QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel. <s> BIB001 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> Sensor networks are distributed event-based systems that differ from traditional communication networks in several ways: sensor networks have severe energy constraints, redundant low-rate data, and many-to-one flows. Data-centric mechanisms that perform in-network aggregation of data are needed in this setting for energy-efficient information flow. In this paper we model data-centric routing and compare its performance with traditional end-to-end routing schemes. We examine the impact of source-destination placement and communication network density on the energy costs and delay associated with data aggregation. We show that data-centric routing offers significant performance gains across a wide range of operational scenarios. We also examine the complexity of optimal data aggregation, showing that although it is an NP-hard problem in general, there exist useful polynomial-time special cases. <s> BIB002 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> Underwater sensor nodes will find applications in oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation and tactical surveillance applications. Moreover, unmanned or autonomous underwater vehicles (UUVs, AUVs), equipped with sensors, will enable the exploration of natural undersea resources and gathering of scientific data in collaborative monitoring missions. Underwater acoustic networking is the enabling technology for these applications. Underwater networks consist of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. In this paper, several fundamental key aspects of underwater acoustic communications are investigated. Different architectures for two-dimensional and three-dimensional underwater sensor networks are discussed, and the characteristics of the underwater channel are detailed. The main challenges for the development of efficient networking solutions posed by the underwater environment are detailed and a cross-layer approach to the integration of all communication functionalities is suggested. Furthermore, open research issues are discussed and possible solution approaches are outlined. � 2005 Published by Elsevier B.V. <s> BIB003 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> This paper discusses hardware design principles for long-term solar-powered wireless sensor networks. We argue that the assumptions and principles appropriate for long-term operation from primary cells are quite different from the solar power case with its abundant energy and regular charging cycles. We present data from a long-term deployment that illustrates the use of solar energy and rechargeable batteries to achieve 24x7 operation for over two years, since March 2005. <s> BIB004 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> This work introduces the concept of a Wireless Underground Sensor Network (WUSN). WUSNs can be used to monitor a variety of conditions, such as soil properties for agricultural applications and toxic substances for environmental monitoring. Unlike existing methods of monitoring underground conditions, which rely on buried sensors connected via wire to the surface, WUSN devices are deployed completely belowground and do not require any wired connections. Each device contains all necessary sensors, memory, a processor, a radio, an antenna, and a power source. This makes their deployment much simpler than existing underground sensing solutions. Wireless communication within a dense substance such as soil or rock is, however, significantly more challenging than through air. This factor, combined with the necessity to conserve energy due to the difficulty of unearthing and recharging WUSN devices, requires that communication protocols be redesigned to be as efficient as possible. This work provides an extensive overview of applications and design challenges for WUSNs, challenges for the underground communication channel including methods for predicting path losses in an underground link, and challenges at each layer of the communication protocol stack. � 2006 Elsevier B.V. All rights reserved. <s> BIB005 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> This paper describes a systematic approach to building micro-solar power subsystems for wireless sensor network nodes. Our approach composes models of the basic pieces - solar panels, regulators, energy storage elements, and application loads - to appropriately select and size the components. We demonstrate our approach in the context of a microclimate monitoring project through the design of the node, micro-solar subsystem, and network, which is deployed in a challenging, deep forest setting. We evaluate our deployment by analyzing the effects of the range of solar profiles experienced across the network. <s> BIB006 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> Wireless underground sensor networks (WUSNs) constitute one of the promising application areas of the recently developed wireless sensor networking techniques. The main difference between WUSNs and the terrestrial wireless sensor networks is the communication medium. The propagation characteristics of electromagnetic (EM) waves in soil and the significant differences between propagation in air prevent a straightforward characterization of the underground wireless channel. To this end, in this paper, advanced channel models are derived to characterize the underground wireless channel and the foundational issues for efficient communication through soil are discussed. In particular, the underground communication channel is modeled considering not only the propagation of EM waves in soil, but also other effects such as multipath, soil composition, soil moisture, and burial depth. The propagation characteristics are investigated through simulation results of path loss between two underground sensors. Moreover, based on the proposed channel model, the resulting bit error rate is analyzed for different network and soil parameters. Furthermore, the effects of variations in soil moisture are investigated through field measurement results. The theoretical analysis and the simulation results prove the feasibility of wireless communication in underground environment and highlight several important aspects in this field. This work will lead to the provision of a generic framework for underground wireless communication and the realization of WUSNs. <s> BIB007 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS challenges in WSNs <s> This paper investigates the probabilistic connectivity of the Wireless Underground Sensor Networks (WUSNs). Due to the unique channel characteristics and the heterogeneous network architecture of the WUSNs, the connectivity analysis is much more complicated than in the terrestrial wireless sensor networks and ad hoc networks. To our knowledge, this connectivity problem in WUSNs was not addressed before. In this paper, a mathematical model is developed to analyze the probabilistic connectivity in WUSNs, which captures the effects of environmental parameters such as the soil moisture and soil composition, and system parameters such as the operating frequency, the sensor burial depth, the sink antenna height, the density of the sensor devices, the tolerable latency of the networks, and the number and the mobility of the above- ground sinks. The lower and upper bounds for the connectivity probability are derived analytically. Simulation studies are performed, where the theoretical bounds are validated, and the effects of several environmental and system parameters on the performance of these networks are investigated. <s> BIB008
WSNs inherit most of the well-known QoS challenges from traditional wireless networks, such as time varying channels and unreliable links  . However, typical characteristics of WSNs, such as severe resource constraints and harsh environmental conditions, pose additional unique challenges for QoS-support. These QoS challenges for WSNs are explained in this section: Resource constraints: WSNs lack of bandwidth, memory, energy and processing capability. However, limited energy is the most crucial one since in many scenarios it is impossible or impractical to replace or recharge batteries of the sensor nodes. Although energy harvesting via solar energy BIB004 BIB006 seems to be a promising solution to energy scarcity, present solar panels are still too large for tiny sensor devices. Eventually, proposed QoS support mechanisms must be lightweight and simple in order to operate on a highly resource constrained sensor node. Node deployment: Deployment of the sensor nodes may be either deterministic or random. In deterministic deployment, sensor nodes are placed by hand and routing can be performed through pre-scheduled paths. In a random deployment, sensor nodes are deployed randomly and organize themselves in an ad hoc manner. Hence, neighbor discovery, path discovery, geographical information of the nodes and clustering are the issues to be solved. Topology changes: Node mobility, link failures, node malfunctioning, energy depletion or natural events like flood or fire can cause topology changes. Moreover, most of the link layer or MAC layer protocols employ sleep-listen schedules and turn the radio of the sensor nodes off temporarily for energy saving. This kind of power management mechanisms also cause frequent topology changes. Inevitably, dynamic nature of the WSN topology introduces an extra challenge for QoS support. Data redundancy: WSNs comprise a large amount of tiny sensor nodes and hence, observed event or phenomena can be detected by several sensor nodes. Although this redundancy helps reliable data transfer, it also causes unnecessary data delivery in the network which consequently yields to congestion. Data aggregation/fusion BIB002  mechanisms may decrease the redundancy but also may introduce additional delay and complexity in the system. Therefore, effective QoS mechanisms are needed to cope with the data redundancy. Multiple traffic types: Sensor nodes which have the capability of sensing or observing various phenomena can generate different types of traffic. For instance, streaming multimedia and location of a detected target or periodic temperature information of an area might be carried at the same time for a specific application. Therefore, applications requiring existence of multiple traffic classes add extra challenging issues to QoS support since requirements of traffic classes differ from each other. Real-time traffic: In some critical applications like natural disaster monitoring or security surveillance, gathered data is valid only for a limited time frame and has to be delivered before its deadline. This type of critical real-time data must be handled by adequate QoS mechanisms. Unbalanced traffic: In a WSN, there is usually a central entity (sometimes multiple of them) that obtains the global view of the sensing environment called the sink node and there may exist middle layer entities for data aggregation and compression named as cluster heads. Therefore, unbalanced traffic flows from sensor nodes to sink nodes or cluster heads are commonly observed in WSNs. Moreover, event-driven applications mostly cause sporadic changes in the traffic pattern in case of event detection. Although smart routing protocols may share the traffic load between different routes, MAC protocol still has to accommodate unbalanced and bursty traffic. Scalability: Most of the WSNs are composed of hundreds or thousands of sensor nodes. As the area of interest or requirements for the quality of observation increase, more sensor nodes need to be deployed. Therefore, designed QoS mechanism must scale well with highly dense or large scale networks. Together with successful deployment examples of traditional terrestrial sensor networks, researchers started to work on using sensor networks in different environments such as underwater and underground. Both Underwater Acoustic Sensor Networks (UW-ASNs) BIB003 and Wireless Underground Sensor Networks (WUSNs) BIB005 differ from traditional terrestrial sensor networks since they operate in diverse environments and communicate through totally different mediums. The diversities in the operating environment and the communication medium have significant effects on the network itself and therefore, pose some additional challenges for QoS support. Except the ones inherited from traditional terrestrial sensor networks, those additional QoS challenges for UW-ASNs and WUSNs can be listed as follows: Underwater/underground channel: Both underwater BIB001 and underground BIB007 BIB008 channels show significant spatial and temporal differences. Also, the propagation delays in underwater and underground are five orders of magnitude higher than the traditional terrestrial channels. Hence, designed QoS mechanisms must take the highly dynamic nature of the channel into account. Higher error rates: High bit error rates (BER) can be experienced due to high communication medium density for both water  and soil. Moreover, connectivity losses occur more frequently due to heavy multipath and fading. Therefore, effective error control mechanisms must be integrated to achieve acceptable level of BER. Extreme environmental conditions: Extreme characteristics of both underwater and underground environments make sensor devices more prone to corrosion and malfunctioning which shortens the network lifetime and decreases the level of reliability. In order to cope with these problems, derived QoS mechanisms must take extreme environmental conditions into account and take the necessary measures beforehand. In WSNs, sensor nodes are generally assumed to be static. However, some recent applications of WSNs, such as medical care and disaster response, utilize mobile sensor nodes and mobility poses another set of unique challenges to be addressed which include topology management, routing, energy management. Since the neighborhood of a node changes frequently due to the mobility, the topology and spatial density of the network also change frequently. Hence, QoS provisioning in mobile sensor networks becomes a more challenging task since envisioned methods must handle highly dynamic node connectivity and density. WSN related challenging issues are highlighted. These challenges make it difficult for providing deterministic QoS guarantees, such as strict bounds on packet delays, guaranteed bandwidth or packet losses in WSNs. However, providing different services for different traffic classes in spite of these challenges are still feasible as we further discuss in the rest of the paper. These mentioned challenging factors must be taken into account during the design of new QoS-support mechanisms and novel techniques have to be adopted in order to cope with them.