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The U.S. Federal Communications Commission (FCC) has allocated 75 MHz of DSRC (Dedicated Short Range Communication) spectrum at 5.9 GHz to be used extensively for V2V(Vehicle to Vehicle) and V2R(Vehicle to Road Infrastructure) communications. The main purpose of this channel allocation is to enable public safety applications that would save lives and improve vehicular traffic flow. Private services are also permitted to lower the maintenance costs and network deployment and to encourage DSRC development and adoption . The DSRC spectrum is divided into seven 10-MHz wide channels, as shown in Figure 2.1.1 Channel 178 is the control channel, which is restricted for safety communications only.
Figure 2.1.1 DSRC Channel assignment in North America.
The two channels at the edges of the spectrum are reserved for future advanced accident avoidance applications and high-power public safety communication usages like future delivery of rich multimedia contents to vehicles at short- to medium range. The rest are service channels and are available for both safety and non safety applications .
IEEE Standards for MAC Protocols for Vehicular Networks
The IEEE has completed the standards IEEE P1609.1, P1609.2, P1609.3, and P1609.4 for vehicular networks . P1609.1 is the standard for the Wireless Access for Vehicular Environments (WAVE) resource manager. It defines the services and interfaces of the WAVE resource manager application as well as the message data formats. It also provides access for applications to the other architectures. P1609.2 defines security, secure message formatting, processing, and message exchange. P1609.3 defines routing and transport services. It provides an alternative to IPv6. It also defines the management information base for the protocol stack. P1609.4 deals mainly with specification of the multiple channels in the DSRC standard. Figure 2.2 shows the IEEE WAVE standards .
Figure 2.2 IEEE WAVE standards.
The WAVE stack uses a modified version of the IEEE 802.11a, known as IEEE 802.11p , for its medium access control (MAC) layer protocol. It uses CSMA/CA as the basic medium access scheme for link sharing and uses one control channel to set up transmissions, which then are carried over some transmission channels. The 802.11p PHY layer is expected to work in the 5.850ââ‚¬"5.925 GHz DSRC spectrum in North America, which is a licensed radio services band in the United States. By using the OFDM system, it provides both V2V and V2R wireless communications over distances up to 1000 m, while taking into account the environment, that is, high absolute and relative velocities (up to 200 km/h), fast multipath fading, and different scenarios (rural, highway, and urban). Operating in 10-MHz channels, it allows data payload communication rates of 3, 4, 5, 6, 9, 12, 18, 24, and 27 Mb/s. By using the optional 20-MHz channels, it allows data payload capabilities up to 54 Mb/s.
As the original IEEE 802.11 standard is designed only for little mobility, the IEEE 802.11p working group addresses important issues such as frequent disconnection and handoff. IEEE 802.11p WAVE defines amendments to IEEE 802.11 to support vehicular network applications. This includes data exchanges between high-speed vehicles and between the vehicles and the roadside infrastructure in DSRC spectrum.
Figure 2.3 Architecture of IEEE protocol for vehicular communications
The current IEEE 802.11p draft aims at providing the minimum set of specifications required to ensure interoperability between wireless devices attempting to communicate in potentially rapidly changing communication environments and in situations where transactions must be completed in a timeframe, much shorter than that of infrastructure or ad hoc 802.11 networks. The WAVE mode basic service set (WBSS) in IEEE 802.11p enhances IEEE 802.11 MAC functions for rapidly changing communication environments. Mobile stations in WAVE mode become members of a WBSS in one of two ways, either as a WBSS provider or as a WBSS user. Mobile stations in WAVE mode typically move much faster than legacy 802.11 mobile stations in infrastructure or ad hoc BSS mode. The most important issue is that mobile stations in WAVE join the vehicular network and transmit or receive data as quickly as possible. Due to these rapidly changing communication environments, the WBSS provider and user should be ready for communications as quickly as possible. For this purpose, WBSS do not require MAC sub layer authentication and association prior to being allowed to transmit data. In a WBSS, a WBSS user only needs to receive the WBSS announcement of a WBSS provider before commencing transmissions [6, 7]. The WBSS provider first transmits WAVE announcement action frames for which the WBSS users listen. That frame contains all information necessary to join a WBSS. Unlike infrastructure and ad hoc 802.11 BSS types, the WAVE users do not perform authentication and association procedures before participating in the WBSS. To join the WBSS, only configuring according to the WAVE announcement action frame is required . In addition, a mobile station in WAVE mode shall generate a CCA (clear channel assessment) report in response to a CCA request to know the time-varying channel state precisely .
2.2 Literature Review
Ad-hoc routing protocols can be classified into two major categories : first one is the topology-based routing protocols and the second one is the geographic position-based routing protocol. The topology-based routing protocols use the information about all the links existing in the network to perform the routing decisions. While in the geographic position-based routing protocol the routing decision is not based on a routing table instead the routing decision is based on geographical positions of current node which is the sender, neighborââ‚¬â„¢s position towards the destination and the position of the receiver .
The topology-based routing protocols can further be divided into Proactive (table-driven) and Reactive (on-demand) routing protocols . The Proactive routing protocols maintains tables for keeping information of all connected nodes in the entire network and hence also called as table-driven. Every node has to periodically exchange route information to maintain a permanent route table of entire topology. In addition to this each node has to update their table from time to time in response to any change in the topology. The advantage of these protocols is lesser amount of delay in determining a path from source node to the destination i.e. path from sender to receiver is available in advance which may be used whenever required without any delay. But one drawback of this category of protocol is unnecessary occupying the available bandwidth for maintaining tables for all possible routes. The standard protocols of this type are (Optimized link state routing) OLSR  and (Destination Sequence Distance Vector Routing) DSDV .The Reactive routing protocols determine any path only when it is requested i.e. on-demand by any node. As a result it overcomes the problem of wastage of bandwidth as in the case of proactive routing protocols. Here the route has to be discovered first for any packet forwarding. Thus it takes time to invent the route from sender to receiver. The standard protocols of this type are Ad-hoc On-Demand Distance Vector (AODV)  routing and Dynamic Source Routing (DSR) .
For dealing with the growing number of routing entries to be maintained at each node due to increasing number of vehicles or nodes the author in  proposed geographic routing. In geographic position-based routing, the packet forwarding decision by a sender node is based on the position of receiver and the position of the senderââ‚¬â„¢s one-hop neighbors. These positions can be determined easily in case of VANETs by the GPS (global positioning system). The main advantage of these routing protocols as compared with topology based protocols is there is no more need for routes establishment and maintenance. Instead the message is forwarded from sender to the one hop neighbor towards the receiver utilizing the position map rather than the tables. So these protocols can be able to handle the scalability issue  as well as the highly dynamic topology and frequent network partitioning in VANETs efficiently . Prominent protocols of this category are Greedy Perimeter Stateless Routing (GPSR) , Distance Routing Effect Algorithm for Mobility (DREAM) , Location Aided Routing (LAR) .
The authors in  evaluated the performance of both position-based and topology-based protocols under different scenarios. Investigation was done using microscopic mobility information based on the road maps. The results show that position-based protocols outperform topology-based protocols. But the authors have used Random Way Point mobility model, which is unsuitable for vehicular ad-hoc networks. In , the authors have compared the performances of two prominent on-demand routing protocols for mobile ad hoc networks - Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV). They used a detailed simulation model with MAC and physical layer models to study Inter-layer interconnections and their performance implications. The authors in  analyzed the performance differences of Ad hoc On-demand Multi-path Distance Vector (AOMDV) and Optimized Link State Routing (OLSR) routing protocols. They evaluated their performance through simulation using network simulator (ns-2). They analyzed the strengths and weaknesses of these two protocols by measuring packet loss rates (%), average end to end delay (sec), and normalized routing load. In , the authors have simulated the Destination Sequenced Distance Vector (DSDV), the table- driven protocol and the Ad hoc On-Demand Distance Vector routing (AODV), an On-demand protocol and evaluated both protocols based on packet delivery fraction and average delay while varying number of sources and pause time. The authors in  presented a number of ways of classification or categorization of routing protocols in VANETs and did the performance comparison of an AODV, DSR and DSDV routing protocols. In , the authors have compared and evaluated the performance of two types of On demand routing protocols- Ad-hoc On-demand Distance Vector (AODV) routing protocol, which is unipath and Ad hoc On-demand Multipath Distance Vector (AOMDV) routing protocol. In this paper they compared the performance of AODV and AOMDV. They found that AOMDV incurred more routing overhead and packet delay than AODV but it had a better efficiency when it comes to number of packets dropped and packet delivery. The authors in  did a survey on data scheduling approaches available to schedule the necessary data considering vehicle to vehicle, infrastructure to vehicle or vehicle to infrastructure communication in Vehicular Networks. They also categorized the various routing protocols available in VANETs. In , the authors have compared reactive routing protocols DSR, AODV and AOMDV in VANETs using User defined graph and Space graph mobility models available in VanetMobisim framework. They evaluated these routing protocols on three performance metrics-packet delivery fraction, end to end delay and normalized routing load. They concluded that AOMDV performed better than DSR and AODV in different mobility models in terms of end to end delay as performance metric. The authors in  categorized various possible applications of vehicular network, along with its features, and implementations in the real world.
In , the authors proposed a Road Map Based (RMB) routing protocol for real-time vehicular ad hoc networks that included some techniques taken from geographical ad hoc routing. It handled mobility using road map and builds stable routing path on the road segments but not on the nodes. Routing path is done through distributed real-time participants of the network. The authors in  analyzed Unicast Routing Protocols like AODV, DSDV and DSR for VANETs using SUMO. They concluded that AODV was better in low mobililty scenarios with higher number of nodes. In , the authors discussed recent trends that lead to the development of comprehensive simulation platforms consisting of many modules: real-world data sources, traffic generator, traffic and network simulators.
The authors in  evaluated the efficiency of two routing protocols OLSR and AOMDV by comparing their performances in the city scenario for the map of city of Arlington, Texas using NS-2. The realistic vehicular mobility traces were generated using Intelligent Driver Model (IDM) in VanetMobiSim. In , the authors simulated multipath, Unipath and hybrid routing protocol in city scenarios using VanetMobisim. The protocols evaluated were AODV, AOMDV and DSDV. The authors in  evaluated AODV, DSR and OLSR using realistic mobility pattern. They studied the effects of the vehicular traffic parameters such as the average speed, vehicle density and road topology on the overall VANET performance. They concluded that DSR gives highest throughput. In , the authors proposed a Real Scenario Mobility Model. This new model can show vehicleââ‚¬â„¢s movement scenario more veritably in routing selection, movement direction of nodes restricted by road network etc. In , the authors have done the performance evaluation of reactive routing protocols like AODV, AOMDV and DSR using space graph mobility model in VanetMobiSim. Their results showed that AOMDV performed better than AODV and DSR. The authors in  presented RBVT, which is a class of VANET routing protocols for city-based environments that takes advantage of the layout of the roads to improve the performance of routing in VANETs. RBVT protocols use real-time vehicular traffic information to create road-based paths between endpoints.