Research On High-throughput And Low-delay MAC Protocol Of Vehicular Ad Hoc Networks

Vehicle ad hoc Networks(VANETs)are important parts of the intelligent transportation system,mainly used to ensuring driving safety,managing traffic and providing entertainment services.Media Access Control(MAC)mainly studies the channel access of nodes and channel resource usage,having an important impact on ensuring proper function in VANETs.However,as the density of vehicles increases,vehicle communication resources become tight,thereby reducing system throughput and prolonging data transmission delay.In order to improve the system throughput and reduce the delay of packets,we propose two MAC protocols for VANETs,which respectively apply for the central VANETs and the distributed VANETs.The main contents are summarized as follows:1)A high-throughput and low-delay MAC(HTLD-MAC)protocol is proposed for center VANETs.First,it devises a timeslot interchange scheme to solve timeslot waste on the overlapping area of roadside units(RSUs).Second,it designs a dynamic timeslot allocation mechanism,in which RSU preferentially allocates timeslots to the vehicle that is about to leave the network for reducing the packet loss rate of system.Finally,it obtains the expression of service channel capacities based on the Nakagami-m fading model and dual slope path loss model for describing the feature of real environment in VANETs better.HTLD-MAC has two striking features.One is that,on the control channel(CCH),the timeslots for non-safety packets are allocated by the RSUs with global information instead of traditional node pairs selfnegotiation timeslots scheme,making the system performance better.The other is that,about the timeslot allocation on the service channels(SCHs),different from traditionally considering throughput,HTLD-MAC takes both throughput and waiting time into account,which not only meets high throughput requirements,but also meets delay demands.It makes the scope of protocol wider.Experimental results show the advantages of HTLD-MAC in terms of throughput and latency.2)A timeslot reservation MAC(TRMAC)protocol is presented for distributed VANETs.Firstly,the structure of node neighbor information table is redesigned.Secondly,the nodes exclude unavailable channels and timeslots according to the table.Next,different from the tradition protocol that nodes randomly select idle timeslot segments,this protocol treats packets differently based on latency.For nodes that want to send delay-sensitive packets,nodes preferentially select long timeslots segments with small number,because they try to complete the transmission within one frame for reducing system delay.For nodes that wants to send delay-tolerant packets,nodes preferentially select short timeslots segments with large number,because they try to transmit in blocks as much as possible for improving the system throughput.Comparing with the nodes of traditional protocols which a successful negotiation on the CCH can only reserve one timeslot segment on the SCH,the nodes of TRMAC protocol only need one successful negotiation on the CCH to reserve a few timeslots on the SCH in the next few frames.It alleviates the congestion of CCH when vehicle density is high.Simulation results show the effectiveness of this protocol in terms of throughput and delay.