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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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2.MAC for Sensor NetworkThree techniques in S-MAC
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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2.MAC for Sensor NetworkThree techniques in S-MAC
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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2.MAC for Sensor NetworkThree techniques in S-MAC
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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2.MAC for Sensor NetworkThree techniques in S-MAC
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.
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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.