# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 1 Introduction  
A Wireless Sensor Network (WSN) is a set of wirelessly interconnected multifunctional devices, called sensor nodes, or just nodes, that are placed in an area of interest with at least one sink node, called base station (BS) [1]. The BS can both apply centralized control over the network it belongs to and communicate with the network end user(s) or/and other wireless or wired networks (Figure 1). Figure 1: An overview of a typical Wireless Sensor Network (WSN) architecture. A typical sensor node is an electronic device, which, as shown in Figure 2, consists of a processing unit, a communication module, a single or many sensing elements, and a power unit. It may also contain optional equipment such as a mobility module or/and a position tracking module [1,2]. A sensor node uses its sensing elements to gather data related to its ambient conditions. In addition, a sensor node uses its processing unit to handle the data it collects, and its communication module to wirelessly exchange data with other sensor nodes and the BS. The power unit is typically a battery. The position tracking module monitors the current location of the sensor node. Finally, the mobility unit provides transportability. In a WSN, the operation of both sensor nodes and the BS(s) is regulated by the so-called protocol stack. As shown in Figure 3, the protocol stack consists of five layers: the physical, the data link, the network, the transport, and the application layer [1]. Specifically, the physical layer performs modulation, transmission, and reception tasks such as the selection of communication frequency, the generation of carrier frequency, the detection and modulation of signals, and the encryption of data. The data link layer deals with data frame detection, data stream multiplexing, medium access and error control in order to achieve consistent point-to-point, and point-to-multipoint connections. The network layer performs the routing of the data delivered by the transport layer. The transport layer is in charge of the retainment of the data flow. The application layer is the layer that adapts its content in order to suit the characteristics of each specific application. In addition, the protocol stack includes five management planes that intend to optimize the operation of a WSN in terms of corresponding performance metrics. They namely are: the power, the mobility, the task, the quality of service (QoS), and the security management planes [3]. Specifically, the power management plane is responsible for diminishing energy consumption. The mobility management plane is responsible for maintaining data routes even when there are movements of nodes. The task management plane is responsible for the assignment of various tasks,  
Figure 3: Overview of the WSN protocol stack. Figure 2: The architecture of a typical sensor node. such as sensing, routing, and data aggregation, to the sensor nodes. The QoS management plane deals with fault tolerance, error control, and performance optimization according to particular QoS metrics. Security management plane records and regulates the network performance in terms of security [4]. A WSN may contain one or more BSs along with some hundreds or even thousands of sensor nodes. The combined use of such a quantity of nodes enables the concurrent acquisition of data related with the ambient conditions at wide areas of interest. This makes WSNs be ideal for the accomplishment of an ever increasing range of tasks such as fire detection, energy management, biomedical applications, environmental and habitat monitoring, surveillance and reconnaissance, home automation, object tracking, traffic control, inventory control, agriculture, machine failure diagnosis, and various military applications [5; 6; 7; 8; 9; 10; 11]. On the other hand, despite the numerous benefits that WSNs offer, there are serious problems that obstruct their operation, such as the presence of congestion, connectivity loss, vulnerable security, deteriorated QoS, and insufficient coverage [12; 13; 14; 15; 16; 17].