## 6 Conclusion and Open Research Issues
Therefore, it is essential to answer the following questions, \"when is re-clustering necessary to select the CHs?\" and \"does re-clustering require re-initiation of complete cluster formation process?\" Several proposals are presented in the literature to provide answers for the aforementioned questions such as replacing the time-based clustering process with an energy-threshold-triggering algorithm, elimination of the unchanged nodes of the former cluster from setup phase of next round clustering, rotation of CH role among the cluster members instead of running a new cluster formation process from scratch and event-driven clustering to eliminate the unnecessary clustering overhead in whole regions of network. * Data aggregation: redundancy in WSNs is necessary to guarantee the level performance of vulnerable sensor nodes. Minimizing this redundancy is one of the main reasons for implementation of cluster-based schemes. Data aggregation in basic clustering algorithms is taken into account only as full aggregation, in which regardless of the number of packets or level of data correlation a single packet is produced by CHs. Nevertheless, this degrades the accuracy and reliability of the aggregated data significantly. Therefore, towards achieving a trade-off between the energy saving and level of accuracy expected by the user, spatial and temporal data correlation should be considered in data aggregation process. Furthermore, it is important to integrate data aggregation into routing protocol to achieve higher data fusion ratio in multi-level data aggregation architecture. * Application dependency: cluster-based algorithms heavily rely on the type of the application. Therefore, the clustering process is affected by the application needs and specifications to adapt to its variety of requirements. Besides, the inter-relation between the environmental conditions, network topology and the requirements of the application presents the clustering organization as a multi-variable-decision that should be addressed by multi-factor evaluation systems such as AHP and FLC. In other words, these evaluation systems are more flexible and modifiable to the variable range of the application needs and can be easily adapted to the new wants of the user or even changes of goals of the application by subtle modification of the decision rules instead of complete alteration of clustering protocols, which seems infeasible especially in large-scale networks. * Balanced clustering: cluster formation in basic clustering approaches is based on minimizing the energy expenditure of cluster members by joining the nearest CH. This method of clustering does not guarantee the creation of balanced clusters. Cluster formation can be regarded as a sub-stage of CH selection phase in which constructing balanced clusters is the consequence of the even distribution of CHs. On the other hand, it can be considered as a separate phase, which can manipulate the cluster sizes by implementing resource-aware algorithms and guarantee the balanced expenditure of energy in network field, through defining threshold constraints for cluster sizes or constructing the cluster in K-hop neighbouring region. * Network connectivity: the clustering scheme has to ensure the intra-cluster and inter-cluster connectivity. The basic clustering schemes assume that cluster members and CHs can directly reach the destination, but given the communication constraints of sensor nodes, the feasibility of establishing the direct route communication especially for inter-cluster data transmission in large-scale scenarios is doubted by researchers. Improving the scheduling algorithm of cluster members in single-hop data transmission and creating a tradeoff between the energy-efficiencyand delay caused by multi-hop transmission are the issues addressed by researchers regarding the intra-cluster data transmission schemes.