1、Vehicular Ad Hoc Networks(VANETs) 车载自组网Internet of Vehicles(IoV) 车联网,1,它的基本思想是在一定的通信范围内的车辆可以相互交换各自的车速、位置等信息以及车载传感器感知的数据，并自动地连接建立起一个移动的网络。 整个车载自组网分为两部分：车与车（V2V，vehicle to vehicle）和车与设施（ V2I， vehicle to infrastructure）。下图显示了车载自组网在真实环境中应用的一个模型示意。,2,可以看到，卫星通信系统分别为车载自组网提供全球定位服务(GPS，global positioning sys
2、tem)和数字多媒体服务(DMB，digital multimedia broad-casting)。 车与车通信使车辆之间能够通过多跳的方式进行自动互联，这好比车与车之间能够像人一样互相交谈，起到提高车辆运行的安全和疏导交通流量等作用。 车载自组网除了可以单独组网实现局部的通信外，还可以通过路灯、加油站等作为接入点的网关(gateway)，连接到其他的固定或移动通信网络上，提供更为丰富的娱乐、车内办公等服务。,3,多年来智能交通系统（Intelligent Transportation System，ITS）一直是研究的热点，在智能交通系统的各个技术领域中，旨在提高车辆行驶的安全性、运输效率
3、以及提供无处不在的无线 Internet 接入的车载自组网（VANETs，Vehicular Ad-hoc Networks）技术又是研究的重中之重。 车载自组网是移动自组网（MANETs，Mobile Ad-hoc Networks）的一个分支，它是建立在配备有无线接口的车辆（即移动节点）之间的，点对点的通信的基础之上的。 与自组网的区别（e.g. Wireless Sensor Networks）面向应用领域的；路由的关键技术,4,5,VANET网络架构,6,车载自组网在交通运输中出现，将会扩展司机的视野与车载部件的功能，从而提高道路交通的安全与高效。典型的应用包括： 行驶安全预警。利用车
4、辆间相互交换状态信息，通过车载自组网提前通告给司机，建议司机根据情况作出及时、适当的驾驶行为，这便有效的提升了司机的注意力，提高驾驶的安全性。 协助驾驶。帮助驾驶员快速、安全的通过“盲区”，例如在高速路出/入口或交通十字路口处的车辆协调通行。,7,8,专用短距离通信技术DSRC，是专门为车载通信开发的技术,乘客办公与娱乐化。,9,最早关于车辆间通信的研究始于 20 世纪 80 年代初的日本；在随后的 90 年代和 21 世纪初，日本的协同驾驶系统 DEMO2000展示了车间通信的另外一个重要应用。目前，欧美也启动了相关的大型科研项目，在车载自组网方面研究较为活跃的几个研究机构有：,10,发展实
5、例, CarTalk2000是一个从 2001 年开始，为期3年的项目。致力于开发一种新的基于车间通信的驾驶员辅助驾驶系统，用于驾驶的安全性和舒适性。 FleetNet，“Internet on the Road”项目是由 6 家公司和 3 所大学为了推动车间通信发展而建立的项目。FleetNet 项目在 2000 年 9 月启动，2003年 12月结束，最终展示了一套成品，通过一系列测试获得了宝贵数据。而紧接着该项目的后续工程NOW，Network on Wheels”从 2004 年开始启动，同时 BMV 和 Volkswagen 这两大世界级的汽车制造商也被吸引到了这一后续项目中来，NO
6、W主要目标是解决在车间通信中的通信协议和如何保证数据安全性。,11, C2C-CC，“ Car2Car Communication Consortium ”是由6家欧洲汽车制造商(BMW、DaimlerChrysler、Volkswagen等)组成，目标是为car2car 通信系统建立一个公开的欧洲标准，不同制造商的汽车能够相互通信。Car2Car通信系统是采用基于无线局域网 WLAN技术，确保在欧洲范围内车间通信的正常运行。 WILLWARN，“ Wireless Local Danger Warning”专注于道路安全性，研究和验证了预防性安全的技术和应用。项目为期 3 年，目标是当任何与
7、车辆安全相关的状况发生时，都能为驾驶员提供告警。这一项目中暴露出的研究难点是车与车、车与基层设施之间的通信问题。,12, ADASE，“Advanced Driver Assistance Systems in Europe”项目的使命是增加欧洲道路行车的安全性，减少交通事故的发生，在碰撞发生之前能够躲避。ADASE 通过主动安全系统，与在道路两旁的基础设施设备相连接，来实现这一目标。 COMCAR， Communication and Mobility by Cellular Advanced Radio”项目提出了一种创新的移动通信网络概念和原型。主要关注的难点是不对称的、交互式的基于 I
8、P 移动服务，这正是车间和车内通信所必需解决的关键性问题。,13, DRiVE, “Dynamic Radio for IP-Services in Vehicular Environments”项目的其中的一个主要目标就是为汽车提供多媒体服务，很方便的就能接入到提供教育或娱乐等服务的网络中。 CHAUFFEUR 2是 CHAUFFEUR 1的一个后续项目。该项目的目标是使车辆能够在安全的距离内行驶在其他车辆的后面。 此外还有日本JSK领导的“Association of Electronic Technology for Automobile Traffic and Driving”,“Gr
9、oup Cooperative Driving”、美国的VII、美国马里兰州立大学的TrafficView项目、法国多个研究机构合作开展的CIVIC等。,14,VANET路由相关的特点,高速变化的拓扑。由于节点之间相对高速的移动，VANET的拓扑一直在改变。e.g. 假设车辆的无线通信半径为250m，在最坏的情况下，车辆之间的相对速度为25m/s，链路的持续时间只有10s。频繁的网络分割。由于与上面类似的原因，VANET网络的连通性也频繁的变化。尤其是在车辆密度较低的时候，网络有很大的概率是不连通的。路由的设计必须要考虑到网络分割的存在。,充足的能量和储存能力。VANET中的无线通信节点为车辆
10、而不是普通的手持设备，所以可以很轻松的提供足够的能量用于支持无线通信和计算，在路由过程中，能量和储存可以认为是无限的。无线通信的地理性。与传统网络中单纯依靠ID号或者IP地址不同，由于车辆的移动性是依赖于已有道路，无线通信的地理特征较大。甚至有些网络应用对于报文的目的区域就是特定的地理空间。,移动模型和可预测性。由于无线节点的高速移动和拓扑的动态性，移动模型和预测是网络路由协议设计的重要考虑依据。车辆节点的运动本质上受限于高速公路、道路或者是街道，所以给予速度和地图，节点的移动在短期内可以被预测。不同的通信场景。在高速公路的通信场景中，环境是简单而且是一维的。而在城市环境中就变得更加复杂，城市
11、中的道路常常被有很多十字路口和多叉路口。,18,19,UNICAST ROUTING,VADD: vehicle-assisted data delivery routing protocol VADD protocol adopted the idea of carry-and-forward for data delivery from a moving vehicle to a static destination. The most important issue is to select a forwarding path with the smallest packet deliv
12、ery delay.,20,A vehicle knows its location by GPS device, and the packet delivery information such as source id, source location, packet generation time, destination location, expiration time, in the packet header. Vehicles can find their neighbors through periodic beacon messages, which also enclos
13、e the physical location of the sender. Vehicles are assumed to be equipped with pre-load digital maps, which provide street-level map and traffic statistics (such as traffic density and vehicle speed on roads at different times of the day),21,22,23,According to the information provided by digital ma
14、ps, VADD protocol proposed a delay model to estimate the data delivery delay in different roads as follows,24,25,VADD Protocols Used in the Intersection Mode Location First Probe (L-VADD) Direction First Probe (D-VADD) Hybrid Probe (H-VADD),26,ROMSGP (Receive on Most Stable Group-Path) routing proto
15、col This paper argues the use of information on vehicles movement information (e.g., position, direction, speed, and digital mapping of roads) to predict a possible link breakage event prior to its occurrence. Vehicles are grouped according to their velocity vectors. This kind of grouping ensures th
16、at vehicles, belonging to the same group, are more likely to establish stable single and multi-hop paths as they are moving together. Setting up routes that involve only vehicles from the same group guarantees a high level of stable communication in VANETs.,27,The scheme presented in the paper also
17、reduces the overall traffic in highly mobile VANET networks. The frequency of flood requests is reduced by elongating the link duration of the selected paths. To prevent broadcast storms that may be intrigued during the path discovery operation, another scheme is also introduced. The basic concept b
18、ehind the proposed scheme is to broadcast only specific and well-defined packets, referred to as “best packets” in the paper.,28,The key idea behind the scheme is to group vehicles according to their velocity headings. This kind of grouping ensures that vehicles that belong to the same group are gen
19、erally moving together. Routes, involving vehicles from the same group, exhibit thus high level of stability. Among these possible routes, communication is set up on the most stable route using the Receive on Most Stable Group-Path (ROMSGP) scheme. Decision of the most stable link is made based on c
20、omputation of the Link Expiration Time (LET) of each path. Obviously, the path with the longest LET is considered as the most stable link.,29,30,Fig depicts the scenario of five vehicles at an intersection where vehicle B is turning onto a new street and the other four vehicles are continuing straig
21、ht on the same road. A connection is established between vehicles A and F. Communication is possible on two routes: one via vehicle B (route A-B-D-F) and the other via vehicle C (route A-CDF). As vehicle B is turning left and vehicle A is continuing straight, the former route is more likely to be ru
22、ptured after a certain time. Consequently, the selection of the latter router is a more appropriate choice and has tendency to add more stability and reliability to the communication path between the two vehicles (A and F).,31,32,Considering the scenario in figure, let as the metric penalty. Assume
23、that no routing metric penalties, all routing metrics equal to one. In such case, both routes ABD and ACD have the same probability of been chosen as route. But if is added to route metric between A to B due to B belongs to different group, the route metric between A to B is 1+ 1 (the route metric b
24、etween A to C) and ACD will be chosen automatically.,33,It is worth noting that in some special situation, such as mountain road, we have to modify our scheme of dividing group. The direction of vehicle changes very frequently so another method should be taken to implement grouping. With the GPS dev
25、ices, users can notice whether they are driving on curved roads. If they do so, grouping can be made among vehicles that are on the same curved roads rather than of their moving directions.,34,35,GVGrid: a QoS routing protocol GVGrid constructs a routing path from source to destination by grid-based
26、 approach, which divides the map into several grids. The RREQ and RREP packets are delivered through different grid to find a routing path through minimum number of grid.,36,The location of a source and a destination are fixed. Each vehicle is equipped with Same Ranged Wireless Device IEEE802.11, et
27、c. Car Navigator (GPS + Digital Map) Accurate geographic information, and roads and direction information. Vehicles exchange the information by hello messages Position, Road, Direction and ID Grid Geographical area into uniform-size squares called grid. Grid size w is determined based on r so that node in every grid can communicate with nodes in neighboring grids.,37,38,
