**3.9. Stability**

**3.7. Scenarios**

236 Contemporary Issues in Wireless Communications

highway scenarios.

bottlenecks.

transmission interval.

**3.8. Cluster size**

In highway scenarios, it is widely recognized that traffic generally follow a platoon pattern according to traffic flow theory. Vehicles in a platoon generally move with similar velocities and are likely to sustain a stable wireless communication in clusters. The clusters are inde‐ pendently controlled and dynamically reconfigured as the vehicles moving. Congestion can occur in highways during an accident so the clustered protocols should be designed to effectively reduce data congestion in high density scenarios, and satisfying QoS requirements. Furthermore, the design of cluster protocol should also consider the market penetration of vehicles enabled with OBU´s. In some cases, there can be a large number of vehicles in road that are not enabled with OBU´s. This creates a large gap between vehicles and resulting in poor communication. The future clustering protocols should consider all the characteristics of

A large number of the available cluster based MAC and routing protocols are purposed for highway environments and does not address the various requirements of the city and urban traffic environments. In city environments, intersections play important roles for information exchange. The vehicle that crosses the intersection before actually receiving a message is defined as the unstable vehicle. As the intersection area is comparatively small and the probability of change of direction is very high, it will be risky to choose an unstable vehicle as the CH from these clusters. Moreover, during rush hours of day intersections are usually the

Vehicles in intersection can take any of the direction Straight (S), Right (R), Left (L) and U-Turn (U) respectively. All the incoming vehicles of two road segments of intersection may be blocked by the red signal, whereas vehicles on the other two road segments flow until the green signal is on. When a vehicle crosses the intersection without having another vehicle arrive at the intersection, a disconnection may occur. Such a situation arises only when a fleet of vehicles has crossed the intersection and when another fleet of vehicles has not been arrived at the intersection. Based on the motion of vehicles, some approaches form clusters S, R, L and U on a particular lane. The created clusters consist of vehicles moving in the same direction. Within the same cluster the vehicles communicate with each other and elect a CH that is responsible for calculating the number of vehicles in its cluster. This information will help to avoid constant

For intersection collision avoidance, the amount of traffic generated by vehicles can be determined by a number of factors such as the cluster size, the number of intersection per cluster, the number of vehicles per intersection per cluster, the size of messages, and the

The size of the cluster is a crucial parameter. To optimize the cluster size is very difficult especially in a highly mobile environment such as VANETs. One of the goals of optimal protocol is to optimize the number of CMs to decrease the end to end delay of messages. If the cluster size is decreased, the channel contention within each cluster decreases. However, the

cluster reconfigurations and overhead by creating another cluster.

Stable clusters are important for a reliable and efficient information exchange. Stable clustering techniques decrease the control overhead of cluster reconfigurations and led to an efficient hierarchical VANET topology. The main condition for stability is the duration of residence´s times of a cluster and its CHs. Stability is also defined by long CH lifetime, and long CM lifetime.
