sokwoo rhee, ph.d. sheng liu, ph.d. - millennial net · about the authors sokwoo rhee, ph.d., was a...
TRANSCRIPT
Sokwoo Rhee, Ph.D.Sheng Liu, Ph.D.
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.
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.
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
Wireless Sensor Networking Source Book
1
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.
Wireless Sensor Networking Source Book
2
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
Wireless Sensor Networking Source Book
3
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
Wireless Sensor Networking Source Book
4
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
Wireless Sensor Networking Source Book
5
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.
Wireless Sensor Networking Source Book
6
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.
Wireless Sensor Networking Source Book
7
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
Wireless Sensor Networking Source Book
8
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.
Wireless Sensor Networking Source Book
9
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.
Wireless Sensor Networking Source Book
10
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.
Wireless Sensor Networking Source Book
11
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.
Wireless Sensor Networking Source Book
12
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
Wireless Sensor Networking Source Book
13
•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-
Wireless Sensor Networking Source Book
14
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.
Wireless Sensor Networking Source Book
15
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
Wireless Sensor Networking Source Book
16
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-
Wireless Sensor Networking Source Book
17
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.
Wireless Sensor Networking Source Book
18
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.
Wireless Sensor Networking Source Book
19
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.
Wireless Sensor Networking Source Book
20
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
Wireless Sensor Networking Source Book
21
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.
Wireless Sensor Networking Source Book
22
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.
Wireless Sensor Networking Source Book
23
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
Wireless Sensor Networking Source Book
24
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
Wireless Sensor Networking Source Book
25
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.
Wireless Sensor Networking Source Book
26
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.
Wireless Sensor Networking Source Book
27
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.
Wireless Sensor Networking Source Book
28
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.
Wireless Sensor Networking Source Book
29
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.
Wireless Sensor Networking Source Book
30
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
Wireless Sensor Networking Source Book
31
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.
Wireless Sensor Networking Source Book
32
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.
Wireless Sensor Networking Source Book
33
•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.
Wireless Sensor Networking Source Book
34
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-
Wireless Sensor Networking Source Book
35
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.
Wireless Sensor Networking Source Book
36
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-
Wireless Sensor Networking Source Book
37
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.
Wireless Sensor Networking Source Book
38
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
Wireless Sensor Networking Source Book
39
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.
Wireless Sensor Networking Source Book
40
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
Wireless Sensor Networking Source Book
41
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.
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.
Millennial Net285 Billerica Rd.
Chelmsford, MA 01824
+1 978-569-1921www.millennialnet.com
© 2011 Millennial Net, Inc. All rights reserved. Persistent Dynamic Routing™ is a trademark, and Millennial Net® and MeshScape® are registered trademarks of Millennial Net, Inc. Modbus is a trademark or registered trademark of Schneider Automation Inc. All other trademarks are property of their respective owners. Information is subject to change.