车载网络综述及相关路由算法分析 Overview of VANET and analysis of relevant routing algorithm

软网1301 王建帮 201192181 软网1301 张凯源

1 车载网络综述 1.1相关概念 随着相关技术的发展，越来越多的无线设备开始被应用在汽车上，如远程钥匙、PDAs、智能手机等，车载网络(英文术语为Vehicular Ad hoc Network，即VANET)的概念因而被提出。在Vehicular ad hoc networks(VANETS):status, results, and challenges一文中，作者从以下四个方面对VANET作出了较为全面的阐述：

1) Intelligent transportation systems (ITSs) VANET中节点可分为vehicles和Roadside Units(RSUs), 它们各自都有接收，存储，转发数据以及路由的功能。两者区别在于，vehicles代表着移动的车辆，其位置是不断变化的，而RSUs则是固定在路边的节点。

Fig. 1 Inter-vehicle communication

Fig. 2 Vehicle-to-roadside communication

Fig. 3 Routing-based communication

由于实际应用的需要，在ITSs中存在三种可能的通信结构（communication configure-tion）:inter-vehicle, vehicle-to-roadside, and routing-based communication。这三者的实现都依赖于有关周围环境的精确且即时的信息，而要获取这样的信息，则需要精确的定位系统（如Bluetooth, Ultra-wide Band, ZigBee等）以及智能的通信协议（如GPS, DGPS）来提供支持。

2) Inter-vehicle communication The inter-vehicle communication configuration (Fig. 1) uses multi-hop multicast/broadcast to transmit traffic related in- formation over multiple hops to a group of receivers.

3) Vehicle-to-roadside communication The vehicle-to-roadside communication configuration (Fig. 2) represents a single hop broadcast where the road- side unit sends a broadcast message to all equipped vehicles in the vicinity.

4) Routing-based communication The routing-based communication configuration (Fig. 3) is a multi-hop unicast where a message is propagated in a multi-hop fashion until the vehicle carrying the desired data is reached.

1.2应用背景 Typically, applications are categorized as safety, transport efficiency, and information or enter-tainment applications.

1) Road safety For safety-related applications, the Vehicle Safety Communications (VSC) consortium identified eight high potential applications : traffic signal violation warning, curve speed warning, emerg-ency electronic brake light, pre-crash sensing, cooperative forward collision warning, left turn assistant, lane-change warning, and stop sign movement assistant.

2) Transportation efficiency For transportation efficiency applications, the Car-to-Car Communication Consortium (C2C- CC) analyzed exemplarily enhanced route guidance and navigation, green light optimal speed advisory, and lane merging assistants.

3) information and entertainment Ideas for information and entertainment applications consist of quite a diverse set: tolling (one of the initial motivators for vehicle-to-infrastructure communications), point-of-interest notifications, fuel consumption management, podcasting, and multi-hop wireless Internet access, to name a few.

2 路由算法简介 针对车载网络路由算法的研究已经持续了很多年，期间有不少创新、有效的成果被陆续提出。相对于移动自组网（Mobile Ad hoc Network，即MANET）来说，车载网络具有网络拓扑变化快，节点运动模式多样，车辆密度分布随时空动态改变，新增与删除节点速度快等特点。因此若直接将以往为移动自组网设计的路由协议应用于车载网络中，显然是不合适的。下面介绍的是在车载网络中讨论较多的三类路由算法：proactive routing protocols, reactive routing protocols, position-based routing。

2.1 Proactive routing protocols Proactive routing protocols employ standard distance-vector routing strategies (e.g., Destination-Sequenced Distance- Vector (DSDV) routing) or link-state routing strategies (e.g., Optimized Link State Routing protocol (OLSR) and Topology Broadcast-based on Reverse-Path Forwarding (TBRPF)). They maintain and update information on rout- ing among all nodes of a given network at all times even if the paths are not currently being used. Route updates are periodically performed regardless of network load, band- width constraints, and network size. The main drawback of such approaches is that the maintenance of unused paths may occupy a significant part of the available bandwidth if the topology of the network changes frequently. Since a net- work between cars is extremely dynamic proactive routing algorithms are often inefficient.

2.2 Reactive routing protocols Reactive routing protocols such as Dynamic Source Routing (DSR), and Ad hoc On-demand Distance Vector (AODV) routing implement route determination on a demand or need basis and maintain only the routes that are currently in use, thereby reducing the burden on the network when only a subset of available routes is in use at any time. Communication among vehicles will only use a very limited number of routes, and therefore reactive routing is particularly suitable for this application scenario.

2.3 Position-based routing 基于位置的路由算法是当今较热门的研究方向。一般来说，基于位置路由至少应当包括三个核心部分：beaconing, location service, forwarding (geographic unicast and geographic broadcast)。