# Evaluation of Green Strategies for Prolonging the Lifespan of Linear Wireless Sensor Networks
## 3 Classification of Energy Management Techniques for Prolonging WSN's Lifetime
### Energy Consumption Analysis of WSN
#### 3.1.1 Analysis of the Energy Consumption of a Sensor Node  
A wireless sensor node comprises a sensing unit, processing unit, communication unit, and power supply unit, as depicted in Figure 4.  
* The sensing unit serves as an interface between the real world and the digital world. It detects different phenomena from the environment, ranging from light, heat, pressure, acceleration, etc., which serve as inputs to the sensor nodes. It is usually made up of transducer and signal conditioning (ADC, filter, and amplifier) parts. The transducer generates an electric signal proportional to the event or condition being monitored or measured [44], and the generated electric signal is typically converted to digital form using the ADC since the processing unit of sensor nodes can only process digital data [45]. * The processing unit is the core of a wireless sensor node, and it is involved with the collection of data from the sensors, processing this data (data filtering, data compression, data aggregation, data routing, etc. ), deciding when and where to send it, reception of data from other sensor nodes, and the setting of actuators' behaviors (if they are present). It has to execute various programs, ranging from time-critical signal processing to communication protocols of application programs. The categories of processing and control units used in a sensor node include the microcontroller (MCU), digital signal processor (DSP), programmable gate arrays (FPGA), and application-specific integrated circuits (ASIC) [32; 44]. * The communication unit is in charge of sending and receiving packets to or from other sensor nodes in the network via wireless communication. The transceiver has different modes of operation, which include transmitting, receiving, and idle/sleep modes, with each state consuming a different amount of energy [46]. The choice of the communication unit is very crucial in determining the sensor node's energy consumption since this unit consumes the highest energy compared to the processing and sensing units [1; 47]. * The power supply unit provides the node with the energy required to cater to the node's operations (sensing, data processing, and communication). Sensor nodes are usually powered via energy stored in batteries or capacitors in applications that require deployment in areas without access to the power grid. The batteries used can either be rechargeable or non-rechargeable. For long-lasting WSN applications where there is a need for sensor nodes to go for long periods unattended and without replacing their energy source, the limited energy storage capacity of batteries is not attractive. Currently, most sensor nodes are designed to have the optional ability to recharge their battery from energy harvested through scavenging techniques such as photovoltaics, temperature gradients, vibrations, pressure variations, the flow of air/liquid, etc. [48; 49]. To model the energy consumption of a sensor node, it is essential to account for the energy consumed by its various components. The energy consumed by a sensor node is the sum of the energy consumed in the active and inactive (sleep) states. \[E_{total}=\sum_{k}\Big{(}E_{Active,k}+E_{Sleep,\ k}\Big{)} \tag{1}\]  
where \(E_{total}\) is the total energy, \(E_{Active,k}\) and \(E_{Sleep,\ k}\) represent the energy consumed in the active and sleep states in a single time step (cycle) \(k\), respectively.