1. Applications Characteristics and Requirements of Sensor Networks

2. 2 Outline WSN Characteristics and Requirements  Goals and Application requirements  Design principles Sensor Node Architecture  Hardware components Energy Consumption 2

3. 3 Sensor Network Topology 3

4. 4 Wireless 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)

5. 5 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

6. 6 Structuring 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)

7. 7 Sensor 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, …

8. 8 Maintenance 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

9. 9 Characteristic 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

10. 10 Characteristic 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

11. 11 Required 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

12. 12 Required 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

13. 13 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, …)

14. 14 Types 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

15. 15 Zigbee Protocol Stack

16. 16 Sensor node architecture

17. 17 Controller 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

18. 18 Communication 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

19. 19 Frequency Bands LFLow FrequencyMFMedium Freq. HFHigh Freq. VHFVery High Freq.UHFUltra High F. SHFSuper High Fr. EHFExtra High FrequencyUVUltra Violet

20. 20 Radio Propagation  Propagation on straight line  Signal strength is proportional to 1/d² in free space  In practice can be modeled by 1/d c, 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)

21. 21 Receiver: 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!

22. 22 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)

23. 23 Distortion 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 pulses multipath pulses © Jochen Schiller, FU Berlin

24. 24 Sharing 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

25. 25 Some 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 transmitter  WSN 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

26. 26 Transceiver 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 = SNR I /SNR O output noise added –Gain? (signal amplification) –Receiver sensitivity: (minimum S to achieve a given E b /N 0 ) –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

27. 27 Transceiver 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!

28. 28 Example radio transceivers A Large variety of transceivers Some -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

29. 29 Wakeup 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

30. 30 Energy 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 s ome 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)

31. 31 Battery examples Energy per volume (Joule per cubic centimeter): Primary batteries ChemistryZinc-airLithiumAlkaline Energy (J/cm 3 )378028801200 Secondary batteries ChemistryLithiumNiMHdNiCd Energy (J/cm 3 )1080860650

32. 32 Energy 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/cm 2 and 15 mW/cm 2  Temperature gradients – 80  W/cm 2 @ 1 V from 5K difference  Vibrations – between 0.1 and 10000  W/cm 3  Pressure variation (piezo-electric) – 330  W/cm 2 from the heel of a shoe  Air/liquid flow (MEMS gas turbines)

33. 33 Energy scavenging – overview

34. 34 Energy consumption  A “back of the envelope” estimation  Number of instructions  Energy per instruction: 1 nJ  Small battery (“smart dust”): 1 J = 1 Ws  Corresponds: 10 9 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!

35. 35 Multiple 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

36. 36 Some 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 idle Sleep mode: 75  W

37. 37 Computation 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, …

38. Thank you ! Q & A