wireless sensor networks (wsn) · wireless sensor networks (wsn) introduction . m. schölzel . ......
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Wireless Sensor Networks (WSN)
Introduction
M. Schölzel
Difference to existing wireless networks
Infrastructure-based networks e.g., GSM, UMTS, …
Base stations connected to a wired
backbone network
Mobile entities communicate wirelessly to these base stations
Traffic between different mobile entities is relayed by base stations and wired backbone
Infrastructure-free networks Try to construct a network without
infrastructure, using networking abilities of the participants
This is an ad hoc network – a network constructed “for a special purpose”
Simplest example: Laptops in a
conference room – a single-hop ad hoc network
IP backbone
Server Router
Gateways
A Wireless Sensor Network
Sensornet
Internet, LAN, GSM-Network, …
Network is embedded in environment
Nodes in the network are equipped with sensing and actuation to measure/influence environment
Nodes process information and communicate it wirelessly
wireless communication wired communication
Roles of Participants in WSN
sources of data: Measure data, report them “somewhere” Typically equip with different kinds of actual sensors sinks of data: Interested in receiving data from WSN May be part of the WSN or external entity, PDA, gateway, … actuators: Control some device based on data, usually also a sink
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WSN Application Examples
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Goal: Collecting data Environmental Monitoring Each node measures temperature Derive a “temperature map”
Intelligent buildings, bridges, or machines Measuring vibrations Control of leakages in chemical plants Monitor mechanical stress (e.g. during
earthquakes) Predictive maintenance
Example: Parking Space Management
Each parking space is equipped with a sensor node AMR sensor senses disturbances on the magnetic field of the earth
(resistance changes, if a car is above the sensor) Hardware:
− 8-bit microcontroller − RF transceiver: CC1120
with 4kbps data rate − 3.6V Li-ion battery
Rather simple network topology: Single hop from sensor node to sink.
Other WSN Application Domains
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Precision agriculture Bring out fertilizer/pesticides/irrigation only where needed
Medicine and health care Wearable sensor nodes monitoring movement of patients Logistics Equip goods (parcels, containers) with a sensor node Track their whereabouts – total asset management E.g. Bosch already provides commercial solutions
Telematics Provide better traffic control by obtaining finer-grained information about traffic conditions Intelligent roadside Cars as the sensor nodes
Examples Car2Car and Car2X communication
Communication Standard WLAN-p already exists
Cars have on-bord-units, infrastructure has road-side-units
Cohda-Box commercially available − 32-bit ARM-like processor − runs a Linux system − up to 27 mbps
Cars and infrastructure can send various kinds of
messages − CAMs: position, direction, speed, etc. − DENMs: Detected hazards − SPATs: State of the traffic light − …
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Cohda OBU
WSN Application Examples
Goal: Building control loops (sensing -> computing -> acting) Communication in industrial environments Communication between mobile machines
Requirement real time demands
− Wireless heart standard much higher reliability of communication required
− probability of packet error rate ~10-9
− typical bit error rate in wireless channels: 10-3 to 10-4
wireless ad-hoc networks: problems
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Without a central infrastructure, things become much more difficult Problems are due to Lack of central entity for organization available Limited range of wireless communication Battery-operated entities Mobility of participants
Problem: Lack of Central Entity
Without a central entity (like a base station), participants must organize themselves into a network (self-organization) Pertains to (among others): Medium access control – no base station can assign
transmission resources, must be decided in a distributed fashion
Finding a route from one participant to another
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Problem: Limited Range
For many scenarios, communication with peers outside immediate communication range is required Direct communication limited because of distance, obstacles,
… Solution: multi-hop network
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Problem: Battery-Operated Entities
Often (not always!), participants in an ad hoc network draw energy from batteries Desirable: long run time for Individual devices Network as a whole
Required: Energy-efficient networking protocols E.g., use multi-hop routes with low energy consumption (energy/bit) E.g., take available battery capacity of devices into account How to resolve conflicts between different optimizations?
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Problem: Mobility
In many (not all!) ad hoc network applications, participants move around (-> mobile ad hoc networks (MANET)) Car2Car-communication Moving robots in industrial environments In cellular network: simply hand
over to another base station
In MANET − Mobility changes neighborhood relationship − Routes needs adaptation − Complicated by scale
• Large number of such nodes difficult to support
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Requirements for WSN (user perspective)
Quality of Service (QoS) Detect events and deliver data reliable
− events may be detected by multiple nodes − reliability of a single node doesn’t matter, system reliability is important!
Deliver data in within a specified delay Lifetime The network should fulfill its task as long as possible – definition depends
on application Lifetime of individual nodes relatively unimportant
Fault tolerance Be robust against faults
− node failures (often permanent): running out of energy, physical destruction − link failures (often temporarily): weather, moving obstacles, …
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Requirements for WSN (administrator perspective)
Deployment Simple deployment even for thousands of nodes (self-organization) Scalability Support large number of nodes (several thousands)
− in existing practical applications < 100
Maintainability WSN has to adapt to changes, self-monitoring, adapt operation Incorporate possible additional resources, e.g., newly deployed nodes Programmability
− Re-programming of nodes in the field might be necessary, improve flexibility, fixing bugs
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Mechanisms to Overcome Problems and to Meet Requirements
Multi-hop wireless communication to overcome limited communication range requires self-organization mechanisms Energy-efficient operation For communication, computation, sensing, actuating Very typical: computation far less energy hungry than communication
− Collaboration & in-network processing • Nodes in the network collaborate towards a joint goal • Pre-processing data in network (as opposed to at the edge) can greatly improve efficiency
− Locality • Do things locally (on node or among nearby neighbors) as far as possible
− Data centric networking • Focusing network design on data, not on node identifies (id-centric networking) • To improve efficiency
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Mechanisms to meet requirements
Auto-configuration Manual configuration just not an option Right after deployment But also periodically during operation to overcome
− link and node failures − mobility problems
Exploit tradeoffs during the design phase E.g., between invested energy and accuracy Design-space-exploration
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Design Space
Due to the many options for tackling problems, WSNs offer a very large design space
Trade-offs between many design space parameters must be found in order to meet the requirements
Design space parameters:
Form Factor
Heterogeneity
Communication Modality
Sensor Coverage
Radio Coverage
Network Topology
Connectivity
Design Space
Form factor of the mote classes: brick, matchbox, grain, dust Depends on application (microscopic to shoebox) Costs may range from cents to hundreds of euro's Impacts
− life time (e.g. size of battery), processing resources, costs Heterogeneity of the WSN classes: homogeneous/heterogeneous First approach: identical nodes only In practice: a variety of nodes can be very useful
− e.g. cluster heads with more resources − special capabilities only required for some nodes (e.g. GPS) − gateways to external networks (GSM, satellite, Internet)
Impacts − Complexity of software
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Design Space
Communication Modality classes: radio (169MHZ, 434MHz, 868MHz, …), visual light communication (VLC), ultrasound, … How do nodes communicate ? most common: radio waves, usually sub-gigahertz bands light beams or laser: smaller, more energy efficient (cf. Smart Dust) Impacts
− data rate, life time, communication range
Connectivity classes: connected / intermittent / sporadic Nodes always connected or only sometimes (regularly or sporadic)? Impacts
− Protocols, data gathering, life time
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Design Space
Sensor-Coverage classes: sparse / dense / redundant sensors could cover only part of area of interest, or all, or the same area is
covered multiple times Impacts:
− observational accuracy, number of nodes, costs
Radio-Coverage classes: from sparse to dense / static /dynamically How many other nodes will be in the communication range? May be adapted at runtime Impacts:
− reliability, energy consumption due to collisions, transmission power, etc., number of nodes
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Design Space
Network Topology classes: infrastructure or ad-hoc (single-hop / star / networked stars / tree / graph) infrastructure-based: motes communicate via base stations only
− often costly
ad-hoc: direct communication between nodes − no expensive infrastructure
diameter is the max number of hops between any two nodes − single hop (d=1), infrastructure based (d=2), multi-hop (d big for ad-hoc networks)
Impacts: − Communication delay, protocol complexity
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Development Flow and Design Space Exploration
Models − used for an early evaluation of possible design
parameters
Simulations: − Evaluate the prediction of the model in the full
dynamic of the network − Test and debug to find software bugs − Not all aspects of the hardware are accurately
reflected
Testbed uses real hardware − Test the software on real hardware − Test and Debug in a distributed system difficult − Systematic test of fault-tolerance techniques not easy
(often the code is instrumented) − real sensors and actors maybe not available →
virtualization
Deployment test in a real setup is required − Not always possible − Test-bed infrastructure not available anymore
• Over-the-air update • power measurement
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Implementation (+Software)
Simulation
Test/Debug Testbed (+Hardware)
Start with requirements
Deployment (+Topology)
Models Evaluation
Test/Debug
Required Knowledge for Developing WSN Applications
Knowledge of Models and Simulators
Microprocessor architecture and programming − Communication with peripherals − Usage of operating systems − Power saving techniques
Architecture and Design of Protocols
− Power saving techniques − Fault Tolerance
• Forward- and backward-error correction