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