**2. Data dissemination algorithms**

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

> Emergency vehicle warning Slow vehicle indication Intersection collision warning Motorcycle approaching indication Emergency electronic brake lights Wrong way driving warning Stationary vehicle - accident Stationary vehicle - vehicle problem

Traffic condition warning Signal violation warning Roadwork warning Collision risk warning

Enhance routing

Fleet management Loading zone management

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

location, road adhesion)

Point of Interest notification

ITS local electronic commerce Media downloading

Insurance and financial services

Regulatory / contextual speed limits notification

Road conditions sensing (rain, visibility, wind, hazardous

Automatic access control and parking management

Traffic light optimal speed advisory

**Category Applications Uses cases**

architecture [9].

Active safety applications Cooperative driving

216 Contemporary Issues in Wireless Communications

Efficiency applications Traffic management

Infotainment applications

**Table 1.** ITS applications on VANETs

assistance applications

Road monitoring

Contextual information Entertainment

> Data dissemination in VANETs has recently received considerable attention. Due to the unique characteristics of VANET, the implementation of reliable data dissemination among vehicles has encountered many challenges. Information dissemination in VANETs provides drivers a way to be aware in real-time of everything that is happening in their surroundings. A wide range of information can be disseminated, including traffic and road conditions, closure and detour information, incident information, emergency alerts, and driver advisories.

Information dissemination schemes in VANETs are commonly categorized into two different groups, according to the type of ITS application: safety and non-safety. During the last years, research community has focused their studies more on safety applications which are highly demanding in terms of message delay and present a challenging field of study. Although in safety applications the frequency of messages is low, the message delay is a key factor because a safety message, e.g., an emergency vehicle warning, has to reach a maximum number of nodes in a given area within a very short time interval, because after this time interval, the message essentially becomes useless.

However, in non-safety applications the message delay loses importance since the message could be useful for a longer time interval, even up to several minutes, e.g., for disseminating traffic road conditions. On the other hand, the frequency of these messages is much higher in this type of applications.

Therefore, data dissemination in VANET is a challenge for the deployment of cooperative services and applications because the dissemination routing protocol has to be suitable both for safety and non-safety applications, and it also has to be aware of the vehicular environment challenges as the high mobility of nodes and the extremely dynamic network topology. Therefore, the design of an efficient information dissemination routing protocol for VANETs is very crucial.

The function of a routing protocol in Ad-Hoc network is to establish routes between different nodes and the main requirement is to achieve minimal communication time with minimum consumption of network resources. The main reasons that make so difficult the design of these routing protocols are the highly dynamic nature of VANETs due to the high mobility of the nodes, and the need to operate efficiently with limited resources, such as network bandwidth. Moreover, routing protocols in VANETs, and generally in every Ad-hoc Networks, are not so good in scalability due to frequently changing network topology, lack of predefined infra‐ structure and limited radio communication range. In the literature, four categories of dissem‐ ination routing protocols for VANETs which are presented: position-based, broadcast, geocast and cluster-based.

Broadcast routing is commonly used in ITS applications in VANETS because it guarantees that every vehicle will receive the message. The simplest way to implement a broadcast service is flooding in which each node re-broadcasts messages to all of its neighbors except the one it got this message from. Flooding performs relatively well for a limited small number of vehicles and is easy to be implemented. Furthermore, this protocol is very reliable in safety applications but it consumes high bandwidth and resources, and it can also provoke a broadcast storm when the number of nodes in the network increases. If multi-hop communications are implemented as each node receives and broadcasts the message almost at the same time, this routing protocol generates contentions and collisions and high bandwidth consumption.

However, there are many studies where they use broadcast, but they design an approach to avoid broadcast storm. In [12], Yang et. al propose a V2V communication protocol for Cooperative Collision Warning application. In this approach when a vehicle has an incident, it becomes an abnormal vehicle (AV) and starts broadcasting periodically Emergency Warning Messages (EWMs), with its geographical position, speed and direction to its surrounding vehicles. If this incident provokes that more vehicles have to stop and, therefore, they become also AV, only one of them is going to send the EWMs to avoid the broadcast storm. In [13], Ferrari et. al use broadcasting protocol with multi-hop communication but to avoid the broadcast storm not every vehicle forward the received messages, only the farthest vehicles from the source forward it.

**Figure 3.** Broadcasting routing protocol

Information dissemination schemes in VANETs are commonly categorized into two different groups, according to the type of ITS application: safety and non-safety. During the last years, research community has focused their studies more on safety applications which are highly demanding in terms of message delay and present a challenging field of study. Although in safety applications the frequency of messages is low, the message delay is a key factor because a safety message, e.g., an emergency vehicle warning, has to reach a maximum number of nodes in a given area within a very short time interval, because after this time interval, the

However, in non-safety applications the message delay loses importance since the message could be useful for a longer time interval, even up to several minutes, e.g., for disseminating traffic road conditions. On the other hand, the frequency of these messages is much higher in

Therefore, data dissemination in VANET is a challenge for the deployment of cooperative services and applications because the dissemination routing protocol has to be suitable both for safety and non-safety applications, and it also has to be aware of the vehicular environment challenges as the high mobility of nodes and the extremely dynamic network topology. Therefore, the design of an efficient information dissemination routing protocol for VANETs

The function of a routing protocol in Ad-Hoc network is to establish routes between different nodes and the main requirement is to achieve minimal communication time with minimum consumption of network resources. The main reasons that make so difficult the design of these routing protocols are the highly dynamic nature of VANETs due to the high mobility of the nodes, and the need to operate efficiently with limited resources, such as network bandwidth. Moreover, routing protocols in VANETs, and generally in every Ad-hoc Networks, are not so good in scalability due to frequently changing network topology, lack of predefined infra‐ structure and limited radio communication range. In the literature, four categories of dissem‐ ination routing protocols for VANETs which are presented: position-based, broadcast, geocast

Broadcast routing is commonly used in ITS applications in VANETS because it guarantees that every vehicle will receive the message. The simplest way to implement a broadcast service is flooding in which each node re-broadcasts messages to all of its neighbors except the one it got this message from. Flooding performs relatively well for a limited small number of vehicles and is easy to be implemented. Furthermore, this protocol is very reliable in safety applications but it consumes high bandwidth and resources, and it can also provoke a broadcast storm when the number of nodes in the network increases. If multi-hop communications are implemented as each node receives and broadcasts the message almost at the same time, this routing protocol generates contentions and collisions and high bandwidth consumption.

However, there are many studies where they use broadcast, but they design an approach to avoid broadcast storm. In [12], Yang et. al propose a V2V communication protocol for Cooperative Collision Warning application. In this approach when a vehicle has an incident, it becomes an abnormal vehicle (AV) and starts broadcasting periodically Emergency Warning

message essentially becomes useless.

218 Contemporary Issues in Wireless Communications

this type of applications.

is very crucial.

and cluster-based.

In the position-based routing protocol the forwarding dissemination decisions are based on location information. This approach makes sense because in VANETs the movements of the vehicles are usually restricted in just bidirectional movements constrained along roads and streets, and the geographical location information of vehicles is taken from street maps, traffic models or even more prevalent navigational systems on-board the vehicles. This protocol is commonly used with multi-hop communications and therefore, nodes usually forward the packet to a node that is geographically closest to the destination. The main advantages of this routing protocol are:


For example, as it is shown in Figure 4, if one vehicle has an accident the information will be only be necessary for the vehicles that are behind the damaged vehicle, not for the ones that are not going to drive again though the point the accident has happened.

However, to use this location-based routing protocol in a built-up city environment is very challenging, due to vehicles are distributed in an irregularly way because they usually are more concentrated on some principal roads than others and the road patterns define their

**Figure 4.** Position-based routing protocol

mobility and add difficulty in the signal reception because of the radio obstacles such as highrise buildings which may lead VANETs unconnected. Furthermore, in general, topology-based routing protocols are considered not to scale in networks with more than several hundred nodes [14].

In order to position-based routing protocol could work, vehicles should send periodically beacon messages to announce their position and enable other nodes to maintain a one-hop neighbor table. This approach is scalable and resilient to topology changes since it does not need routing discovery and maintenance; however, periodic beaconing creates a lot of congestion in the network [15]. This beaconing frequency can be configured according to different scenarios or traffic situations, but if this beaconing frequency is not enough the inaccuracy of position information is higher and a neighbor selected as a next hop may no longer be in transmission range implying to a significant decrease in the packet delivery rate.

Therefore, the key ideas we have to take into account to select one position-based routing protocol are:


There are three different kinds of position-based protocols which are restricted directional flooding, greedy and hierarchical routing protocols. The most used routing position-based protocol is the greedy in which they use forwarding to route packets from a source to the destination. This strategy do not establish and maintain the routes between the source and the destination; on the other hand, a source node define the approximate position of the destination and add this data in the data packet and selects the next hop depending on the optimization criteria of the algorithm; for example, as it is shown in Figure 5, one criteria could be the closest neighbor to the destination [16],[17]. In the same way, each intermediate node selects a next hop node until the packet reaches the destination, as it is shown in Figure 4 Position-based routing protocol.

**Figure 5.** Greedy routing protocol

The main characteristics of Greedy algorithms are:

**•** Loop free

mobility and add difficulty in the signal reception because of the radio obstacles such as highrise buildings which may lead VANETs unconnected. Furthermore, in general, topology-based routing protocols are considered not to scale in networks with more than several hundred

In order to position-based routing protocol could work, vehicles should send periodically beacon messages to announce their position and enable other nodes to maintain a one-hop neighbor table. This approach is scalable and resilient to topology changes since it does not need routing discovery and maintenance; however, periodic beaconing creates a lot of congestion in the network [15]. This beaconing frequency can be configured according to different scenarios or traffic situations, but if this beaconing frequency is not enough the inaccuracy of position information is higher and a neighbor selected as a next hop may no longer be in transmission range implying to a significant decrease in the packet delivery rate. Therefore, the key ideas we have to take into account to select one position-based routing

**•** Loop-freedom: routing protocols should be inherently loop-free and should avoid recovery strategies using timeouts of old packets and memorizing packets that have been seen before

nodes [14].

**Figure 4.** Position-based routing protocol

220 Contemporary Issues in Wireless Communications

protocol are:

**•** Path strategy

**•** Memorization

**•** Scalability **•** Robustness

**•** Metrics

**•** Distributed operation

**•** Guaranteed delivery


In restricted directional flooding, the sender will broadcast the packet to all single hop neighbors towards the destination. The node which receives the packet checks whether it is within the set of nodes that should forward the packet (according to the used criteria). If it is, it will forward the packet. Otherwise the packet will be dropped. In restricted directional flooding, instead of selecting a single node as the next hop, several nodes participate in forwarding the packet in order to increase the probability of finding the shortest path and to be robust against the failure of individual nodes and position inaccuracy.

**Figure 6.** Restricted directional flooding routing protocol

The main characteristics of Restricted Directional Flooding are:


The third forwarding strategy is to form a hierarchy in order to scale to a large number of mobile nodes. This strategy tries to reduce the complexity of the information each vehicle has to handle and also improves the scalability of the network. The two main strategies used to combine nodes location and hierarchical network structures are the zone-based routing and the dominating set routing [18].

Geocast routing is a location-based routing but in a multicast way, so each message is broad‐ casted to every vehicle inside a defined area. In Figure 7 it is shown that the defined area are the vehicles which receive the yellow messages. Geocast can be implemented with a multicast service by simply defining the multicast group to be the certain geographic region.

**Figure 7.** Geocast routing protocol

it will forward the packet. Otherwise the packet will be dropped. In restricted directional flooding, instead of selecting a single node as the next hop, several nodes participate in forwarding the packet in order to increase the probability of finding the shortest path and to

The third forwarding strategy is to form a hierarchy in order to scale to a large number of mobile nodes. This strategy tries to reduce the complexity of the information each vehicle has to handle and also improves the scalability of the network. The two main strategies used to combine nodes location and hierarchical network structures are the zone-based routing and

Geocast routing is a location-based routing but in a multicast way, so each message is broad‐ casted to every vehicle inside a defined area. In Figure 7 it is shown that the defined area are the vehicles which receive the yellow messages. Geocast can be implemented with a multicast

service by simply defining the multicast group to be the certain geographic region.

be robust against the failure of individual nodes and position inaccuracy.

**Figure 6.** Restricted directional flooding routing protocol

222 Contemporary Issues in Wireless Communications

**•** Path strategy: flooding / multipath

**•** Not loop free

**•** Localized operation

**•** Metric: Hop count

**•** No guarantee of delivery

the dominating set routing [18].

**•** Memory

**•** Not scalable **•** Not robust

The main characteristics of Restricted Directional Flooding are:

Most geocast routing methods are based on directed flooding, which tries to limit the message overhead and network congestion of simple flooding by defining a forwarding zone and restricting the flooding inside it. With this routing protocol we consume less network resources than broadcast routing but we also guarantee that every vehicle will receive the message. However, we continue having the broadcast storm problem unless we only use one-hop communications. Geocast routing is divided into three types which are: Routing with simple flooding, direct flooding and no flooding [19].

The Geocast routing based on simple flooding was not created for geocast routing but it is used as a basic unit and for the comparison with other protocols. In this method, the source vehicle delivers the packet to all other nodes in the network and all receivers have to check whether they are within the destination area. This is a very straightforward approach but is not a wellorganized approach. In this approach, information of location is not used.

In the Geocast routing based on direct flooding the packet is forwarded to a defining region called "forwarding zone". In this approach a packet is only forwarded to forwarding zone by the source node and not to all nodes in the network. In other words, this protocol is based on flooding but avoids flooding the whole network by defining a forwarding zone, and therefore, outside the forwarding zone the packet is discarded. There are two types of forwarding zone, the first one is the rectangular forwarding zone and the other one is distance-based forwarding zone.

The Geocast routing without Flooding is a simple geocast routing protocol that uses a regular unicast routing protocol between the sender and the destination region. Inside the destination region, flooding can be used, as well as any other routing protocol that can be independent of the protocol used outside the destination region, but the main difference is that it does not use flooding outside the forwarding zone.

But the most used routing protocol for vehicular environment is the cluster-based, where vehicles are grouped into different clusters according to some parameters. These parameters differ from one algorithm to another and are the key factor to build stable clusters. Some of those parameters could be the location, speed or inter-vehicle distance. Other parameters, as the IEEE 802.11p wireless coverage area of each vehicle, could affect in the size of clusters which could vary from one cluster to another in the same network depending on the location of nodes.

Therefore, clusters are virtual groups selected by a clustering algorithm where at least there is Cluster Head (CH) and some Cluster Members (CMs). The main advantage of cluster-based solution is that it can achieve good scalability for large networks, but, on the other hand, the delay and overhead involved in the formation and maintenance of clusters has to be taken into consideration.

The highway, urban, city and intersection scenarios require different characteristics for selection of CHs and for formation of clusters.

**Figure 8.** Clustering routing protocol

The cluster-based routing solution could be designed in three different ways depending on how vehicles discover the CH. It could be in a proactive, reactive or hybrid way. In the proactive solution beacon messages are constantly broadcast and flooded among vehicles since every vehicle should maintain updated their neighbor table to know which the next hop node toward a certain destination is. The advantage of the proactive routing protocols is that there is no route discovery since route to the destination is maintained in the background and is always available upon lookup. Despite its good property of providing low latency for realtime applications, the periodically beacon sending for the maintenance of the neighbor table requires a significant part of the available bandwidth, especially in highly mobile VANETs.

In the reactive approach the configuration phase is initiated by the vehicle because it starts a communication when it needs to communicate with another vehicle. It maintains only the routes that are currently in use, thereby reducing the burden on the network. Reactive routings typically have a route discovery phase where query packets are flooded into the network in search of a path. The phase completes when a route is found.

In a mixed approach vehicles also send periodic proactive beacon messages to have the neighbor table updated but they are also able to create a new communications on demand when they need to send any message to another vehicle.

To sum up, it is not very obvious which is best routing protocol for data dissemination in VANETs because it depends on application and the characteristics of the scenario like the position of the vehicles, speed, direction of movement, potential communication duration and potential number of communication neighbours, among others. Therefore, research commun‐ ity should continue researching on the development of new dissemination data routing protocols.
