## 6 Conclusion and Open Research Issues
Handling the heavy tasks like exchanging the control messages, data aggregation and data communication by CHs has led to wide acceptance of CH role rotation strategies. Further research showed that several characteristics of basic clustering schemes such as fixed probabilistic or round robin fashion CH selection schemes, basic cluster formation strategies and direct single-hop data transmission cannot guarantee the optimum performance of the homogeneous sensor networks. Therefore, in depth investigations are done for improving the issues and attributes of basic clustering schemes in the following directions:  
* Selectivity: remaining energy level of an individual node, average residual energy of nodes, dissipated energy in last round, number of neighbouring nodes in a defined radius, distance from the BS and sensing coverage are the parameters taken into consideration in the literature to optimize the selection of CHs and thus to improve the network lifetime and performance. However, selection of the nodes having the highest energy level is the most favorable and widely accepted feature among other parameters. * Even distribution of CHs: guaranteeing the fair dispersion of CHs within the network area is an important issue, which is not provided by the basic clustering schemes. Self-organized schemes address this issue by considering density dispersion parameter in CH selection process or by selecting CHs based on regional merit, although location awareness and learning the resources information of neighbouring nodes are the extra costs in implementation of these schemes. Another solution to ensure the even placement of CHs is the supervision of the CH selection process by generating a global view of the network and controlling the numbers and position of the selected nodes for the CH role by using BS assisted schemes. However in centralized schemes, sensor nodes should be able to reach the BS to provide the necessary information regarding position or their updated resources status. By considering the limited transmission range and battery resources of the sensor nodes, direct communication of all the nodes or even CHs with a BS located in far distances from the sensing area is not feasible in large-scale fields. Furthermore, using multi-hop schemes to transmit this information entails implementation of a temporary routing protocol in initial stages that affects the scalability and real-time operation of the network and increases the algorithm complexity and overhead. Therefore, a trade-off between the costs and benefits of centralized algorithms should be noted. * Failure recovery: the failure of CH as the coordinator of a cluster causes interruption in performance of a great part of the network. In basic clustering schemes, this failure persists up to next re-clustering round, and this is the reason that the nodes having higher level of energy are selected as CH to minimize the chance of CH failure due to depletion of energy resources in most schemes. However, there are other reasons of failure such as territorial conditions, physical attacks (jamming or tampering) or node mobility. Thus, it seems essential to consider failure-management systems to improve the performance of the network and to reduce the transition time in failure recovery situations. * Re-clustering: although rotation of CH role is one of the bases of clustering algorithm to distribute the energy consumption among all the nodes, exchange of control messages in clustering process to re-construct the clusters is itself an energy intensive process.