Network Reliability And Connectivity Oriented Research For VANETs

Vehicular ad hoc networks(VANETs),is one of the key technologies of the intelligent transportation system which can provide an ideal and useful communication networking technology for intelligent transportation system by ensuring traffic safety,efficiency,commerce,and entertainment.Due to broad application prospect of VANETs,the VANETs are of profound significance to the development of traffic information.At present,research on safety applications in VANETs is great interest to the academia and industry.Network reliability and connectivity are the basis and premise of applications of VANETs.It includes physical connectivity and logical connectivity,involving the physical link connectivity,the effectiveness of MAC and routing protocols.This thesis mainly studies the physical link connectivity for VANETs,providing the theoretical reference for the application and implementation of VANETs.To analyze the network connectivity in VANETs,the main work and contributions include the following four aspects:First,a simplified approach is conducted that is mainly based on a road-type environment for a VANET network.A heuristic approach was proposed for calculating the reliability based on the highest probability paths using the Dijkstra algorithm and the inclusion-exclusion approach for calculating the reliability of a given path.For vehicle-to-vehicle(V2V)communication,short-range protocols were considered-ZigBee(802.15.4),WiFi(IEEE 802.11),and Bluetooth(802.15.1)-as well as their standards for data rates,association time,and transmission range.On the other hand,the IEEE 802.11b was used for vehicle-to-roadside(V2R)communications.Another parameter that was considered was the speed limit of the road environment,and three types of road environments were evaluated:highway,urban,and mixed.Other factors that were considered were the number of vehicles,the number of roadside units,and the type of message that was transmitted.The effects of all of these elements on the connectivity of the network were studied.Furthermore,as a contribution first time the heuristic approach is incorporated with inclusion-exclusion approach in order to simplify the calculation of reliability in the context of VANETs.Second,an analytical model was proposed for highway scenarios where the traffic congestion could have the vehicles reduce their speed instead of blocking the flow of traffic.In this model,nearby vehicles must be informed when traffic congestion occurs before reaching the toll plaza so they can reduce their speed in order to avoid traffic congestion.Once they have crossed the toll plaza they can travel on at their normal speed.The road was divided into two or three sub-segments to help analyze the performance of connectivity.The proposed analytical model considered various parameters that disturb the connectivity probability,including traveling speed,communication range of vehicles,vehicle arrival rate,and road length.The simulation results show that the proposed method is more conform to the actual influences of toll plaza,and is more accurate and more realistic to reflect the specific impacts of VANET environment.Third,another network reliability model based on safe distance under the tunnel has been proposed.In the tunnel,effect of vehicle transmission range Rtran and vehicle density p was considered on the connectivity probability.In addition,a reliability model which takes into account minimal safe headway Sh among nearby vehicles at highway tunnel was proposed.Global Positioning System(GPS),a component of onboard unit(OBU)needs a rich line of sight for perfect services as GPS would not function properly due to signal interference.Although,in case of a fully connected network,there would be more chances of danger between vehicles which are close to each other,so the network would not be safe,thus accidents and collisions might occur.Hence,maintaining the minimal safe headway distance under the tunnel would be interesting and useful for VANET.The results obtained by simulations prove that the deviation of the safe distance will bring substantial changes in the network reliability when transmission range of vehicles is less than 150m.Hence,while designing the network reliability models,the safe headway cannot be ignored.Last,a new formula has been proposed,where terms correspond 1 by 1 to a class special ST route(SRORT).The new formula terms are much lesser than the inclusion-exclusion principle.An algorithm for the ST reliability has also been presented.The algorithm produced Source-to-Terminal reliability or computed a Source-to-Terminal reliability expression by calculating a class of special networks of the given network.Also,a mobility metric called universal speed factor(USF),was proposed which is an extension of existing speed factor,which was supposed to maintain the same speed of vehicles at all times.The USF describes an exact relation between the relative speed of consecutive vehicles and the headway distance.The connectivity of vehicles in different mobile situations was analyzed using USF i.e.,slow mobility connectivity,static connectivity,and high mobility connectivity.It was observed that probability of connectivity pc is directly proportional to the mean speed μv till specified threshold μτ,and then decreases.Simulation results show that the extremely congested traffic environment will be extremely connected.The connectivity will be increasing until a given threshold.