**1. Introduction**

Although nowadays wireless networks are a regular and familiar framework for sharing information among devices, the way in which these nets are organized and managed is evolving day by day due to the requirements of the scenarios in which they are deployed. Since those first experiments carried out by the WECA (Wireless Ethernet Compatibility) association in late 90s, the application areas and use cases in where wireless communications are applied has been changing.

Many of the wireless networks that we use daily at home, at the office or when se use a cellular are based on those first approaches, in which an Access Point is needed to have connectivity. These setups are called 'Infrastructure Mode' and use a fixed and wired backbone to address information from the source AP to the destination AP. But in some situations these networks are limited by their own nature due to their need for an AP, a base station, some routers or switches and so on. It is in these scenarios where a ''Infrastructure-less Mode'' can overcome these drawbacks, allowing the nodes of a network to routing and forwarding information for other nodes, without relying on centralized administrator. These types of networks are called wireless ad hoc networks [1].

Now, if we have into consideration the current trends in technology, it can be said that mobility and ubiquity are common characteristics to all the new gadgets launched to the market. Users want to be online anytime and everywhere and to obtain information from all the surrounding elements. Then, we talk about Mobile Ad hoc Networks (MANETs), that is, wireless networks with a dynamic shape, a shifting number of nodes, a defined bandwidth and other character‐

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**Figure 1.** Infrastructure based networks (left) and Ad hoc networks (right)

istics, where the nodes can be any kind of devices with communications and networking capability that communicate with each other without a centralized coordinator [2]. In this scenario, each node can play the role of a router, hosting the network topology dynamically, because as it was mentioned above, the shape and the topology of the net can change as well as the nodes on it. The main characteristics of MANETs can be summarized as follows [3]:


In this context, thanks to the rapid increase and improvement of the mobile computing a wide set of wireless devices have proliferated, making possible that traditional hardware as digital cameras, thermostats, cooking ovens or washing machines are provided with communications and computing functionalities so they can be part of a MANET. This new paradigm is known as Internet of Things [4], that is, a scenario in which all the objects beyond computers, mobiles or touch screens have the ability of generating, sharing and processing information in a pervasive manner [5]. With all of this, technologies must have evolved to new standards, architectures, protocols, hardware, services and facilities that will make possible a control of the way in with all the nodes access to the net to share their information.

One scenario that represents perfectly the characteristics and it is a perfect case of study of MANETs is the Vehicular Ad hoc Networks (VANETs), a subset of MANETs, which creates wireless networks between vehicles [6]. In a VANET each vehicle is a moving node which creates wireless networks with surrounding vehicles [7], thanks to the On-Board Unit (OBU), a hardware with communications and computing capabilities that allows drivers to receive information about events that can affect his driving. Then, the main function of the OBU is to exchange information with other vehicles or Road Side Units (RSUs), elements located at the infrastructure that act as gateways between the VANET and other networks or agents as Traffic Management Centers (TMC). These centers are placed far away the VANET and play an important role in the applications developed in the area of VANETs, coordinating the infor‐ mation that is shared among VANETs that are deployed in different geographical areas.

In VANETs they can be distinguish two types of links: vehicular-to-vehicular communication (V2V), based on an Ad hoc architecture, vehicles exchange directly messages without a central coordinator; and vehicle-to-infrastructure or infrastructure-to-vehicle (V2I or I2V), where the messages are shared between the vehicles and the RSUs. VANETs are designed for a huge range of cooperative applications, that is, services that provide information to the drivers thanks to the data shared among all the vehicles on the net. These can be safety and non-safety applications, which allow several added services as infotainment, traffic management, toll payment, and geographical based services and so on [8]. That is, VANETs make possible to deploy applications that help to improving the transport services and traffic conditions using collaborative systems based on V2X Ad hoc networks.

**Figure 2.** Vehicular Ad hoc Network Scenario

istics, where the nodes can be any kind of devices with communications and networking capability that communicate with each other without a centralized coordinator [2]. In this scenario, each node can play the role of a router, hosting the network topology dynamically, because as it was mentioned above, the shape and the topology of the net can change as well as the nodes on it. The main characteristics of MANETs can be summarized as follows [3]:

**•** Dynamic topologies: network topology can change quickly due to the nodes can move freely

**•** Bandwidth constrains: compare with wired networks, the capacity of a MANET is relatively

**•** Energy constrains: although many of the nodes can be plugged to the power line or they can be equipped with big batteries, some of them use small power supplies, so during the network design it is necessary to consider how to save power in order to assure the stability

**•** Limited physical security: although the decentralized nature of MANETs provides robust‐ ness against the single points of failure, these nets must be protected against eavesdropping,

In this context, thanks to the rapid increase and improvement of the mobile computing a wide set of wireless devices have proliferated, making possible that traditional hardware as digital cameras, thermostats, cooking ovens or washing machines are provided with communications and computing functionalities so they can be part of a MANET. This new paradigm is known as Internet of Things [4], that is, a scenario in which all the objects beyond computers, mobiles or touch screens have the ability of generating, sharing and processing information in a pervasive manner [5]. With all of this, technologies must have evolved to new standards, architectures, protocols, hardware, services and facilities that will make possible a control of

One scenario that represents perfectly the characteristics and it is a perfect case of study of MANETs is the Vehicular Ad hoc Networks (VANETs), a subset of MANETs, which creates wireless networks between vehicles [6]. In a VANET each vehicle is a moving node which creates wireless networks with surrounding vehicles [7], thanks to the On-Board Unit (OBU), a hardware with communications and computing capabilities that allows drivers to receive information about events that can affect his driving. Then, the main function of the OBU is to exchange information with other vehicles or Road Side Units (RSUs), elements located at the

small and also it is sensitive to interferences, noise, and signal fading effect.

in the net.

and longevity of the network.

214 Contemporary Issues in Wireless Communications

spoofing, and the injection of malicious data attacks.

**Figure 1.** Infrastructure based networks (left) and Ad hoc networks (right)

the way in with all the nodes access to the net to share their information.

This introduces the definition of Intelligent Transport Systems, where each vehicle is a sender, a receiver and a router at the same time, so it can broadcast the information to the VANET, which uses this information to provide these safety and non-safety services to the drivers. The OBU is the hardware in charge of processing these data and it also enables these short range wireless ad hoc networks (the coverage area is around 300 meters) but it also must dispose other systems that permit to report position information such as Global Positioning System (GPS) or a Differential Global Positioning System (DGPS) receiver if more accuracy position information is required. This information is quite important because most of the services that are available in a VANET depend on the geographical position of the source and the destina‐ tion. Table 1 presents a classification of ITS applications that can be deployed using the VANET architecture [9].


**Table 1.** ITS applications on VANETs

These applications can be deployed on urban or motorway scenarios, each one with its own particularities. In an urban scenario, many of the times there is not line of sight between the nodes so fading and communication disruptions are frequents. In a motorway, the high density of vehicles can overload the radio channels in which the VANETs work. Yes, although maybe users are not aware about that, the radio spectrum (the physical interface used by wireless communications networks) is a limited resource that it must be shared among all OBUs and RSUs that shape the VANET. Commonly, ISM (Industrial Scientific Medical) radio bands with frequency ranges 2.40–2.4835 GHz and 5.15–5.875 GHz are used by wireless networks for license-free communications [10]. The definition of these standards is crucial in order to attend the increase on the demand of the spectrum channels and to make possible that different networks can coexist in the same radio band.

Although WLAN (IEEE 802.11a/b/g/n) could be the technology used in VANETs, most of the applications included at Table 1 require time-critical communications, a continuous handover among different RSU in V2I/I2V links, and as these standards use CSMA (Carrier Sense Multiple Access), so many of the nodes cannot have success in channel access due to the high density of some scenarios. Due to the limitations of these standards in mobile scenarios as VANETs, a new extension has been developed: IEEE 802.11p, designed specifically for vehicular environment in which high reliability and low delay characteristics are mandatory. This new standard, known as Dedicated Short Range Communication (DSRC) uses the physical layer of IEEE 802.11a working on the 5.9 GHz band and quality of service enhance‐ ments of IEEE 802.11e. Network and transport layers are in the scope of WAVE (Wireless Access in the Vehicular Environment) standard which defines the protocols and services that support multi-channel wireless connectivity between IEEE 802.11 Wireless Access in Vehicular Environments devices [11].

Once the access to the medium is defined under the frame of the IEEE 802.11p standard, in a situation in which many nodes have information to transmit to different destinations in a network that is geographically distributed, it is quite important to determine the protocols that allow to organize the addressing of the information and to assure that all the nodes have the chance of transmitting and receiving data. The nature of MANETs, and specifically of VANETs implies that the maintenance, management and routing task of the network must be done by all the nodes, making these kind of networks more difficult or more complex to other wireless networks. Therefore, advances techniques of management and arrangement should be applied to organize the network and assure its effective implementation and its fairness and reliability for all the nodes.

In the next sections of these chapter are analyzed the main techniques used to disseminate data in VANETs, with an special emphasis in clustering, a control scheme that can take into consideration the speed and distance difference among neighboring nodes in the VANET to group them in order to assure a stable cluster structure and then enhance the stability of the network topology.
