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### 车载自组织网络通信协议 #### 概述车载自组织网络（VANETs）作为智能交通系统（ITS）的关键组成部分，在近年来获得了广泛关注。这些网络旨在支持车辆间以及车辆与基础设施间的无线通信，从而实现各种创新应用，如实时路况更新、紧急车辆优先通行、自动收费等。本文将基于提供的文档摘要，深入探讨车载自组织网络中的关键技术点，包括无线通信基础、道路及交通建模、广播/地理广播以及路由技术。 #### 无线通信与移动计算车载自组织网络利用无线通信技术进行数据传输。在该领域内，已经发展出了多种低级别的标准和技术，如IEEE 802.11p/WAVE、DSRC（专用短程通信）等，为车辆间的数据交换提供了可靠的基础。这些技术不仅支持高速移动环境下的通信需求，还能够应对复杂多变的交通状况。 #### 智能交通系统架构智能交通系统的架构主要包括以下几个层次：感知层、网络层、应用层。其中，感知层负责采集交通环境中的各种数据；网络层负责数据的传输；而应用层则是实现具体服务的部分。为了确保系统的高效运行，不同层次之间需要紧密协作。 #### 应用潜力车载自组织网络的应用范围非常广泛，从安全驾驶辅助到娱乐信息系统都有其身影。例如： - **交通安全**：通过车辆间的实时通信，可以提前预警潜在的危险情况，如前方事故或紧急刹车。 - **交通管理**：利用车辆收集的实时数据，交通管理中心可以优化信号灯控制策略，减少拥堵现象。 - **信息服务**：提供路况信息、天气预报、紧急救援等服务，提高出行效率。 - **自动驾驶**：车载自组织网络是实现自动驾驶车辆之间通信的重要手段之一，对于实现完全自动驾驶至关重要。 #### 道路与交通建模为了更好地理解和预测车辆在网络中的行为，研究人员通常会采用两种主要的建模方法：微观建模和宏观建模。微观建模关注个体车辆的行为特征，而宏观建模则侧重于整体交通流的特性。 - **微观建模**：通过模拟单个车辆的动作，如加速、减速、转向等，来预测车辆在网络中的运动轨迹。 - **宏观建模**：研究整个交通系统的流动特性，如流量、速度分布等，适用于大规模的交通网络分析。 #### 数据传播机制车载自组织网络中的数据传播机制主要有广播和地理广播两种形式。 - **广播**：向所有邻近节点发送消息。这种方法简单易行，但在高密度网络环境中容易导致信道拥塞。 - **地理广播**：只向特定地理位置的节点发送消息，可以有效减少不必要的数据传输，提高网络效率。 #### 路由技术车载自组织网络中的路由技术必须考虑车辆的高度动态性，因此需要设计专门的算法来应对这一挑战。常见的路由协议包括： - **地理位置路由**：基于地理位置信息进行路由决策，适用于高度动态的网络环境。 - **概率路由**：根据统计学原理，选择最优路径进行数据转发。 - **混合路由**：结合地理位置路由和概率路由的优点，提供更加灵活的路由方案。 #### 结论车载自组织网络作为一种新兴的无线通信技术，已经在智能交通系统中展现出巨大的应用潜力。随着相关技术的不断进步和完善，未来的车载自组织网络将会更加高效、可靠，为人们的出行带来更多的便利。同时，如何进一步提高网络性能、降低能耗、增强安全性等问题，也将成为未来研究的重点方向。

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WIRELESS COMMUNICATIONS AND MOBILE COMPUTING

Wirel. Commun. Mob. Comput. (2009)

Published online in Wiley InterScience

(www.interscience.wiley.com) DOI: 10.1002/wcm.879

Communication protocols for vehicular ad hoc networks

Arnaud Casteigts

∗,†

, Amiya Nayak

∗

and Ivan Stojmenovic

∗

School of Information Technology and Engineering, University of Ottawa, Canada

Summary

Vehicular networks are envisioned for large scale deployment, and standardization bodies, car manufacturers, and

academic researchers are solving a variety of related challenges. After a brief description of intelligent transportation

system (ITS) architectures and the main already-established low-level standards, this tutorial elaborates on four

particular aspects of vehicular networks, which are (i) the potential for a large set of innovative applications, (ii) a

review of the main modeling approaches used for both roads and trafﬁc, and ﬁnally two important communication

primitives, that are (iii) data dissemination via broadcasting/geocasting, and (iv) routing in both highway and urban

environments. A particular emphasis is on recent protocols that realistically consider the inherently complex nature

of vehicular mobility, such as intermittent connectivity, speed variability, and the impact of intersections. Copyright

KEY WORDS: vehicular ad hoc networks; wireless communications; road modeling; trafﬁc modeling; broadcast-

ing; geocasting; routing

1. Introduction

Mobile ad hoc networks are networks that self-

organize over an evolving topology and rely on

multi-hop communication instead of using a ﬁxed

infrastructure. In the past two decades, a plethora of

communication protocols were proposed to target var-

ious speciﬁc contexts ranging from robotic networks,

pedestrians networks, low earth orbit satellites, or

military units in a battleﬁeld. Despite a large set of

potential applications in these environments, vehicular

networks are likely to be the very ﬁrst deployed

large-scale instance of mobile ad hoc networks. The

years to come will witness development of these

networks, as vehicles will start to be equipped with

wireless communication capabilities and able to run

∗

Correspondence to: Arnaud Casteigts, Amiya Nayak or Ivan Stojmenovic, School of Information Technology and Engineering,

University of Ottawa, Canada.

†

E-mail: casteig@site.uottawa.ca; anayak@site.uottawa.ca; ivan@site.uottawa.ca

dedicated protocols to communicate with each other

and with the infrastructure along the roads.

Vehicular networks have the potential to assist in

coping with a continually increasing trafﬁc demand

and accident statistics worldwide. Building new roads

is an expensive way of increasing limited capacity

of existing roads. Hence, the integration of vehicles

as active communication and computation agents of

the road management into Intelligent Transportation

System (ITS) offers unprecedented improvement

opportunities, ranging from selecting routes with

up-to-the-minute information, to giving priority to

response teams, notifying vehicles and drivers about

road incidents (see Figure 1, where operations 1 and 2

are related to trafﬁc safety, while operation 3 improves

trafﬁc efﬁciency), delivering contextual services to

A. CASTEIGTS, A. NAYAK AND I. STOJMENOVIC

Fig. 1. A typical collision scenario leading to various reac-

tions: (1) fast forward of instant warnings to arriving cars to

avoid rear-end collisions, (2) infrastructure assisted warning

delivery toward incoming trafﬁc to make it slowing down

ahead of time, and (3) notiﬁcation to allow vehicles to take

an alternative route.

drivers, reducing fuel consumption and greenhouse gas

emissions, controlling the ﬂow of vehicles based on

real-time trafﬁc monitoring and congestion detection,

dynamically adapting signal light schedules to the

trafﬁc conditions, and so forth. The main advantage

of using ad hoc communications between vehicles

is the easy and low-cost deployment this solution

offers, compared to the prohibitive installation and

maintenance cost of a full coverage infrastructure.

On the other hand, this also comes with a number

of challenging problems for both networking and

transportation research communities.

The basic requirement of such systems is a strong

set of standards allowing all vehicles to communicate

with each other regardless of their brands and models.

Standardization bodies, car manufacturers, public

sector players and academics have thus conducted

much effort ahead of this standardization, including

GM-CMU [1], MIT CarTel [2], or Berkeley PATH [3]

in the United States. Other examples include European

initiatives like Network-On-Wheels [4] or PReVENT

[5], whose results are now being integrated by the Car

2 Car Communication Consortium [6], and Asian ones

such as the Toyota InfoTechnology Center [7] and the

Vehicle Information and Communication System [8]

in Japan, or the large-scale trafﬁc sensing that were

performed in China (more than 4000 GPS-enabled

taxis tracked in Shanghai to study the so-generated

network, SUVnet [9]). The ﬁrst set of standards

concerning the access layers (PHY & MAC) are now

released and being implemented. Upper layers, such as

network and transport layers, are still under discussion

and open to new research contributions.

The design of reliable and adaptive protocols in

vehicular context is challenging, especially due to the

high dynamicity of the underlying topology and its

intermittent connectivity in most scenarios. Yet, the

movement of cars is constrained by the road structure

and this fact can be exploited to improve networking

tasks. It is also expected that a partial infrastructure

is still to be available at some strategic places (e.g.,

at intersections inside cities) to improve the connec-

tivity and provide dedicated services to drivers and

passengers. Besides safety and efﬁciency applications,

enabling value-added business applications is certainly

one of the most determining factor for a quick and

successful adoption of these technologies, which is

in turn crucial for safety and efﬁciency applications

that require a signiﬁcant penetration rate to function

properly and bring beneﬁts to drivers.

This tutorial is organized as follows. The next section

extends the introduction by presenting an overview of

the ITS architecture and brieﬂy presenting the family of

standards and technologies that were chosen for access

layers (e.g., DSRC and WAVE). Section 3 discusses

the main applications one may expect from these

networks, classiﬁed as trafﬁc safety, trafﬁc efﬁciency,

and value-added applications. In Section 4, we present

the variety of models and parameters that can be used

to represent physical roads and vehicular trafﬁc. The

choice for a proper model is of paramount importance

because protocols can hardly be tested in real contexts,

and their evaluation thus rely mainly on simulations.

The two last sections are dedicated to communication

protocols, and more particularly to broadcasting and

geocasting protocols (Section 5) and routing protocols

(Section 6). The emphasis is set on protocols that con-

sider realistic aspects of vehicular networks, such as

their intermittent connectivity. Some important topics

like security (encryption, channel abuse, privacy) are

not discussed in this tutorial. We refer the reader to [10]

for a survey on security issues, and to [11] for a com-

prehensive overview of other related topics including

trafﬁc engineering and human factors studies.

2. Architecture Overview

The whole system is generally seen as the integration

of four classes of components [12]: vehicles, personal

devices, road-side equipment, and central equipment.

Road-side equipments (or RSUs for Road-Side Units)

are physical devices deployed along the road to

perform local tasks related to the trafﬁc. They can be

for example traditional devices like trafﬁc lights or

variable message signs enhanced with computational

and wireless communication capabilities, or dedicated

devices such as curve warning emitter or multi-hop

relays

(that aims at increasing the connectivity between

vehicles). These units can stand alone or be connected

with each other through a private network. They can

also be connected to the Internet and linked to central

servers. However, due to high cost of their deployment,

DOI: 10.1002/wcm

COMMUNICATION PROTOCOLS FOR VEHICULAR AD HOC NETWORKS

and connecting them to other infrastructure, they are

normally assumed to be absent, or sparsely deployed

but mostly individually isolated. Central servers are

intended to concentrate most of the ITS management

and perform tasks like collecting trafﬁc information,

predicting congestions, initiating actions (possibly

relayed on the road by some RSUs), coordinating

trafﬁc lights, etc. They are usually considered as being

reachable through the Internet (or sometimes via some

RSUs). Finally, personal devices are attached to the

vehicle by the user. This includes for example GPS-

based navigation devices or in-car multimedia devices.

Mobile phones may also be counted in this category if

either they use the vehicle access to the Internet, or pro-

vide their own access to the vehicle (see below). They

are not considered as part of the system otherwise.

2.1. Communications in ITS

There are several kinds of interaction a vehicle may

have. Most of them are depicted on Figure 2. The

main is certainly the interactions between vehicles

themselves, generally referred to as Vehicle-to-Vehicle

communication (V2V), or Inter-Vehicle Communi-

cation (IVC). These are pure ad hoc interactions

and are most appreciated in time critical safety

applications. Others interactions are generally denoted

as Vehicle-to-Infrastructure interactions (V2I), which

comprise different kinds of interactions. For dedicated

road-side equipment related to trafﬁc applications,

the communication is expected to take place in

the same ad hoc domain as the cars, using also the

same access technology (802.11p, discussed below).

The term Vehicle-to-Roadside (V2R) is sometimes

used to point out the particular subset of these interac-

tions that does not involve any communication beyond

the RSU itself (e.g., within its private network or the

Internet).

The integration of vehicles in the IPv6 framework

will be considered in the early stage of market intro-

duction to enable the large potential of applications

private network

ITS server

Internet

802.11 p

RSU RSU

HotSpot

802.11 p

802.11

(a/b/g)

2G/3G

Fig. 2. Overview of the main set of interactions envisioned

between vehicles and the infrastructured network.

of the Internet and motivate people to equip existing

vehicles. Car manufacturers and device vendors there-

fore consider providing network devices with double

interfaces that support both ( DSRC or 802.11p and

WiFi protocols (802.11a/b/g). Such vehicles would

be able to access the infrastructure through HotSpots

and communicate with servers through the Internet

(e.g., to get map updates or weather conditions).

Finally, because many modern smart phones behave

like modems (generally through bluetooth commu-

nication), they are sometimes considered to provide

2G/3G cellular network access to the vehicles in sparse

locations like mountains or countrysides, where no

road-side equipment is available. Other radio technolo-

gies such as FM, WiMax, and shorter range mediums

(infrared, ultrasonic or millimeter-wave sensors) were

also studied for speciﬁc use cases such as pre-crash

sensing, and their integration within the car system

was in particular studied by the PReVENT project [5]).

In V2V interactions, we can distinguish between

single-hop and multi-hop communications. Using

single-hop communications means that a vehicle can

only send messages to (and receive message from)

its direct neighbors, while multi-hop communications

enable the exchange of messages with remote vehicles,

using the vehicles in-between as relays. Both types of

communications are used for different kinds of applica-

tions and protocols. For example, diffusing information

using periodic exchanges of hello messages is a single-

hop communication scheme, while most broadcasting

and routing protocols reviewed in Section 5 are multi-

hop ones.

2.2. Access Layers and Physical Devices

We now describe the access standard that has been

established for V2V and V2I communications on the ad

hoc domain. Vehicles are envisioned to communicate

with each other and with the road-side equipment using

the Dedicated Short-Range Communications (DSRC)

standard. More precisely, DSRC is a short to medium

range wireless communication channel speciﬁcally

designed for V2V and V2I communications, along

with a set of protocols speciﬁcations. It operates in the

5.9 GHz band in the US (5.8 GHz in Europe and Japan),

and was meant to provide very high data transfer rates

in circumstances where latency is critical. Its Medium

Access Control (MAC) and Physical Layer (PHY) spec-

iﬁcations extend 802.11a (into 802.11p) for the speciﬁc

need of ITS applications. This standard, together with

the upper layers deﬁned by the IEEE 1609 series, is

called WAVE [13].

DOI: 10.1002/wcm

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