A survey of transport protocols for wireless sensor networks <s> Performance Metrics <s> Wireless sensor networks (WSN) are event based systems that rely on the collective effort of several microsensor nodes. Reliable event detection at the sink is based on collective information provided by source nodes and not on any individual report. Hence, conventional end-to-end reliability definitions and solutions are inapplicable in the WSN regime and would only lead to a waste of scarce sensor resources. However, the absence of reliable transport altogether can seriously impair event detection. Hence, the WSN paradigm necessitates a collective phevent-to-sink reliability notion rather than the traditional end-to-end notion. To the best of our knowledge, reliable transport in WSN has not been studied from this perspective before.In order to address this need, a new reliable transport scheme for WSN, the event-to-sink reliable transport (ESRT) protocol, is presented in this paper. ESRT is a novel transport solution developed to achieve reliable event detection in WSN with minimum energy expenditure. It includes a congestion control component that serves the dual purpose of achieving reliability and conserving energy. Importantly, the algorithms of ESRT mainly run on the sink, with minimal functionality required at resource constrained sensor nodes. ESRT protocol operation is determined by the current network state based on the reliability achieved and congestion condition in the network. If the event-to-sink reliability is lower than required, ESRT adjusts the reporting frequency of source nodes aggressively in order to reach the target reliability level as soon as possible. If the reliability is higher than required, then ESRT reduces the reporting frequency conservatively in order to conserve energy while still maintaining reliability. This self-configuring nature of ESRT makes it robust to random, dynamic topology in WSN. Analytical performance evaluation and simulation results show that ESRT converges to the desired reliability with minimum energy expenditure, starting from any initial network state. <s> BIB001 </s> A survey of transport protocols for wireless sensor networks <s> Performance Metrics <s> There exist several applications of sensor networks wherere liability of data delivery can be critical. While the redundancy inherent in a sensor network might increase the degree of reliability, it by no means can provide any guaranteed reliability semantics. In this paper, we consider the problem of reliable sink-to-sensors data delivery. We first identify several fundamental challenges that need to be addressed, and are unique to a wireless sensor network environment. We then propose a scalable framework for reliable downstream data delivery that is specifically designed to both address and leverage the characteristics of a wireless sensor network, while achieving the reliability in an efficient manner. Through ns2 based simulations, we evaluate the proposed framework. <s> BIB002
Transport protocols for WSNs should provide end-to-end reliability and end-to-end QoS in an energy-efficient manner. Performance of transport protocols for WSNs can be evaluated using metrics such as energy efficiency, reliability, QoS (e.g., packet-loss ratio, packet-delivery latency), and fairness. Energy Efficiency -Sensor nodes have limited energy. As a result, it is important for the transport protocols to maintain high energy efficiency in order to maximize system lifetime. Packet loss in WSNs can be common due to bit error and/or congestion. For loss-sensitive applications, packet loss leads to retransmission and the inevitable consumption of additional battery power. Therefore, several factors need to be carefully considered, including the number of packet retransmissions, the distance (e.g., hop) for each retransmission, and the overhead associated with control messages. Reliability -Reliability in WSNs can be classified into the following categories: • Packet reliability: Applications are loss-sensitive and require successful transmission of all packets or at a certain success ratio. • Event reliability BIB001 : Applications require only successful event detection, but not successful transmission of all packets. In addition, BIB002 defines destination-related reliability. For example, messages might need to be delivered to sensor nodes in a specific subarea or to the nodes that cover a specific subarea, or to nodes that are equipped with a particular sensor type. QoS Metrics -QoS metrics include bandwidth, latency or delay, and packet-loss ratio. Depending on the application, these metrics or their variants could be used for WSNs. For example, sensor nodes may be used to transmit continuous images for target tracking. These nodes generate high-speed data streams and require higher bandwidth than most eventbased applications. For a delay-sensitive application, WSNs may also require timely delivery data. Fairness -Sensor nodes are scattered in a geographical area. Due to the many-to-one convergent nature of upstream traffic, it is difficult for sensor nodes that are far away from the sink to transmit data. Therefore, transport protocols need to allocate bandwidth fairly among all sensor nodes so that the sink can obtain a fair amount of data from all the sensor nodes.