In recent years research on active safety systems with the main focus on environment sensing has been done. Therefore vehicles are equipped with sensors which record information about the nearby traffic situation and possible hazards. For anticipatory driving additional information outside of the range of the sensors' area needs to be collected. This can be done by introducing data transfer between vehicles, which allows to gather data of a larger area and therefore improves the anticipatory capabilities of the system.One main aspect in the data exchange between vehicles is the organization of the access to the medium. It has to be able to cope with all the problems of mobile ad-hoc networks. Especially the hidden station problem is an important factor in vehicular ad-hoc networks (VANETs). But also the high density of nodes, high dynamics and a limited data rate require a fair and efficient access to the medium.In this paper we present a medium access scheme for vehicular ad-hoc networks which is based on clustering of the vehicles. Thus the effect of hidden stations can be minimized, which leads to a more reliable data transfer than IEEE 802.11 based systems can provide.
In this paper we present a cross-layer design between packet routing and medium access control (MAC) for a vehicular ad hoc network (VANET) in order to guarantee special quality of service (QoS) requirements. As precondition we consider vehicles equipped with driving-assistant-systems that exchange environmental object information through the VANET. The object information is collected by onboard sensors and gets obsolete after some time because of the fast changing surroundings. Therefore it should be distributed to other vehicles with a lower delay than the data gets outdated. This introduces a special low delay QoS requirement the VANET has to provide to the application. As a solution we propose a cross-layered combination of MAC and routing mechanisms. In detail we use advantages of a cluster-based forwarding mechanism and adapt them to the MAC. This helps to decrease collisions and to control packet forwarding. On the other hand the MAC protocol provides cluster and neighbor node information to the routing algorithm. We show that this QoS mechanism satisfies the required packet delay for a desired information propagation area. Finally VANET simulations of certain traffic scenarios validate the proposed cross-layer protocol and show the performance of this mechanism.
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