# Sink-Type-Dependent Data-Gathering Frameworks in Wireless Sensor Networks: A Comparative Study
###### Abstract
Unlike a ground-based mobile sink, an aerial mobile sink can fly along a guided route within a shorter time due to its three-dimensional movement capability [36]. These frameworks have the ability to perform without the help of any infrastructure, which enables data gathering from inaccessible terrain or even from sea surfaces [37]. Network congestion decreases as the aerial sink works as a data mule. Every time it collects and delivers data from the sensors to the base station (BS), the aerial mobile robot can be recharged. Considering these advantages, many researchers have proposed aerial robot-like, UAV-based data-collection optimization in WSNs [38; 39]. Hybrid approaches have also been reported, which assume the presence of a static infrastructure in addition to a mobile sink. Instead of an aerial sink moving toward all the sensor nodes to collect data, one leader node is chosen by utilizing clustering or chain construction techniques. An aerial sink can fly directly to the WSN deployed zone and collect data from the cluster head (CH) or leader node [40]. Several review studies have been published on data-gathering techniques in WSNs. In [41], five energy-efficient data-gathering protocols for UAV-aided WSNs are discussed and compared qualitatively, mostly focusing on the main ideas and performance evaluation techniques. Focusing on the energy efficiency of WSNs, various data-collection approaches are discussed [42; 43]. However, they did not consider mobile robot-based data collection. An overview of the mobile robot-based data-gathering process in WSNs and the corresponding issues and challenges are provided in [44]. The necessity of mobile robot trajectory optimization in the performance improvement of WSNs is discussed in [1] based on applications, trajectory techniques, and domains used to formulate the trajectory. Various data-gathering techniques in WSNs are presented, focusing on the optimization of important performance metrics such as network lifetime, latency, data accuracy, and energy consumption. A classification of mobile sink-based data dissemination strategies in WSNs along with their advantages and disadvantages is discussed in [45]. Considering data gathering based on a mobile sink, a survey on routing protocols in this area is presented in [46]. A range of techniques related to mobile robots in WSNs is reviewed in [47]. A qualitative review of fast and efficient mobile robot-based data-collection schemes in WSNs is provided in [48]. A survey on aerial mobile sink-based WSNs is presented in [49] in terms of different aspects of the integration of a mobile sink and a WSN. However, none of these surveys compare static- and mobile-sink-based data-gathering techniques. In addition, most of the survey works have been carried out based on qualitative performance analysis. Although a simulation-based analysis of the energy efficiency of WSNs with static and mobile sinks is described in [50], the aerial mobile robot-based sink is not explicitly considered. The main ideas of the exiting surveys on data gathering in WSNs are summarized in Table 1. However, there has been no quantitative performance analysis published to date, which has compared the performances of data-gathering schemes in WSNs considering the three types of sinks (i.e., static, ground mobile, and aerial mobile sinks). On the contrary, our paper reviews such data-gathering frameworks based on the three different types of sinks, analyzing the data-gathering frameworks both qualitatively and quantitatively. The use of WSNs is increasing daily. However, collecting sensor data remains a major challenge. The lifetime of the WSN depends on the manner in which the perceived data are collected. Depending on the sink type, the lifetime of the sensor nodes and the quality of services (QoS) parameters of the network will vary. Therefore, suitable application scenarios and expected outcomes should be analyzed before choosing the sink type. Static sink-based solutions can be suitable options for long-term implementation.