# Data Gathering Techniques in WSN: A Cross-Layer View
## 2 WSN Architecture--Arising Platforms and Novel Infrastructure Concepts
### Application-Oriented  
Many sensor platforms are application-oriented. Occasionally, their suggested architecture can be applied to other applications; however, their design and evaluation are typically aimed at a specific one. Hence, in many cases, both hardware and software technological developments are introduced for effective functioning. One of the most common tasks of WSN is the obvious one of monitoring a terrain. There are many variants of WSN monitoring. For example, the requirement can be to monitor every point in the Field of Interest (FoI) vs. monitoring a limited number of specific locations or targets (aka target coverage) vs. just monitoring a border of a region to detect intruders (aka barrier coverage). The coverage problem typically involves selecting a subset of sensors that fulfill  
Figure 2: Schematic description highlighting the main topics covered in the section. the monitoring objective while maintaining network connectivity. The sensors' capabilities and the monitoring objective determine the network topology. We present several recent examples that mainly concentrate on connectivity and data gathering under the constraints of the monitoring objective. [PERSON] et al. [25] focus on energy-efficient data gathering in target coverage problem, in which an \(n\) sensor WSN needs to monitor \(T\) specific targets, and there exists a route (multi-hop) from each source to the sink. The paper assumes that the source nodes that sense the targets and initiate data packets into the network are known, and deals with the forwarding of these packets to the sink. The paper proposes a distributed data gathering algorithm in which after each node discovers its neighbors and their hop-count to the sink, it will forward data packets (when required) to its neighbor with maximum remaining energy and a lower hop count to the sink (the remaining energy is assumed to be known). [PERSON] [26] focuses on the \(k\)_-coverage_ problem in which each point in the FoI is required to be covered by at least \(k\) sensors at any time, and each active sensor participating in the monitoring task is required to be connected to the sink (possibly via a multi-hop route). The paper assumes that the sensors are heterogeneous (they do not have the same characteristics) and mobile, hence the sensors can move toward any region of interest in the deployment field to participate in any deficient \(k\)_-coverage_ area and can also act as mobile proxy sinks that collect sensed data from the sensors and deliver them to the sink. [PERSON] [26] partitions the problem into two problems which are solved sequentially. Namely, the _mobile \(k\)-coverage_ problem, which selects a minimum subset of active sensors that solve the \(k\)_-coverage_ problem and the _data gathering_ problem, and devise a forwarding scheme from the active sensors to the sink such that the energy consumption due to sensor mobility and communication is minimized. [PERSON] and [PERSON] [27] utilize mobile sensors to achieve area coverage. These mobile sensors can be moved and relocated to cover holes after the random deployment. The authors suggest a two-phase approach. According to the first one, the monitoring area after the initial random deployment is identified (by the BS), and mobile nodes are relocated to cover the monitoring holes detected after the initial deployment, trying to ensure full coverage of the AoI by the static and relocated sensors. At the second stage, the proposed algorithm schedules the sensors' activity (awakening and transmission times) that minimizes the energy consumption of the nodes while collecting and sending data to the base station. To this end, the paper distinguishes between \"normal\" nodes and cluster heads. A survey that reviews algorithms and techniques related to the connectivity-coverage issues in WSN can be found in [PERSON] and [PERSON] [28]. Occasionally, WSN architectures and designs are more application-oriented. For example, [PERSON] et al.