Transcript Slide 1

Wireless Sensor Networks
(WSN)
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Comparing to MANET
• Similarities
– No infrastructure
– Multi-hop communication
• Differences
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Nodes are more resource-constrained and more prone to failure
More nodes (up to hundreds or thousands) in a network
Random deployment
Unattended
Longer life time
Trust relationships between sensor nodes (typically belong to the
same organization)
– Application-specific
– …
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Usage of Sensor Networks
• Environmental observation
– Water/air pollution detection
– Forest fire detection
– Animal habitat monitoring
• Military monitoring
– Battlefield surveillance
– Vehicular traffic monitoring
– Tracking the position of the enemy
• Building monitoring
– Monitoring climate changes/vibration
• Healthcare
– Being implanted in the human body to monitor medical problems
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Some Real Examples
• Great Duck Island
– A prototype sensor network is deployed to monitor the nesting
grounds of elusive seabirds
– Biologist get information they need with minimal human
disturbance
• Vineyard
– Embedded sensors are deployed to monitor temperature in a
vineyard in Oregon’s Willamette Valley.
• Golden Gate Bridge
– 200 motes organized in an ad hoc sensor network are used for
tracking stress on the bridge
• Proactive Health Research Project (Intel)
– Help seniors age with dignity and independence, by developing
sensor network-based in-home technology prototypes.
• Preventive maintenance on an oil tanker in the North
Sea (Intel and BP)
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Sensor Nodes
• MICA2(Motes): a popular research platform at the moment
(J. Hill, et al., “The platforms enabling wireless sensor networks,”
Comm. of the ACM, June 2004/Vol 47. No. 6, pages 41-46)
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Sensor Nodes: MICA2
• The core is a small, low-cost, low-power computer
– Atmel Atmega 128L processor (4MHZ), 128 KB on-board flash
memory
– As powerful as 8088 CPU (in original IBM PC)
– Power consumption
• 8 milliamps (running), 15 micro-amps (sleep)
• One or more sensors can be mounted
• Connect to the outside world with radio
– Transmission range: 10-200 feet, Rate: 76bps
– Power consumption
• 25 milliamps (Trans), 10 milliamps (Recv), <1 milliamps (off)
• Power supply: 2 AA batteries
– 2,000 milliamps-hours
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(http://computer.howstuffworks.com/mote.htm)
Sensor Nodes
• Four classes
(J. Hill, et al., “The platforms enabling wireless sensor networks,”
Comm. of the ACM, June 2004/Vol 47. No. 6, pages 41-46)
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Physical layer and MAC sublayer
• Related Standard
– IEEE 802.15.4 Wireless Medium Access Control (MAC) and
Physical Layer (PHY) Specifications for Low-Rate Wireless
Personal Area Networks (LR-WPANs)
• Physical layer
– 2.4-2.4835 GHZ (worldwide)
– 902-928 MHz (North America) or 868-868.6 MHz (Europe)
• MAC Sublayer
– Contention Access
• CSMA-CA
– Contention Free
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Other MAC Protocols (not complete)
Protocol
Summery
PAMAS
Power Aware Multi Access protocol with Signaling for Ad Hoc
Networks: uses two separate channels, one for control
packets and one for data packets. It puts the nodes to sleep
when the neighbor is transmitting.
S-MAC
Periodic Listen and Sleep, Collision and Overhearing
Avoidance, Message Passing, Implemented over Berkeley
Mote.
TDMAbased MAC
protocol
Assumes a gateway per cluster where the gateway assigns
the time slots for each node.
Adaptive
Rate Control
Random
Delay
Good for per node fairness. Uses adaptive rate control to
adapt the originating data and route-thru traffic. Rate control
mechanism uses linear increase and multiplicative decrease
where each one. Their CSMA scheme uses sensor sampling
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phase shift to avoid capturing effect.
Deployment
• Random deployment
– Large number of nodes; target area may be remote
and/or hostile  manual deployment is impossible in
many cases
• Problems caused by random deployment
– Localization
• Sensor nodes must discover their locations after deployment
– Coverage
• For sensing quality, a certain level of sensing coverage should be
achieved.
– Security
• It is hard to store various encryption keys on nodes, since the
neighborhood cannot be know a priori.
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Localization
• Basic idea of existing schemes
– Initial configuration
• A small number of beacon nodes
– Know their locations by using GPS or being set manually
• A large number of nodes (non-beacon nodes) that do not know
their locations
– Localization process
• Beacon nodes send beacon signals to a set of non-beacon nodes
• A non-beacon node obtains
– Locations of the beacon nodes
– Some features related to the distance to these beacon nodes
» Received signal strength indicator, Time to arrival, etc.
• The non-beacon node estimate its own location based on the
obtained information
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Sensing Coverage
• Objectives
– High sensing coverage (full coverage is ideal)
– Energy efficient and low cost
• Samples of existing work
– Sleep scheduling (Ye et al. 03’, Gui et al. 04’)
• Over deploying sensor nodes and make spare nodes sleep
• Waking up sleeping nodes following certain sleep planning
methods
– Employing mobile sensor nodes (Wang et al.03’&05’)
• Deploy a large fraction of stationary nodes and a small fraction of
mobile nodes
• Stationary nodes detect sensing holes and notifies mobile nodes
• Mobile nodes move to heal the holes
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Network Architectures: Flat
Sink (data collector)
•A large number of sensor nodes form a peer-to-peer ad hoc network
•They forward message for each other
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Network Architecture: Hierarchical
Sink
Cluster head
•The network is divided into clusters, each cluster has a head (logically or
physically)
•Ordinary node  cluster head; cluster heads form a backbone
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Network Architecture: Hierarchical
(J. Hill, et al., “The platforms enabling wireless sensor networks,”
Comm. of the ACM, June 2004/Vol 47. No. 6, pages 41-46)
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Communication Patterns
• A sensor network is a content-based (or datacentric) network
– In WSN, networking take place directly on contents (or
data)
• In Internet/MANET, networking protocols use identifiers of nodes.
– Contents can collected, processed and stored in the
network
• Desirable interaction paradigm in WSN:
Publish/Subscribe
– Entities can publish data under certain names
– Entities can subscribe to updates of such named data
(H. Karl, “Ad hoc and sensor networks Chapter 12: Data-centric and
content-based networking”)
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Naming
• Content based naming
– Tasks are named by a list of attribute – value
pairs
– Task description specifies an interest for data
matching the attributes
• Animal tracking:
Request
Interest ( Task ) Description
Type = four-legged animal
Interval = 20 ms
Duration = 1 minute
Location = [-100, -100; 200, 400]
Reply
Node data
Type =four-legged animal
Instance = elephant
Location = [125, 220]
Confidence = 0.85
Time = 02:10:35
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Communication Patterns
• External storage-based pattern
– fixed subscriber
• Sink-initiated pattern
– subscriber initiated
• Source-initiated pattern
– publisher initiated
• Rendezvous-based pattern
– Intermediate entities help to match publishers and subscribers
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External Storage-based Pattern
source
sink
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Sink-initiated Pattern: Directed Diffusion
• The sink periodically broadcasts interest
messages to each of its neighbors
• Every node maintains an interest cache
– Each item corresponds to a distinct interest
– No information about the sink
– Interest aggregation : identical type, completely
overlap rectangle attributes
• Each entry in the cache has several fields
– Timestamp: last received matching interest
– Several gradients: data rate, duration, direction
(Intanagonwiwat et al., “Directed Diffusion for wireless sensor
networking,”, IEEE/ACM Transactions on Networking (TON), Feb, 2003)
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Directed Diffusion
• Set up gradient: Constrained or Directional flooding
based on location.
Gradient
Source
Interest
Sink
(Intanagonwiwat et al., “Directed Diffusion for wireless sensor
networking,”, IEEE/ACM Transactions on Networking (TON), Feb, 2003)
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Directed Diffusion
Gradient
Source
Data
Sink
(Intanagonwiwat et al., “Directed Diffusion for wireless sensor
networking,”, IEEE/ACM Transactions on Networking (TON), Feb, 2003)
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Source-initiated Patterns
Dissemination Node
Data Announcement
Source
Data
Sink
Query
Immediate
Dissemination
Node
(Ye et al., “TTDD: A Two-tier Data Dissemination Model for
Large-Scale Wireless Sensor Networks,” Mobicom’02)
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Rendezvous-based pattern
• Data with the same name are stored at the
same place
PDA
(Ratnasamy et al., “Data-centric Storage in Sensor Networks
with GHT,” Mobile Networks and Applications, 2004.)
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Rendezvous-based pattern
• Fault tolerance consideration
• If a storing fails, it is replaced by another node closest
to itself
• To protect the stored data, data can be replicated in
multiple nodes
(Ratnasamy et al., “Data-centric Storage in Sensor Networks
with GHT,” Mobile Networks and Applications, 2004.)
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Routing
• Address-centric (AC) protocol
– Each source independently sends data along the shortest path to sink
based on the route that the queries took (“end-to-end routing”)
• Data-centric (DC) protocol
– The sources send data to the sink, but routing nodes en-route look at
the content of the data and perform some form of
aggregation/consolidation function on the data originating at multiple
sources.
(Krishnamachari et al., “Modeling data-centric
routing in wireless sensor networks”, 2002.)
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Data Aggregation
• Types
– Removing redundancy
– In-network processing
After aggregation,
the redundancy is
removed
Sensing range of A
Redundancy
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Sensing range of B
Data Aggregation: in-network processing
• Aggregation is implemented by three functions
– Merging function f, initializer i and evaluator e.
• General form <z> = f (<x>,<y>)
– <x> and <y> are multi valued partial state records
– <z> is partial state record resulting from application of
f to <x> and <y>
f(<3,1>,<5,1>) = <8,2>
• E.g. f is the merging function of AVERAGE
f(<S1,C1>,<S2,C2>)=<S1 + S2, C1 + C2>
i(x) = <x,1>
e(<S,C>) = S/C
where S and C are Sum and Count.
f(<8,2>,<7,1>) = <15,3>
e(<15,3>) = 15/3 = 5
i(7)=<7,1>
<3,1>
i(3)=<3,1>
<5,1>
i(5)=<5,1>
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Security Issues
• Major challenges
– Resource constraints in computation, storage
and communication
• Public key is too expensive
– Private key operations in MICA2: a few milliseconds
– RSA-based public key operations: tens of seconds (5060)
– ECC-based public key operations: tens of seconds
(~30s)
• Security mechanisms must be low-cost
– Unattended deployment & lack of temper
resistance
• Must address node compromise
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Security Issues
• Existing research
– Low-cost key management
• Pair-wise key & group-wise key
– Message and entity Authentication
– Securing protocols in sensor networks
• Localization
• Data aggregation
•…
– Privacy/Anonymity
• Protecting data source
(details will be discussed in later classes)
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Standardization of Sensor Networks
• Zigbee Aliance
– The ZigBee Alliance is an association of companies
working together to enable reliable, cost-effective,
low-power, wirelessly networked, monitoring and
control products based on an open global standard.
– Focus
• Defining the network, security and application software
layers
• Providing interoperability and conformance testing
specifications
• Promoting the ZigBee brand globally to build market
awareness
• Managing the evolution of the technology
– Members: more than 100 companies
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Zigbee Specification (June 2005)
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