**3.6. Challenges of clustering**

algorithm is based on hierarchical and geographical data collection and dissemination mechanism. The cluster formation is based on the position of the vehicles at a particular segment instead of the individual positions. However, this type of protocols incurs more overheads for V2V and V2I communication. In some other approaches, each vehicle entering into the network collects the neighbor vehicles information, assuming precedence to each vehicle and polls each vehicle individually (according to precedence) to check whether it is CH or not and then joins the cluster. Also every vehicle in the network collects 2-hop neighbor's information along with 1-hop neighbor's information from the CH through periodic polling.

Some clustering algorithms estimate the future mobility of nodes predicting the probability that the current neighborhood of a mobile node will remain the same. The drawback of the prediction method is the lack of accuracy in some cases. In some of clustering algorithms, the clusters are formed based on mobility metric and the signal power detected at the receiving vehicles on the same directed pathway. Through such method this type of protocol helps in forming stable clusters. However, it does not consider the losses prevalent in the wireless channel. In practical scenario effects of multi path fading are bound to affect the cluster formation method and thus the stability. These effects of multi path fading are taken into account in the density based clustering algorithm. The cluster formation is based on the weight metric which takes into consideration the link quality and the traffic conditions. It can be seen

In some clustering approaches considers the behavior of the vehicles, using the speed and

The cluster formation is based on direction of vehicle at the approaching intersection. In other approach, cluster is formed based on distance and direction of vehicle it takes after crossing the junction. Some of the research enforces a weight cluster mechanism with a backup manager. These algorithms operate in similar way. Algorithms consider the position, direction, speed and range of the nodes to perform the algorithm. On the other hand, some takes into consideration the number of neighbors based on the dynamic transmission range, the direction of vehicles, the entropy, and the distrust value parameters. They works with an adaptive allocation of transmission range (AATR) technique, where hello messages and density of traffic around vehicles are used to adaptively adjust the transmission range among them. The

In some approaches, the cluster formation interval is constant, which implies a synchronous creation of clusters. This does not allow for effective cluster reorganization. The directional based clustering algorithm are based on the following mobility metrics (a) moving direction (b) leadership duration (c) projected distance variation of all the neighboring vehicles over time. In practical scenario effects of multi path fading are bound to affect the cluster formation method and thus the stability. Some approaches take into account the destination of the vehicles to arrange the clusters and implements an efficient message mechanism to respond in real time and avoid global re-clustering. There might be a problem with knowing the final destination a priori as drivers usually do not use navigation system for known routes. Some algorithms are proposed for calculating the density of vehicles in a particular region around

These two information collection leads to more overhead in V2V communication.

that the stability is improved compared to other approaches.

destination of the vehicles is used as a parameter to arrange clusters.

direction parameters.

234 Contemporary Issues in Wireless Communications

One of the numerous challenges clustering algorithms in VANETs is the mobile and dense communication topology. The main problem in clustering is the control overhead introduced to elect the CH and to maintain a stable cluster. The cluster structure assures the scalability of VANETs, where high mobility of the moving vehicles within the road causes lots of challenges to face. Location services might not provide the needed accuracy everywhere or will not be available at all so more work is needed on location independent clustering solutions. Providing highly accurate digital maps that are needed by some solutions presents a challenging task and could slow down the implementation so advantages and disadvantages of map based solutions should be researched.

In many papers the correlations between the transmission range and the VANETs density, packet transmit rate, packet size, data rate and channel conditions have been researched. However, the different network simulators should also be evaluated and presented with all the relevant parameters including MAC, transmission range, packet size, bit rate etc. Since each vehicle in the VANET has its own view of the network density and channel conditions, finding the optimal network parameters is difficult. The research should focus to the optimi‐ zation of cluster size and transmission range that maintains a high VANET stability and reliability, increases the life time of a connection link, and at the same time decreases the time required for a safety message to reach its intended destination. Presented clustering protocols are designed for different aims e.g. overhead minimization, fast cluster creation, cluster stability, etc. The most important parameter among them is the cluster stability. Their tradeoffs and effects between them should also be analyzed and presented.

The vehicles with relatively high mobility, can pose difficulties for flat networks stability. Many of the presented protocols use metrics derived from the same input parameters where among them position and radio signal strength(RSS) are the most important. More research effort should be put in defining and ranking the aims that clusters and clustering protocols should try to achieve. One of the goals is to optimize the mean number of created clusters and the number of CHs at each time step.

For performance evaluations of clustering protocols common parameters used are cluster stability, CH election, cluster size, cluster delay, cluster reconfiguration and cluster overhead etc. These terms are quite generic so their definition and explanation with VANET specifics is needed to provide consistency between different researches. More focus should be put on evaluation of those common parameters to highlight the most useful ones, merge similar ones etc. This would help researchers to concentrate their research on extending and designing the most prospective ones. Fair comparison of different clustering protocols is a hard task due to non-existent standard testing procedures and scenarios so more work and standardization is needed in this area. The characteristics of different scenarios of VANETs and different parameters are explained in detail in later sections.
