# Data Gathering Techniques in WSN: A Cross-Layer View
## 1 Introduction
In particular, the survey comprises the following topics:  
The device's platform, which accommodates the sensing unit, can highly impact the performance of the application utilizing it and specifically the data-gathering application, and vice versa, the application (e.g., data gathering) can impact the platform architecture when designed in application-oriented manner or when some of the essential features and requirements are taken into account in the platform design process. The same mutual effect also applies to the WSN infrastructure and the network architecture (e.g., topology, system organization). We start with reviewing studies pertaining to the general device's platform and infrastructure-related novelties (Section 2). We cover new domains which were only recently exposed to WSN and IoT networks and introduced new opportunities for algorithm design in such networks. Some of these novel technologies have revolutionized the way applications can utilize each particular device as well as the shared network and have enabled new algorithm opportunities and design challenges across the entire protocol stack which we describe throughout this survey. Next follows Section 3 which provides a focused summary of recent advances in compressed sensing--a signal processing technique that can take advantage of sparsity and redundancy of the data. In the context of data gathering procedures, compressed sensing is utilized to reduce report payload at several levels, which include reduction of the sensed data size and the transmitted report size, by pruning the devices that need to send reports and by compressing the combined relayed data before forwarding it toward its destination (the sink). We provide a basic compressed sensing background and review the state-of-the-art in the context of data gathering in WSN. Section 4 considers the medium access control (MAC) sublayer. In wireless communication, channel utilization is critical and broadly influences several performance aspects such as throughput, latency, power utilization, delivery ratio, and more. Over the years, numerous algorithms and protocols have been suggested for wireless channel access in general, and WSNs with their specific characterization in particular. In Section 4, we review only a small fraction of the countless MAC protocols that have been designed for WSNs. We mainly concentrate on protocols that highlight a conceptual approach or trend and review some of the more recent MAC protocols in data gathering in WSN and IoT networks, which address new challenges such as highly dense networks, congested channels, and very limited resources. We ascend the protocol stack and in Section 5 we address routing aspects. As with the MAC sublayer, routing protocols in multi-hop wireless networks have also been extensively studied. We start by providing several milestones in the context of data gathering in WSNs. We continue with more recent studies which mainly include enhancements to the aforementioned protocol, taking into account new challenges such as scalability demands and energy-related advances which present new opportunities yet impose new constraints. We continue with studies that leverage the multi-hop topology to realize a network-codingmechanism. Finally, we discuss a new paradigm that extends the traditional setup in which the sensed data need to be routed toward a static central monitoring station (sink), and utilize a mobile sink (or sinks) that traverse the terrain and can help in collecting the devices' reports. We review several state-of-the-art schemes in this mobile sink paradigm. The final section of this survey is dedicated to wearable technology in the form of smart devices that are attached to the human body to monitor the user and their environment. Wearable technology involves challenges in all the aspects discussed in the previous sections, yet they introduce new opportunities for high-demand applications with unique performance requirements and constraints.