**3. Clustering algorithms**

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

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

The highway, urban, city and intersection scenarios require different characteristics for

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

of nodes.

consideration.

selection of CHs and for formation of clusters.

224 Contemporary Issues in Wireless Communications

**Figure 8.** Clustering routing protocol

Clustering is a technique for grouping vehicles in the geographical vicinity together, making the network more robust and scalable. Under a cluster structure from Figure 9, vehicles may be assigned a different status or function, such as cluster head (CH), gateway (GV), or cluster member (CM). A CH normally serves as a local coordinator for its cluster, performing intracluster transmission arrangement, data forwarding, and so on. A GV is a non-CH vehicle with inter-cluster links, so it can access neighboring clusters and forward information between clusters an RSUs. A CM is usually called an ordinary vehicle, which is a non-CH vehicle without any inter-cluster links.

Cluster-based solutions may be a realistic approach in supporting reliable and scalable multihop communication for VANETs [20]. Clustering has been shown to effectively reduce data congestion [26], and can support Quality of service (QoS) requirements [21] for both delaytolerant (e.g. road/weather information) and delay-intolerant (e.g. safety messages). According to [22] clustering provides three basic benefits.


Clustering can be done in a centralized or decentralized way. In centralized way, RSU elect CHs and forms clusters based on periodic message. As a fixed infrastructure, the RSU should be fully utilized to collect information and use this information to perform central control. It acts as backbone of all data transmissions. However, it does not work in network where there are no RSUs. Decentralized clustering is based on the "hello message" exchange between the vehicle and it forms clusters and elects its CHs. Additionally, most protocols only use peer to peer communication to gather and transmit information, so those data can hardly be converged and processed in centralization. This is further discussed in detail in coming section.

**Figure 9.** (a) Highway and (b) City scenario.

#### **3.1. Infrastructure centric clustering**

Infrastructure based clustering is a centralized clustering where it gathers information from all the vehicles in the road, including speed, direction, positions, and further traffic related information. Infrastructure divides vehicles in the road into different cluster groups, it coordinates in the election of CH, routing of packets and allocation of the channel to its CMs. As a fixed infrastructure, it computes the collected information to perform central control. Moreover, using V2V clustering some algorithms require additional devices for computation to fulfill the aim, which will raise the vehicles cost and reduce the feasibility of algorithms. Infrastructure based clustering is used to solve the above-mentioned shortcomings and to achieve high stability. Overall, the amount of data to be sent is comparatively small (the position, speed, direction of each vehicle), but the communication reliability is vital.

Some approaches shown in Table 2 uses infrastructure for centralized channel allocation in order to reduce channel allotment time and control overhead. It can be seen infrastructure divides the spectrum allocated to a particular area into prefixed overlapping spatial clusters. The medium in each cluster is divided into time slots and each time slot is allocated to a vehicle in accordance to the priority of the message and availability of the time slot. However, due to centralized allotment the reliability and fairness is lowered. In another approach, infrastruc‐ ture allocates channels to the moving vehicles based on their clusters and enables channel reuse in non-adjacent clusters. The infrastructure broadcast is heard by all the neighboring vehicles in the infrastructure region and this solves the issue of hidden/exposed vehicles. Furthermore, broadcast helps to avoid contention and results in efficient utilization of the allocated band‐ width. The lack of contention for channel acquisition and priority list at the infrastructure allows the protocol to ensure predictable delivery of safety messages. Nevertheless, these types of algorithms may not scale at high density and would not function in ad hoc mode in regions where there are no infrastructures.


**Table 2.** Comparison between various infrastructures based protocols.

**3.1. Infrastructure centric clustering**

**Figure 9.** (a) Highway and (b) City scenario.

226 Contemporary Issues in Wireless Communications

Infrastructure based clustering is a centralized clustering where it gathers information from all the vehicles in the road, including speed, direction, positions, and further traffic related information. Infrastructure divides vehicles in the road into different cluster groups, it coordinates in the election of CH, routing of packets and allocation of the channel to its CMs. As a fixed infrastructure, it computes the collected information to perform central control. Moreover, using V2V clustering some algorithms require additional devices for computation to fulfill the aim, which will raise the vehicles cost and reduce the feasibility of algorithms.

ROAD SIDE UNIT(RSU)

CLUSTER HEAD (CH) CLUSTER MEMBER (CM)

(b) City Scenario

(a) Highway Scenario

GATEWAY

Vehicular motion are confined to strait jacket roads and travels at high velocity and the enter/ exit infrastructure area in short interim's of time. At a given period of time, the total number of vehicles in an infrastructure area can vary significantly from a small density of vehicles to a large density of vehicles in a very short interim of time. Algorithms must be distributed or should require partial infrastructure assistance with an efficient hand-off from one infrastruc‐ ture to another to meet these attributes. The vehicular movement is predetermined to road structure and directional antenna would be suitable for communication via infrastructures. The vehicle broadcast radio frequencies with transmission channels, each one considered as a common medium over which two neighboring vehicles cannot transmit simultaneously because a transmission collision occurs. So, in order to efficiently share the medium, MAC protocol is needed and is beset by contention delay. However, a protocol must ensure that safety messages are delivered within a prescribed time frame. The protocol must not suffer without the hidden/exposed terminal or deafness problem to ensure reliable message delivery. Although the infrastructure is an extra, it will be furnished on the highways extensively and applied in VANET in the near future. Therefore, compared with great and lifelong benefit, the infrastructures expense is of trifling importance at all. The efficient cluster based MAC and routing protocols can provide a more stable communication than a solution using V2V clustering. The optimum protocol should that take the advantages of fixed infrastructure and optimize the problem.
