Sokwoo Rhee, Ph.D. Sheng Liu, Ph.D....Sokwoo Rhee, Ph.D., was a research associate at MIT focusing...

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Sokwoo Rhee, Ph.D. Sheng Liu, Ph.D.

Transcript of Sokwoo Rhee, Ph.D. Sheng Liu, Ph.D....Sokwoo Rhee, Ph.D., was a research associate at MIT focusing...

Page 1: Sokwoo Rhee, Ph.D. Sheng Liu, Ph.D....Sokwoo Rhee, Ph.D., was a research associate at MIT focusing on wireless biomedical instrumentation. His research led to the development of a

Sokwoo Rhee, Ph.D.Sheng Liu, Ph.D.

Page 2: Sokwoo Rhee, Ph.D. Sheng Liu, Ph.D....Sokwoo Rhee, Ph.D., was a research associate at MIT focusing on wireless biomedical instrumentation. His research led to the development of a

Wireless Sensor Networking Source BookA Guide to the Fundamentals of Wireless Sensor Networks

by Sokwoo Rhee, Ph.D.and

Sheng Liu, Ph.D.

Version 1.0 January 2005

© 2005 Millennial Net, Inc. All rights reserved.

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About the authors

Sokwoo Rhee, Ph.D., was a research associate at MIT focusing on wireless biomedical

instrumentation. His research led to the development of a sensor ring that measures the

wearer’s vital signs. The practical application for this ring was nursing home resident care.

Residents would wear the rings to monitor their temperature, heart rate, and oxygen satura-

tion; the data would be transmitted continuously to a base station. The small size require-

ments of the ring necessitated a small battery. While a coin cell battery would fit the bill in

terms of size, there were power consumption and mobility challenges still to be met. For

practical reasons, the battery would need to run for months not days, the transmission range

would need to cover the entire nursing home facility, and the data transmission would need

to support mobile residents. Sokwoo began researching mesh networking as the approach to

address the requirements and bring the application from a good idea to a practical solution.

Sheng Liu, Ph.D., spent five years as a research scientist at MIT, directing industry-spon-

sored research programs in the areas of controls, robotics, simulations, signal processing,

and mechatronics. He then went to Raytheon where, as Senior Development Engineer, he

played a critical role in designing receiver spread-spectrum decoding algorithms for differen-

tial-GPS based aircraft precision approach and landing systems.

In 2000, Sokwoo and Sheng joined forces. Sheng’s experience in autonomous systems anal-

ysis, dynamic programming, and algorithm development combined with Sokwoo’s work on

the practical biomedical sensor application resulted in the development of a set of innovative

techniques to meet the practicality needs of the ring sensor. These techniques broke ground

in reducing power consumption, extending transmission distance, and managing a dynamic,

mobile network with a high degree of reliability. The result was a wireless sensor networking

protocol that was applicable across a wide spectrum of applications. With this breakthrough

protocol, Sowkoo and Sheng founded Millennial Net, Inc. and developed and commercialized

the Millennial Net wireless sensor networking platform.

Today, as chief technology officer and vice president of research respectively, Sokwoo and

Sheng continue to lead the industry in developing innovative technologies to enable Millen-

nial Net’s customers develop practical wireless sensor networking-enabled applications.

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Wireless Sensor Networking Source Book

1. Introduction ..................................................................................1Purpose of This Source Book ....................................................................................1 Symbols Used in this Book .....................................................................................2Defining Wireless Sensor Networks ...........................................................................3Opportunities ........................................................................................................5

Replacing Traditional Wired Networks ................................................................................. 5New Opportunities ........................................................................................................... 5

2. Wireless Sensor Networking Overview ..........................................6System Modules ....................................................................................................6

Application Platform ......................................................................................................... 7Gateway ........................................................................................................................ 7Mesh Node Module .......................................................................................................... 8End Node Module ............................................................................................................ 8Sensor/Actuator .............................................................................................................. 8

System Software ...................................................................................................9Module Firmware ............................................................................................................. 9API ................................................................................................................................ 9Network Monitoring System ............................................................................................ 10

3. Network Design Considerations ..................................................11Design Drivers ..................................................................................................... 11Range ................................................................................................................ 12

Shout Versus Whisper .................................................................................................... 13Environmental Concerns ................................................................................................. 14Radio Frequency ........................................................................................................... 16Radio Transmission Techniques ....................................................................................... 16

Power ................................................................................................................. 18Data Rate ........................................................................................................... 18

Raw Data Rate .............................................................................................................. 18Network Throughput ...................................................................................................... 19

Duty Cycle .......................................................................................................... 19Scalability ........................................................................................................... 20Mobility .............................................................................................................. 21

Mobile Sensors .............................................................................................................. 21Mobile Gateways ........................................................................................................... 22

4. Topologies and Data Models ........................................................23Network Topologies .............................................................................................. 23

Star ............................................................................................................................. 23Mesh ........................................................................................................................... 24Star-Mesh Hybrid .......................................................................................................... 25

Data Models ........................................................................................................ 26Data Collection Models ................................................................................................... 27Bi-Directional Dialogue Data Models ................................................................................. 28

5. Routing Techniques .....................................................................30Efficient Protocol .................................................................................................. 30

Proactive Protocols ........................................................................................................ 31Reactive Protocols ......................................................................................................... 32

Routing Protocol Design ........................................................................................ 326. The Millennial Net System ...........................................................34

Persistent Dynamic Routing™ Protocol .................................................................... 34Highly Responsive ......................................................................................................... 34Reliable ........................................................................................................................ 35Extremely Power Efficient ............................................................................................... 35Scalable ....................................................................................................................... 35

Build vs. Buy ....................................................................................................... 35A Complete System .............................................................................................. 36

System Software ........................................................................................................... 36Hardware Modules ......................................................................................................... 36Development and Management Tools ............................................................................... 36

Evaluation Kits .................................................................................................... 37Glossary ..........................................................................................38

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Wireless Sensor Networking Source Book

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1. Introduction

The Wireless Sensor Networking Source Book provides a meth-odology for selecting and implementing a wireless sensor net-work.

Your company’s project requires integrating a wire-

less sensor network between a network of sensors

and the application used to monitor and control them.

You have been put in charge of selecting the wireless

sensor network to use. You’re familiar with some con-

cepts of wireless networks, but don’t feel comfortable

enough to make an informed decision on this particu-

lar type of wireless system. What do you need to

know? What questions need to be asked? Where do

you start? You start here with the Wireless Sensor

Networking Source Book. This guide is for engineers

and decision makers that will be designing, specify-

ing, selecting, or implementing a wireless sensor net-

work.

Purpose of This Source Book

The source book provides a broad understanding of

the technology fundamentals and design consider-

ations that affect function and performance of a wire-

less sensor network. As opposed to a high-data-rate

wireless systems used in LAN applications (WLAN), a

low-power wireless sensor network is specifically

designed for low-data-rate applications. This guide is

designed to provide you with the information neces-

sary to make an informed decision when selecting

and integrating such a network system. Table 1-1

provides a quick reference to the information you will

find in this guide.

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Table 1-1: Information provided in this guide

Symbols Used in this Book

The symbols shown in table 1-2 are used in this book

to illustrate sensor networking concepts:

Table 1-2: Symbols used in this guide

Guide Section Information Provided

Chapter 2: Wireless Sensor Networking Overview

This chapter provides a basic understanding of the wireless sensor network building blocks as a prereq-uisite to a discussion of fundamental network design considerations outlined in Chapter 3.

Chapter 3: Network Design Consider-ations

The information presented in this chapter will help you assess the feasibility of a wireless sensor net-work in your application, to make important scoping and sizing decisions, and to establish a framework to assess different options in specifying and selecting a wireless sensor network system for your application.

Chapter 4: Topolo-gies and Data Models

This chapter provides a look at three “textbook” topologies and discusses the different data models used by wireless sensor networks to collect and manage data.

Chapter 5: Routing Techniques

In this chapter, you’ll learn the advantages and dis-advantages associated with the different routing techniques developed specifically for wireless sensor networks.

Chapter 6: The Mil-lennial Net System

This chapter provides a brief overview of Millennial Net’s wireless sensor networking platform with Per-sistent Dynamic Routing™.

Symbol Description

Sensor/actuator

Application

End node

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Defining Wireless Sensor Networks

Typical wireless sensor network applications share three common requirements: small form factor, long bat-tery life, and dynamic operating environment.

Until recently, networks designed for monitoring and

controlling sensors or actuators on a network were

limited in application and scope due to a major net-

work design consideration—the cables required to

connect the various sensors and actuators to a cen-

tralized collection point. In addition to the costs asso-

ciated with installing and maintaining communication

cables (fiber optic or copper), this type of network

infrastructure prevents sensor mobility and severely

limits the feasible applications of such a network.

Thanks to significant advances in low-power radio

and digital circuit design, self-organizing wireless

sensor networks are now a reality. Sensors of all

types (temperature, motion, occupancy, vibration,

etc.) can now be wirelessly enabled and deployed

inexpensively and quickly.

Wireless sensor networks fundamentally change the

economics of deploying and operating a sensor net-

work, unlocking opportunities to achieve new effi-

ciencies in production processes, building control, or

monitoring, to name just a few. Wireless sensor net-

works also enable the development of a brand new

Mesh node

Gateway

Symbol Description

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class of applications and services not previously pos-

sible with wired sensor networks.

There are no adminis-trative duties associ-ated with establishing and maintaining an ad hoc network.

As illustrated in Figure 1-1, wireless sensor networks

form what is called a wireless ad hoc network, which

refers to a network’s ability to self-organize and self-

heal. This means there are no administrative duties

associated with establishing and maintaining a wire-

less sensor network. By comparison, a wired infra-

structure network, such as the LAN found in most

office environments, requires a significant amount of

overhead to install and maintain in terms of cabling

and administrative time.

In an ad hoc network, sensor nodes consisting of a

sensor attached to a wireless module can be ran-

domly placed and moved as needed. If the network

needs to scale up, additional sensor nodes are easily

added. The new sensor nodes and surrounding net-

work will do the work of discovering each other and

establishing communication paths through single-

and multi-hop paths. All this is made possible

through the use of robust, efficient network protocols

developed specifically for wireless sensor networks.

Figure 1-1: Untethered, mobile ad hoc network nodes

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Opportunities

Looking forward, wire-less sensor networks will unlock new and exciting applications and services.

Today, wireless sensor networks are being used in a

number of low-power, low-data-rate applications aid-

ing digital precision instruments on the factory floor,

collecting water and gas meter readings, monitoring

shipments through the supply chain, and reporting on

the vital signs of individual wearers. Looking forward,

wireless sensor networks will unlock new and exciting

applications and services.

Replacing Traditional Wired Networks

Sensors and actuators can now be monitored and

controlled wirelessly, obviating the expensive instal-

lation and maintenance of copper or fiber optic

cables. For instance, wireless sensor networks are

now being installed in building maintenance systems,

replacing the traditional RS-485 cables used to con-

nect the building controller with the various thermo-

stats located throughout a building. The wireless

sensor network is transparent to the controller and

thermostats, that up until now used the RS-485

cables to communicate with each other.

New Opportunities

The emerging technology behind wireless sensor net-

works is opening the door to a new world of opportu-

nities in data collection and system monitoring

applications—opportunities where traditional wired

networks made them economically or physically

impossible to consider. Today, applications as varied

as monitoring water usage in large apartment com-

plexes to unobtrusively monitoring a patient’s blood

sugar level are now possible. Wireless sensor net-

works will allow companies to develop new sources of

revenue and cut or eliminate waste.

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2. Wireless Sensor Networking Overview

This chapter provides you with a basic understanding

of the wireless sensor network building blocks as a

prerequisite to a discussion of fundamental network

design considerations outlined in Chapter 3.

System Modules

The modules of a wireless sensor network enable

wireless connectivity within the network, connecting

an application platform at one end of the network

with one or more sensor or actuator devices at the

other end. As shown in Figure 2-1, the gateway and

end node modules create a transparent, wireless data

path between the application platform and sensor.

Figure 2-1: Basic wireless sensor network components

Exchange of analog or digital information between an

application platform and one or more sensor nodes

takes place in a wireless fashion. In this example, the

data path between the gateway and end node is

referred to as a single-hop network link.

To extend the range of a network or circumvent an

obstacle, a wireless mesh node module can be added

between a gateway and an end node as shown in Fig-

ure 2-2.

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Figure 2-2: Adding a mesh node module

This particular example represents a multi-hop data

path, in which data packets are handed off from one

module to the next before reaching their destination

(gateway-to-mesh node-to-end node and vice versa).

More elaborate network layouts are discussed later in

“Network Topologies,” but for now, we’ll take a closer

look at each of the network components shown in

Figure 2-2.

Application Platform

This is the network device (PC, handheld, etc.) used

to monitor and control the actions of the various sen-

sors and actuators that are connected to the wireless

sensor network. The application platform is capable

of making decisions based on the information it gath-

ers from the network. Typically, the wireless sensor

network will come with an API (application program-

ming interface) and/or a GUI (graphical user inter-

face) used to interface with the wireless modules.

Gateway

The gateway is the interface between the application

platform and the wireless nodes on the network. The

gateway can be a discrete module, or it can be inte-

grated onto a Flash card form factor for a handheld

device, for example. All information received from the

various network nodes is aggregated by the gateway

and forwarded on to the application platform. In the

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reverse direction, when a command is issued by the

application program to a network node, the gateway

relays the information to the wireless sensor network.

The gateway can also perform protocol conversion to

enable the wireless network to work with other indus-

try-standard network protocols.

Mesh Node Module

Hardware design will affect a module’s power-efficiency.

Considered full-function devices (FFD), mesh node

modules (sometimes called routers) are used to

extend network coverage area, route around obsta-

cles, and provide back-up routes in case of network

congestion or device failure. In some cases, mesh

nodes may also be connected via analog and digital

interfaces to sensors and actuators, providing the

same I/O functionality of an end node module. Mesh

nodes can be battery powered or line powered.

End Node Module

Considered reduced-function devices (RFD), end

nodes (sometimes called endpoints) provide the

physical interface between the wireless sensor net-

work and the sensor or actuator that it is wired to.

End nodes will usually have one or more I/O connec-

tions for connecting to and communicating with ana-

log or digital sensor or actuator devices. End nodes

are typically battery powered.

Sensor/Actuator

These are the devices you ultimately wish to monitor

and/or control. An example is a sensor monitoring

the pressure in an oil pipeline.

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System Software

The software required to integrate and operate a

wireless sensor network resides as firmware in the

system modules and in the application platform as a

set of API functions or network monitoring system

(NMS).

Module Firmware

Firmware design will affect a module’s power-efficiency.

Module firmware is a small, efficient piece of code

that incorporates the module into a larger ad hoc net-

work. It “drives” the module's operation as part of

the larger ad hoc network.

The firmware is also responsible for packaging the

analog and digital sensor data into digital packets and

delivering them across the wireless sensor network.

API

An API, or application programming interface, is a set

of commonly used functions for streamlining applica-

tion development. Used by application developers, an

API provides hooks to integrate the application plat-

forms with the modules on the wireless sensor net-

work. API functions are grouped into “libraries.”

In wireless sensor networks, there two different API

libraries:

•High-level Library: These functions are used to integrate the application with the gateway mod-ule.

•Low-level library: These functions are used to integrate the sensor/actuator with the end node module.

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Network Monitoring System

A network monitoring system (NMS) is software used

to interface with a particular wireless sensor network,

eliminating the need for any programming. Through

the NMS’s graphical user interface (GUI), network

operators are able to see the various nodes of their

wireless sensor network. Depending on the type of

network, control commands can also be issued

through the NMS. For example, a pin on a digital

interface between an end node and an actuator can

be set to high to change the state of the actuator.

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3. Network Design Considerations

The information presented in this chapter will help

you assess the feasibility of a wireless sensor network

in your application, make important scoping and siz-

ing decisions, and establish a framework to assess

different options in specifying and selecting a wireless

sensor network system.

In this chapter, the major design drivers associated

with wireless sensor networks are described. Most

importantly, you’ll learn about the performance

trade-offs that may need to be considered during the

network design process. Understanding how the

design drivers are inter-related will help if and when

a trade-off decision needs to be made. Ultimately,

this will provide you with the tools needed to design a

wireless sensor network that will operate at its opti-

mal level of performance.

Design Drivers

Table 3-1 contains a matrix used to develop a profile

of your particular wireless sensor network applica-

tion. The profile matrix lists the important network

design drivers and will help you determine how

important each driver is in the overall design and

operation of your network. This exercise will also help

determine what design trade-offs may need to be

made with each wireless sensor networking system

you are investigating.

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Table 3-1: Application profile

Range

The term “range” can be used to describe either of

the following:

•Network Range: The total physical area covered by a wireless sensor network.

•Module Range: The distance that data can be transmitted between two modules on a network. Two communicating modules represent the most basic building block in designing a wireless sen-sor network.

Factors that affect the range of a network or network

module include:

•Number of supported network nodes as deter-mined by the manufacturer.

Design Driver

Level of Importance

Minimal Moderate Critical

Range

Short distance between modules

Long distance between modules to minimize

hops

Maximum distance between modules

desired

Power

External power source available

Long battery life desired to minimize battery replacement

Single battery must provide power for mul-

tiple years

Data rate

Very low data rate Moderate data rate High data rate

Duty cycle

Low duty cycles Moderate duty cycles High duty cycles

Scalability

Small network size Moderate network size Large network size

Mobility

Modules stationary and data paths stable

Modules mobile and/or data paths changeable

Modules extremely mobile and data paths

highly changeable

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•Power associated with the radio frequency used.

•Environmental issues, such as walls, electrical interference, etc.

By understanding some of the range-related concepts

and issues associated with module range, you’ll

understand how to efficiently attain the desired net-

work range.

Shout Versus Whisper

Even with the addi-tional modules, the multi-hop whisper method consumes much less power to move data between the two points on a net-work.

When transmitting data between two distant points

on a network, more power is not always the best

answer to bridging the distance between them. Fig-

ure 3-1 illustrates two different methods for transmit-

ting data between two points on a network.

Figure 3-1: Shout versus whisper links

With the “shout” method, the modules use high out-

put power to transmit data packets between them.

While the two modules are able to communicate

effectively, they are not doing it in a very power-effi-

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cient manner. The “whisper” method illustrates how

multiple modules using low output power are used to

bridge the same distance. Even with the additional

modules, the multi-hop whisper method consumes

much less total RF transmit power to move data

between the two points on a network. Figure 3-2

illustrates the relationship between power and dis-

tance for the two methods.

Figure 3-2: Power/distance relationship

Multi-hopping is a technique also used in wireless

sensor networks to extend the range of a network far

beyond the limits of the radio frequency used. If for

example, the frequency being used restricted the dis-

tance between network modules to no more than 50-

100 feet, this distance could be extended by inserting

one or more mesh node modules. The data would

then “hop” from source to destination modules using

the mesh nodes as stepping stones.

Environmental Concerns

The maximum range at which two modules on a net-

work can communicate is affected by a number of

environmental and network characteristics.

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Items blocking the line of sight between network

modules, such as walls and floors, will limit wireless

communications. The type of building material used

in such obstacles will affect how well a radio fre-

quency can penetrate the object.

Figure 3-3: Penetrating line-of-sight obstacles

In cases where obstacles must be circumvented to

provide radio connectivity between modules, one or

more mesh node modules can be inserted for this

purpose as shown in Figure 3-4.

Figure 3-4: Circumventing radio-frequency obstacles

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Radio Frequency

Each module on the network contains a radio trans-

mitter used to communicate with the other wireless

modules on the network. Wireless sensor networks

generally use one of the license-free ISM frequency

bands.

Lower radio frequen-cies, such as 916 MHz which is license-free in North America, require less power and are bet-ter at penetrating objects such as walls or doors.

Typically, the radio or RF components consume more

than 70% of the total power in full-operation mode,

sometimes consuming even more while receiving

(RX) than transmitting (TX) data. The RF components

also burn significant amounts of power during TX/RX

switching or waking up. So, many different scenarios

must be considered in the power budget.

Radio Transmission Techniques

For applications where environmental noise is an

issue, the modulation scheme of the radio should also

be considered as a way of working around such prob-

lems. There are typically two modulation schemes or

techniques used for transmitting radio signals over a

wireless sensor network—one uses narrowband sig-

nals while the other transmits wideband or spread-

spectrum signals.

Narrowband Signals

These radio signals use a very narrow portion of the

radio frequency bandwidth as shown in Figure 3-4.

Spread-Spectrum Signals

Spread-spectrum sig-nals are resistant to interference and hard to intercept.

A spread-spectrum transmitter takes a narrowband

signal and spreads it across a broad portion of the

radio frequency in a predefined method. Destination

devices receiving the signal understand the pre-

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defined method and de-spread the signal before the

data can be interpreted.

Spread-spectrum signals are usually created using

the direct sequence spread spectrum (DSSS) method

or the frequency hopping spread spectrum (FHSS)

method shown in Figure 3-4.

The DSSS method spreads the narrowband signal out

over a broad portion of the frequency band. The

FHSS method spreads its signal by “hopping” the nar-

rowband signal across a broad frequency range as a

function of time.

Figure 3-4: Narrowband, DSSS, and FHSS signals

RF circuitry power consumption is highly dependent

on the modulation scheme. Spread-spectrum RF

chips consume much more power than typical nar-

rowband radios because of the complex base-band

processing. Although spread-spectrum radios offer

better immunity to interference, for many sensor net-

work applications, narrowband radios remain a prac-

tical and more power-efficient choice.

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Power

Power efficiency is a critical design factor for wireless sensor net-work components.

The importance of how efficiently the modules in a

wireless sensor network manage their power

resources can vary with each application. In some

applications, power consumption efficiency is not an

issue as access to local power resources is readily

available to each module. Modules integrated with

the thermostats of a building automation system, for

example, can draw their power from the same 24

VAC source used by the thermostats. In other appli-

cations, wireless sensor network modules are located

in areas where access to local power is not possible,

either because of module location or mobility issues.

In such instances, the modules typically draw their

power from small, coin-cell batteries, making efficient

use of power critical. Being able to operate efficiently

for long periods of time using battery power is a

major advantage of wireless sensor networks over

wired networks and a critical design factor.

Data Rate

Data rate refers to the amount of data the wireless

sensor network is capable of carrying or supporting.

Expressed in bits per seconds (bps), data rate is eval-

uated in two ways: the raw data rate and the actual

network throughput.

Raw Data Rate

This value is determined by the radio used in the

wireless sensor network modules and is less relevant

to wireless sensor network design than network

throughput described below.

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Network Throughput

This value refers to the actual data rate a network

can support. Network throughput is always less than

the raw data rate, and will vary based on many fac-

tors including:

•The size of the network (number of nodes).

•The density of the modules within the wireless sensor network. Modules must negotiate for transmission (TX) time, therefore, the fewer the number of modules within communication range of each other, the faster data can be exchanged.

Figure 3-5: Network node density

Duty Cycle

The duty cycle of a module refers to the percentage of time

the module is active versus inactive, and is determined

using the following equation:

module time on ÷ time period = duty cycle (%)

When in an active state, a module is transmitting data,

receiving data, or simply “listening” to the network. Since

modules consume power when transmitting or receiving

data, it is important to keep the duty cycle to a minimum to

achieve the greatest level of power efficiency.

When inactive, some end node or mesh node mod-

ules can be configured to enter into a sleep mode,

conserving valuable energy. These modules will wake

up to either issue a network “heartbeat” on a regular

basis or when needed for data transmission and

High-Density Network

Modules all competing for transmission time.

Low-Density Network

Little competition between modules for transmission.

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reception. The heartbeat is the end node’s way of let-

ting the network know it is still there.

The duty cycle for the modules on a network should be configurable, especially for power-conscious applications.

The duty cycle for the modules on a network should

be configurable, especially for power-conscious appli-

cations. In some applications, input from a module

might only be required every few hours, allowing the

module to remain in sleep mode for extended periods

of time. The module will wake up briefly to transmit

the data it has collected, then return to a power-con-

serving sleep mode.

For applications where module input is required very

frequently, keeping the duty cycle low by putting the

module in sleep mode—even if only for a few sec-

onds—enables the modules to conserve a significant

amount of energy compared with being constantly in

an active or awake state.

One key design challenge in reducing duty cycle of

mesh nodes modules is to ensure these nodes can

wake up in time to route for other mesh nodes. A

poorly coordinated sleep/wake-up schedule among

mesh nodes can lead to excessive latency or even

loss of data. Therefore, reducing duty cycle of mesh

nodes must be implemented intelligently in order to

save power without sacrificing responsiveness and

robustness.

Scalability

In typical wireless sen-sor networks, there is an inverse relationship between network size and latency.

In typical wireless sensor networks, at any given

sampling rate, there is an inverse relationship

between network size and latency. In other words, it

becomes more difficult to build a responsive ad hoc

network as the number of nodes increases. This is

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due to the network overhead that comes with the

increased size of the network.

In ad hoc networks, the network is formed without

any predetermined topology or shape. Therefore, any

node wishing to communicate with other nodes

should generate more packets than its data packets;

these extra packets are generally called “control

packets” or “network overhead.” As the size of the

network grows, more control packets will be needed

to find and keep the routing paths. In typical ad hoc

networks, the overhead increases exponentially as

the network size grows. In a small network, the

amount of control packets is almost negligible. But

when the network size starts increasing, the over-

head increases rapidly.

Mobility

Mobility refers to the ability of the network to handle mobile nodes and changeable data paths.

Mobility refers to the ability of the network to handle

mobile nodes and changeable data paths. High net-

work responsiveness is a pre-requisite for supporting

mobility. There are two kinds of mobility that a wire-

less sensor network must support: mobile sensors

and mobile gateways.

Mobile Sensors

The self-configuring nature of ad hoc sensor networks

enable them to be able to recognize sensors entering

and exiting the network. This enables the network to

monitor and control dynamic environments where

sensors are not stationary. It also provides low-main-

tenance scalability. Adding a new sensor to the net-

work requires only placing the sensor node within the

network; no further configuration or set up is

required.

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Mobile Gateways

Gateway mobility enables a gateway device to enter

the network, automatically bind to that network and

gather data, then leave the network. One mobile

gateway can bind to multiple networks and multiple

mobile gateways can bind to a given network.

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4. Topologies and Data Models

This chapter provides a look at three “textbook”

topologies and discusses the different data models

used by wireless sensor networks to collect and man-

age data.

Network Topologies

The architectures used to implement wireless sensor

network solutions include star, mesh, and star-mesh

hybrid topologies. Each of these topologies presents

its own set of challenges, advantages, and disadvan-

tages as shown in Table 4-1 and discussed below.

Table 4-1: Network topologies

Star

A star topology, as shown in Figure 4-1, is a single-

hop system in which all wireless sensor nodes com-

municate bi-directionally with a gateway.

The gateway can be a PC, PDA, dedicated building

control device, embedded Web server, or other gate-

way to an application platform or another network.

The end nodes are identical and the gateway serves

both to communicate data and commands among

end nodes, and to transfer data to an application or

other network, such as the Internet. The end nodes

Topology Power usage Range

Star Low Short

Mesh High Long

Star-mesh hybrid Low Long

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do not pass data or commands to each other; they

use the gateway as a coordination point.

Figure 4-1: Star topology

Among wireless sensor networking topologies, the

star topology is the lowest in overall power consump-

tion, but is limited by the transmission distance of the

radio in each end node back to the gateway. This dis-

tance can range from ten to hundreds of meters.

Notice also, that there are no alternate communica-

tion paths between any of the end nodes and the

gateway. Should a path become obstructed, commu-

nication with the associated end node may be lost.

Mesh

Mesh topologies are multi-hopping systems in which

all wireless sensor nodes are mesh nodes and com-

municate directly with each other to hop data to and

from the gateway and to pass commands to each

other. This is illustrated in Figure 4-2.

A mesh network is highly fault tolerant because each

sensor node has multiple paths back to the gateway

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or to other nodes. The multi-hop system allows for a

much longer range than a star topology.

Figure 4-2: Mesh topology

Star-Mesh Hybrid

A star-mesh hybrid seeks to take advantage of the

low power and simplicity of the star topology, as well

as the extended range and self-healing nature of a

mesh network topology. As shown in Figure 4-3, a

star-mesh hybrid organizes end nodes around mesh

nodes which, in turn, organize themselves in a mesh

network. The mesh nodes serve both to extend the

range of the network and to provide fault tolerance.

Since end nodes can communicate with multiple

mesh nodes, if a mesh node fails or if a radio link

experiences interference, the network will reconfigure

itself around the remaining mesh nodes.

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Figure 4-3: Star-mesh hybrid topology

Data Models

The data model is a function of the applica-tion and describes the flow of data and how that data is used.

The data model characterizes and describes the way

in which data flows through and is used in the net-

work, or stated a different way, the interaction

between the sensors and the application. Unlike the

topology which is a function of the network protocol,

the data model is a function of the application. You

will need to determine the data model most appropri-

ate for your application based on the application’s

requirements. Broad categories of data models

include data collection and bi-directional dialogue

models.

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Data Collection Models

Data collection models describe monitoring applica-

tions where the data flows primarily from the sensor

node to the gateway.

Periodic Sampling

For applications where certain conditions or pro-

cesses need to be monitored constantly, such as the

temperature in a conditioned space or pressure in a

process pipeline, sensor data is acquired from a num-

ber of remote sensor nodes and forwarded to the

gateway or data collection center on a periodic basis.

The sampling period mainly depends on how fast the

condition or process varies and what intrinsic charac-

teristics need to be captured.

In many cases, the dynamics of the condition or pro-

cess to be monitored can slow down or speed up from

time to time. Therefore, if the sensor node can adapt

its sampling rates to the changing dynamics of the

condition or process, over-sampling can be minimized

and power efficiency of the overall network system

can be further improved.

Another critical design issue associated with periodic

sampling applications is the phase relation among

multiple sensor nodes. If two sensor nodes operate

with identical or similar sampling rates, collisions

between packets from the two nodes is likely to hap-

pen repeatedly. It is essential that sensor nodes can

detect this repeated collision and introduce a phase

shift between the two transmission sequences in

order to avoid further collisions resulting in optimal

network operation and minimized power usage.

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Event Driven

There are many cases that require monitoring one or

more crucial variables immediately following a spe-

cific event or condition. Common examples include

fire alarms, door and window sensors, or instruments

that are user activated. To support event-driven

operations with adequate power efficiency and speed

of response, the sensor node must be designed such

that its power consumption is minimal in the absence

of any triggering event, and the wake-up time is rela-

tively short when the specific event or condition

occurs. Many applications require a combination of

event driven data collection and periodic sampling.

Store and Forward

In many applications, data can be captured and

stored or even processed by a sensor node before it

is transmitted to the gateway or base station. Instead

of immediately transmitting every data unit as it is

acquired, aggregating and processing data by remote

sensor nodes can potentially improve overall network

performance in both power consumption and band-

width efficiency. One example of a store-and-forward

application is cold-chain management where the tem-

perature in a freight container carrying produce or

pharmaceuticals, for instance, is captured and

stored; when the shipment is received, the tempera-

ture readings from the trip are downloaded and

viewed to ensure that the temperature and humidity

stayed within the desired range.

Bi-Directional Dialogue Data Models

Bi-directional dialogue data models are characterized

by a need for two-way communication between the

sensor/actuator nodes and gateway/application.

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Polling

Controller-based applications, such as those found in

building automation systems, use a polling data

model. In this model, there is an initial device discov-

ery process that associates a device ID with each

physical device in the network. The controller then

polls each device on the network successively, typi-

cally by sending a serial query message and waiting

for a response to that message. For example, an

energy management application would use a polling

data model to enable the application controllers to

poll thermostats, variable air volume (VAV) sensors,

and other devices for temperature and other read-

ings.

On-Demand

The on-demand data model supports highly mobile

nodes in the network where a gateway device enters

the network, automatically binds to that network and

gathers data, then leaves the network. With this

model, one mobile gateway can bind to multiple net-

works and multiple mobile gateways can bind to a

given network. An example of an application using

the on-demand data model is a medical monitoring

application where patients in a hospital wear sensors

to monitor vital signs and doctors access that data

via a PDA that is a mobile gateway. A doctor enters a

room and the mobile PDA automatically binds with

the network associated with that patient and down-

loads vital sensor data. When the doctor enters a sec-

ond patient's room, the PDA automatically binds with

that network and downloads the second patient's

data.

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5. Routing Techniques

In this chapter, you’ll learn the advantages and dis-

advantages associated with the different routing

techniques developed specifically for wireless sensor

networks.

A wireless sensor network relies on its network

layer's routing algorithm to discover routes and

deliver data packets from sources to destinations.

The routing layer protocol is also responsible for

maintaining and repairing routes when radio links (or

hops) along established routes are broken, due to

relocation or failure of nodes, sever RF interference,

or congestion. It is the routing algorithm that enables

a wireless sensor network to self-organize and self-

heal.

Routing always has some degree of overhead associ-

ated with it. The size of this overhead directly affects

the responsiveness and scalability of the network. To

build and implement highly responsive, efficient, and

scalable networks, you need a protocol that is very

efficient and minimizes the overhead needed to

accomplish its tasks.

Efficient Protocol

The routing protocol is designed to find the most efficient data path route(s) to use between network mod-ules and to dynami-cally find new paths when conditions within the network change.

The embedded routing protocol used by each of the

network modules affects a number of characteristics

within the wireless sensor network—from the way in

which modules self-organize to the way in which data

is transmitted from source to destination node. The

routing protocol is designed to find the most efficient

data path route(s) to use between network modules

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and to dynamically find new paths when conditions

within the network change.

The more efficient the protocol, the more efficiently

the wireless sensor network operates, which results

in less power being consumed by each module.

A number of highly intelligent routing techniques

have been developed over the years for both wired

and wireless networks. During the 80s and 90s, sub-

stantial advances were made to support the explosive

growth of computer networks and particularly, the

Internet. Prevalence of wireless communication in

recent years further spurs the development of net-

work routing techniques. In general, these routing

techniques fall into two main categories: proactive

and reactive protocols.

Proactive Protocols

In proactive protocols, such as the link-state routing

and the destination-sequenced distance vector

(DSDV) routing, each node in the network maintains

route information to every other node, typically in the

form of routing tables. These routing tables are

updated periodically to account for changes in net-

work topology and link conditions. The main advan-

tage of proactive routing is that route information is

constantly updated and, therefore, valid routes are

always readily available. The route update process in

proactive algorithms, however, requires a significant

amount of overhead, consuming network bandwidth.

This overhead grows exponentially with network size.

Hence, for bandwidth-limited applications such as

wireless networks, proactive routing cannot scale.

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Reactive Protocols

As opposed to proactive routing, reactive routing

algorithms establish and maintain routes on demand

(i.e., only at the request of nodes that have traffic to

send to specified destination nodes). Reactive routing

does not require constant updating of route informa-

tion, hence, reducing the overhead associated with

the update process. The on-demand, dynamic nature

of reactive routing makes it particularly effective for

ad hoc wireless networks where network nodes can

be highly mobile and network connection can be

formed in an ad hoc manner without the need of any

prescribed infrastructure. Popular routing algorithms,

such as dynamic source routing (DSR) and ad hoc on-

demand distance vector (AODV) routing, have been

applied to an increasing number of ad hoc wireless

networking applications such as mobile PC networks

(WiFi) and battlefield radio networks. Both DSR and

AODV have been adopted as part of the solution

framework provided by the Mobile Ad hoc Networks

(MANET) task group within the Internet Engineering

Task Force (IETF).

Routing Protocol Design While the existing routing algorithms are effective for

various wired and wireless networking applications,

they are not well suited for wireless sensor networks

due to their unique characteristics and application

requirements that are intrinsically different from the

traditional networks. Among others, the main differ-

ences between wireless sensor networks and tradi-

tional networks are as follows.

•Nodes in wireless sensor networks run on limited power resources, such as low-capacity batteries.

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•Radios used in wireless sensor networks support relatively low data rates, typically in the range of 100K - 200K bits per second.

•In most cases, wireless sensor nodes operate in license-free radio frequency bands with relatively low output power. As such, radio links among nodes can be easily interfered with and cor-rupted, especially in popular bands such as the 2.4GHz band used by WiFi, Bluetooth and cord-less phones.

•Wireless sensor nodes typically employ low-cost microcontrollers with limited computation capac-ity and memory.

•Many applications require a relatively large-scale network with hundreds of wireless sensor nodes to be densely deployed.

•Topology of a wireless sensor network may change frequently.

These unique characteristics pose significant chal-

lenges to routing protocol design for wireless sensor

networks.

Limited channel data rate and radio output power require a highly efficient routing proto-col.

With stringent resource constraints, routing in wire-

less sensor networks must be implemented with min-

imal duty cycle and overhead. Limited channel data

rate and radio output power require a highly efficient

routing protocol to carry data with low overhead and

direct traffic through reliable, multi-hop routes. In a

dynamic network with mobile nodes and strong inter-

ference, radio links are constantly broken, and the

routing protocol must allow network nodes to quickly

repair routes and adapt to changing topology. The

routing protocol must also be highly scalable to sup-

port formation and maintenance of large-scale net-

works.

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6. The Millennial Net System

The Millennial Net wireless sensor networking system

delivers a robust, reliable, scalable networking proto-

col and a complete system for fast and cost-effective

time to deployment.

Persistent Dynamic Routing™ Protocol

Millennial Net has developed and optimized its proto-

col to address the unique characteristics and chal-

lenges associated with wireless sensor networking.

The end result is a networking system and associated

protocol that is highly scalable, ultra-efficient, and

extremely responsive and resilient in dynamic

environments. The Millennial Net protocol for wireless

sensor networks that provides the industry’s longest

battery life at sensor nodes while delivering data over

fault-tolerant links with end-to-end redundancy.

The Millennial Net protocol is based on Persistent

Dynamic Routing—a set of patented techniques for

reliable and scalable wireless sensor networks—which

has been designed specifically to meet all critical

challenges of wireless sensor networks. When form-

ing an ad hoc sensor network, Persistent Dynamic

Routing requires minimal overhead for requesting

and establishing connectivity without relying on the

bandwidth-consuming flooding technique.

Highly Responsive

Self-configuration is initiated from the end nodes (not

the gateway) for ultra-efficient, light-weight topology

discovery and re-discovery providing high respon-

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siveness for mobile sensors in a dynamic environ-

ment.

Reliable

Persistent routing techniques ensure data packet

delivery for highly reliable data transmission required

for mission-critical applications.

Extremely Power Efficient

Dynamic route discovery ensures that the best data

route is determined on the fly for efficient bandwidth

yielding low power consumption and high battery life.

Scalable

Low overhead yields high scalability to support a net-

work infrastructure with the headroom to grow and

adapt.

Build vs. Buy

Your ability to take advantage of the benefits of wire-

less sensor networking is only as good as your ability

to quickly and cost-effectively develop and deploy

your wirelessly networked sensor application. Many

so-called solutions available today are simply chips

and stacks, leaving you to source components and

fabricate PCBs, optimize networking software, as well

as perform integration and application development

from scratch. Millennial Net delivers a complete sys-

tem that lets you concentrate on your application

requirements. With the complete system software

delivered on hardware modules and APIs to stream-

line sensor and host application integration, your

time to market is fast and your development effort is

extremely cost-effective.

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A Complete System

Millennial Net delivers a complete system of software,

hardware modules, and open system interfaces for

fast deployment of robust wireless sensor networks.

System Software

The Millennial Net system software, based on pat-

ented Persistent Dynamic Routing™ technology forms

the foundation of a wireless sensor network that is

efficient, responsive, and resilient. Topology discov-

ery is ultra-efficient, light-weight, and highly respon-

sive to mobile sensors and a dynamic environment.

Persistent routing techniques ensure reliable data

transmission for mission-critical applications.

Dynamic route discovery makes the platform

extremely scalable and power-efficient providing long

battery life. The system is architected to minimize

overhead, enabling the network to scale very effec-

tively.

Hardware Modules

End nodes provide a direct interface to analog and

digital sensors. The gateway moves data between

end nodes and the host, and monitors data links,

devices, and battery status. Mesh nodes extend the

range of the network, route around obstacles, and

form redundant routes.

Development and Management Tools

The Sensor Integration API provides sensor-specific

functions to streamline the integration process and

provide data reduction benefits. The Application Inte-

gration API enables quick development of applica-

tions to integrate, display, and report on the data

collected. The Network Monitoring System is a graph-

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ical application for configuring and monitoring the

network.

Evaluation Kits

Millennial Net offers an Evaluation Kit that lets devel-

opers install a wireless sensor network prototype in

less than one day. The kit contains everything

needed including software (with Millennial Net’s pat-

ented Persistent Dynamic Routing protocol), hard-

ware, and accessories. More information is available

online at www.millennialnet.com/EvalKit.

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Glossary

API Application Programming Interface: A set of definitions of the ways

in which one piece of computer software communicates with

another. It is a method of achieving abstraction, usually (but not

necessarily) between lower-level and higher-level software. One of

the primary purposes of an API is to provide a set of commonly-

used functions-for example, to poll a wireless network for active

network nodes (mesh nodes and end nodes). Programmers can

then take advantage of the API by making use of its functionality,

saving them the task of programming everything from scratch. APIs

themselves are abstract: software that provides a certain API is

often called the implementation of that API.

ad hoc network A group of wireless sensors connected as an independent wireless

network, communicating directly with each other without the use of

an access point.

bandwidth The amount of data that can be transmitted in a fixed amount of

time. For digital devices, the bandwidth is usually expressed in bits

per second (bps) or bytes per second. For analog devices, the band-

width is expressed in cycles per second, or Hertz (Hz).

Bluetooth An industrial specification for wireless personal area networks

(PANs). Bluetooth provides a way to connect and exchange infor-

mation between devices like personal digital assistants (PDAs),

mobile phones, laptops, PCs, printers and digital cameras via a

secure, low-cost, globally available short range radio frequency.

Bluetooth lets these devices talk to each other when they come in

range, even if they're not in the same room, as long as they are

within 10 meters (32 feet of each other).

data model As it pertains to wireless sensor networks, the data model charac-

terizes and describes the way in which data flows through and is

used in the network. Common data model categories include data

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collection models (periodic sampling, event driven, and store and

forward) and bi-directional dialogue data models (polling and on-

demand).

DSSS Direct Sequence Spread Spectrum: Spread spectrum method of

spreading a narrow band signal. This method uses special pseudo

noise codes to expand the narrow band signal out across a broad

portion of the radio band. (See also FHSS and spread spectrum.)

duty cycle The duty cycle of a module refers to the percentage of time the

module is active versus inactive.

end node The network module that provides the physical interface between

the wireless sensor network and the sensor or actuator that it is

wired to. Sometimes called a Reduced Function Device (see RFD).

endpoint See end node.

FFD Full Function Device: A term referring to a device that can act as an

intermediate mesh node, passing data from other devices. (See

also RFD.)

FHSS Frequency Hopping Sequence Spread Spectrum: Spread spectrum

method of spreading a narrow band signal out across a broad por-

tion of the radio band. This method “hops” the signal as a function

of time. (See also DSSS and spread spectrum.)

gateway The network module that provides the interface between the appli-

cation platform and the modules on the wireless sensor network.

IEEE Institute of Electrical and Electronics Engineers: Organization of

engineers, scientists, and students that is known for developing

standards for the computer and electronics industry.

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IEEE 802.11.4 Standard developed by IEEE that defines the lower protocol layers

(PHY and MAC) for low-data-rate wireless Personal Area Networks

(PANs).

ISM The industrial, scientific, and medical (ISM) radio bands were origi-

nally reserved internationally for non-commercial use of RF electro-

magnetic fields for industrial, scientific and medical purposes. In

recent years they have also been used for license-free error-toler-

ant communications applications such as wireless LANs, Bluetooth,

and wireless sensor networks:

•900 MHz band

•2.4 GHz band

•5.8 GHz band

latency In networking, the amount of time it takes a packet to travel from

source to destination. Together, latency and bandwidth define the

speed and capacity of a network.

mesh node The module on the wireless sensor network used to extend network

coverage area, route around obstacles, and provide back-up routes

in case of network congestion or device failure. The mesh node can

also provide a direct physical interface to a sensor or actuator.

Sometimes called a Full Function Device (see FFD).

mesh topology A wireless sensor networking architecture consisting of a gateway

and mesh nodes that provides extended area coverage, routing

around obstacles, and back-up data paths.

narrowband Radio signal that contains all of its power within a very narrow por-

tion of the radio frequency band.

OSI Open System Interconnection: An ISO standard for worldwide com-

munications that defines a networking framework for implementing

protocols in seven layers. Control is passed from one layer to the

next, starting at the application layer in one station, proceeding to

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the bottom layer, over the channel to the next station and back up

the hierarchy.

packet A piece of a message transmitted over a packet-switching network.

One of the key features of a packet is that it contains the destina-

tion address in addition to the data.

personal area net-work

A personal area network (PAN) is the interconnection of information

technology devices within the range of an individual person, typi-

cally within a range of 10 meters.

protocol The protocol defines a common set of rules and signals that devices

(nodes) on the network use to communicate.

protocol stack A set of network protocol layers that work together. The OSI refer-

ence model that defines seven protocol layers is often called a

stack.

RFD Reduced Function Device: A term referring to a device that is just

smart enough to talk to the network (see also FFD).

router See mesh node.

sensor node A wireless sensor network node consisting of a sensor or actuator

device attached to a wireless module. The wireless module provides

the interface between the sensor device and the wireless network.

SNR Signal-to-noise ratio: the ratio of the amplitude of a desired analog

or digital data signal to the amplitude of noise in a transmission

channel at a specific point in time. SNR is typically expressed loga-

rithmically in decibels (dB). SNR measures the quality of a trans-

mission channel or an audio signal over a network channel. The

greater the ratio, the easier it is to identify and subsequently isolate

and eliminate the source of noise. A SNR of zero indicates that the

desired signal is virtually indistinguishable from the unwanted

noise.

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Wireless Sensor Networking Source Book

42

spread spectrum (wideband)

Technique for taking a narrowband signal and spreading it across a

broader portion of the radio frequency band. Spread-spectrum sig-

nals are more resistant to interference than narrow band signals.

The two basic methods for spreading a narrowband are direct

sequence and frequency hopping. (See also DSSS and FHSS.)

star topology A wireless sensor networking architecture consisting of a gateway

and end nodes that is extremely power efficient for short-range net-

works.

star-mesh hybrid topology

A wireless sensor networking architecture consisting of a gateway,

mesh nodes, and end nodes that optimizes range and power effi-

ciency of the network.

topology As it pertains to wireless sensor networks, the geometric arrange-

ment of the modules (gateway, mesh nodes, and end nodes) within

a network. Common topologies include star, mesh, and star-mesh

hybrid.

ZigBee A standard developed by the ZigBee Alliance for wireless sensor

networks to define the upper protocol layers.

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