## 2 Related Work
Hierarchical cluster control (HCC) was proposed to improve various parameters in clustering. These are cluster size, connectivity between CHs and cluster joining. However, the entire network must be traversed before it can be computed. In an emergent algorithm for highly uniform cluster formation (ACE) [9], one way of CH selection is when many nodes in its neighborhood do not belong to any cluster, then it elects itself as a CH. The two functional components of ACE are the generation of new clusters and the migration of existing ones. ACE uses a protocol to cluster the WSNs into a constant number of iterations. Unfortunately, it is hard to detect the number of iterations required to cluster WSNs. Moreover, additional overheads are caused by the migratory mechanism in ACE. In brief, total energy consumption in multi-hop data delivery in clustered WSNs should be analyzed comprehensively. Several MAC protocols have been proposed by various researchers to reduce the energy consumption during the idle listening state. The energy efficiency is achieved by allowing sensor nodes to enter the sleep state periodically or aperiodically [10, 11]. These MAC protocols are either based on contention, contention-free or hybrid approaches. Some of the recently proposed contention protocols, such as ShortPreamble MAC Protocol (X-MAC) [12] and Low Power Down link MAC Protocol (WiesMAC) [13], improve the energy efficiency by minimizing the length of the listening state and duty cycle. These are achieved by using an adaptive preamble sampling scheme [14]. This scheme can reduce the energy used for transmitting packets. However, WiesMAC consumes more energy in multi-hop communications. In X-MAC [12], the energy efficiency can be achieved at both ends of data communications. This energy efficiency is achieved by using a shortened preamble sampling and an adaptive duty cycle for different channel loads at the receiver. However, the simulation results of X-MAC are not good enough to prove the effects of the algorithm. Contention-free protocols, such as energy-efficient MAC protocol (S-MAC) [10], adaptive energy-efficient MAC protocol (T-MAC) [15] and utilization based duty cycle tuning MAC protocol (U-MAC) [16], can specify when nodes are awake and asleep within a period. These contention-free MAC protocols can be divided into periodic and event-driven based on the frame. Some of the periodic protocols are [10, 15, 17, 18]. The nodes in these protocols synchronize the frame and time slots with each other. Additionally, nodes periodically wake up, transmit and receive messages. Then, nodes go to sleep until the next period to minimize energy consumption. However, the length of the listening state is not considered. In T-MAC [15], the authors shortened the length of the listening state. Then, nodes go to the sleep state if no data are received during this time period. However, it will not converge in the case of event-driven traffic. U-MAC was proposed to solve this problem by tuning the duty cycle according to traffic load. However, some nodes may have higher data traffic in a frame considered with respect to other nodes. These nodes will have less sleeping time due to the periodic frame. Event driven TDMA Protocol (ED-TDMA) [19] alleviates this problem by changing the length of the TDMA frame. The length of the frame depends on the number of source nodes. However, if a CM needs more than one time slot to transmit data, the CH cannot allocate more than one time slots in one frame. [PERSON] _et al_. [20] propose a distributed TDMA scheduling protocol based on coloring algorithm (TDMA-CA) utilizes spatial reuse of the wireless channel. The TDMA-CA protocol assigns distinct colors for conflict sensors in the WSNs and arranges different slots for data transmission for each color. However, additional overheads are created for synchronization among all nodes in the network. In research of data aggregation schemes, there are different methods of data aggregation based on tree- and cluster-based aggregation.