**Power Considerations for Sensor Networks**

Khadija Stewart1 and James L. Stewart2 <sup>1</sup>*DePauw Universtity*

<sup>2</sup>*Purdue University USA*

#### **1. Introduction**

Wireless sensor networks (WSNs) are networks composed of small, resource-constrained and collaborative devices. WSNs are used in a plethora of domains including environmental and agricultural monitoring, military operations, in the health care field and in building automation. The three main functions of wireless sensor nodes (also called motes) are to sense the environment, perform computations, store intermediate results and communicate with other motes in the network.

This chapter focuses on power considerations for all aspects of wireless sensor networks. It covers software, hardware and networking aspects of the motes. The main limitation of wireless sensor motes is that they operate on battery power. In many WSN applications, the motes are placed in remote areas and deployed for the lifetime of the network. During this time the only power resource the motes have access to is their battery. An example of such a deployment is the Mount St. Helens project developed to study volcanic activities on Mount St. Helens (were volcanic eruptions can occur at any time with very little warning). The sensors were placed on the mountain using helicopters and work at length to continually sense seismic activity and relay information to a data center. For such applications, the battery lifetime is the main factor that dictates the lifetime of the network. It is therefore imperative to develop wireless sensor mote platforms that minimize the power consumption and/or maximize the lifetime of the network as a whole.

Several works in the literature address one or two aspects of the mote's architecture and/or functionality but to the authors' knowledge, no work has combined all said aspects and addressed them as a homogeneous unit. This chapter studies and analyzes each hardware component of the mote's architecture, all the main protocols used in the mote's stack layer, discusses the work that has been done in terms of reducing the power consumption, increasing the battery lifetime and or increasing the lifetime of the entire network as a whole.

The chapter is organized as follows: Section 2 gives an overview of wireless sensor networks, their applications and general architecture. Section 3 focuses on the hardware architecture of the motes (the CPU, communication infrastructure, memory and sensors). Section 4 introduces the layered protocol stack of the sensor motes (application, transport, network, link and physical layers). Section 5 summarizes the chapter and suggest paths forward.

organizations to provide a real time data collection system that can forecast floods (ALERT,

Power Considerations for Sensor Networks 131

Another use for WSNs is in intelligent building management. In fact, they have been used in HVAC, lighting, climate control, fire protection, energy monitoring and security applications among others. In Canada for example, the National Research Council launched a three-year project to develop wireless sensor networks to do just that. The project started in 2008 and is

A very important application of WSNs is in the healthcare field. WSNs can be used to provide continuous, remote, inexpensive, instantaneous and non-invasive monitoring of a patient's vital signs. This technology can be used to allow the elderly to remain in their own residences

All these WSN applications consist of deploying the network for an extended period of time on a single battery charge. It is therefore imperative that the motes be power efficient and that

The hardware of wireless sensor motes consists of sensors (analog and/or digital), a microcontroller, also referred to as a microprocessor or Central Processing Unit(CPU), memory, RF communication module (transceiver) and battery. The design of each component of a WSN mote should take into consideration the power metrics (power consumption and voltage requirements) of the component. Additionally, the integration/interface of all the components as a whole should be studied for power consumption (having analog sensors means that an ADC component in the CPU should be required to convert the sensor readings

To reduce power consumption, several works suggest the introduction of sleep and wake up cycles for the motes. Other schemes suggest a better integration of the functionality of hardware components (using cross-layer principles). Another consideration in the design of the CPU is the clock component. Several applications of WSNs require some level of time synchronization. Clock choices and designs affect the amount of drift that a sensor mote's clock can experience requiring more or less time synchronization operations when the mote is

Of the five main units, the sensing unit is the most application specific. Meaning the type of sensor used will depend on the application. For instance, wireless sensors used for structural health monitoring may consist of materials apt for monitoring strain, acceleration (accelerometer) and linear and angular displacement. Other application specific sensors may measure, vehicular movement, soil consistency, blood alcohol levels, humidity, noise levels and so on. These sensors should then report some signal indicative of their acquisition. Temperature (thermo-coupler outputting voltage or thermistor outputting resistance), force or pressure (piezoelectric outputting voltage, strain gauge outputting resistance), position (linear variable differential transformers (LVDT) outputting alternating current) or light intensity (photodiode outputting current) all need to report information regarding their surroundings

n.d.).

anticipated to continue through 2011.

**3. The WSN hardware architecture**

to a digital format etcÉ)

deployed (Akyildiz et al., 2002).

to a processing unit (Wilson, 2004).

**3.1 Sensors**

but still be able to continuously check their vitals.

the lifetime of the network as a whole be as long as possible.
