30 IEEE POTENTIALS0278-6648/13/$31.00©2013IEEE

Ubiquitous and pervasive communication services have rapidly gained popularity over the last few years. The main objective of these services is to allow

information exchange in all environments. While this information exchange conventionally takes place between hand-held devices carried by the end users, a similar information exchange between vehicles is being envisaged. Vehicular communication is becom-ing increasingly popular in both the research and development arenas due to its far-reaching benefits. In a nutshell, it is concerned with giving vehicles the ability to detect and convey information regarding different traffic incidents. Vehicles can send information messages either to other vehicles within close range or to the roadside base stations (BSs). This idea is not entirely new because it previously existed in the form of “telematics.” Telematics deals with the use of telecommunications for the information exchange between remote objects. The term became known in the late 1970s but did not gain popular-ity because the enoromous size of communication devices was a serious limitation to their widespread use. However, with the recent developments in chip design and integrated systems, housing low-power, small-sized devices inside vehicles has become possible. Vehicular communication has now become a distinct possibility and is being looked at from research and development perspectives.

The need of revolutionizing the transportation system has also fueled the progress in vehicular communication. Road accidents give rise to thousands of fatalities every year

across the world. The World Health Organization predicts that traffic injuries will become the third largest burden of disease if necessary steps are

not taken. Issues like traffic congestion affect the quality of life and also impose a heavy financial burden. It has

been reported that traffic congestion costs around £20 billion every year on United Kingdom roads,

while the same on the roads in the United States is ten times higher. Motivated by

these facts, the research and develop-ment communities are exploring the use of vehicular communication to improve passenger safety on roads and highways. Projects such as intelligent transportation systems (ITSs) have been active for some time, which focus on developing innovative applications for vehic-ular scenarios.

Classification and applications

Vehicular communication is concerned with the information

exchange between vehicles and between vehicles and outside

infrastructure. Based on “who” a vehicle communicates with, vehicu-

lar communication is classified into two main types.

Vehicle-to-vehicle communicationsVehicles communicating with each other in

small groups give rise to a vehicular ad hoc net-work (VANET). This type of vehicular communica-

tion does not require a central entity to liaison informa-tion exchange between the vehicles. Vehicles moving on roads

form small groups and share information using different routing algorithms. Since the flow of information is directed from one vehicle to another, this kind of communication is often termed as vehicle-to-vehicle (V2V) communication. A V2V Date of publication: 22 July 2013

Digital Object Identifier 10.1109/MPOT.2012.2225172

Vehicular communication and sensor networks

Syed Faraz HaSan

© ISTOckPhOTO/LINdA buckLIN

© cAN STOck PhOTO/kEO

The

Ubi

quito

us M

achine

juLy/AuguST 2013 31

communication scenario is similar to ad hoc networks, which also employ rout-ing protocols to exchange information between nodes that are multiple hops away from each other. However, since the vehicular scenario changes very rapidly due to frequent arrivals and exits of vehicles on different road seg-ments, making effective use of VANETs opens various research avenues. For instance, developing routing algorithms that can convey information from one vehicular node to another with mini-mum duplication is an active area of research. Also, since the vehicular envi-ronments change very rapidly, reducing the latency in transmitting and receiving a message is also an interesting area.

The main purpose of V2V communi-cation is to exchange time-sensitive traf-fic information between vehicles. Such messages may be meant to inform other vehicles, for example, about the lead vehicle’s intent of changing lanes or taking a turn. While visual aid in the form of blinking indicators is available to serve this purpose, it fails to convey visual information at blind spots. V2V communication, on the other hand, can convey this information even if the lead vehicle is not directly visible to the rest of the vehicular nodes.

Another useful application of V2V communication is to alert the driver if the intervehicular distance (IVD) gets dangerously small. In order to ensure safety, vehicles are required to keep a standard distance between each other while traveling on roads and highways. A vehicle is envisaged to monitor IVD and alert the driver as soon as it crosses a certain threshold. A vehicle can share this information with the following vehi-cles to exercise caution. Figure 1 shows two cases where IVD is being monitored by a sensor on board the vehicle. This sensor issues alert messages to the driver as soon as IVD falls below a predeter-mined threshold [see Fig. 1(b)]. In addi-tion to information sharing, researchers are considering giving vehicles the ability to react to potential hazards. In the sample applica-tion shown in Fig. 1, a sensor may be tuned to issue an alert command or apply emergency brakes based on the prevailing IVD.

Vehicle-to-infrastructure communications

Note that the range of V2V communication is limited, and

the information is shared between vehi-cles in local groups. Vehicles forming a VANET are usually in close vicinity and thus the communication range is restricted to a particular segment of the road. The second type of vehicular com-munication, vehicle-to-infrastructure (V2I) communication, is concerned with information exchange over a larger scale. A V2I scenario is also referred to as road-side-to-vehicle (R2V) or V2R communi-cation. V2I communication allows the vehicles to exchange information with a roadside BS that is usually one hop away from them. This roadside BS is con-nected with other BSs that are located on different junctions and road seg-ments. Using this network of roadside BSs, a vehicle can convey information from one part of the city to another. As an example, suppose a vehicle faces traf-fic congestion in a certain part of the city. It can convey this information to other vehicles via the roadside units so that the incoming vehicles can take alter-nate routes toward their destination. This has been graphically shown in Fig. 2. Keeping the incoming vehicles updated about prevailing traffic conditions can help in avoiding traffic buildup on the already congested road segments. Use of a portal (or server) as shown in Fig. 2, which holds global traffic information, has also been proposed. Roadside units can query this portal to see whether there had been any accidents on the route a vehicle is about to take. Similarly, portal-driven roadside BSs can also be

used for sharing vacant parking space information with the vehicles.

V2V2I communicationMore recently, a third type of vehicu-

lar communication has been introduced: V2V2I communication. As the name sug-gests, V2V2I is a hybrid of V2V and V2I communication scenarios. It allows the vehicles to form a local group and select one vehicle as the “super vehicle.” While other vehicles can communicate with each other using routing protocols, the super vehicle can also communicate with the outside network. Figure 3 depicts V2V, V2I, and V2V2I communi-cation environments. As can be seen from the figure, two cars (shown in black) have gotten into an accident. If only V2V is used in this context, the information about the accident will be circulated only among the vehicles on that road. Similarly, if only V2I is used, this information will be shared globally but not with all vehicles on the same road (due to the limited transmission range of the BS). On the other hand, if V2I and V2V are combined together, accident information can be shared locally as well as globally. The overall network will then be known as a V2V2I network, in which the vehicle communi-cating with the BS is recognized as the super vehicle.

Note that in this application and most others, such as traffic congestion moni-toring and exchanging intervehicular distance, a vehicle needs to house the appropriate detection ability. In these applications, a vehicle first “senses” an event and then communicates the same to other nodes. Sensing mechanisms are important for effective use of vehicular communication.

Wireless sensor networks for vehicular communication

According to its legacy definition, a sensor converts a physical phenomenon into electrical parameters for the ease of

measurement. Sensors have been used to convert, for exam-ple, mechanical, optical, and chemical signals into electricity. A sensor placed on an automo-bile engine monitors its temper-ature and reports the same in electrical form for maintenance and other activation purposes. Sensor systems are typically large in size with considerable power requirements. This limi-tation has made it difficult to

Fig. 1 An on-board sensor monitors intervehicle distance and alerts the driver.

IVD > Threshold

(a) (b)

IVD < Threshold

Onboard SensorAlerts Driver

Regardless of the communication environment, the

communication technology used for conveying

information should support high data rates with

reduced network latency.

32 IEEE POTENTIALS

employ these in low-cost setups. With the recent advents in the small-sized, low-power electronics, it has been possi-ble to monitor certain phenom-enon using a set of wireless sensors.

A so-called wireless sensor network (WSN) is composed of sensor nodes that are conven-tionally spread across an area of interest. These sensor nodes are responsible for detecting con-cerned events and communicat-ing relevant information to a central BS over the wireless links. WSNs have found appli-cations in environmental moni-toring, military systems, and structural and agricultural activi-ties. More recently, WSNs have been used in transportation sys-tems to sense and communicate potential traffic hazards on roads. The idea is to spread sensor nodes across road seg-ments, which can sense a par-ticular stimulus and convey rel-evant information to a nearby unit. Figure 4 shows a sample application of WSNs in which roadside sensors send warning messages to vehicles approach-ing each other at a blind spot. These sensors detect incoming vehicles and convey this infor-mation to the nearest BS over wireless links. In return, this BS sends warning messages to the incoming vehicle. Note that for such applications, a large number of sensor nodes must be deployed across the area of interest. This incurs significant deployment, labor, and mainte-nance costs.

Vehicular sensor networks (VSNs) are an evolved version of WSNs in which sensor nodes are placed on board the vehi-cle. This way, a moving vehicle serves as a mobile sensor that no longer stays static at a par-ticular location. The sample scenarios shown in Fig. 1 and Fig. 2 are examples of VSNs where sensors are placed on the vehicles. The advantages of using VSNs are obvious: a set of static sensor nodes placed around a road segment can be replaced by a single vehicle carrying one sensor node. However, since vehicles are mobile, one particular location cannot

be monitored over a long period. In other words, unlike static sensors, VSNs cannot offer temporal coverage of the area of interest.

The tasks performed by a sensor node, either static or mobile, are not lim-

ited only to “sensing.” A sensor is composed of at least five units that perform their own separate tasks. In addition to the sensing unit, a typical sensor node houses modules for processing, communica-tion, storage, and battery, as shown in Fig. 5. A sensor module monitors different parameters that are analyzed by the processing unit to yeild meaningful results. Once the processed results suggest that an event has occured, an infor-mation signal is sent by the communication module to the nearest BS. In VSNs, if a BS is not readily available, the infor-maiton signal is stored in the memory module and is retrived upon getting a connection with the BS. The battery module suffices for the power requirements of the sensor node. In VSNs, a sensor node mounted on a vehicle can be powered by the vehicle’s battery.

After a vehicular sensor detects an event of interest, it uses wireless communication technologies to share this infor-mation with the nearby nodes.

V2V and V2I communication technologies

It has been previously dis-cussed that vehicular communi-cation allows both single-hop and multihop information exchange. Single-hop commu-nication takes place in V2I environments where the road-side BS is one hop away from a group of vehicles. On the other hand, V2V communication uses multihop communication as information arrives at the desti-nation via multiple intermedi-ate nodes. Regardless of the communication environment, the communication technology used for conveying information should support high data rates with reduced network latency.

IEEE has standardized 802.11p for use in both V2V and V2I communication setups. Also called Wireless Access in Vehicular Environments (WAVE), IEEE 802.11p is a modified version of the legacy IEEE 802.11a standard. The main modifications

BS-1

BS-2

PortalOther BSs

Vehicle ChangesDirection on Receiving

Traffic Update from BS-2

Traffic Build-UpReported to BS-1

Fig. 2 Sample application: A vehicle changes its direction upon getting information on traffic congestion.

V21 Communication

Two Cars Meetan Accident

V2V21 Communication

AccidentAhead

Accident onRoad-X

V2V

Com

mun

icat

ion

Fig. 3 A V2V, R2V, and V2V2I communication scenario.

InformationSignal

SensedStimulus

Wireless SensorNetwork

Fig. 4 Roadside sensor nodes warn the approaching vehicles at the blind spot.

juLy/AuguST 2013 33

have been made on the PHY and MAC layers of 802.11a. On the PHY layer, bandwidth has been reduced from 20 to 10 MHz so as to allow larger guard band. On the MAC layer, a new mode of operation known as the WAVE mode has been introduced. WAVE mode allows faster con-nection establishment between vehicles for quick commence-ment of information exchange.

While 802.11p has been specifically designed for vehic-ular communication, other wireless technologies have also been considered for use in the vehicular context. For exam-ple, Zigbee and Bluetooth communication have been studied for a V2V communication scenario. The debate on choosing an appropriate communication technology for V2I environments has received more atten-tion. This is because V2I environments require a number of roadside units to exchange information with a group of vehicles. A suitable technology for V2I setup should have well-deployed BSs that can support reasonable data rates. If the BSs are not already deployed, their deployment cost should not pose a financial limitation. Among various communication technologies, the fol-lowing have been explored in consid-erable depth.

The cellular networks (Global System for Mobile Communication, Universal Mobile Telecommunication System, and others) are suitable for vehicular context because of their almost ubiquitous pres-ence. Cellular BSs are widely deployed in almost all cities and cover a large geo-graphical area. However, cellular net-works suffer from low data rates in com-parison with other networks such as IEEE 802.11b/g wireless LANs (WLANs). Emerging cellular technologies such as long-term evolution are offering higher data rates. WLANs have also seen massive

deployment ever since their introduction in the late 1990s. While 802.11g and 802.11n offer data rates as high as 54 Mb/s and 600 Mb/s, respectively, these networks have a small coverage region

and are inherently designed for indoor use only. The coverage area of a WLAN access point is typically 200–300 m, which is very small for use in vehicular context.

Second, unlike cellular systems, the deployment of WLANs is h ighly unplanned, which results in disrupted network services when 802.11 net-works are used over a larger mobility domain. In addition to cellular and

802 . 1 1 ne two rk s , 8 02 . 1 6 WiMAX has also been consid-ered for use in vehicular com-munication. WiMAX BSs have a larger coverage area and, unlike 802.11 WLANs, WiMAX BSs have been designed for out-door use. On the down side, the penetration of WiMAX BSs across most cities is not wide-spread enough to support con-tinuous services in the vehicu-lar environments. Dedicated deployment of WiMAX BSs have significant cost con-straints. Table 1 summarizes the key characteristics of these technologies.

Read more about it • A. D. Joseph, “Works in progress: Intelligent transportation systems,” IEEE Pervasive Comput., vol. 5, no. 4, pp. 63–67, 2006. • E. Strom, H. Hartenstien, P. Santi, and W. Wiesbeck, “Vehicular commu-nications: Ubiquitous networks for sus-tainable mobility,” in Proc. IEEE, 2010, pp. 1111–1112. • J. Miller, “Analysis of vehicle lane changes for determining fastest paths in the V2V2I ITS architecture,” in Proc. IEEE Int. Conf. Intelligent Transportation Systems, 2008, pp. 1207–1212. • S. F. Hasan, N. H. Siddique, and S. Chakraborty, “Extended MULE con-cept for traffic congestion monitoring,” J. Wireless Pers. Commun., vol. 63, no. 1, pp. 65–82, 2012. • J. Erikkson, H. Balasubramanian, and S. Madden, Cabernet: Vehicular content delivery using Wi-Fi, in Proc. ACM Int. Conf. Mobile computing Net-work, 2008, pp. 199–210.

• G. Meijer, Smart Sensor Systems. New York: Wiley, 2008. • I. S. Ansari, “An implementation of traffic light system using multi-hop ad hoc networks,” in Proc. IEEE Int. Conf. Network-Based Information Sys-tems, 2009, pp. 177–181.

About the authorSyed Faraz Hasan (faraz@skku.edu) is

an assistant professor at Sungkyunkwan University. He earned his Ph.D. from the University of Ulster in 2011 and his B.E. from NED University of Engineering and Technology in 2008. His research interests include vehicular communications, wire-less networks, positioning techniques, and stochastic modeling.

Table 1. Key parameters of wireless communication technologies.

Cellular Systems WLAN WiMAX

Data rates 7.2–14.4 Mb/s 54–600 Mb/s 75 Mb/s

Deployment Well planned Random Random

Population density High High Low

Deployment location Outdoor Indoor Outdoor

SensingModule

ProcessingModule

StorageModule

Vehicular Sensor Node

PowerSupply

CommunicationModule

Fig. 5 A block diagram of a vehicular sensor node.

Vehicles moving on roads form small groups and share information using different

routing algorithms. Since the flow of information is directed from one vehicle to another, this kind of communication is often termed as vehicle-to-

vehicle communication.