applications characteristics and requirements of sensor networks
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Applications Characteristics
and Requirements of Sensor Networks
Applications Characteristics
and Requirements of Sensor Networks
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OutlineOutline
WSN Characteristics and Requirements Goals and Application requirements Design principles
Sensor Node Architecture Hardware components
Energy Consumption
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Sensor Network TopologySensor Network Topology
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Wireless Sensor NetworksWireless Sensor Networks
Sensor nodes can be devices close to a human user, interacting with humans
Alternative concept: Instead of focusing interaction on humans, focus on interacting with environment 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 sensor networks (WSN) Or: Wireless sensor & actuator networks (WSAN)
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Roles of Participants in WSN 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, Smart Phone,
Smart Device …
Actuators: Control some device based on data, usually also a sink
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Structuring WSN Application TypesStructuring WSN Application Types
Interaction patterns between sources and sinks classify application types Event detection: Nodes locally detect events (maybe jointly with nearby
neighbors), report these events to interested sinks
Event classification additional option Periodic measurement Function approximation: Use sensor network to approximate a function
of space and/or time (e.g., temperature map) Edge detection: Find edges (or other structures) in such a function (e.g.,
where is the zero degree border line?) Tracking: Report on the current position of an entity (e.g, identify and
track intruders)
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Sensor DeploymentSensor Deployment
Static sensors Random Deployment
The objective is typically to distribute uniformly sensors across an area
• Dropped from aircraft
Planned Deployment Sensors are deployed in a predefined structure to meet some
application objectives
Area coverage, preventive maintenance
• Mobile sensors Typically used to augment network capabilities: compensate for
deployment shortcomings Move actively to seek out areas of interest Moved passively by surrounding elements: wind, water, …
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Maintenance OptionsMaintenance Options
Feasible and/or practical to maintain sensor nodes? E.g., to replace batteries? Or: unattended operation? Impossible but not relevant? Mission lifetime might be very small
Energy supply? Limited from point of deployment? Some form of recharging, energy scavenging from environment?
E.g., solar cells
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Characteristic and Requirements for WSNsCharacteristic and Requirements for WSNs
Type of service of WSN Not simply moving bits like another network Rather: provide answers (not just numbers) Issues like geographic scoping are natural requirements, absent from other
networks
Quality of service Traditional QoS metrics may not necessarily be relevant WSN must provide “right answer at the right time”
Robustness Sensors nodes are prone to various failures: running out of energy,
physical destruction, software corruption, security attacks
Extended lifetime The network is expected to operate over long periods of time,
depending on the type of application Lifetime of individual nodes is relatively unimportant
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Characteristic and Requirements for WSNsCharacteristic and Requirements for WSNs
Scalability Support large number of nodes
Wide range of densities Vast or small number of nodes per unit area, very application-dependent
Programmability Re-programming of nodes in the field might be necessary, improve
flexibility
Maintainability WSNs are self-monitoring networks
Necessary to adapt to changes as they occur
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Required Mechanisms to Meet RequirementsRequired Mechanisms to Meet Requirements
Multi-hop wireless communication
Energy-efficient operation For communication and computation, sensing and actuating
Auto-configuration Manual configuration is typically not feasible
Nodes in the network collaborate towards a joint goal Nodes in the network collaborate towards a common task In-network processing, as opposed edge-processing, greatly improves
the network efficiency
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Required Mechanisms to Meet RequirementsRequired Mechanisms to Meet Requirements
Data centric networking Focusing network design on data, not on node identities (id-centric
networking) To improve efficiency
Locality Processing is performed locally, within a node or among nearby
neighbors as much as possible
Tradeoff-driven design principles Striking a balance between invested energy and accuracy of results
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Enabling Technologies for WSN Enabling Technologies for WSN
Cost reduction For wireless communication, simple microcontroller, sensing, batteries
Miniaturization Some applications demand small size “Smart dust” as vision
Energy harvesting Recharge batteries from ambient energy (light, vibration, …)
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Types of Radio NetworksTypes of Radio Networks
Cellular Networks base stations distributed over the field each base station covers a cell used for mobile phones WLAN can be seen as a special case
Mobile Ad Hoc Networks self-configuring network of mobile
nodes node serve as client and router no infrastructure necessary
Sensor Networks network of sensor devices with
controller and radio transceivers base station with more resources
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Zigbee Protocol StackZigbee Protocol Stack
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Sensor node architectureSensor node architecture
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ControllerController
Main options: Microcontroller – general purpose processor, optimized for embedded
applications, low power consumption, cheap DSPs - Optimized for signal processing tasks, not suitable here FPGAs – not optimized for energy consumption ASICs – best solution, but very expensive
(FPGA: field programmable gate array, ASIC: Application specific integrated circuit)
Example microcontrollers Texas Instruments MSP430
16-bit RISC core, up to 4 MHz, versions with 2-10 kbytes RAM, several DACs, RT clock, prices start at 0.49 US$
Atmel ATMega 8-bit controller, larger memory than MSP430, slower
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Communication deviceCommunication device
Device type depends on the transmission medium Electromagnetic: radio frequencies Electromagnetic: light Ultrasound
Radio transceivers transmit a bit or byte stream as a radio wave The received bit is converted back into bit or byte stream
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Frequency BandsFrequency Bands
LF Low Frequency MF Medium Freq. HF High Freq.VHF Very High Freq. UHF Ultra High F. SHF Super High Fr.EHF Extra High Frequency UV Ultra Violet
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Radio PropagationRadio Propagation
Propagation on straight line Signal strength is proportional to 1/d² in free space
In practice can be modeled by 1/dc, for c up to 4 or 5
Energy consumption for transmitting a radio signal over distance d in empty space is d²
Basic properties Reflection Refraction (between media with slower speed of propagation) Interference Diffraction Attenuation in air (especially HV, VHF)
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Receiver: DemodulationReceiver: Demodulation
The receiver looks at the received wave form and matches it with the data bit that caused the transmitter to generate this wave form Necessary: one-to-one mapping between data and wave form Because of channel imperfections, this is at best possible for digital
signals, but not for analog signals
Problems caused by Carrier synchronization: frequency can vary between sender and receiver
(drift, temperature changes, aging, …) Bit synchronization (actually: symbol synchronization): When does
symbol representing a certain bit start/end? Frame synchronization: When does a packet start/end? Biggest problem: Received signal is not the transmitted signal!
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Transmitted signal received signal!Transmitted signal received signal!
Wireless transmission distorts any transmitted signal Received <> transmitted signal; results in uncertainty at receiver about which bit sequence
originally caused the transmitted signal Abstraction: Wireless channel describes these distortion effects
Sources of distortion Attenuation – energy is distributed to larger areas with increasing distance Reflection/refraction – bounce of a surface; enter material Diffraction – start “new wave” from a sharp edge Scattering – multiple reflections at rough surfaces Doppler fading – shift in frequencies (loss of center)
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Distortion effects: Non-line-of-sight pathsDistortion effects: Non-line-of-sight paths
Because of reflection, scattering, …, radio communication is not limited to direct line of sight communication
Effects depend strongly on frequency, thus different behavior at higher frequencies
Different paths have different lengths = propagation time
Results in delay spread of the wireless channel Closely related to frequency-selective fading
properties of the channel With movement: fast fading
Line-of-sight path
Non-line-of-sight path
signal at receiver
LOS pulsesmultipathpulses
© Jochen Schiller, FU Berlin
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Sharing the MediumSharing the Medium
Space-Multiplexing Spatial distance Directed antennae
Frequency-Multiplexing Assign different frequencies to the senders
Time-Multiplexing Use time slots for each sender
Spread-spectrum communication Direct Sequence Spread Spectrum (DSSS) Frequency Hopping Spread Spectrum (FHSS)
Code Division Multiplex
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Some transceiver design considerationsSome transceiver design considerations
Strive for good power efficiency at low transmission power Some amplifiers are optimized for efficiency at high output power To radiate 1 mW, typical designs need 30-100 mW to operate the
transmitterWSN nodes: 20 mW (mica motes)
Receiver can use as much or more power as transmitter at these power levels
! Sleep state is important Startup energy/time penalty can be high
Examples take 0.5 ms and 25 mW to wake up Exploit communication/computation tradeoffs
Might payoff to invest in rather complicated coding/compression schemes
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Transceiver characteristicsTransceiver characteristics
Capabilities– Interface: bit, byte, packet
level? – Supported frequency range?
• Typically, somewhere in 433 MHz – 2.4 GHz, ISM band
– Multiple channels? – Data rates?– Range?
Energy characteristics– Power consumption to
send/receive data? – Time and energy consumption
to change between different states?
– Transmission power control?– Power efficiency (which
percentage of consumed power is radiated?)
Radio performance– Modulation? (ASK, FSK, …?)– Noise figure? NF = SNRI/SNRO
• output noise added– Gain? (signal amplification)– Receiver sensitivity: (minimum S
to achieve a given Eb/N0)– Blocking performance (achieved
BER in presence of frequency-offset interferer)
– Out of band emissions – Carrier sensing & RSSI
characteristics• Received Signal Strength
Indication– Frequency stability (e.g.,
towards temperature changes)– Voltage range
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Transceiver modesTransceiver modes
Transceivers can be put into different operational modes, typically: Transmit mode Receive mode Idle – ready to receive, but not doing so
Some functions in hardware can be switched off, reducing energy consumption a little
Sleep – significant parts of the transceiver are switched off
Not able to immediately receive something Recovery time and startup energy to leave sleep state can be
significant
A challenging issue is how to wakeup receivers via radio when they are in sleep mode Contradictory, to some extent!
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Example radio transceiversExample radio transceivers
A Large variety of transceiversSome -examples
– RFM TR1000 family• 916 or 868 MHz• 400 kHz bandwidth• Up to 115.2 kbps• On/off keying or ASK • Dynamically tuneable output
power• Maximum power about 1.4 mW• Low power consumption
– Chipcon CC1000• Range 300 to 1000 MHz,
programmable in 250 Hz steps • FSK modulation• Provides RSSI
– Chipcon CC 2400• Implements 802.15.4• 2.4 GHz, DSSS modem • 250 kbps• Higher power consumption
than above transceivers – Infineon TDA 525x family
• E.g., 5250: 868 MHz• ASK or FSK modulation• RSSI, highly efficient
power amplifier• Intelligent power down,
“self-polling” mechanism • Excellent blocking
performance
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Wakeup receiver – Energy ImpactWakeup receiver – Energy Impact
Transition between idle and RECEIVING states must be designed carefully Transitioning from idling to ready to receive consumes considerable
amounts of power
A critical question: when to switch on a receiver? Contention-based MAC protocols ─ Receiver is always on
Wasted energy
TDMA-based MAC protocols ─ Receiver wakes during its assigned slot Synchronization overhead; Inflexible
Desirable: receiver (only) checks for incoming messages Upon signal detection, wake up main receiver for actual reception
Ideally, the wakeup receiver can already process simple addresses Not clear if this can actually built
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Energy supply of mobile/sensor nodesEnergy supply of mobile/sensor nodes
Goal: provide as much energy as possible at smallest cost/volume/weight/recharge time/longevity In WSN, recharging may or may not be an option
Options Primary batteries – not rechargeable Secondary batteries – rechargeable, only makes sense in combination with
some form of energy harvesting
Requirements include Low self-discharge Long self live Capacity under load Efficient recharging at low current Good relaxation properties (seeming self-recharging) Voltage stability (to avoid DC-DC conversion)
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Battery examplesBattery examples
Energy per volume (Joule per cubic centimeter):
Primary batteries
Chemistry Zinc-air Lithium Alkaline
Energy (J/cm3) 3780 2880 1200
Secondary batteries
Chemistry Lithium NiMHd NiCd
Energy (J/cm3) 1080 860 650
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Energy scavengingEnergy scavenging
How to recharge a battery? A laptop: easy, plug into wall socket in the evening A sensor node? – Try to scavenge energy from environment
Ambient energy sources Light solar cells – between 10 W/cm2 and 15 mW/cm2 Temperature gradients – 80 W/cm2 @ 1 V from 5K difference Vibrations – between 0.1 and 10000 W/cm3 Pressure variation (piezo-electric) – 330 W/cm2 from the heel of a shoe Air/liquid flow
(MEMS gas turbines)
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Energy scavenging – overview Energy scavenging – overview
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Energy consumptionEnergy consumption
A “back of the envelope” estimation
Number of instructions Energy per instruction: 1 nJ Small battery (“smart dust”): 1 J = 1 Ws Corresponds: 109 instructions!
Lifetime Or: Require a single day operational lifetime = 24 60 60s =86400 s 1 Ws / 86400s = 11.5 W as max. sustained power consumption!
Not feasible!
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Multiple power consumption modesMultiple power consumption modes
Way out: Do not run sensor node at full operation all the time If nothing to do, switch to power safe mode Question: When to throttle down? How to wake up again?
Typical modes Controller: Active, idle, sleep Radio mode: Turn on/off transmitter/receiver, both
Multiple modes possible, “deeper” sleep modes Strongly depends on hardware TI MSP 430, e.g.: four different sleep modes Atmel ATMega: six different modes
scavenge
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Some energy consumption figuresSome energy consumption figures
Microcontroller TI MSP 430 (@ 1 MHz, 3V):
Fully operation 1.2 mW Deepest sleep mode 0.3 W – only woken up by
external interrupts (not even timer is running any more) Atmel ATMega
Operational mode: 15 mW active, 6 mW idleSleep mode: 75 W
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Computation vs. communication energy costComputation vs. communication energy cost
Tradeoff? Directly comparing computation/communication energy cost not
possible Energy ratio of “sending one bit” vs. “computing one instruction”:
Anything between 220 and 2900 in the literature To communicate (send & receive) one kilobyte
= computing three million instructions!
Hence: try to compute instead of communicate whenever possible
Key technique in WSN – in-network processing! Exploit compression schemes, intelligent coding schemes, …
Thank you ! Q & A
Thank you ! Q & A
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