# Data Gathering Techniques in WSN: A Cross-Layer View
## 7 Concluding Remarks  
Data gathering in modern WSN and IoT networks encompasses many challenges, which span the entire communication stack. Many techniques, protocols, and solutions have been proposed over the years, but as technology advances, new challenges and new opportunities arise. In this survey, we reviewed these main challenges and opportunities, as well as recent advances, and related them to specific data-gathering research domains. We provided a comprehensive state-of-the-art data-gathering literature review in modern WSN networks, distinguishing between the communication layers and the research domain. We first summarized general architectural novelties and emerging architectures. We reviewed several new technological advances and their influence on sensing device design, the platform carrying it, and the transceiver. We reviewed the effect of these architectural advantages on specific applications, such as agriculture, smart cities, smart homes. We showed how cloud computing drives new WSN types, which introduces new directions, and discussed how WSN can coexist within social network domains. Compressed sensing was summarized next. We provided an overview of this important scheme and reviewed its utilization in WSNs. We proceeded with the MAC layer. Since the performance of many innovations in the higher layers rely on the underlay MAC protocol and since many data gathering schemes utilize traditional WSN MAC protocols, we provided an overview of these traditional protocols and mentioned state-of-the-art works in WSN and IoT. These works devise new MAC approaches for data gathering. Next, we covered the recent advances in routing. Similarly, we opened with an overview of the traditional routing protocols utilized for data-gathering and reviewed several recent enhancements. We reviewed the utilization of network coding for data gathering and explored the facilitation of UAV and mobile-sink in collecting the data from the sensors. Lastly, we turned our attention to the area of wearables which opens new research horizons for human health and activity surveillance and discussed the new paradigm of Wireless Body Area Networks. In the spirit of the times, we concluded with several studies that utilize some of the techniques discussed in this survey to aid in combating the COVID pandemic. While we provided the general background to the research areas we covered, we mainly focused on cutting-edge research works. Yet we note that even seemingly exhausted topics, such as MAC and Routing protocols, provide new technological developments and present opportunities for new research domains. As implied throughout this survey, there are several research areas that attract a lot of attention and anticipation for future developments. For example, technological innovations in manufacturing more compact sensing units with yet superior transmission, reception and processing capabilities are extremely in demand in several disciplines which include wearables, smart homes, IoT-related domains and others. All the more so, this is relevant when dealing with healthcare applications and implants. Such technological innovations will require in turn enhancements to other domains across the communication stack in order to adjust to the new opportunities and limitations. Energy acquisition is still a fruitful research domain. In this respect, finding new sources of energy harvesting (EH), better utilization of existing energy resources and energy storage are still challenging research fields. Similarly, new EH methods impose multiple new challenges on the entire protocol stack, which are correlated with the EH method, e.g., different EH methods dictate constraints on the MAC design, which in turn, impact the routing protocol which affects the performance end eventually the application utilizing the infrastructure.