| The explosive growth of traffic and ubiquitous information service have madecommunication networks closely integrate into the vehicular networks, so that theVehicular Communication Networks emerged as the times require and became aresearch hotspot. The communication between vehicles formed a self-organized andopen-structured network, called Vehicular Ad Hoc Networks, which not only has thecharacteristics of mobile ad hoc networks, such as dynamic, distributed, multi-hop, etc,but also has its own unique attributes, such as high speed of vehicles and frequentchanges of topology. These attributes affect the transmission performance in VANETdirectly.There are many applications for vehicle safety in VANET, so it is necessary tochoose a reliable transport protocol for these applications. In the high dynamicenvironment of VANET, due to the poor quality of channel and frequent changes ofroute, the reasons for package loss are complicated, so that the traditional TCP protocolcan not work very well. At the same time, there are only a few researches on TCPmechanism in VANET environment currently. Based on this, this paper analyzed thereasons that the existing TCP is not suitable for the Vehicular CommunicationNetworks through simulation experiments, and introduced the cross-layer design andthe fuzzy control to improve TCP transmission performance in VANET environment.In this paper, we first proposed an improved VC-TCP based on cross-layer design,which combined the network layer and the transport layer for cross-layer optimization.Firstly, by providing useful movement information of vehicles to the network layer, thestate of the link can be predicted in the routing process at network layer; and then byadding VC bits in the TCP header to indicate link state, the network layer can recordlink state into VC bits using cross-layer communication, so that the TCP sender candistinguish package loss reasons rapidly according to VC bits; finally, based on thefour-state transition diagram, the traditional TCP congestion control algorithm wasimproved, which can apply appropriate congestion control strategy under differentconditions.Subsequently, the theory of fuzzy control is introdced in this paper, and theVF-TCP is proposed based on the researches above. The VF-TCP made use of therelative speed and the distance between neighbor nodes, the relative variation rate ofround-trip time and short-term throughput as the input parameters of fuzzy reasoning, and then established a fuzzy control rule base, in order to determine the reasons ofpacket loss through fuzzy reasoning, finally, improved the TCP congestion controlstrategy and route handover strategy based on cross-layer scheme.The simulation results show that both the improved TCP congestion controlstrategy can improve the average throughput and end-to-end delay in VehicularCommunication Networks, especially when the channel quality is poor and the nodesmove in high-speed. |
