*3.3.1. Clustering in MAC layer*

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

V2V based clustering is a decentralized clustering where clusters are formed based on communication between vehicles. Additionally, the CH election will be based on V2V communication. There are several advantages of using V2V-centric clustering as compared with the infrastructure-centric VANETs. V2V-centric clustering can avoid the short commu‐ nication link period, high frequent hand-offs, fast channel fading, etc., that are caused by the high relative-speed difference between the fixed infrastructure and the fast-moving vehicles. Finally, the V2V-centric clustering performs better in active safety applications, which only requires exchanging messages among one hop vehicles within their transmission range.

V2V communications are expected to significantly improve transportation safety and mobility on the road. Several applications of V2V communications have been identified, from safety and warning applications, up to traffic control and driver assistance applications. In infra‐ structure centric clustering, all the communications is done via the infrastructure which causes a lot of control overhead and additional delay. Furthermore, it would be very cost intensive to build an infrastructure based communication all along the road structure. The V2V based clustering technique avoids the use of stationary base stations by building up VANETs, where all vehicles in a common transmission radius can exchange messages. However, CH selection carried out through V2V communications has some shortcomings, e.g., highly complex protocols, large computation and communication cost, need of additional devices and so on. Another important issue is that the connection between two adjacent CHs may be lost due to vehicles high speed, which drastically reduces the link quality. The hidden terminal problem where two vehicles are outside of each other's transmission radius, but both attempt to transmit to a vehicle that is within the radius of both. This issue is likely in pure V2V scenarios where there is no centralized communication system. The result of the hidden terminal problem is data collisions. By enabling vehicles to transmit/receive messages with each other via V2V as well as with infrastructure communications, VANETs could contribute to more safer and congestion free roads by providing correct and timely message to neighboring

Clustering can simplify essential functions like bandwidth utilization, routing, and channel access. In MAC layer, it can provide a fairer and reliable channel access to all vehicles in network. This can lead to increase in the reliability of packets and scalability of the network. In network layer, clustering for routing can find the closest vehicles to intended destination.

optimize the problem.

**3.2. V2V based clustering**

228 Contemporary Issues in Wireless Communications

vehicles and other related departments.

**3.3. Clustering in layers**

Introducing a cluster scheme already on the MAC layer additionally provides the possibility of a fairer medium access. When clustering applied in VANETs, it brings interesting research point such as broadcast storm that occurs when several vehicles are passing at a specific region at the same time, causing network congestion, packet collisions and delays in the medium access layer. A cluster-based MAC scheme is needed in V2V communication to overcome the lack of specialized hardware for infrastructure and the mobility to support network stability and channel utilization. In this case the CH can take over the responsibility to assign bandwidth to the CMs and therefore even QoS support can be improved. As the bandwidth can be assigned centrally fewer collisions have to be expected which consequently increases the reliability.

Many researchers have proposed cluster based multi-channel medium access control protocols to improve the performance and reliability of VANETs. In these protocols, clustering is used to limit channel contention and provide fair channel access within the cluster. On the other hand, multi-channel is used to increase the network capacity by the spatial reuse of the network resources and reduce the effect of the hidden terminal problem. Moreover, to optimize the communication range and the cluster size is very difficult especially in a highly dynamic environment such as VANETs. However, in order to overcome this situation some approaches divide the service area into a set of region units, and limit the number of vehicles in each region unit for the contentions of radio channels. Each region unit is then associated with a nonoverlapping radio channel pool. Since the number of vehicles in each region unit is limited, the contention period is reduced and the throughput is increased. However, these types of approach have low bandwidth utilization in case of sparse traffic. Some of clustering algo‐ rithms try to minimize the total number of clusters by creating hierarchical clusters with a diameter of at most four hops. In this section of the chapter, we compare well known cluster based MAC protocols in Table 3.

The MAC layer is divided into different cycles and each cycle is divided into contention based or contention free. In the current literature, several MAC protocols have been proposed to VANETs. Some of the well-known MAC protocols are ADHOC MAC [32], SDMA [33], VeMAC [34], DMMAC [35], STDMA [36], VeSOMAC [37] etc. These protocols are proposed for various scenarios and have many drawbacks such as hidden terminal problem, time unbounded, unreliability etc. There is a need for new MAC protocol in VANETs that can provide mobility (i.e., the MAC protocol should support vehicles to leave and join inter-vehicle communications at high speed), delay bounded (i.e., the communication must be delay bounded and real-time), scalability (i.e., VANET should scale itself according to the number of vehicles present), bandwidth efficiency (i.e., the radio resource should be utilized in an efficient and fair manner), cost (i.e., for cost-efficient and reliable communications, VANET should be fully decentral‐ ized), and fairness (i.e., every vehicle should get a fair chance to get the radio channel).The


**Table 3.** Comparison between various cluster based MAC protocols.

challenge of successfully deploying VANET services is to ensure timely and reliable data delivery for mobile vehicles.
