# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 1 Introduction
However, it is commonly accepted that, the most important, by far, weakness of WSNs is the extremely limited lifetime of their sensor nodes, due to their strict energy constraints. This is due to the fact that the batteries that typically supply the energy to the sensor nodes are usually impractical to be either replaced or recharged because sensor nodes are usually located at difficult to reach places. The limited energy sufficiency of sensor nodes is the main cause for the termination of their operation and thus the gradual elimination of the overall lifetime of WSNs. Therefore, the accomplishment of energy saving is an issue of crucial importance for the effective operation of WSNs [18]. This is why energy inefficiencies that exist at each one of the five layers of the protocol stack of sensor nodes have to be eliminated. Actually, a sensor node consumes its energy primarily during its wireless communications and only partially during sensing and data processing [19]. For this reason, there are numerous research efforts that pursue energy conservation at the network layer of the protocol stack through the attainment of energy efficient route setup and consistent data communication among the sensor nodes themselves and the BS. Consequently, the main goal of routing protocols in WSNs is to preserve the energy of sensor nodes, in order to keep them workable for as much time as possible, and thus prolong the network lifetime and retain network connectivity. In addition, given that neighboring sensor nodes may produce similar sensing data, routing protocols in a WSN need to identify and eliminate the redundant information. Furthermore, in WSNs, the most of data flow is directed from many sources to one single sink, while IP based organization is not forthrightly applied and sensor nodes have limited storage and processing capacity. On the other hand, routing protocols proposed for wired networks do not examine issues like the aforementioned. Instead, their primary concern is the achievement of high Quality of Service (QoS). Hence, they are considered to be unsuitable for WSNs. For all aforementioned reasons, numerous protocols for energy efficient routing especially in WSNs have been proposed during the last two decades. Many of them are collectively presented in relevant surveys [20; 21; 22; 23; 24; 25; 26; 27]. For taxonomy purposes, the use of specific categories has been suggested in order to classify protocols. This work both adopts and extends the classification proposed in [22; 23], as shown in Figure 4. According to this, protocols are classified into four categories depending on (i) how the network is structured, (ii) how data are exchanged, (iii) whether location information is used or not, and (iv) whether Quality of Service (QoS) or multiple paths are supported or not. The intention of this article is to provide a collective presentation of characteristic protocols that belong to each one of these categories, both classic and modern ones, highlight their main features, make a comparative study on their advantages and disadvantages, provide a discussion on general and specific considerations raised, and denote future research issues. Compared to previous similar reviews, the work presented in this article, provides not only an up-to-date but also a more exhaustive survey of energy efficient routing protocols. In addition, it both extends the existing taxonomies and considers more performance criteria for the comparative study of the protocols studied. In accordance with this perspective, the rest of this article is organized as follows. In Section 2, protocols classified according to Communication Model are presented. In Section 3, protocols classified according to Network Structure are described. Section 4 refers to protocols classified according to Topology. Section 5 analyzes Reliable Routing protocols. In Section 6, main findings in this survey are discussed and concluding remarks are extracted. Finally, Section 7 identifies open research issues.