## 2 State of the Art  
A fundamental problem of WSN systems lies in the relatively high energy consumption. The most demanding component in energy terms is perhaps the radio, usually the batteries are not big enough to support long time and long distance communications because of the needed emission power. The microcontroller also affects the consumption, specially when it is executing a computation intensive task. Notwithstanding, energy consumption for communications is much higher than for computing. In order to reduce the energy consumption, some researches are focused on the data transmission [12; 13]. [PERSON] et al. compared different communication protocols with their own protocol based on mote positions and subdivision into regions to improve the data transmission energy consumption. [PERSON] et al. used a node cut strategy to dynamically generate filtering tuples that have little computational overhead when collecting query results, instead of issuing queries with filters. This reduces the comparison frequency and the computation time. Other researchers optimize the energy efficiency by using optimal cluster head selection, forming chains of nodes or balancing the load, i.e., focusing on the topology of the WSN. [PERSON] et al. [14] suggested implementing an optimized solution to the problem of arranging objects in an scalable IoT. [PERSON] et al. [15] discussed about the use of different protocols for clustering in WSN providing a complete vision of the problem. Likewise, there are researches targeting in minimizing the energy consumption taking into account the time between transmissions. [PERSON] et al. [16] suggest an energy-aware adaptive sampling method that allows each mote in the network to adapt the sampling rate to its sensors according to the available energy in the system. Moreover, there are power management schemes in the motes that attempt to control the radio depending on the state of the data transmission, called wake-up protocols. These protocols are grouped into three categories: duty-cycling, non-cycled wake-up and path reservation wake-up protocols [17] that can be available in the Media Access Control (MAC) layer. The duty-cycling protocols are mechanisms that turn the radio on and off to conserve energy [18], meanwhile a sleep-wake up cycle is employed to save energy reducing the energy consumption in idle listening, it increments the waste power due to control packet overhead and collisions. Non-cycled wake-up protocols use a low power wake-up radio that is active all the time or use a passive circuit that should be powered from the wake-up signal, which is only able to receive the wake-up message [19]. Finally, path reservation wake-up protocols exploit parallel data communication and channel reservation [20]. In this case, communications are in multi-loop links and the wake-up signal is independent to data transmissions using separate radio channels. In duty-cycling mechanisms, depending on the tolerance of the scenario, i.e., message delay, collision rates and control packet overhead (synchronization or asynchronous transmission), duty-cycling may work efficiently or not. As a consequence of the different scenarios, there are different approaches of duty-cycling: synchronous, asynchronous or semi-synchronous approach. In the synchronous approach motes exchange synchronization information through the network to archive a degree of synchronization [21; 22]. This approach is useful when each mote has its own data exchange time window. The semi-synchronous approach groups neighbor motes in synchronous clusters that exchange information asynchronously among them [23; 24].