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Traf c Models. Traf c modeling is a well-known research area in civil engineering. It is important to model vehicular traf c during the design phase of new roads and intersections. There are a number of traf c models that accurately mimic vehicle traf c, and a good survey focused at VANETs can be found in reference 20. With respect to the movement of the vehicles, two distinct strategies are common. In the rst model, each vehicle, upon entering an intersection, randomly makes a decision on whether to go straight or turn left or right. This model is somewhat similar to Brownian motion mobility models. In a second model, each vehicle randomly chooses a destination somewhere else on the map and attempts to go there on the shortest possible route. Once it reaches the destination, it pauses and then chooses another random destination. This model is close in spirit with the commonly used RWP model encountered in MANETs. One important aspect in a traf c model is the driver behavior. The reaction of the drivers in different situations will affect, for example, the traf c throughput. A driver must decide when to overtake a slow-moving vehicle, when to change lanes on a multilane highway, and when to slow down or accelerate. The driver behavior will affect, indirectly, the performance of a VANET and must be taken into account in the modeling process. 15.6.2 Simulation Issues Due to the cost and dif culties involved in deploying large vehicular testbeds, the vast majority of the proposed VANET protocols and systems are evaluated via simulations or with small testbeds. Usually, the simulation of a VANET system includes two stages. In the rst stage, the vehicle movements are determined, usually using a traf c simulator for example, CORSIM [42]. The input of the traf c simulator includes the road model, scenario parameters (including maximum speed, rates of vehicle arrivals and departures, etc.). The output from the traf c simulator is a trace le where every vehicle s location is determined at every time instant for the entire simulation time. In the second stage, the trace le is used as input in a network simulator for example, ns-2 [40]. Each vehicle becomes a node in a MANET with the trace le specifying the movements of each node. However, there is a clear need for integrated traf c and network simulators, to evaluate of the performance of VANETs. To begin with, it is cumbersome to rst use a traf c simulator (e.g., CORSIM) to simulate the vehicle movements, and then a network simulator (e.g., ns-2), to simulate the network behavior. Second, it can be argued that the movement of the vehicles should change as a function of the information they receive from other vehicles. For example, if a vehicle receives information about traf c congestion or an accident on a road segment, it could re-route and go another way. To our knowledge, there has so far only been one attempt to build a completely integrated traf c and network simulator for VANETs. The simulator Jist/SWANS [7]
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is a Java-based network simulator for MANETs that has an integrated street mobility model STRAW [10]. However, signi cant more work is needed for this simulator to gain wide acceptance in the community. 15.6.3 Communication Channel Models Accurate models for the communication channel are a prerequisite for meaningful simulation results. Simulators for MANETs usually implement a circular propagation model, with no obstacles. There are two features of VANET that are not incorporated in the commonly used channel models for MANETs. First, vehicles will move with, sometimes, very high velocity. The channel models used today usually assume no or very low mobility. In reference 55 the physical layer of DSRC was modeled in detail and used in the simulations. The results showed high bit error rates due to the high velocity of the vehicles. Second, the roadside obstacles will affect transmission footprint. In a city scenario, buildings will block the radio waves and cause shorter transmission ranges than in a rural highway scenario. In reference 18, experiments showed that the radio range for IEEE 802.11 was much smaller in cities than on highways. 15.7 OPEN ISSUES Despite considerable advances in VANET protocol design, the commercial deployment of VANETs is still several years ahead. Many issues remain to be addressed; in this section we will discuss some of them. 15.7.1 Access Network DSRC and the future 802.11p has been proposed as the access protocol of choice. However, several challenges remain both in the physical as well as in the MAC layers. In the physical layer, only very few papers [46] considered the effects of the very high mobility on an 802.11a-like OFDM modulation scheme. While the typical issues have been thoroughly studied for the WLAN case of stationary users (or users with low mobility), VANETs are substantially different. Furthermore, since OFDM is not used in any cellular telephony standard, not even estimates of how this modulation will behave in a high-speed environment are available. At the MAC layer, there are several issues to be resolved. To begin with, the MAC layer of 802.11 supports two distinct modes: infrastructure mode and ad hoc mode. In infrastructure mode, a central coordination point manages a single cell of the associated station. This mode ts well for roadside-to-vehicle communications. However, in ad hoc mode, the design of the MAC protocol assumes that all nodes share a single collision domain that is, that every node is capable of communicating directly with any other node (in one hop). This is clearly not the case for VANETs, and it has been shown that several coordination
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