Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
Node Architecture
The wireless sensor nodes are the central element in a wireless sensor
network (WSN). It stores and executes the communication protocols and
the data-processing algorithms. The quality, size, and frequency of the
sensed data that can be extracted from the network are influenced by the
physical resources available to the node. Therefore, the design and
implementation of a wireless sensor node is a critical step.
The node consists of sensing, processing, communication, and power
subsystems. The designer has a plethora of options in deciding how to build
and put together these subsystems into a unified, programmable node.
Sensor node is shown in the figure 1.
Figure 1: Architecture of a wireless sensor node
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Date: Wednesday, March 16, 2016
Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
I. The Sensing Subsystem
The sensing subsystem integrates one or more physical sensors and
provides one or more analog-to-digital converters as well as the
multiplexing mechanism to share them.
Nowadays, there are a plethora of sensors that measure and quantify
physical attributes at a cheap price. A physical sensor contains a
transducer, a device that converts one form of energy into another form
of energy, typically into an electrical energy (voltage). The output of this
transducer is an analog signal having a continuous magnitude as a
function of time. Therefore, an analog-to-digital converter is required to
interface a sensing subsystem with a digital processor.
Analog-to-Digital Converter
The analog-to-digital converter (ADC) converts the output of a sensor –
which is a continuous, analog signal into a digital signal. This process
requires two steps: (a) the frequency and magnitude of the signal; and (b)
the available processing and storage resources.
1. The analog signal has to be quantized (i.e., converted from a continuous
valued signal into a discrete valued signal; discrete both in time and
magnitude). The most important decision at this stage is to determine the
number of allowable discrete values. This decision in turn is influenced by
two factors:
2. The sampling frequency. In communication engineering and digital
signal processing, this frequency is decided by the Nyquist rate (the
minimum sampling rate should be twice the bandwidth of the signal) in
wireless sensor networks, however, the Nyquist rate does not suffice.
Oversampling is required because of noise.
The prevailing consequence of the first step is the quantization error while
the second is aliasing.
An ADC is specified, among other things, in terms of its resolution,
which is an expression of the number of bits that can be used to encode the
digital output. For example, an ADC with a resolution of 24 bits can
represent 16,777,216 distinct discrete values. The resolution of an ADC
can also be expressed in volts – since the output of most
MicroelEctroMechanical Systems MEMS sensors is analog voltage. The
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Date: Wednesday, March 16, 2016
Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
voltage resolution of an ADC is equal to its overall voltage measurement
range divided by the number of discrete intervals. In other words:
Where
Q: - is the resolution in volts per step (volts per output code).
Epp: - is the peak-to-peak analog voltage;
M: - is the ADC’s resolution in bits.
Example
Consider an industrial process whose thermal property ranges from −20 to
+80◦ C. The choice of the physical sensor as well as the ADC depends on
the type of thermal change that is of interest. If, for example, a change of
0.5 ◦C is required, an ADC with a resolution of 8 bits is sufficient. If, on
the other hand, a change of 0.0625 ◦C is required, then the ADC should
have a resolution of 11 bits.
II. The Processor Subsystem
The processor subsystem brings together all the other subsystems and
some additional peripherals. Its main purpose is to process (execute)
instructions pertaining to sensing, communication, and self-organization.
It consists of a processor chip, a nonvolatile memory (usually an
internal flash memory) for storing program instructions, an active memory
for temporarily storing the sensed data, and an internal clock, among other
things.
Whereas a wide range of off-the-shelf processors are available for
building a wireless sensor node, one has to make a careful choice, as it
affects the cost, flexibility, performance, and energy consumption of
the node.
If the sensing task is well defined from the outset and does not change
over time, a designer may choose either a field programmable gate array
or a digital signal processor. These processors are:
1- Very efficient in terms of their energy consumption;
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Date: Wednesday, March 16, 2016
Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
2- For most simple sensing tasks, they are quite adequate However
As these are not general-purpose processors, the design and
implementation process can be complex and costly. Most existing sensor
nodes at present use microcontrollers. There are some justifications
besides those just mentioned.
WSNs are emerging technologies; and the research community is still
active with research for developing energy-efficient communication
protocols and signal-processing algorithms. As this requires dynamic code
installation and update, the microcontroller is the best option.
Microcontroller
A microcontroller is a computer on a single integrated circuit,
consisting of a comparatively simple central processing unit and
additional components such as high-speed buses a memory unit, and an
external clock. Microcontrollers are integrated in many products and
embedded devices. Today such simple systems as elevators, ventilators,
office machines, household appliances, power tools, and toys
ubiquitously employ microcontrollers
Microcontroller Board
Advantages and Disadvantages
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Date: Wednesday, March 16, 2016
Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
A microcontroller can be chosen over other types of small-scale processors
because of:
1- Programming flexibility it offers.
2- Its compact construction and small size.
3- Low power consumption.
4- Low cost make it suitable for building computationally less intensive,
standalone applications.
5- There are development environments that offer an abstraction of all the
functionalities of a microcontroller. This enables application
developers to program microcontrollers without the need to have a lowlevel knowledge of the hardware.
Microcontrollers are not as powerful and as efficient as some custom
made processors such as digital signal processors (DSPs) and field
programmable gate arrays (FPGAs). Moreover, for applications which
demand simple sensing tasks but large-scale deployments (such as in
precision agriculture and active volcano monitoring), one may prefer to use
architecturally simple but energy- and cost-efficient processors such as
application specific integrated circuits.
III. Communication subsystem
The communication subsystem consists of a wireless transceiver and an
antenna that are used to transmit and receive messages one bit or symbol
at a time.as shown in figure bellow.
XBEE Communication Modules
Wireless Sensor Network Gateway
The functions available in most transceivers are the selection of a
frequency channel and a transmit power, the modulation transmitted and
demodulation of received data, symbol synchronization and clock
generation for received data.
A transceiver may also include additional functions, which reduce the
processing requirements of Micro-Controller Unit (MCU).
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Date: Wednesday, March 16, 2016
Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
For example, an IEEE 802.15.4 synchronization of an incoming frame,
clear channel assessment for detecting ongoing traffic in a frequency
channel, Received Signal Strength Indicator (RSSI) and Link Quality
Indication ) LQI) for measuring signal strength and estimating link quality
to neighboring nodes, Cyclic Redundancy Check (CRC) calculation for
checking bit errors on received frames, data encryption/decryption for
improving network security and automatic acknowledge transmissions
after received frames. Since these features are implemented most
efficiently in physical layer, they can improve overall network energyefficiently. Yet, the increased complexity raises hardware cost. In practice,
the lowest power COTS (commercial off-the-shelf) transceivers available
today include only some of these features.
A wireless transceiver can be based on acoustic, optical or RF waves.
1- Acoustic communication: is typically used for under water
communications or measuring distances based on time-of-flight
measurements. The disadvantages are long and variable propagation delay,
high path loss, noise, and very low data rate. Also, a large external antenna
is needed.
2- Optical communication: has low energy consumption especially in
reception mode, and it can utilize very small antenna. A transmitter can be
implemented by a Light Emitting Diode (LED) or a laser, and a receiver
by a photo diode. However, radiation is directional and a Line-of-Sight
(LOS) is required. Hence, the alignment of a transmitter to a receiver is
difficult or even impossible in large-scale WSN applications.
3- RF communication combines the benefits of high data rate, long range
and nearly omnidirectional radiation, making it the most suitable
communication technology for WSNs. Disadvantages are large antenna
size and higher energy consumption compared to the optical technology.
In general, an RF transceiver (radio) has four operation modes:
transmit, receive, idle, and sleep. Radio is active in transmit and receive
modes, when power consumption is also the highest. In idle mode, most of
circuitry is shut down, but the transition to the active mode is fast. The
lowest power consumption is achieved in sleep mode when all circuitry is
switched off.
Most short-range radios utilized with WSNs operate in the 433 MHz,
868 MHz ,915 MHz, and 2.4 GHz license-free Industrial Scientific
Medical (ISM) frequency bands. The 2.4 GHz band is the widest providing
more channels, while obstacles have least effect on lower frequency bands.
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Date: Wednesday, March 16, 2016
Wireless Sensor Network
University of Babylon
Mehdi Ebady Manaa
College of IT
2rd class – Department of Network
Depending on the frequency band and antenna type, operating range with
1mW transmission power is from few meters to hundreds meters.
IV. Power subsystem
The power subsystem stores supply energy and converts it to an
appropriate supply voltage level. The subsystem consists of an energy
storage, a voltage regulator, and optionally an energy scavenging unit.
1- Energy Storage
The energy storage can be a non-rechargeable (primary) battery, a
rechargeable (secondary) battery, or a super capacitor. Primary batteries
are cheap and have the highest energy density. They are the most common
power source for WSNs. Secondary batteries have lower energy density
and are more expensive, but they can be recharged only 500 - 1000 times.
Compared to secondary batteries, super capacitors have lower energy
density and they are more expensive. However, their lifetime is in the
order of a million charging/discharging cycles. Super capacitors are
suitable to be used with an energy generator, since energy is typically
generated in peaks during short periods of time, and the amount of stored
energy can be relatively low
2- Energy scavenging
As energy storages have finite capacity, there is a requirement for
self-powered devices. In energy scavenging, a node collects energy from
its surrounding environment. this enables more active and longer term
operation by reducing the dependency on batteries or eliminates the need
for them completely. Installation costs are reduced as self-powered
wireless sensors do not require wires or conduits, and are therefore very
easy to install.
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Date: Wednesday, March 16, 2016