Network-Wide Broadcast Service in Wireless Sensor Networks

Network-Wide Broadcast Service in Wireless Sensor Networks

Feng Wang (Simon Fraser University, Canada) and Jiangchuan Liu (Simon Fraser University, Canada)
DOI: 10.4018/978-1-61520-701-5.ch018
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Abstract

Network-wide Broadcast is one of the most fundamental services in wireless sensor networks (WSNs). It facilitates sensor nodes to propagate messages across the whole network, serving a wide range of higher-level operations and thus being critical to the overall network design. A distinct feature of WSNs is that many sensor nodes alternate between the active state and the dormant state, so as to conserve energy and extend the lifetime of the network. Unfortunately, the impact of such cycles has been largely ignored in existing network-wide broadcast implementations that adopt the common assumption of all sensor nodes being active all over the whole broadcast process. In this chapter, we first provide a brief survey on previous research works on network-wide broadcast services. We then revisit the network-wide broadcast problem by remodeling it with active/dormant cycles and showing the practical lower bounds for the time and message costs, respectively. We also propose an adaptive algorithm named RBS (Reliable Broadcast Service) for dynamic message forwarding scheduling in this context, which enables a reliable and efficient broadcast service with low delay. The performance of the proposed solution is evaluated under diverse network configurations. The results suggest that the proposed solution is close to the lower bounds of both time and forwarding costs, and it well resists to the network size and wireless loss increases.
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Introdcution

Network-wide broadcast is one of the most fundamental services in wireless sensor networks (WSNs) (Akyildiz, Su, Sankarasubramaniam & Cayirci, 2002). It facilitates sensor nodes to propagate messages across the whole network, serving a wide range of higher-level operations: During the networking configuration, control messages may be broadcast from the sink to all the sensor nodes; For data collection, the interest or query messages may be broadcast within the network; Upon observing an event, a sensor node may broadcast a message to coordinate with other sensor nodes for tracking the event and storing the sensed data; to name but a few. Hence, implementing an effective network-wide broadcast service is critical to the overall performance optimization of a WSN.

Flooding and gossiping (Akyildiz, Su, Sankarasubramaniam & Cayirci, 2002) are two commonly used broadcast approaches, though their basic forms are known inefficient. If we assume that all sensor nodes in the network are active during the broadcast process (referred to as all-node-active assumption), ideally every sensor node needs to receive and forward the broadcast message at most once. Significant efforts thus have been made toward enhancing the efficiency of the basic flooding or gossiping, while retaining their robustness in the presence of the error-prone transmissions (Guo, 2004; Kyasanur, Choudhury & Gupta, 2006; Stann, Heidemann, Shroff & Murtaza, 2006).

The all-node-active assumption is valid for wired networks and for many conventional multi-hop wireless networks. It however fails to capture the uniqueness of energy-constrained wireless sensor networks. In a WSN, the sensor nodes are often alternating between the dormant state and the active state (Gu, Hwang, He & Du, 2007; Liu, Zhao, Cheung & Guibas, 2004; Wang, Xing, Zhang, Lu, Pless & Gill, 2003; Yan, He & Stankovic, 2003); in the former, they go to sleep and thus consume little energy, while in the latter, they actively perform sensing tasks and communications, consuming significantly more energy (e.g., 56 mW for IEEE802.15.4 radio plus 6 to 15 mW for Atmel ATmega 128L micro-controller and possible sensing devices on a MicaZ sensor mote). Define duty-cycle as the ratio between the active period and the full active/dormant period. A low duty-cycle WSN clearly has a much longer lifetime for operation, but breaks the all-node-active assumption. In such a network, if the number of sensor nodes is very small, it may be possible to wake up all sensor nodes for message broadcast through global synchronization with the customized active/dormant schedules. For larger scale WSNs, however, synchronization itself remains an open problem. More importantly, the duty-cycle schedules are often optimized for the given application or deployment, and a broadcast service accommodating the schedules is thus expected for the cross-layer optimization of the overall system.

In this chapter, we revisit the network-wide broadcast problem in low duty-cycle WSNs. Their scale, together with their application/deployment-specific duty-cycles, renders the all-node-active assumption impractical. This in turn introduces a series of new challenges toward implementing the network-wide broadcast service. From a local viewpoint, since the neighbors of a node are not active simultaneously, a node would have to forward a message multiple times at different time instances; From a global viewpoint, since the topology is time-varying with no persistent connectivity, if not well-planned, the latency for a broadcast message to reach all sensor nodes can be significantly prolonged. The error-prone wireless links further aggravate these problems. The experiments later in this chapter have shown that, for ultra low duty-cycles (less than 0.4), a conventional broadcast strategy would simply fail to cover all the sensor nodes within an acceptable timeframe.

Key Terms in this Chapter

Wireless Sensor Network: The network that contains sensor nodes for sensing the ambient environment. Each sensor node may also perform tasks such as processing data and communicating with neighboring sensor nodes. The communication is often done by wireless communications. And often there is a base station in the network, where sensing data are sent for further utilization.

Dormant State: The state where a sensor node turns off most of its functioning components so as to greatly reduce energy consumption. In this state, often a count down timer is used so as to wake the sensor node up when its dormant state is finished.

Duty-Cycle: In a wireless sensor network, sensor nodes are often designed to be able to switch between the active state and the dormant state. Duty-cycle is the ratio between the active period and the full active/dormant period.

Network-Wide Broadcast: The communication mode that a host in a network sends a message to all the other hosts in the same network.

Reliability: In the network-wide broadcast scenario, it means the ratio of hosts that have successfully received the broadcast message to all the hosts that are supposed to receive the message when the broadcast process finishes.

Forwarding: The communication mode that a host transmits a message to its neighboring (one-hop) hosts. Due to the broadcast nature of the wireless communication, a message forwarded by a sensor node may be received by all its neighboring sensor nodes.

Active State: The state where a sensor node is fully functioning. In this state, a sensor node may perform tasks such sensing the ambient environment, processing data or communicating with neighboring sensor nodes.

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