– Draft – ( DO NOT DISTRIBUTE ) Wireless Sensor Networks : A Survey Revisited

With the recent advances in Micro ElectroMechanical Systems (MEMS) technology and wireless communications; the implementation of lowcost, lowpower, multifunctional sensor nodes that are small in size and communicate untethered in short distances has become feasible. The ever-increasing capabilities of these tiny sensor nodes enable the realization of wireless sensor networks (WSN) based on the collaborative effort of a large number of nodes. However, in order to realize the existing and envisioned applications and hence take the advantages of the potential gains of WSN necessitate effective communication protocols which can address the unique challenges posed by the WSN paradigm. Since the time these challenges had been been first pointed out in the literature, there has been a great deal of research effort focused on addressing them. Furthermore, the promising results of the research efforts since then have enabled the development and realization of practical sensor network deployment scenarios. In this paper, a survey of the applications, developed communication protocols, and real deployment scenarios proposed thus far for WSN is revisited. The objective of this survey revisit is to provide a contemporary look at the current state-of-the-art in WSN and discuss the still-open research issues in this field. I. I NTRODUCTION With the recent advances in Micro Electro-Mechanical Systems(MEMS) technology, wireless communications, and digital electronics; the construction of low-cost, low-power, multifunctional sensor nodes that are small in size and communicate untethered in short distances has become feasible. The ever-increasing capabilities of these tiny sensor nodes, which consist of sensing, data processing, and communicating components, enable the realization of wireless sensor networks (WSN) based on the collaborative effort of a large number of nodes. Wireless Sensor Networks have a wide range of applications such as environmental monitoring [187], biomedical research [166], human imaging and tracking [53], [54], and military applications [117]. In accordance with our vision [3], WSN are slowly becoming an integral part of our lives. Recently, considerable amount of research efforts have enabled the actual implementation of sensor networks tailored to the unique requirements of certain sensing and monitoring applications. In order to realize the existing and potential applications for WSNs, sophisticated and extremely efficient communication protocols are required. WSNs are composed of a large number of sensor nodes, which are COMPUTER NETWORKS JOURNAL (ELSEVIER SCIENCE) 2 densely deployed either inside a physical phenomenon or very close to it. In order to enable reliable and efficient observation and initiate right actions, physical phenomenon features should be reliably detected/estimated from the collective information provided by sensor nodes [3]. Moreover, instead of sending the raw data to the nodes responsible for the fusion, sensor nodes use their processing abilities to locally carry out simple computations and transmit only the required and partially processed data. Hence, these properties of WSN impose unique challenges for development of communication protocols in such an architecture. The intrinsic properties of individual sensor nodes, pose additional challenges to the communication protocols in terms of energy consumption. As will be explained in the following sections, WSN applications and communication protocols are mainly tailored to provide high energy efficiency. Sensor nodes carry limited, generally irreplaceable power sources. Therefore, while traditional networks aim to achieve high Quality of Service(QoS) levels, sensor network protocols focus primarily on power conservation. Moreover, the deployment of the WSN is another constraint that is considered in developing WSN protocols. The position of sensor nodes need not be engineered or pre-determined. This allows random deployment in inaccessible terrains or disaster relief operations. On the other hand, the random deployment constraints of WSN result in self-organizing protocols to emerge in the WSN protocol stack. In addition to the placement of nodes, the density in the network is also exploited in WSN protocols. Since generally, large number of sensor nodes are densely deployed in WSN, neighbor nodes may be very close to each other. Hence, multihop communication in sensor networks is exploited in communication between nodes since it leads to less power consumption than the traditional single hop communication. Furthermore, the dense deployment coupled with the physical properties of the sensed phenomenon introduce correlation in spatial and temporal domain. As a result, the spatio-temporal correlation-based protocols emerged for improved efficiency in networking wireless sensors. After the first and the most comprehensive survey on WSN [3] which was published three years ago, the research on the unique challenges of WSN has accelerated significantly. The promising results of the existing research that has been developed in the last three years have enabled the development and production of mature products, which have eventually created a brand new market empowered by the WSN phenomenon. Throughout these three years, the deployment of WSN has become a reality. Consequently, research community has gained significant experiences out of these WSN deployment cases. Furthermore, many researchers are currently engaged in developing schemes that address he unique challenges of WSN. In this paper, we present a survey of existing products, developed protocols, and r search on algorithms proposed thus far for WSN. Our aim is to provide a contemporary look at the current state of the art in WSN since its initial steps [3] and discuss the still-open research issues in this field. The remainder of the paper is organized as follows. In Section II, we present existing applications and ongoing research efforts on some sensor network applications which show the usefulness of sensor networks. The existing work on the WSN protocol stack is surveyed in Sections VI, VII, VIII, and IX for transport, routing, data link and physical layers, respectively. Moreover, the open research issues are discussed for each of the protocol layer. Furthermore, the synchronization and localization problems in WSN are investigated in Section XI and Section X, respectively, along with the existing olutions and open research issues. The existing evaluation approaches for WSN including physical testbeds and software simulation environments are overviewed in Section XIII. We conclude our paper in Section XIV. II. W IRELESSSENSORNETWORK APPLICATIONS The emergence of WSN paradigm has triggered extensive research on many aspects of the sensor networking. However, the applicability of sensor networks has long been discussed with the emphasis on potential applications that can be realized using wireless sensor networks. In this section, we present the existing commercial and academic products using the sensor networking concept and provide an extensive survey on the existing applications of WSN. It has been stated that WSNs may consist of many different types of sensors such as seismic, low sampling rate magnetic, thermal, visual, infrared, acoustic and radar, which are able to monitor a wide variety f ambient conditions [3]. As a result, wide range of applications are possible using the WSN paradigm. This spectrum of applications includes homeland security, monitoring of space assets for potential and man-made threats in space, ground-based monitoring on both land nd water, defense intelligence gathering, environmental monitoring, urban warfare, weather and climate analysis nd prediction, battlesphere monitoring and surveillance, exploration of the solar system and beyond, monitoring of seismic acceleration, strain, temperature, wind speed a d GPS data. The heterogeneity in the available sensor technologies and applications, hence, requires a common standardCOMPUTER NETWORKS JOURNAL (ELSEVIER SCIENCE) 3 ization to achieve the practicality of sensor networks applications for industrial purposes. For this purpose, IEEE 802.15.4 [81] standards body is formed for a specification for low data rate wireless transceiver technology with long battery life and very low complexity. Three different bands are chosen for communication, i.e., 2.4GHz (global), 915Mhz (Americas) and 868Mhz (Europe). While the PHY layer uses BPSK in the 868/915 MHZ bands and O-QBPSK at 2.4 GHz band, the MAC layer provides communication for star, mesh and clustertree based topologies with controllers. The transmission range of the nodes is assumed to be 10-100m with data rates 20Kbps to 250Kbps [80]. Applications for the IEEE 802.15.4 standard include sensor networks, industrial sensing and control devices, building and home automation products, and even networked toys. Along with IEEE 802.15.4, ZigBee [223], an international, non-profit industry consortium of leading semiconductor manufacturers, and technology providers has been formed. ZigBee was created to address the market need for cost-effective, standards-based wireless networking solutions that support low data rates, low power consumption, security and reliability [223]. Moreover, Wireless Industrial Networking Alliance (WINA) was formed in 2003 to stimulate the development and promote the adoption of wireless networking technologies and practices to help increase industrial efficiency. As a first step, this ad hoc group of suppliers and endusers is working to define end-user needs and priorities for industrial wireless systems [206]. It is widely recognized that standards such as Bluetooth and WLAN are not suited for low power sensor applications. On the other hand, standardization attempts such as ZigBee and WINA, which specifically address the typical needs of wireless control and monitoring applications, will enable rapid improvement of WSN in the