Wireless Sensor Network

1. T.Y.B.Sc.C.S
Chapter 2 : Sensor Node Hardware and Network
Architecture
By
Mrs.Vidhi Khedekar
Assistant Professor,
Dnyandeep College of Science and Commerce,
Morvande-Boraj,Khed.

2.  Agenda:
1. What is Single-node architecture?
2. What are the hardware components & design constraints?
3. Introduction to TinyOS and nesC.

3.  A sensor is a device that detects and responds to some type of input from the physical
environment.
 The specific input could be light, heat, motion, moisture, pressure, or any one of a great number
of other environmental phenomena.
 The output is generally a signal that is converted to human-readable display at the sensor
location or transmitted electronically over a network for reading or further processing.
Example:
 In a mercury-based glass thermometer, the input is temperature. The liquid contained expands
and contracts in response, causing the level to be higher or lower on the marked gauge, which is
human-readable.
Sensor node:
 A sensor node, also known as a mote, is a node in a sensor network that is capable of performing
some processing, gathering sensory information and communicating with other connected nodes
in the network.
 A mote is a node but a node is not always a mote.

4.  Main Architecture of Sensor Node:
The main architecture of sensor node includes
following components:
 Controller module
 Memory module
 Communication module
 Sensing modules
 Power supply module

5. Main components of a WSN node :
 Controller-
 Processes all relevant data
 Capable of executing arbitrary code
 Communication device(s) -
 Device for sending and receiving data over a wireless channel
 Sensors/actuators -
 The actual interface to the physical world;
 Devices that can observe or control physical parameters of the environment.
 Memory -
 Stores data and programs
 Often different types of memory are used for programs and data
 Power supply -
 Some form of batteries to provide energy
 Sometime recharging by obtaining energy from the environment,
e.g. solar cells

6.  Controller-
 The controller is the core of a wireless sensor node, it process all relevant data, capable of
executing arbitrary code.
 It has to execute various programs, hence it is a Central Programming Unit (CPU) of the node.
 These,
-Collects data from the sensors
-Process the collected data
-Decides when and where to send the data
-Receives data from the other sensor nodes
-Decides on the actuator’s behavior

7.  Microcontroller:
◦ For the sensor nodes mostly microcontroller are used
◦ It is a general purpose processor, optimized for embedded applications, low power consumption
◦ Examples
 Intel StrongARM
- High-end processor often used in PDAs
- SA-1100 model: 32-bit RISC core, running @206MHz
 Texas Instruments MSP 430
- Intended for usage in embedded applications
- 16-bit RISC core, up to 4 MHz, 2-10 kB RAM, several DACs, RT clock
 Digital signal processor(DSPs)
- These are familiar because of their architecture and instruction set for processing large amount
of vector data
 FPGA(Field Programmable Gate Arrays)
- Flexible, can be reprogrammed to adapt to a changing set of requirements.
- But take time and energy.
 ASIC(Application Specific Integrated Circuit)
- These are used only when peak performance is needed
- Ex: High speed routers and switches

8.  Memory is required to store the programs and intermediate data; usually different types of
memory are used for program and data.
 RAM (Random Access Memory)
- To store intermediate sensor readings, packets from other nodes and so on.
- Is fast, but looses content if power supply is interrupted
 ROM (Read-Only Memory)
- To store fixed programs; not writeable
 EEPROM (Electrically Erasable Programmable)or Flash Memory
- Programs are stored
- Enables overwriting of data
- Can be used as intermediate storage if RAM is insufficient or if the RAM’s power supply should
be turned-off
- BUT: long read/write access delays, high energy requirement

9.  To turn nodes into a network a device is required for sending and receiving information over the
wireless channel.
 Communication device is used to exchange data between individual nodes.
 Choices of transmission medium:
 Wireless communication chooses any one of the following as transmission medium
- Radio frequencies(most used in WSN)
- Optical communication
- Ultra sound
- Magnetic inductance(less used)
 Among the above Radio Frequency(RF) is best because it provides,
-Long range
-High data rates
-Acceptable error rates at reasonable energy expenditure
-Does not require line of sight between sender and receiver

10.  - Both a transmitter and a receiver are required in a sensor node
- For practical purposes these two tasks are often combined in one entity, the so called transceiver
- It convert a bit stream coming from a microcontroller(or a sequence of bytes or frames) and
convert them to and from radio waves.
- A range of low cost transceiver is commercially available.
- It includes all the circuitry required for transmitting and receiving modulation, demodulation,
amplifiers, mixers, filters and so on.
- Usually it uses half duplex mode because transmitting and receiving at the same time is
impractical in the wireless medium
Bit Stream Radio Waves
Radio
Transceiver

11.  Transceivers can be put into different operational states:
 Transmit
- In this state transceiver is active and the antenna radiates energy
 Receive
- In this state receive part is active
 Idle
- In this state transceiver ready to receive but currently no receiving
 Sleep
- Significant parts of the receiver are switched off
 A fairly common structure of transceivers is into the Radio Frequency (RF) front end and the
baseband part:
- The radio frequency front end performs analog signal processing in the actual radio frequency
band.
- The baseband processor performs all signal processing in the digital domain and communicates
with a sensor node’s processor or other digital circuitry.

12.  Some important elements of an RF front ends architecture are:
- The Power Amplifier (PA) accepts up converted signals from the IF or baseband part and
amplifies them for transmission over the antenna.
- The Low Noise Amplifier (LNA) amplifies incoming signals up to levels suitable for further
processing without significantly reducing the SNR.

13.  The actual interface to the physical world: The devices that can observe or control physical
parameters of the environment.
 Sensors can be roughly categorized into three categories:
 Passive, omnidirectional sensors
 These sensors can measure a physical quantity at the point of the sensor node without actually
manipulating the environment by active probing -In this sense, they are passive.
 Passive, narrow-beam sensors
 These sensors are passive as well, but have a well-defined notion of direction of measurement.
 A typical example is a camera, which can “take measurements” in a given direction, but has to be
rotated if need be.
 Active sensors
 This last group of sensors actively probes the environment.
 For example, a sonar or radar sensor or some types of seismic sensors, which generate shock waves
by small explosions.
 Actuator:
 a device that converts an electrical signal to a physical output
 to open or close a switch or a relay or to set a value in some way. Whether this controls a motor, a
light bulb, or some other

14. There are essentially two aspects:
◦ First, storing energy and providing power in the required form;
◦ Second, attempting to replenish(refill) consumed energy by “scavenging” it from some node-external
power source over time.
1.Storing energy :Batteries
 Traditional batteries: The power source of a sensor node is a battery, eighter non-
rechargeable(Primary) or rechargeable (Secondary).
 Upon these batteries the requirements are
o Capacity
o Capacity under load
o Self-discharge
o Efficient recharging
o Relaxation
o DC-DC conversion: Unfortunately, batteries alone are not sufficient as a direct power source for a
sensor node.
 One typical problem is the reduction of a battery’s voltage as its capacity drops. Consequently, less
power is delivered to the sensor node’s circuits, – a node on a weak battery will have a smaller
transmission range than one with a full battery.

15.  For this energy scavenging is used which is the process of recharging the battery with energy
gathered from the environment like solar cells or vibration-based power generation.
1. Photovoltaics
◦ The well-known solar cells can be used to power sensor nodes
2. Temperature gradients
◦ Differences in temperature can be directly converted to electrical energy.
3. Vibrations
◦ General form of mechanical energy is vibrations.
4. Flow of air/liquid
◦ Another often-used power source is the flow of air or liquid in wind mills or turbines.
◦ The challenge here is again the miniaturization, but some of the work on millimeter scale.

16. • Why was TinyOS needed?
• Introduction to TinyOS
• Goals/Objectives behind TinyOS
• Requirements of WSN for Operating System
• TinyOS as a Solution
• TinyOS Model
▫ Data Model
▫ Thread Model
▫ Programming Model
▫ Component Model
▫ Network Model
• Example Application

17.  Problems with traditional OS
◦ Multithreaded Architecture not useful
◦ Large Memory Footprint
◦ Does not help to conserve energy and power
 Requirements for Wireless Sensor Networks
◦ Efficient utilization of energy and power
◦ Small Footprint
◦ Should support diversity in design and usage
◦ More emphasis on Concurrent execution

18.  TinyOS began as a collaboration between University of California, Berkeley and Intel Research.
 It is a free open source operating system designed for wireless sensor networks.
 It is an embedded operating system written in NesC
 It features a component based architecture.
TinyOS as a Solution
• Component based architecture allows frequent changes while still keeping the size of code
minimum.
• Event based execution model means no user/kernel boundary and hence supports high
concurrency.
• It is power efficient as it makes the sensors sleep as soon as possible.
• Has small footprint as it uses a non-preemtable FIFO task scheduling.

19. • Static Memory Allocation
▫ No Heaps or any other dynamic
structures used.
▫ Memory requirements determined
at compile time.
This increases the runtime efficiency.
• Global variables
▫ Allocated on per frame basis.
• Local Variables
▫ Saved on the stack
▫ Defined in the function/method

20. • Power-Aware Two-levels Scheduling
▫ Long running tasks and interrupt events
▫ Sleep unless tasks in queue, wakeup on event
• Tasks
▫ Time-flexible, background jobs
▫ Atomic with respect to other tasks
▫ Can be preempted by events
• Events
▫ Time-critical, shorter duration
▫ Last-in first-out semantic (no priority)
▫ Can post tasks for deferred execution

21.  Separation construction/composition
 Construction of Modules
◦ Modules implementation similar to C coding
◦ Programs are built out of components
◦ Each component specifies an interface
◦ Interfaces are “hooks” for wiring components
 Composition of Configurations
◦ Components are statically wired together
◦ Increases programming efficiency (code reuse) an runtime efficiency (static defs.)

22.  Components should use and provide bidirectional interfaces.
 Components should call and implement commands and signal and handle events.
 Components must handle events of used interfaces and also provide interfaces that must implement
commands.
Component Model : Hierarchy
 Commands
◦ Flow downwards
◦ Non Blocking requests
◦ Control returns to caller
 Events
◦ Flow upwards
◦ Post task, signal higher level events, call lower level cmds
◦ Control returns to signaler
 To avoid cycles
◦ Events can call commands
◦ Commands can NOT signal events