No problem with power
Ø Slow migration rate
Ø High possibility to be fragmented
Ø Crucial effect of security and privacy
2.4 Research Challenges in VANETs
When deploying of a vehicular networking system, a number of issues have to be determined, often from distant fields of expertise, ranging from applications improvement up to efficient issues. VANET could be considered as an instantiation of MANETs (Mobile Ad hoc Networks); however their behavior is fundamentally different. These unique characteristics of these networks are as follows:
Ø Rapid topology changes and fragmentation, resulting in small effective network diameter
Ø Virtually no power constrains
Ø Variable, highly dynamic scale and network density
Ø Driver might adjust his behavior reacting to the data received from the network, inflicting a topology change
Here we briefly mention some of the core research challenges that need to be discussed.
2.5 Wireless Access technology
There are several wireless access standards that could be used as a foundation for VANET technology. In general the major seek is to provide a set of air interface protocols and parameters for high-speed vehicular communication by mean of one or more different media.
2.5.1 Cellular technology (2/2.5/3G)
The key role of 2/2.5G i.e. cellular technology are coverage and security, and 3G, slowly but steadily coming over 2/2,5G, provides enhanced and better capacity and bandwidth. Several telematic and fleet management projects already uses cellular technology (e.g. SMS reports), on the other hand it is comparatively more expensive, together with limited bandwidth and latency make it impossible to use as a main communication means.
2.5.2 IEEE 802.11p based technology
IEEE is working on a variation of 802.11 standards that would be applied to support communication between vehicles and the roadside, or, alternatively, among vehicles themselves, operating at speeds up to 200 km/h, handling communication ranges as high as 1,000 meters. PHY and MAC layers are based on IEEE 802.11a, shifted to the 5.9 GHz band (5.850-5.925 GHz within US). The technology is promoted by the car industry both in Europe and US. Estimated deployment cost is foreseen to be relatively low due to large production volumes. C. Combined wireless access one of the most significant and important efforts in combining those wireless access technologies is done by ISO TC 204 WG16. It builds on the top of IEEE 802.11p, using additional set of interface protocols. Currently supported standards include: Cellular Systems: GSM/GPRS (2/2.5G) and UMTS (3G), Infrared Communication and wireless systems in 60 GHz band. Using all those interfaces in a single, uniform system would result in increased flexibility and redundancy, thus improving applications' performance. Apart from interoperability issues, CALM is also engaged in the standardization of the protocols, network layer and the management services.
2.6 WAVE Architecture
WAVE system architecture is totally a set of WAVE standards that describes the communication stack of vehicular nodes and the physical air link between them. Any RSU may have two interfaces, one for the WAVE stack or architecture or wireless networks and the other for external interfaces like wired line Ethernet that may be used to get access to internet and for connection to internet it is mainly used. Similarly, each OBU may have two interfaces, one for the wireless WAVE stack and the other for sensor-connections and human interaction.
OBU is not full-duplex so, therefore, it cannot transmit messages simultaneously, so DSRC is half-duplex. The RSU and OBU can send messages only when the channel becomes idle and also confirmed that it is idle. If the channel is busy, RSU and OBU need to wait and if the channel is idle then RSU or OBU will send the signal Request to Send (RTS) to control channel. The control channel will allocate the channel on the basis of high priority first followed by low priority. The high priority messages are those messages related to public safety.
The WAVE architecture is defined by the IEEE 1609 family of standards and uses the IEEE 802.11p amendment to extend the use of 802.11 to vehicles. The IEEE 1609 family is composed of four standards describing the resource manager, security services, networking services and multi- channel operations.
WAVE standard consists of five complementary parts
Ø 802.11p “Wireless Access in Vehicular Environments (WAVE)”  which is an amendment to the well known IEEE 802.11 Wireless LAN Standard and covers the physical layer of the system.
Ø 1609.1 “Resource Manager”  that covers optional recommendations for the application layer. , 
Ø 1609.2 “Security Services for Applications and Management Messages”  that covers security, secure message formatting, processing and exchange. , 
Ø 1609.3 "Networking Services”  that covers the WAVE communication stack. , 
Ø 1609.4 “Multi-Channel Operation”  that covers the arrangement of multiple channels and how they should be used. , 
The most evident part is its dual stack. Whereas there is a well-known stack, called TCP/IP stacks and on the other hand there is a stack, called WAVE Short Message stack. The function of the WAVE Short Message stack is to provide a connectionless transport protocol i.e. without checking the connection that whether connection is made or not, similar to UDP but on a single-hop basis. The safety applications are supposed to use this stack only while non-safety applications can use both. It should be noted that the devise or design of this approach is focused on non-safety applications and considers safety as a black box.
2.6.1 IEEE 1609.1 - Resource Manager
The IEEE 1609.1 standard defines the architecture and data flows of WAVE. It also describes command messages and data formats. , . The standard explains how data communication between road side units and vehicle on board units occurs. The discussion of this standard's operation will be based on the standard defines applications residing on the on board unit as Resource Command Processors and those residing in road side units or elsewhere as Resource Manager Applications. The Resource Manager is the focus of this standard and is also the application that is responsible for managing communication between multiple Resource Manager Applications and Resource Command Processors. , 
WAVE communication imitates a client-server architecture that is managed by the Resource Manager. For example, in the case where a company wants to provide traffic updates by analyzing vehicle speed statistics in a stretch of highway, the application that analyzes the traffic data (a Resource Manager Application) would reside on the road side unit or a remote server that is connected to a road side unit. When the Resource Manager Application sends a request for the speed of the vehicle the Resource Manager application in the road side unit receives the request then forwards it to the vehicle's Resource Command Processor application using WAVE. The vehicle then replies to the Resource Manager which forwards the message to the Resource Manager Application. If another passing vehicle asks for traffic updates by sending a request to the road side unit, the roles of client and server from the previous case are switched.
WAVE is designed to provide secure communications and minimize the cost of on board units by minimizing the amount of processing required by them. All only desired information relevant to road safety will be transferred.
2.6.2 IEEE 1609.2 - Security Services
The IEEE 1609.2 standard defines secure message formats and processing and infers circumstances for using secure message exchange. , . It deals with security services for applications and management messages. Security is important in WAVE because vehicles transmit sensitive information that could constitute a violation of privacy if accessed by unauthorized parties. The efficacy and reliability of a system where information is gathered and shared among autonomous entities raises concerns about the authenticity of the received data. For example, a bad actor could misrepresent its observations in order to gain advantage (e.g. a vehicle V falsely reports that its desired road R is stopped with traffic, thereby encouraging others to avoid R and providing a less-congested trip for V on R). More malicious actors could impersonate other vehicles or road-side infrastructure in order to trigger safety hazards. Vehicles could reduce this threat by creating networks of trust, and ignoring, or at least distrusting, information from un-trusted senders. , 
A trusted communication generally requires two properties are met:
Ø The sender is conclusively identified as a trusted source.
Ø While in transit, the contents of the sender's message are not tampered.
WAVE maintains security by ensuring confidentiality and authenticity in message transmissions. The final standard is expected to address privacy issues with the current version. WAVE ensures confidentiality through encryption. WAVE systems use both symmetric and asymmetric encryption methods. In symmetric encryption, the same key is employed to both encrypt and decrypt the message. In asymmetric encryption (also known as public key cryptography) a widely distributed public key is usually used to encrypt messages while a private key that is kept secret by the receiver is used to decrypt messages. A good private key is defined such that it is computationally impractical to be determined from the public key. Since asymmetric encryption often requires more computation than symmetric encryption. WAVE uses asymmetric encryption to establish communications and then utilize Advanced Encryption Standard, a symmetric encryption standard for actual communications.
WAVE ensures accurate identification by authenticating transmitting units before opening an encrypted communication session. When a vehicle initiates a transmission to another vehicle, it applies its private key to encrypt a digital signature to identify itself in the message. The receiving vehicle can then use that vehicle's public key for decryption to verify the signature. In order to prevent another vehicle from acting like the sender, the public key is certified with a trusted certificate authority.
WAVE communications present some privacy issues that must be solved before widespread implementation. When a vehicle sends a broadcast to other vehicles, the vehicle should somehow be authenticated but not have its identity leaked to other vehicles. The current standard does not address this issue. The standard also recommends the utilization of permanent MAC addresses for each vehicle. This potentially allows a vehicle to be tracked by observing the location of its transmissions over time. While there are ongoing discussions related to periodically assigning new addresses to solve this issue, we should recognize that WAVE deployment is still an area of concern for privacy advocates.
2.6.3 IEEE 1609.3 - Networking Services
The IEEE 1609.3 standard contains transport and network layer services such as addressing, routing and WAVE data exchange. , . It also contains the behavior of the system when dealing with messages of different application priorities (not channel priorities).
Every WAVE device is given a 48-bit MAC address for each interface and at least one globally unique IPv6 address. Since on board units only have a single wireless interface, they have one MAC address. Road side units typically require a MAC address for each of their wired and wireless interfaces. IPv6 addressing allows WAVE to handle a large network without worrying about address exhaustion. IPv6 is designated as the future replacement for today's internet addressing scheme. It supports 2128 unique addresses and is not expected to be completely used in the foreseeable future. WAVE designers have also allowed the use of other types of network addresses through the use of the Wave Short Message Protocol (WSMP).
WAVE defines two types of channels: control and service channels. The control channel is designed to have high speed and low latency to support high priority, safety applications. This channel is also used to establish communications for uncritical applications before switching to a service channel. The service channels are designed to handle general purpose communications. Unlike control channels that only support WSMP. The service channels support both WSMP and IPv6 for transmissions. WSMP is optimized to operate over the WAVE network. It allows units to directly control the channel and power used during transmission. During heavy traffic, reducing transmission power can lower the amount of interference experienced by other vehicles allowing WAVE to handle large vehicular networks. WSMP also allows non-IPv6 traffic to be encapsulated in messages.
2.6.3 IEEE 1609.4 - Multi-channel Operations
IEEE 1609.4 is focused on lower layer networking functions. It describes in detail how WAVE deals with control and service channels. Additionally, it describes aspects of the IEEE 802.11p amendment. , .
We require the following services on the road to get maximum benefits,
220.127.116.11 Spectrum issues
The intended usage period for V2V communication system is estimated for at least 20 years and within this time the spectrum availability has to be guaranteed. In the USA the FCC has already allocated 75 MHz of spectrum at 5.9 GHz (from 5.850 to 5.925 GHz) band. As agreed by VSC and VII Consortiums, the best technology available for the communications systems using this spectrum would be a derivative of IEEE 802.11. Thus the before mentioned IEEE 802.11p WG and ISO TC204. Unfortunately a continuous spectrum of 75 MHz in DSRC band is not available in Europe. Hence the Car2Car CC has proposed a derivative of the US approach. The proposal allocates the 5.9 GHz band range (5.875 – 5.925 GHz) for primary use of safety critical applications, and 2 x 10 MHz at 5.855 –5.875 GHz for non safety applications. Those bands are used in the US as well, and their allocation in Europe would allow for world-wide harmonization. Earlier the 5.9 GHz band was allocated for military radar systems and fixed satellite services; however recently the CEPT/ECC Short Range Device Maintenance Group (SRD/MG) has recommended placing the 10 MHz control channel in 5.885 - 5.895 GHz, to align with the US approach, 5.875 GHz) to take also into account radiolocation services below 5.85 GHz. The decision about bandwidth assignment and the second 10 MHz channel in the upper part of the ISM band (5.865 - for safety applications were taken by CEPT/ECC in March 2008).
Department of Transportation could designate regional (e.g. Michigan Department of Transportation) or municipal agencies to create and manage certificates.
18.104.22.168 Privacy Issues
Because each vehicle periodically broadcasts a fixed authenticator and fixed identifier, it can be tracked throughout the road network. This allows substantial privacy compromise, because an adversary can track a user as they drive around, and may even be able to associate a vehicle's identifier with a user identity. An adversary can potentially hack into the OBU, OBU of a vehicle being then able to access information by reading communication logs. The point of attack is the fact that every node carries a unique identifier that is utilized for both, to establish a session between nodes, and for forwarding along the routing path. When the identifier represents an electronic license plate an adversary can even link the identifier to the personal data of the registered vehicle owner. We can easily identify, locate and track vehicle's position using exclusive identifiers for communication. As a technical solution to protect driver's privacy, the use of changing pseudonyms (randomly chosen changing identifiers) has been proposed. But problem will arise in this case Imagine your friends were constantly changing their phone number without letting you know in advance. How would you know which number to call if you wanted to talk to one of them? The same happens in VANETs if vehicles keep changing the addresses. Therefore, it gets difficult for system to resolve each time before transaction.
Wireless computer communication has become a nearly ubiquitous technology. Recently years wireless data networks or technology have become very popular due to its high data rates and also with low cost hardware. The processing of Information is not longer bound to stationary computer systems and many new and interesting applications have been enabled by mobile data processing. The key point for the success in the wireless data networks or technology is the arrival of IEEE 802.11 standard for wireless LAN (WLAN). Today it is unquestionable that the most successful standard for networking computers wirelessly. This standard first enabled us for the usage of wireless connectivity that we often granted. It is popular because of its simplicity and robustness against different kinds of failures that is much comparable to wired networks. Operation in unlicensed radio bands allows wide-spread public use and easy adoption, while at the same time requires data transmission to be resilient against interference and thus provide robust connectivity. Distributed medium access control allows uncomplicated deployment and smooth operation of multiple networks in the same area. The initial 802.11 standard was approved and published by the Institute of Electrical and Electronics Engineers (IEEE) in 1997. Much of the fundamental service definition including the basic MAC functions, like DCF and AP association rules, was already part of the initial standard, is still valid and in general use today. Other parts were subsequently extended in different supplements and amendments. These provide, for example, faster data transmission, quality of service (QoS) extensions and adaptation to specific locations and environments. Table-3.1 shows a selection of currently approved or proposed 802.11 amendments and includes a short description of their individual aims.
Each vendor had his own set of protocols, transmission modes and parameters. Incompatibility between these solutions prohibited wide-spread adoption and lead to definition of 802.11, which is the prevalent standard for medium range packet radio networking today.
3.1 802: The Big Picture
The IEEE 802.11 standard is part of the IEEE 802 standards family, which deals with local and metropolitan area networks. The 802 standards define services and protocols for the lower two layers of the ISO/OSI layer reference model, the data link and physical layer. The number 802 was simply the next available IEEE standard number at that time.
Among the members of this standard family are 802.3 for Ethernet, 802.5 for Token Ring, 802.11 for Wireless LAN and the upcoming 802.16 for WiMAX. There are also some less successful or retired standards like 802.4 for Token Bus and 802.8 for Fiber Optic networks. The 802 family also includes a set of base standards like 802.1 for network bridging and management, 802.2 for logical link control (LLC) and 802.10 for interoperable LAN security mechanisms. The 802.11 wireless standards defines a service point compatibly with 802 medium access control (MAC) requirements, the same as 802.3 wired Ethernet fulfills. It therefore provides an interface to the upper layers that is equivalent to that of other network technologies and ultimately yields a user experience very similar to wired networks.
Initial standard, 1 or Mbps using FHSS or DSSS in 2.45 GHz band.
Up to 54 Mbps using OFDM in 5 GHz band.
Up to 11 Mbps using HR/DSSS with CCK in 2.45 GHz band.
TPC and DFS for 5 GHz band in Europe.
ERP using OFDM, CCK, DSSS and others for up to 54 Mbps in 2.45 GHz band.
Security mechanism WPA and WPA2 using broken WEP encryption.
QoS extension: HCF with EDCA and HCCA.
Revised standard incorporating all preceding amendments.
Radio Quality measurement and Network information.
Fast handoff for transition between BSS
In work for 2009
WAVE – Wireless Access for Vehicular Environments.
In work for 2009
MIMO, channel bonding and frame aggregation for higher throughput.
Mesh networking for infrastructure and ad-hoc, multi-hop connectivity.
Table-3.1: Selection of 802.11 standards and amendments
3.2 Outline of 802 and 802.11 Layers
This section gives a very broad overview of the components defined in the 802.11 standard. 3.1 illustrates these sub layers and how they build up the 802.11 architecture. The medium access control (MAC) layer provides a service entry point for higher entities to exchange message packets between addressable wireless stations. To support this service the MAC utilizes the underlying physical layer (PHY) services provided by the pair of PLCP and PMD. The MAC layer defines a transmission frame format and different data, control and management frames for exchange between wireless stations (STAs) and access points (APs). To manage frame exchanges on the shared medium, the MAC defines the coordination functions DCF which regulate how stations may access the shared wireless medium. Conceptually the MAC layer contains a sub layer called MAC sub layer management entity (MLME), which provides operational features like AP scanning and association, encryption set up and configuration. The PHY layer is split up into two sub layers. The physical medium dependent (PMD) layer defines how a specific medium is accessed by the transceiver, while the physical layer convergence procedure (PLCP) provides adaptation to a common PHY interface. The 802.11 standard defines different PMD/PLCP pairs for communication in the 5 GHz U-NII radio band and using infrared light. Integrated into the PHY layer is another management layer, the physical layer management entity (PLME), which exports configurable aspects of the used transmission modes and other information. Both management layers are accessed by the station management entity (SME) cross layer, which is not explicitly defined by the 802.11 standard. Its purpose is to provide an interface for higher level system management. Possible services of the SME are configuration of 802.11 components by the user, gathering of statistics or forwarding of user requests.
3.3 MAC Layer
The purpose of the MAC layer is, as its name suggests, coordinating access to the medium. However, the MAC layer in a single wireless device can, in effect, only control packets sent by it. To prohibit another device from sending packets is not in its direct control. Therefore, a group of independent wireless stations must collaboratively coordinate packet transfer by following a set of rules. This set of distributed rules is put down in the 802.11 standard and all equipment must follow them for smooth operation. At the core, wireless packet-based medium access is about agreeing on distributed algorithms or protocols that determine when and which station may send a packet. How the packets themselves are actually sent on the medium is determined by the PHY layer below. The packet sending interface of the PHY has only broadcasting semantics: a transmitted packet is heard by all receivers within range. The MAC layer, however, attempts to provide an interface to higher layers that is indistinguishable from other wired 802 layers like Ethernet or Token Ring. Due to the differences of wireless communication this aim cannot (and should not) be completely fulfilled, but a common interface that enables wireless devices to be used side-by-side with other network devices on a system is possible.
This network interface has many attributes commonly known from Ethernet devices. For example, each wireless device has a 48 bit MAC address, which is globally unique and allocated from the same pool as Ethernet devices. Using the MAC addressing scheme, unicast packet delivery semantics are defined within a BSS just like with Ethernet.
However, the medium used by the PHY layers of wireless devices has characteristics fundamentally different from traditional wired media. The MAC layer must incorporate advanced functionality to deal with these differences. Some of the differences and mechanisms to overcome them are listed below
Ø Communication using a wireless medium is significantly less reliable than wired communication. Therefore, the MAC uses positive acknowledgement for most packet transfers.
Ø Full connectivity between all stations cannot be assumed, e.g. due to obstacles packets are therefore relayed through a central access point.
Ø Radio propagation between two stations is generally asymmetric.
Ø Communication is unprotected from other signals on the medium. Particularly in unlicensed bands, the grades of quality of service (QoS) guarantees that can be provided are severely limited.
3.4 Communication Context
Wireless stations are grouped together to form a communication context called a basic service set (BSS). All data transfer between stations takes place within a BSS, except if the new upcoming 802.11p standard is used. There are two kinds of BSS: independent BSS (IBSS) and infrastructure BSS with an AP. Independent BSS are more commonly referred to as ad-hoc networks and communication within an IBSS is limited to direct passing of packets between stations. An infrastructure BSS on the other hand contains an AP, which is connected to further networks usually via a wired network interface. The AP operates as a bridge between wired and wireless communication by forwarding packets as necessary. Even packets sent between wireless stations are relayed via the AP.
3.5 CSMA/CA using CS and NAV
802.11 use a scheme, called as Carrier sense multiple accesses with collision avoidance (CSMA/CA) to coordinate access to the wireless medium and is similar to carrier sense multiple access with collision detection (CSMA/CD) which is used by Ethernet. In both cases the carrier requires to sense the medium for multiple accesses to the medium.
However, it is difficult and also expensive to built radio transceivers that can do both jobs that are to listen and send simultaneously that is full duplex. Therefore 802.11 devices are not required to work in full duplex. With half duplex transceivers, however, collision detection as employed by Ethernet while sending is not possible. Instead, collision avoidance is attempted through distributed coordination protocols.
There are two different carrier sensing mechanisms used by 802.11: physical carrier sense and virtual carrier sense.
A physical carrier sense indication is raised when another signal is detected on the medium. However, this signal detection is rather difficult because most wireless modulation schemes are only distinguishable from noise if the receiver is properly synchronized and has the capabilities required to decode the signal. The different PHYs in 802.11 require individual carrier sensing mechanisms and the standard leaves device manufacturers great flexibility in designing these mechanisms. Virtual carrier sense is defined by the duration field in the MAC header, which is attached to every packet sent. The duration field contains a time value (in microseconds) for which the medium is reserved after the current packet. This medium reservation is called the network allocation vector (NAV). As a first basic coordination rule, a station may not send a packet if either physical or virtual carrier sense indicates that the medium is busy. Using this rule, collisions are already avoided in most cases.
However two important collision scenarios remain:
The first one occurs when two stations wish to send a packet and both detect the medium as idle. The issue is to determine when a station is allowed to start sending. 802.11p use this to prioritize access to the medium for different transmission methods. Section 3.8 discusses these access priorities in detail.
And the second collision scenario, which is a special case of the first, is called the “hidden terminal” constellation. This happens when two stations are too far away from each other to sense their transmission and thus both detect the medium as idle. If both attempt to send to a third station located in between, their transmissions interfere and packets will be lost.
Figure-4.1 shows the hidden terminal scenario. If A and B attempt to send a packet to C at nearly the same time, both packets are superimposed at the receiver. Under most circumstances both packets are decoded incorrectly and are dropped. If however they receive power of one signal is small relative to the other, the stronger signal can still be correctly decoded.
3.6 RTS/CTS Exchange
To avoid interference in the hidden terminal scenario, 802.11 uses a protection scheme. First a request to send (RTS) control frame is sent, which must be answered by the destination station with a clear to send (CTS) frame.
Important in this scenario is that all other stations within range receive the RTS, the CTS or both frames. Thus all interferers in range of both stations are prohibited from sending during the data frame. However, the RTS/CTS protection sequence comes at the cost of two additional frames. These introduce delay and reduce the maximum throughput by adding overhead. Hence the RTS/CTS sequence should only be used to large data frames. For large data frames the cost of collision is reduced because the RTS frame is much shorter and thus medium recovery time is shorter. RTS/CTS do not reduce the probability of collision. We are not applying RTS/CTS because we are dealing with mobile vehicle which are changing their positions frequently so it get difficult to keep RTS/CTS record for the long time.
The most commonly used coordination mode today is distributed coordination function (DCF). The time a station must wait is uniformly randomized. The range from which the random waiting time is chosen is called the contention window (CW).
The rules of DCF are following: if a station is newly initialized then it may send the first frame immediately if the medium is idle or free. If the medium is not idle, then a back off time is uniform randomly chosen from the interval [0 . . . CW], where CW=CWmin. Once the medium is idle the station must still wait further and decrement the back off counter for each Slot Time interval. If the medium is seized by a different station in that time, the back off counter decrementing is stopped. Decrementing proceeds once the medium is idle again. When the back off counter reaches zero, the station may send the waiting frame.
Figure-3.3 shows an example time line of DCF's packet coordination. The example contains three stations all within range of each other and initially station A is sending a packet. While A is sending, the higher layers in both B and C queue a packet for transmission. B and C determine that the medium is busy and must pick a random back off. In the example, B picks 5 and C's back off is randomized to 8. When A finishes,
Its broadcast frame at t1, it too must start a back off timer in this case with initial value 9.
All stations start decrementing their back off counters. The small rectangles each depict an interval of Slot Time. After 5 Slot Time, station B wins the distributed competition and is allowed to send its frame at t3. Because all other stations sense that the medium is busy, they stop their back off counters. Once B has finished its frame, it starts a new back off to transmit an additional frame and this time must wait 4 slots. At t5 all station again start decrementing and this time C's counter, which still contained 3, first reaches zero. The frame transmitted by C finishes at t7 and a new back off is started for C. Because both A's and B's counters contain a remaining back off of 1 slot, both transmit their packets after 1 Slot Time at t9. Due to the interference of both packets, they will probably not be received correctly and a retransmission might be necessary.
As the example shows, DCF does not prohibit collisions; it only avoids those using random back offs. By increasing the CW interval, DCF automatically adapts to higher network load.
DCF is statistically fair to all stations with respect to the time they have to wait before sending. However, it is not fair with respect to the amount of bytes transmitted by each station; packet payload size or transmission time is not taken into account by DCF. Thus byte throughput is not fairly coordinated by DCF. Obviously DCF is not designed to provide QoS i.e. all traffic is delivered with best effort semantics.
In contention periods (CPs) of HCF, TXOPs are allocated by EDCA using a set of distributed rules, which are based on those defined in DCF. Similar to DCF, EDCA is very likely to become the most widely used channel access mechanism, because it is simple and easy to implement and hassle free to employ due to its distributed nature. EDCA defines four access categories (ACs) with different medium access priorities. The four ACs are labeled AC_VO for “voice” class traffic, AC_VI for “video” class traffic, AC_BE for best effort traffic and AC_BK for background traffic. For packets received from higher layers that are tagged with user priorities (UPs) defined in 802.1D a mapping to ACs is defined in the standard. The four classes are enumerated with an access category index (ACI) i.e. 3 for AC_VO, 2 for AC_VI, 0 for AC_BE and 1 for AC_BK.
Each AC has a separate queue and enhanced distributed channel access function (EDCAF) (see figure-3.4). Note the difference between EDCA as a sub mechanism of HCF and an EDCAF, which defines when packet transmission is allowed for each AC. Thus there are four EDCAF in a QoS station.
An EDCAF is very similar to the original DCF. The differences can be summarizing with following modifications:
Like DCF, the medium must be idle for a specific time interval before the EDCAF is granted access or starts decrementing the back off counter. This interval, denoted as AIFS [AC], corresponds to DIFS in DCF. For backwards compatibility with PCF, the value of AIFSN [AC] must be greater or equal to 2 for non-AP stations.
If a transmission needs to be delayed, a random back off is chosen from [0 . . . CW]. As with DCF, the CW value starts at CWmin [AC] and is increased for each retransmission, up until CWmax [AC] is reached. Each EDCAF has a separate CW variable. The back off counter is decremented only after the medium is idle.
When the back off counter reaches zero, the AC is granted an EDCA-TXOP. This TXOP has a maximum duration specified by the parameter TXOPLimit [AC]. A TXOPLimit = 0 allows a single frame at any data rate. Note that the TXOP is granted to the AC, not to the station. Last modification is the addition of an admission control mandatory (ACM) parameter, which specifies whether a station needs permission from the AP to use the AC.
A QoS-enabled station holds four EDCAFs, so it can occur that two or more back off counters reach zero simultaneously. This is called an internal collision and is resolved by granting access only to the EDCAF with highest priority. The other EDCAF must initiate the usual back off procedure just as if an “external” collision on the medium is detected.
3.8.1 Default EDCA Parameters
EDCA can be used with or without an AP as central controller. If EDCA is used within a BSS controlled by an AP supporting QoS, the EDCA parameters used by all associated stations are defined in the beacon. This allows the AP to tune and limit the QoS provided by EDCA depending on administrative requirements and other QoS constraints.
If EDCA is used in an ad-hoc BSS (IBSS) or outside the context of a BSS (802.11p), then a default predefined parameter set is used. This default parameter set is different for each PHY layer and is defined in the standard by calculations from other parameters. There is no acknowledgement in the case for broadcast and multicast frames.
3.10 PHY Layers
The 802.11 standard specifies different PHY layers for transmitting data over different media. All PHY layers have a common interface to the higher MAC layer, which coordinates packet transfer on the medium. Each PHY layer definition contains all details of the low level aspects of wireless data transfer: how bits are encoded, modulated and how the wireless medium is multiplexed. Regulatory aspects of the 5 GHz and 5.9 GHz radio bands used by the different PHY layers are discussed in the following section 3.10.1. The original standard from 1997 defines two PHY layers for the 2.45 GHz ISM radio band. Both PHY layers operate at two data rates: 1 Mb/s or 2 Mb/s. Two different spread spectrum techniques are used to encounter the challenge of transmitting in unlicensed bands: frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). The original standard also contained a PHY layer for infrared light, which was not as successful as the radio PHY layers. In 1999, the 802.11a amendment was approved by the IEEE and it introduced new specifications for up to 54 Mb/s in the 5 GHz U-NII bands. These high data rates were possible by employing orthogonal frequency division multiplexing (OFDM). The speed increase of 802.11a and 802.11b was a major break through at that time and made wireless LAN a serious contender to wired connections for many scenarios. By adapting the 802.11a PHY, operating with OFDM, for the 2.54 GHz radio band, the higher transmission rates of up to 54 Mb/s were made possible in all of Europe. With the future 802.11p, the same OFDM transmission schemes are being made available in the 5.9 GHz band.
3.10.1 The ISM and U-NII Bands
Wireless devices use parts of the radio spectrum as communication medium by sending electromagnetic signals. The used radio frequency band is the key resource to wireless communication. However, since the radio spectrum is a limited resource, most governments regulate its use by mandating frequency allocation and requiring operational limits of equipment. Different national regulatory bodies are commissioned for individual jurisdictions, like the Federal Communications Commission (FCC) for the United State, the Conference of Postal and Telecommunication Administrations (CEPT) for Europe and the Bundesnetzagentur for Germany. For purposes of harmonization and common equipment almost all countries follow the radio regulations issued by the International Telecommunication Union (ITU), nevertheless regional difference exist and must be dealt with.
Allocation No. 5.138
Allocation No. 5.150
6.765 – 6.795 MHz
13.553 – 13.567 MHz
433.05 – 434.79 MHz
26.957 – 27.283 MHz
61.0 – 61.5 GHz
40.66 – 40.70 MHz
122 – 123 GHz
902 – 928 MHz
244 - 246 GHz
2.400 – 2.500 GHz
5.725 – 5.875 GHz
24.00 – 24.25 GHz
(a) ISM band defined by the ITU
5.15 – 5.25 GHz
5.25 – 5.35 GHz
5.470 – 5.725 GHz
5.725 – 5.825 GHz
(a) U-NII bands defined by the FCC
Table-3.2: ISM and U-NII bands
The industrial, scientific and medical (ISM) bands (see table 3.2) are parts of the electromagnetic spectrum designated by the ITU for “industrial, scientific and medical applications”, meaning non-communication applications. For communication applications this advantage, however, is a double-edged sword since unlicensed, secondary users must cope with interference from primary users. The whole idea behind regulation is to keep interference from other equipment at a minimum, like traditional AM/FM-radio broadcasting stations. To mitigate the interference problem, devices that use the 2.45 GHz ISM-band are required by regulation to limit power to 30dBm (before the antenna) in the USA and to 20dBm (at the antenna) in most of Europe.
Major drawback of the 2.45 GHz ISM band is that there are already many applications using this range. Bluetooth, various cordless mice and other devices also operate in these bands. In 1999, the FCC allocated the 5.850 – 5.925 GHz frequency range, in short called the 5.9 GHz band, for a variety of Dedicated Short Range Communication (DSRC) uses in the transportation system. Envisioned applications include increased traffic safety, traffic monitoring with congestion detection and avoidance, and traffic light control and possible preemption by emergency vehicles. An example for a system operating in the 5.9 GHz band, which is already in wide use, is the drive-by truck toll collection system in large parts of Europe. The upcoming 802.11p amendment adapts the 802.11 standard for the 5.9 GHz band.
3.10.2 802.11a – OFDM
Since the 802.11a communication specifications are of special interest in this thesis, they are reviewed and explained in great detail in the following subsections. Eight different transmission modes are carried by 802.11a. The different schemes of data rates depend on how the used 5 GHz band is partitioned into channels. Table 3.3 shows the data rates in Mb/s for 20, 10 and 5MHz channels. Obviously the data rates for 20MHz channels is twice as large as for 10MHz channels, but then e.g. the U-NII low range 5.15– 5.25 GHz can be partitioned into only five channels. To cope with interference in the unlicensed bands, 802.11a employs OFDM. For OFDM the used frequency range is split up into 48 sub carriers and each sub carrier transmits 1, 2, 4 or 6 coded bits, depending on the keying modulation used. Furthermore, convolution encoding is used to make transmission more robust against bit errors by adding redundancy. And finally, because data bits tend to contain long sequences of zeros or ones, the data bits must be scrambled prior to encoding. This series of transformations is detailed step by step in the following sections. At the beginning there is a sequence of plain data bits waiting for transfer.
Coding Rate (R)
Coded bits per subcarrier NBPSC
Coded bits per OFDM symbols NCBPS
Data bits per OFDM symbol NDBPS
Data Rate (Mbps) for 20 MHz channels
Data Rate (Mbps) for 10 MHz channels
Data Rate (Mbps) for 5 MHz channels
Table-3.3: Data rates and modulation parameters for 802.11a
Simulation and Results
We are considering a total of 10Km of highway on which vehicles are entering by a Poisson process mean vehicles entrances and movement on the highway is independent of each other and also random. We cannot prejudge when and how much vehicles are going to enter a highway at a certain time. Also we have no knowledge of the speed of the vehicles. Though we can get the average number of vehicles entering in a highway in a certain hour by statistical data but we cannot exactly pinpoint the number at a certain time.
From 10km highway we have selected 1km part of it as a highway where we are going to implement vehicle ad hoc network. It will be able to broadcast its message up to 1km only. Vehicles within 1km range will only get chance to transmit their packet according to random time allocated to them.
In our work we have checked what is the time taken by vehicles to access channel and what is the frequency of channel access. Channel access mean a time elapsed since the vehicle has asked for the access and finally it get it. As far as the frequency of channel access is concerned it means that how many time vehicles get access to the channel. More the access more will be the chance to transmit. We are not concerned that how much data certain vehicle transmits after it get accesses to channel.
4.1 Flow Diagram
4.1.1 Flow Diagram Description
Ø Total number of vehicles
It represents the total number of vehicles present on the highway when simulation was run. In our case we are considering the length of highway as 10Km.
Ø Allocating different positions above vehicles
This represents the positions of vehicle before the simulation started. As in real case vehicle movement is cannot be controlled so we have randomize their positions to get more close to real scenario. In fact we do random positioning of vehicles on 10km highway.
Ø Check how many vehicles are within 1000 or 500meter range
Here we are checking that how many vehicles are within range of 1000 or 500 meters. In fact we are assuming that any vehicle can broadcast message in a 1000 or 500 meters range successfully as according to 802.11p which is described above. To make task realistic we are considering the first one kilometer patch of highway and are checking how many vehicle are within that patch. We will study the effect of this area only because this area is making ad-hoc network.
Ø Allocate different random time to vehicles that are within range
As in real situation each vehicle can ask for the channel at any time so that is why we have allocated different random time to all the vehicles that have succeeded to get inside 1000 or 500meter range. Now this random time determines that how much a certain vehicle has to wait before it transmits its data.
Ø Check whether any random time has decremented to zero or not
This provides us with the facility of switch statement because it has two states yes or no. For yes it will show that its random time has decremented to zero and no represent that random time has not yet decreased to zero.
Ø Set these flags to 1
Here we are setting flags of all those vehicles to 1 whose random time has decreased to zero it can be none at a time or may be one at a time or may be more than one at a time.
Ø If all flags are zero
If this condition will get true it means no vehicle was ready to transmit. All the vehicles were struck in a randomized back off. We will call this route as Ideal because in this case channel remained ideal.
o Increment Idle Counter
Here we will simply increment the idle counter which we have initialized to zero before the simulation began. This will provide us the record that how many times we have gone into the idle scenario.
o Decrement all the times of node
In this step we will decrement the timer of each vehicle so as to bring them towards zero. After that we will return to main step and will check again check whether any random time has decremented to zero or not.
Ø If only one flag is 1
If this condition gets true it represent that only one vehicle has got its random time decreased to zero and is ready for the transmission. We are calling this route as success because we are successfully transmitting data.
o Transmit Packet of this Vehicle only
As only a prescribed vehicle has succeeded in getting access to channel so it will able to transmit its data now. In fact it was this vehicle whose timer reached zero.
o Increment the Success Counter
Here we will simply increment the success counter which we have initialized to zero before the simulation began. This will provide us the record that how many times we have gone into the success scenario.
o Clear Flag (0)
After the vehicle has successfully transmitted its data we will reset its flag to zero so that it should get access again in a frame time for which we are running simulation.
o Randomize time of this node
After the vehicle has successfully transmitted then it means its counter has gone to zero. To allow it given to become part of network we will have to assign it some new random time. After that we will return to main step and will check again check whether any random time has decremented to zero or not.
Ø If two or more flags are 1
In this case it will represent that more than two vehicles got ready for transmission at the same time so collision occurred. That is why we have called this route as contention.
o Increment Contention Counter
Here we will simply increment the contention counter which we have initialized to zero before the simulation began. This will provide us the record that how many times we have gone into the contention scenario.
o Randomize only those flags
As the contention has occurred for the above vehicles because their random time had decreased to zero so we will randomize the time of all those nodes whose random time has reduced to zero simultaneously. After that we will return to main step and will check again check whether any random time has decremented to zero or not.
This process or simulation will remain continued until a specified time provided by the user has elapsed. After the time has elapsed we will come out of the loop and deduce results in the form of graphs and statistical data.
4.2 Simulation Parameters
Ø Number of vehicles
Ø Sensing range
Ø Time for which simulation runs
We have used Matlab 2007a and we have utilized different libraries of it. We are going to vary the above three parameters and will get results. We have varied one variable at a time and kept the other two constant to ensure good approximation. We have changed the number of vehicles between 100 - 500 and have deduced some results. Sensing range has been changed from 500 to 1000 meters. We have taken two ranges to know how VANET perform in these two ranges. Simulation time is an independent variable and its value is user dependent. In our case we have varied the simulations time between 10 to 1000 units.
4.3 Graphical plots
We are plotting Poisson distribution because we are dealing with random event occurrence in a given amount of time. Sample applications that involve Poisson distributions include the number of people walking into a store in an hour, and the number of vehicles crossing a certain road in a hour. The Poisson distribution is a one-parameter discrete distribution that takes nonnegative integer values in a given interval. Also Poisson distribution is a best estimate for random events.
From the figure-4.6 and figure-4.7 we can deduce that for lesser number of vehicles we get access quickly. The shows that we have a sparse traffic situation mean the traffic density is less. If we compare between above two figures then we will observe that for the 500m sensing range we get early access to channel. This is because the number of vehicles within the range has reduced so contention has reduced. This also shows that VANET perform well in small ranges because it become easy for vehicles to communicate with each other in this range. Physical interfarance and probability of hidden terminals also decreases because in small ranges mostly vehicle location will be known to other vehicles.
From the 4.6 and 4.8 we observe that as the number of vehicles have increased from 100 to 200 with same sensing range that is 1000m will result in increase channel access delay from 80 to 100 units, so with due to the change in these condition the probability of channel access has decreased. This is due to increase in the number of vehicles trying to access the channel so contention will increase.
This whole thesis was about ad-hoc network which is a non infrastructure. Vehicles are going to utilize 802.11p protocol for communication between them. We have checked that how vehicles are going to communicate with each other.
From the graph and simulation results we have come up with the following conclusion:
When we increase the number of vehicles time to access the channel increases and frequency of channel access decrease. This is because when number of vehicles increases the chance of collision increases hence vehicles get less chance to get access to the channel as a result vehicles consume more time in accessing channels and also their probability decreases.
When we increase the sensing range of vehicles this will reduce the probability of channel access and also vehicle will take more time in accessing the channel. This is because as the range increases more vehicles come into the prescribed ad-hoc network hence contention between the vehicles increases.
When we increase the time of simulation, frequency of vehicles to get channel access will increase (probability of channel access) and also the time to access channel will decrease. This is because in more time larger number of vehicle will get chance to transmit.
5.3 Future work
Currently researchers are working on the bottom two layers that is MAC and Physical layer. These two layers are basic unit on which the whole structure will operate. We have to maximize our capacity and SNR; also results from simulations do not predict the true picture so we have to consider some extra parameters in real scenario which we have not considered yet this will make our network more complex. Some of the researchers are focusing on topology related problems such as range optimization, routing mechanisms, or address systems, as well as security issues like traceability or encryption. In addition, there are some specific research interests such as the effects of directional antennas for VANETs and minimal power consumption for sensor networks. In short the following fields require a lot of work to be done
Ø Physical layer prototypes for inter-vehicle communications.
Ø Routing protocols for inter-vehicle communications
Ø Real-time traffic information system(fading, shadowing)
Ø In-vehicle database management system
 Dedicated Short Range Communications (DSRC) Home, http://www.leearmstrong.com/Dsrc/DSRCHomeset.htm
 Intelligent Transportation Systems (ITS), http://www.its.dot.gov/. VSC capability. http://www.rita.dot.gov/press_room/press_releases/dot137_09/html/dot137_09.html
 STRONG VANET SECURITY ON A BUDGET By Yih-Chun Hu and Kenneth P. Laberteaux ESCAR 06 (http://www.escar.info/) on November 14, 2006
 Ad Hoc Peer-to-Peer Network Architecture for Vehicle Safety Communications by Wai Chen, Telcordia Technologies, Inc. Shengwei CAI, Toyota InfoTechnology. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01421912
 The WAVE Communications Stack: IEEE 802.11p, 1609.4 and, 1609.3 http://www.ieeevtc.org/plenaries/vtc2007fall/34.pdf
 IEEE 1609 Working Group Revised Architecture Diagram by Richard Roy on 2008-10-16 http://vii.path.berkeley.edu/1609_wave/oct08/presentations/Wave%20Station%20Architecture_v0.1.ppt