Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Hybrid Beaconing Approaches <s> Vehicular ad hoc networks play a critical role in enabling important active safety applications such as cooperative collision warning. These active safety applications rely on continuous broadcast of self-information by all vehicles, which allows each vehicle to track all its neighboring cars in real time. The most pressing challenge in such safety-driven communication is to maintain acceptable tracking accuracy while avoiding congestion in the shared channel. In this article we propose a transmission control protocol that adapts communication rate and power based on the dynamics of a vehicular network and safety-driven tracking process. The proposed solution uses a closed-loop control concept and accounts for wireless channel unreliability. Simulation results confirm that if packet generation rate and associated transmission power for safety messages are adjusted in an on-demand and adaptive fashion, robust tracking is possible under various traffic conditions. <s> BIB001 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Hybrid Beaconing Approaches <s> In this paper, we present a new scheme for dynamic adaptation of transmission power and contention window (CW) size to enhance performance of information dissemination in Vehicular Ad-hoc Networks (VANETs). The proposed scheme incorporates the Enhanced Distributed Channel Access (EDCA) mechanism of 802.11e and uses a joint approach to adapt transmission power at the physical (PHY) layer and quality-of-service (QoS) parameters at the medium access control (MAC) layer. In our scheme, transmission power is adapted based on the estimated local vehicle density to change the transmission range dynamically, while the CW size is adapted according to the instantaneous collision rate to enable service differentiation. In the interest of promoting timely propagation of information, VANET advisories are prioritized according to their urgency and the EDCA mechanism is employed for their dissemination. The performance of the proposed joint adaptation scheme was evaluated using the ns-2 simulator with added EDCA support. Extensive simulations have demonstrated that our scheme features significantly better throughput and lower average end-to-end delay compared with a similar scheme with static parameters. <s> BIB002 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Hybrid Beaconing Approaches <s> In many active safety applications, each vehicle must periodically disseminate a beacon message including the status information, such as position, speed, steering, etc., so that a neighbor vehicle can better perceive and predict the kinematics of the vehicle sending this message. However, a simple broadcasting of beacon message may lead to a low message reception as well as an excessive delay. To resolve this problem, we consider the requirements of an active safety application, a channel load and a vehicle mobility in a highway scenario of vehicular networks. Based on these requirements, we analyze the impact of the following three key parameters of the beacon dissemination on the performance of vehicular networks: beacon period, beacon transmission power, and contention window (CW) size. We first derive a beacon period which is inversely proportional to the vehicle speed. Next, we mathematically formulate the maximum beacon load to demonstrate the necessity of the transmission power control. We finally present an approximate closed-form solution of the optimal CW size that leads to the maximum throughput of beacon messages in vehicular networks. <s> BIB003 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Hybrid Beaconing Approaches <s> In this paper, we address the beacon congestion issue in Vehicular Ad Hoc Networks (VANETs) due to its devastating impact on the performance of ITS applications. The periodic beacon broadcast may consume a large part of the available bandwidth leading to an increasing number of collisions among MAC frames, especially in case of high vehicular density. This will severely affect the performance of the Intelligent Transportation Systems (ITS) safety based applications that require timely and reliable dissemination of the event-driven warning messages. To deal with this problem, we propose an original solution that consists of three phases as follows; priority assignment to the messages to be transmitted /forwarded according to two different metrics, congestion detection phase, and finally transmit power and beacon transmission rate adjustment to facilitate emergency messages spread within VANETs. Our solution outperforms the existing works since it doesn't alter the performance of the running ITS applications unless a VANET congestion state is detected. Moreover, it ensures that the most critical and nearest dangers are advertised prior to the farther and less damaging events. The simulation results show promising results and validate our solution. <s> BIB004
The basic principle of vehicle to vehicle communication is to improve the vehicle perception and predict the information of the other vehicles in the vicinity. This perception can be achieved through beaconing. In the near future, many VANET applications will rely on beaconing to enhance information sharing. In safety related applications, vehicles broadcast beacon message, which includes position, direction, velocity and VANET related information, to neighbour vehicles. However, the performance network might be deteriorated in terms of excessive packet delay and unreliability of beacon message dissemination in case of using naive beacon broadcasting service. To alleviate the problem of simple broadcasting, three key parameters can be adapted to vehicular network scenarios; beacon generation rate, beacon transmission power and contention window of the Medium Access Control (MAC) layer BIB003 , BIB004 , BIB002 . The authors in BIB004 proposed a beaconing congestion solution due to periodic beacon transmission that consumes large amount of available bandwidth resources. The proposed solution consists of three phases. The first phase assigns different levels of priority to the received beacon messages in order to differentiate their importance. When a vehicle receives a beacon message from nearby vehicles, its MAC layer ranks them based on the content of the messages and the hop count. The content of beacon messages are classified into immediate danger notification, warning message and driving information announcement. A vehicle receives more than one safety related messages and should assign priority to each message based on their contents. If the content of the messages have the same priority, the hop count should be considered in priority of message transmission. The priority of the message is inversely proportional with hop count metric. The network is considered as congested if the number of packets in the network is higher than its processing capacity. In this case, the performance of the network is deteriorated due to high packet drops. As a consequence, the authors considered congestion detection mechanism in the second phase of the proposed solution. In VANET, congestion can be detected based on the continuous monitoring of the following metrics: When the network is congested, a vehicle can control the beaconing load by adjusting its transmission power and beaconing generation rate. The transmitted power can be adapted based on the minimum transmission power used by the vehicle itself and vehicles in the vicinity and distance between the vehicle to next packet forwarder of the warning message. At the same time, a vehicle can adapt its beacon generation rate by equ.5 b = (BF) − B(emergency)) B size (5) where BF is the bandwidth fair share and B(emergency) is the estimated bandwidth that is enough to emergency related situations and B size denotes the beacon size. Fig.7 shows the general overview of the proposed beacon adaptation solution. The performance evaluation was implemented using a OPNET [46] and in 10 km bidirectional four lane road section. In the simulation scenario, vehicles are travelling with a speed of 60 to 120 km/h and 10 to 6 vehicle/km/lane. Results of the simulation show that the proposed solution possesses higher beacon delivery ratio and emergency message reception as well as lower total delay as compared to only power control or beacon rate control solutions. Thus the combined power and beacon rate control is promising solution as compared to the existing solutions. However, in this study obstacle modelling is an imperative need to verify and validate the practical emergency message dissemination. Since most of active safety applications require optimal broadcast of beacon frame in order to maintain accurate positional information while avoiding control channel congestion, the authors in BIB001 