**3.8. Cluster size**

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 number of CHs is increased, so that the resulting virtual network formed by these CHs will become more complex. There is then a tradeoff between the cluster size and the number of CHs. Cluster size is variable according to vehicle density, speed and required minimum bandwidth or QoS where parameters can be predefined or provided on the fly from vehicle sensors and application profiles.

The cluster size can be controlled by a predefined transmission range between a CH and its CMs. Optimal cluster size and hence the transmission range that maintains a high VANET reliability, stability and scalability, increases the life time of a communication link, and at the same time reduces the end to end delay for a safety message to reach its intended destination. Optimal cluster size is both related to the radio transmission power and vehicle traffic density. Therefore, cluster size may limit radio efficiency and throughput. For the cluster protocols, we have so far assumed that transmission power is fixed and is uniform across the VANET. There are different methods to reduce the cluster size by reducing transmission power. There is different power control protocols proposed but most of them are oscillating because of the fast varying vehicles densities in VANETs. Selection of optimal power control algorithm and vehicular densities will reduce the end to end delay, reliability and fairness.

Optimal cluster size is also determined by the correlation between spatial reuse of the medium (which leads to small numbers) and end to end delay minimization (which lead to large numbers). Other parameters also apply, such as geographical area and power consumption.
