ERTP: Energy-efficient and Reliable Transport Protocol for data streaming in Wireless Sensor Networks

Abstract

Emerging data streaming applications in Wireless Sensor Networks require reliable and energy-efficient Transport Protocols. Our recent Wireless Sensor Network deployment in the Burdekin delta, Australia, for water monitoring [T. Le Dinh, W. Hu, P. Sikka, P. Corke, L. Overs, S. Brosnan, Design and deployment of a remote robust sensor network: experiences from an outdoor water quality monitoring network, in: Second IEEE Workshop on Practical Issues in Building Sensor Network Applications (SenseApp 2007), Dublin, Ireland, 2007] is one such example. This application involves streaming sensed data such as pressure, water flow rate, and salinity periodically from many scattered sensors to the sink node which in turn relays them via an IP network to a remote site for archiving, processing, and presentation. While latency is not a primary concern in this class of application (the sampling rate is usually in terms of minutes or hours), energy-efficiency is. Continuous long-term operation and reliable delivery of the sensed data to the sink are also desirable.

This paper proposes ERTP, an Energy-efficient and Reliable Transport Protocol for Wireless Sensor Networks. ERTP is designed for data streaming applications, in which sensor readings are transmitted from one or more sensor sources to a base station (or sink). ERTP uses a statistical reliability metric which ensures the number of data packets delivered to the sink exceeds the defined threshold. Our extensive discrete event simulations and experimental evaluations show that ERTP is significantly more energy-efficient than current approaches and can reduce energy consumption by more than 45% when compared to current approaches. Consequently, sensor nodes are more energy-efficient and the lifespan of the unattended WSN is increased.

Introduction

Many applications in Wireless Sensor Networks (WSNs) produce streaming data [19], [22], [23], [24]. In this class of applications, each sensor node periodically samples and relays the sensed data to a central data collection node, referred to as the base station or the sink. In such applications, two important requirements are: end-to-end reliability and long-term operation. A data packet needs to be reliably relayed to the sink. Typically, the end-to-end transmission latency is not a primary concern in this class of applications, but energy-efficiency is. WSNs are expected to operate independently for weeks, or even for months. Our recent Wireless Sensor Network deployment in the Burdekin delta, Australia, for water monitoring [19] is one such example. Our application involves streaming sensed data such as pressure, water flow rate, and salinity periodically from many scattered sensors to the sink node which in turn gets relayed via an IP network to a remote site for archiving, processing and presentation. In our deployment, we observed that the channel quality of radio links between sensor nodes is unreliable and the packet error rate in each link varied considerably over time [19]. Similar observation has also been reported in the literature [16], [17], [25].

Often, the reliability requirements for data streaming applications are not absolute but rather statistical in their nature. That is, the reliability is determined by the quantity of data packets delivered to the sink rather than the reliability of each data packet. For example, in the Burdekin deployment, we would like to study sensor readings over a reasonable scale of time such as a week or a day, but not over the scale of a minute or a second. In fact, we require that at least 75% of data packets per day from each sensor node are received at the sink [19] for offline processing. Using a statistical reliability metric when designing a reliable transport protocol guarantees delivery of enough information to the users, and also reduces the number of transmissions when compared to an absolute reliability metric. Previous studies in [8], [20] have also shown that the statistical reliability approach can significantly reduce energy consumption.

While many transport protocols for WSNs have been studied in the literature, most of them do not address both requirements, i.e., transmission reliability and energy-efficiency, for data streaming applications. Current work focuses on either providing reliability for data transmission, or minimizing energy consumption, but not both. In this paper, we discuss the design and implementation of an Energy-efficient Transport Protocol (ERTP) that ensures statistically reliable delivery of sensor data to the sink for data streaming applications in WSNs. To reduce energy consumption, ERTP achieves end-to-end reliability by controlling the reliability at each hop dynamically. ERTP uses Stop-and-Wait Hop-by-Hop Implicit Acknowledgment (SW HBH iACK) for loss recovery. In wireless links, the transmitter can overhear forwarding transmissions and interprets them as iACKs. Obviously, when a packet reaches the sink, there will be no further forwarding so the sink node needs to send an explicit ACK (eACK). The transmitter retransmits the packet if, after a certain timeout, no acknowledgment has been received. The primary contributions of the paper are summarized as follows:

• We present an analysis of the trade-off between energy consumption and end-to-end reliability for ERTP, in which HBH iACK approach and duplicate detection are used at each sensor node. To balance energy consumption and reliability, ERTP dynamically controls the maximum number of retransmissions at each sensor node.

• We propose a distributed algorithm for retransmission timeout estimation in ERTP. Determining how long the node should wait for an iACK is non-trivial since iACK timeout depends on the time it takes a packet to be forwarded by the downstream node. The simulation results in Section 4 show that the proposed retransmission timeout algorithm is significantly more energy-efficient than other approaches. To the best of our knowledge, ours is the first work which investigates adaptive retransmission timeout estimation for the class of HBH iACK protocols in WSNs.

• We design, implement, and evaluate ERTP in TinyOS [6] for real-world sensor networks. Our extensive evaluations show that ERTP can reduce energy consumption by more than 45% when compared to current approaches. Consequently, sensor nodes are more energy-efficient and the lifespan of the unattended WSN is increased.

The remainder of the paper is organized as follows. Related studies are described in Section 2. The protocol details are described in Section 3. The simulation and implementation results are presented in Sections 4 Simulation, 5 Implementation and experimental evaluation. The paper is concluded in Section 6.