chapter6 sensor network

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Sensor Network


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1.Introduction

Goal

Wireless Sensor Network

Ubiquitous Computing

Ubiquitous Network Society

Human-centric


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1.IntroductionUbiquitous

Ubiquitous

7A

Anytime

Anyone

Anywhere

Any Device

AffordableAll Security

Any Information/Service


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1.IntroductionGeneral Purpose

A wireless sensor network (WSN) is a

wireless network using sensors to

cooperatively monitor physical or

environmental conditions

The development of wireless sensor

networks was originally motivated by

military applications.

Wireless sensor networks are now used in

many wide-range application areas.


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1.IntroductionSensors

Image Sensor Modules (8 8 5.7mm)

Ultrasonic Magnetic Sensor (22.5 22.5 39mm)

WII sensor (240 35 15mm)


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1.Introduction

sensor characteristics

Wireless sensors are small devices that

gather information.

Pressure, Humidity, Temperature

Speed, Location

Wireless sensors have some

characteristics:

Low power

Small size

Low cost


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1.Introductionsensor network characteristics

Primary Function

Sample the environment for sensory

information

Propagate data back to the infrastructure

Traffic pattern in sensor network

Low activity in a long period

Bursting data in short time

Highly correlated traffic


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1.Introductionsensors categories

Sensors can be classified into two

categories:

Ordinary Sensors

Data gathering

Ordinary Sensors require external circuitry to

perform some dedicated tasks like data analyzing.

Smart Sensors Data gathering and processing

Smart Sensors have internal circuitry to perform

dedicated tasks.


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1.IntroductionRelated Work

Related work

CSMA

To improve the energy consumption by avoiding

overhearing among neighboring nodes

TDMA

No contention-introduced overhead and collisions Not easy to manage the inter-cluster

communication and interference

Not easy to dynamically change its frame length

and time slot assignment


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1.IntroductionRelated Work

PAMAS Power off radio when not actively transmitting

and receiving packet.

Zigbee

Combined with IEEE 802.15.4 (Low-RateWireless Personal Area Network, LR-WPAN)

Low rate: 250kbps

Short distance: 50-300m

Low power consumption frequency band:

Global: 2.4GHz ,16 channels

America: 915MHz, 10 channels

Europe: 868MHz, 1 channel.


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1.IntroductionZigbee stack

Zigbee Platform Stack and IEEE802.15.4

PHY Layer

MAC Layer

Network / Security

Layers

Application Framework

Application/Profiles

IEEE

802.15.4

ZigBee or OEM

ZigBee

Alliance

Platform


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1.IntroductionZigbee Application

Reference: NTPZigBee


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2.MAC for Sensor NetworkSensor Network MAC Protocol

Carrier Sensing

Only during low traffic load.

Backoff

Backoff in application layer is desired other

than in MAC layer.

Contention

RTS-CTS only during high traffic load.


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2.MAC for Sensor NetworkSources of Energy Wastage

The major sources of energy wastage are:

Collisions

Overhearing

Control packet overhead

Idle listening

Achieving good scalability and collision

avoidance capability is necessary.


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2.MAC for Sensor NetworkS-MAC

Sensor-MAC (S-MAC): Medium Access Control

for Wireless Sensor Networks

S-MAC is a medium-access control (MAC)protocol designed for wireless sensor networks.

Sensor networks are deployed in an ad hocfashion, with individual nodes remaining largely

inactive for long periods of time, but then

becoming suddenly active when something is

detected.


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2.MAC for Sensor NetworkS-MAC

These characteristics of sensor networks

and applications motivate a MAC that is

different from traditional wireless MACs

such as IEEE 802.11 in almost every way

Energy conservation and self-configuration

are primary goals.

Per-node fairness and latency are lessimportant.


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2.MAC for Sensor NetworkThree techniques in S-MAC

S-MAC uses three techniques to reduce

energy consumption.

Nodes go to sleep periodically.

Nearby nodes form virtual clusters to

synchronize their wake-up and sleep periods

to keep the control packet overhead of thenetwork low.

Message passing is used to reduce the

contention latency and control overhead.


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Periodic Listen and Sleep:

Nodes do not waste energy by listening to an

empty channel or when a neighboring node is

transmitting to another node.

Nodes use RTS and CTS to talk to each other

and contend for the medium.


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Collision and Overhearing Avoidance:

S-MAC adopts a contention-based scheme to

avoid collisions.

A duration field is introduced in each

transmitted packet which indicates how much

longer the transmission will last.

When a node receives a packet, it will not transmitany packets for at least the time that is specified in

the duration field.


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Collision and Overhearing Avoidance:

Overhearing is avoided by letting the nodes,

which get RTS and CTS packets which are

not meant for them, go to sleep.

All immediate neighbors also go to sleep till

the current transmission is completed after a

sender or receiver receives the RTS or CTSpacket.


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Message Passing:

Long messages are fragmented into smaller

messages and transmitted in a burst.

To avoid the high overhead and delay encounteredfor retransmitting when message is lost.

ACK messages are used to indicate if a

fragment is lost at any time.

The sender can resend the fragment again.

The ACK message also have the duration field to

reduce overhearing and collisions.


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3. Challenges

Challenges:

1. Energy Efficiency:

Power consumptions are crucial to wireless sensor

network applications because sensor nodes arenot connected to any energy source.

Energy efficiency is a dominant consideration no

matter what the problem is.

Sensor nodes only have a small and finite sourceof energy. Many solutions, both hardware and

software related, have been proposed to optimize

energy usage.


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3. Challenges

2. Ad hoc deployment:

Most sensor nodes are deployed in regions whichhave no infrastructure.

We must cope with the changes of connectivityand distribution.

3. Unattended operation:

Generally, once sensors are deployed, there is nohuman intervention for a long time.

Sensor network must reconfigure by itself when

certain errors occur.


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3. Challenges

4. Dynamic changes:

As changes of connectivity due to addition of more

nodes or failure of nodes, Sensor network must beable to adapt itself to changing connectivity.


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4.Coverage

Coverage can be classified into threetypes:

Area coverage

deployment of sensors to cover a given area

Point coverage

deployment of sensors to cover a set of points

Barrier coverage

The goal is to minimize the probability ofundetected penetration through the barrier.

To find a path in a region

For any point on the path, the distance to the

closest sensor is minimized.


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4.Coverage

Area coverage

Areacoverage

deployment

of sensorsto cover a

given area


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4.Coverage

Point coverage

Point

coverage

deployment of

sensors tocover a set of

points


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4.Coverage

Point coverage

Barrier

coverage

To find a path

from A to B

For any point

on the path, the

distance to theclosest sensor

is minimized.

A

B


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5.Localization

In sensor networks, nodes are deployed

without priori knowledge about their

locations.

Estimating spatial-coordinates of the node

is referred to as localization.


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5.LocalizationGPS

Global Positioning System (GPS) is an

immediate solution.

There some factors against the usage of

GPS:

GPS can work only outdoors.

GPS receivers are too expensive to

unsuitable for wide-range deployment.

It cannot work in the presence of obstructions.


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5.LocalizationCategories

Localization can be classified into two

categories:

Fine-grained

Based on timing / signal strength

Coarse-grained

Based on proximity


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5.LocalizationProximity base localization

Trilateration / Multilateration technique

Proximity based localization:

Some nodes which can know their position through

some technique (ex. GPS) broadcast their positioninformation.

Other nodes listen to these broadcast messages

and calculate their own position.

A simple method would be to calculate its positionas the centroid of all the positions it has obtained.

This method leads to accumulation of localization

error.


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5.LocalizationTrilateration Example

Trilateration

A is 5m from B

A is 10m from C

A is 8m from D

B

C

D

A


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5.LocalizationTrilateration

Trilateration is a geometric principle which

allows us to find a location if its distance

from other nodes are known.

The same principle can be extended to

three-dimensional space.

Four spheres would be needed to locate

certain point in 3D space.


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5.Localization

Fine-grained method

Signal strength method

Attenuation happens when signals are

propagated. We can use the degree of

attenuation to calculate the distance.

Timing method

The distance between two nodes isdetermined by the time of flight of the signal.


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6.RoutingThree types in sensor network

Because of the energy constrained nature

of sensor networks, conventional routing

protocols have many limitations when

being applied to sensor networks.

Three types of routing protocol in sensor

network:

Data-centric

Hierarchical

Location-based


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6.RoutingData-centric

Data-centric: Managers broadcast a Query message to the network.

If a sensor observes some events related to the

Query message, it sends the data to the data center. Data aggregation:

sensor1

sensor2

Relay

node1

Data

Center

Data A

Data A

Data A


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6.Routing

Data-centric

Data centric: Flooding

Flooding is one of basic data transmitting

methods.

If any sensor receives or generates some

packets, it will broadcast these packets to

all its neighbors.

Nodes may receive duplicate data.

More power consumption.


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6.Routing

Data-centric

Data centric: SensorProtocols for

Information via Negotiation (SPIN)

There are three messages in SPIN:

Advertisement (ADV): When a node has some

data to send, it sends an ADV message to its

neighbors containing data descriptor (meta-data).

Request (REQ): When a node wants to receive

some data. It sends an REQ message first.

DATA: Actual data message with a meta-data

header.


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6.Routing

Data-centric

Node1 Node6

Node3

Node2

Node4

Node5

Node7

ADV

(meta data A)

ADV

(meta data A)

ADV

(meta data A)

REQ

(meta data A)

DATA

(meta data A)

ADV

(meta data A)

ADV

(meta data A)

ADV

(meta data A)

REQ

(meta data A)

DATA

(meta data A)

SPIN:


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6.Routing

Data centric

Directed Diffusion

The propagation of data and its aggregation

at intermediate nodes on the way to the

request originating node are determined bythe messages which are exchanged between

neighboring nodes within some distance.

6


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6.Routing

Data centric

Node5

Node4

Node3

Node2

Node1

Node7

interest

interest

interest

interest

interest

interest

Node6

interest

interest

interest

interest

Gradient

Gradient

Gradient

Gradient

Gradient

Gradient

Gradient

Gradient

Gradient

Gradient

Node5

Node4

Node3

Node2

Node1

Node7

Node6

return path (Gradient)Forward interest

Directed Diffusion:

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6.Routing

Data centric Directed Diffusion:

Sender can choose the best return path.

EX: minimum response time, least hops

Node5

Node4

Node3

Node2

Node1

Node7

Node6

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6.Routing

Hierarchical

Hierarchical: Low EnergyAdaptive

Clustering Hierarchy (LEACH)

LEACH is a two-tier protocol.

Cluster head

Cluster member

Every node runs a random algorithm

periodically to decide its identity. (cluster heador not)

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6.Routing

Hierarchical

LEACH

All cluster heads broadcast Advertisement

(ADV) message and other nodes decide

which cluster they belong to according thestrength of ADV message.

Cluster members only send data to their

cluster head. Then, cluster heads reply data

to Sinks.


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6.Routing

Location-based

Location-based: GeographicAdaptiveFidelity (GAF)

GAF divides the network into several

virtual grids. For adjacent virtual grids A and B, every node

in A can directly connect with every node in B.

In GAF, every node has three types ofstatus:

Active

Discovery

Sleep

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6.Routing

Location-based

GAF:

Initially, every node is in discovery status and

tries to find out nodes belong to the same grid

with itself. Every node in discovery status sets a timer

Td. Once the Td timer ends, Nodes

broadcast discovery message and get into

active status.

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6.Routing

Location-based

GAF:

When a node is in active status, it will start a

timer Ta.

Data transmission is allowed until Ta timer ends. In active status, nodes will periodically broadcast

discovery message at Td intervals.

Once Ta timer ends, nodes return to discovery

status.

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6.Routing

Location-based

GAF:

If a node in discovery status receives a

discovery message sent from the node which

is in the same grid and has higher ranking, itwill get into sleep status.

After a Ts timer, it will return to discovery status.

Ranking can be done by remaining power or ID

sequence.

chapter6 sensor network

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