WSN

Wireless Sensor Networks
CSC 687
Jerry Yoakum

Outline
 Overview
– Environment Monitoring
– Medical application
 Data-dissemination schemes
 Media access control schemes
 Distributed algorithms for collaborative processing
 Architecture for a Wireless Sensor Network
 Final Remarks

Overview
 A Wireless Sensor Network (WSN) consists
of base stations and a number of wireless
sensors (nodes).

Overview (Cont.)
Wireless Sensor Node: Components

Overview (Cont.)
Characteristics of Wireless Sensor Networks
 Requirements: small size, large number, tether-less, and low cost.
Constrained by
– Energy, computation, and communication
 Small size implies small battery
 Low cost & energy implies low power CPU, radio with minimum
bandwidth and range
 Ad-hoc deployment implies no maintenance or battery
replacement
 To increase network lifetime, no raw data is transmitted

Overview (Cont.)
Ad Hoc Wireless Networks
 Large number of self-organizing static or mobile
nodes that are possibly randomly deployed
 Near(est)-neighbor communication
 Wireless connections
– Links are fragile, possibly asymmetric
– Connectivity depends on power levels and fading
– Interference is high for omnidirectional antennas
 Sensor Networks and Sensor-Actuator Networks are
a prominent example.

Overview (Cont.)
Distinguishing Features
WSNs are ad hoc networks (wireless nodes that self-organize into
an infrastructureless network).
BUT, in contrast to other ad hoc networks:
 Sensing and data processing are essential
 WSNs have many more nodes and are more densely deployed
 Hardware must be cheap; nodes are more prone to failures
 WSNs operate under very strict energy constraints
 WSN nodes are typically static
 The communication scheme is many-to-one (data collected at a
base station) rather than peer-to-peer

Overview (Cont.)
Lifetime
 Nodes are battery-powered
 Nobody is going to change the batteries. So, each operation
brings the node closer to death.
"Lifetime is crucial!”
To save energy:
 Sleep as much as possible.
 Acquire data only if indispensable.
 Use data fusion and compression.
 Transmit and receive only if necessary. Receiving is just as
costly as sending.

Overview (Cont.)
Scalability and Reliability
WSNs should
 self-configure and be robust to topology changes (e.g., death of
a node)
 maintain connectivity: can the Base Station reach all nodes?
 ensure coverage: are we able to observe all phenomena of
interest?
Maintenance
 Reprogramming is the only practical kind of maintenance.
 It is highly desirable to reprogram wirelessly.

Overview (Cont.)
Data Collection
 Centralized data collection puts extra burden on nodes close to
the base station. Clever routing can alleviate that problem
 Clustering: data from groups of nodes are fused before being
transmitted, so that fewer transmissions are needed
 Often getting measurements from a particular area is more
important than getting data from each node
 Security and authenticity should be guaranteed. However, the
CPUs on the sensing nodes cannot handle fancy encryption
schemes.

Overview (Cont.)
Power Supply
 AA batteries power the vast majority of existing platforms. They
dominate the node size.
 Alkaline batteries offer a high energy density at a cheap price.
The discharge curve is far from flat, though.
 Lithium coin cells are more compact and boast a flat discharge
curve.
 Rechargeable batteries: Who does the recharging?
 Solar cells are an option for some applications.
 Fuel cells may be an alternative in the future.
 Energy scavenging techniques are a hot research topic
(mechanical, thermodynamical, electromagnetic).

Overview (Cont.)
Radio
 Commercially-available chips
 Available bands: 433 and 916MHz, 2.4GHz ISM bands
 Typical transmit power: 0dBm.
Power control
 Sensitivity: as low as -110dBm
 Narrowband (FSK) or Spread Spectrum communication. DS-
SS (e.g., ZigBee) or FH-SS (e.g., Bluetooth)
 Relatively low rates (<100 kbps) save power.

Overview (Cont.)
CPU
 The Microcontroller Unit (MCU) is the primary choice for
in-node processing.
 Power consumption is the key metric in MCU selection.
 The MCU should be able to sleep whenever possible,
like the radio.
 Memory requirements depend on the application
 ATmega128L and MSP430
are popular choices

Overview (Cont.)
Sensors
 The power efficiency of the sensors is also
crucial, as well as their duty cycle.
 MEMS techniques allow miniaturization.

Overview (Cont.)
Applications of Wireless Sensor Networks
 Military and national security application
 Environment monitoring (examples coming)
 Medical application (example coming)
 Nearly anything you can imagine

Environment monitoring (1)
Great Duck Island
• 150 sensing nodes deployed throughout the island relay data
temperature, pressure, and humidity to a central device.
• Data was made available on the Internet through a satellite link.

Environment monitoring (2)
Zebranet: a WSN to study the behavior of zebras
• Special GPS-equipped collars were attached to zebras
• Data exchanged with peer-to-peer info swaps
• Coming across a few zebras gives access to the data

Medical application
 Intel deployed a 130-node network to monitor
the activity of residents in an elder care
facility.
 Patient data is acquired with wearable
sensing nodes (the “watch”)
• Vital sign monitoring
• Accident recognition
• Monitoring the elderly

Data-dissemination Schemes
Conventional Methods
 Direct communication with the base station
– Sensor nodes communicate with the base station directly.
– Energy consuming.
 Multi-hop Scheme
– Transmit through some other intermediate nodes.
– Energy consuming.

Clustering Hierarchy
Data-dissemination Schemes (Cont.)

Low Energy Adaptive Clustering
Hierarchy (LEACH)
 Designed for sensor networks where an end-user wants to
remotely monitor the environment. Where the data from the
individual nodes must be sent to a central base station, often
located far from the sensor network.
 Desirable properties for protocols on these networks:
– Use 100’s – 1000’s of nodes
– Maximize system lifetime
– Maximize network coverage
– Use uniform, battery operated nodes
 The use of distributed cluster formation and local processing to
reduce global communication along with randomized rotation of
the cluster-heads allows LEACH to achieve the desired properties
while being energy-efficient.
Data-dissemination Schemes (Cont.)

Media Access Control (MAC) Scheme
Two categories of MAC schemes for wireless networks
 Contention-based schemes
– Designed for minimum delay and maximum throughput.
– Require transceivers to monitor the channel at all times.
 Reservation-based or schedule-based schemes
– Detect the neighboring radios before allocating collision-free
channel to a link.
– TDMA —a natural choice for sensor networks.

MAC Scheme (Cont.)
TDMA-based solutions
 The self-organizing “super frame” algorithm
– Super frame = TDMA period + BOOTUP period + unused
bandwidth
– Performs well only under the specific conditions.
 Power Aware Clustered TDMA (PACT)
– Divide the TDMA structure into control slot and data slot.
– Hard to maintain the cluster when there are mobile nodes.

MAC Scheme (Cont.)
Other solutions
 Sensor-MAC (SMAC)
– Nodes periodically sleep.
– A node sleeps during transmission period of other
nodes.
– Not suitable for time-critical applications.

Architecture for a WSN
Special addressing requirement
 Local unique addresses
 Data-centric
 Example: Each node has an unique number.
Attribute-based naming architecture
 Data is named by one or more attributes.
 Example: Each node is distinguished by an
attribute – GPS sensors are practical for this.

Architecture for a WSN (cont.)
An address-free architecture (AFA)
 Advantage
– Randomly select probabilistically unique identifier for each
transaction.
– Spatial locality.
– Temporal locality.
 Drawback
– Not applicable when static addressing of nodes is needed.
– Identifier conflict.

Final Remarks
 Can you think of any applications where a
wireless sensor network would be the best
solution?
 Do you foresee wireless sensor networks
becoming ubiquitous within the next ten
years? During your lifetime?

WSN

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Editor's Notes