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Wireless Sensor Network
FundamentalsSteven Lanzisera
Environmental Energy Technologies Division, LBNL8 February 2010
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Overview
Wireless sensor networks
The fundamentals
Network lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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What We Wont Discuss
Particular applications
Jargon soup
IEEE 802.15.4, IEEE 802.11a/b/g/n
Zigbee, Bluewave, etc.
Companies and their technologies
Dust Networks, Arch Rock, Sensicast, etc.
Specific protocols
Latency, feedback control, etc.
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Overview
Wireless sensor networks
The fundamentals
Network lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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Wireless Sensor Networks: Vision
Ubiquitous connectivity
Smart everything
Incredibly cheap Last for years
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Wireless Can Change Everything
Adapted from K. Pister, UC Berkeley / Dust Networks
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Industrial Process Monitoring
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B90 Monitoring Network
Dave Watson et al.
~40 wireless sensor nodes Provide power meter and temperature data Adding ~40 nodes from Sensicast (separate network)
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Why Arent They Everywhere?
Data reliability (% of data that gets to the server) Typical research grade network: ~80%
Best in class: >99.9%
Network stability, packet loss, etc Interoperability, ease of use
No two major companies can talk
Network lifetime Multi-year lifetime promised
Several month lifetime often realized
Poor market experiences
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Overview
Wireless sensor networks
The fundamentals
Network lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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Battery Powered Nodes
Virtually every node uses a battery
Mains power is expensive & annoying
Energy harvesting is vaporware Lifetime must be years
Real cost is in labor to deploy, maintain
Cost to replace 500 batteries each month?
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Energy in Wireless Sensor Networks
Basic networks listen all the time (Zigbee, etc)
5 day lifetime with best in class AA batteries
Turn the radio off! 1% duty cycle gives 500 days
But how do you know when to turn it on?
Much of WSN research has focused on energy How to make radios consume less power
How to determine when to turn the radio on
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Overview
Wireless sensor networks
The fundamentals
Network lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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The network stack is about abstraction
Provide standard interfaces so you can work at
one layer without knowledge of everything else
The Network Stack
Application Layer
Physical Layer
Link Layer
Internet Layer
Transport Layer
Bits in the air or on a wire
Medium access, data reliability, flow control
Routing (IP)
Data reliability, flow control (TCP, UDP, etc)
Application protocol (HTTP, FTP, etc)
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Payload, Headers, & Packets
Physical
Link
Network
Transport
Application
Physical
Link
Network
Transport
Application
headerpayload
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Link Layer
Often interchangeable with Medium Access
Control (MAC)
When (or where) to send data
Dealing with contention
Low level addressing (hardware address)
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MAC: Aloha
All devices at the same frequency all the time If you need to talk, send a packet, otherwise, listen
Only works with extremely low traffic loads
Two problems
Radio is always on (poor lifetime)
Collisions cause data loss
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MAC: Preamble Sampling
Nodes sleep most of the time (radio off) Wake up to listen once in a while
When they need to talk, they send a long preamble
Nodes hear preamble, stay awake, get packet
Contention is a problem
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MAC: Carrier Sense Multiple Access
Carrier Sense Multiple Access is CSMA
Listen to hear if the channel is busy
If no, transmit your packet
If yes, wait awhile and check again
Designed for wired
Everyone can hear everyone else
What if they cant all hear one another?
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Hidden Terminal In Wireless Networks
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MAC: Time Division Multiple Access
Each node assigned a time to talk No contention
Nodes must know when to wake up to listen
Entire network must be time synchronized Frequency hopping is almost free
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Internet Layer
Routing packets from source to destination
Defines logical network connections
Determines which physical links to use
Wired networks have stable links
Define a path from A to B and it will work later too
Send a packet and it will most likely get there
Wireless networks can have unstable links
Links come and go over time
Packet delivery rates on good links are low
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Network Topologies
Tree Network Topology
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Mesh Network Topology
Network Topologies
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Overview
Wireless sensor networks
The fundamentals
Network Lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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Wireless Communication: 10km View
B sends energy with information to A
Environment, design determines the energy required
A & B must speak the same basic language
Carrier frequency
Modulation (OOK, AM, FM, etc)
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EnergyNoise
bitperEnergyEfficiency1logBandwidthCapacity 2
Bits/second Hertz(1/seconds)
ModulationConstant (1)
(Unitless)
Signal toNoise Ratio
(Unitless)
How to Get Data From B to A
Enough energy must be received to decode data
The modulation determines:
The bandwidth
The efficiency
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Signal Propagation
Radio waves travel from B as an
electromagnetic wave
Just like light, travels at 3x108 m/s
EM wave is a sinusoid in both time, space
Distance, Time
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Signal Propagation: Free Space
Transmitted power is spread over a sphere
Power falls off with surface area
Communication range up to 1km?
Usually calculated using this theory Even outdoors this is far from a good estimate
2R
P
PTX
RX
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Signal Propagation: Indoors
Radio waves bounce off and go through objects
Interference occurs at receiver
Unpredictable, geometry dependent
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Signal Propagation: Fading
Distance from Wall
log(RFPo
wer) wavelength
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Combating Fading
Diversity in space or frequency
Spatial diversity: multiple antennas
Watteyne, Lanzisera, Mehta, Pister, ICC 2010
Antenna 2Antenna 1
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Combating Fading
Diversity in space or frequency
Spatial diversity: multiple antennas
Frequency diversity
Wideband modulation (WiFi, CDMA cellphones)
Frequency hopping (Bluetooth)
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Fading and Diversity
ch.11
ch.13
ch.15
ch.17
ch.12
ch.14
ch.16
ch.18
ch.19
ch.21
ch.23
ch.25
ch.20
ch.22
ch.24
ch.26
Diversity improves performance
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Wireless Communication Key Points
Communication range is not a sphere
Lots of things cause this
Multipath fading
Diversity can help
Space: multiple antennas
Frequency: channel hopping
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Overview
Wireless sensor networks
The fundamentals
Network lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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Radios of the World
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RF Transceivers
Sensitivity better than 1pW (-90dBm) common
Dynamic range of 109 to 1013 common
Equal to or better than human hearing, eye sight
Can see bright things or dim things but not both
Yet, we find that we can only communicate several
meters.
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Transceiver Design
Data
AntennaDigital toAnalogConverter
BasebandFilter
Mixer PowerAmplifier
Low NoiseAmplifierMixerBasebandFilter
Analog to
DigitalConverter
Synthesizer
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Power in Transceivers
Large power overhead
Dominates in low
power systems
Non-ideal power amp
Efficiency 15% - 50%
1W (30 dBm) out of antenna takes 5W out of battery!
Low noise amplifier: the noise = 4x the power
Total power in TX and RX about the same in WSN
PA changes this so that PTX >> PRX
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Silicon Implementation
4% Increase
Kluge, ISSCC 2006
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Things That Mess Up Communication
Noise
Interference
2.4 GHz
Channels 11-26
2.4835 GHz
5 MHz
2.4 GHz
PHY
Relative Noise Power
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Overview
Wireless sensor networks
The fundamentals
Network lifetime
Wireless networking
Wireless communication
The radio frequency transceiver
Interoperability
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Interoperability
Normally not all layers need to be the same
Two computers can talk even if one is wireless and
one is wired
But we want to buy a sensor node from A anduse it in a wireless network from B
Need several layers to be the same
Physical, link and network layers
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Interoperability
Physical layer is standard (IEEE 802.15.4)
Link layer is not standard
Even the standard contains several choices
Network layer is becoming standard (6loWPAN)
Glue between link and network layer is not standard
Devices continue to not work together
Vendors trying to decide what to do
But there is some hope
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Summary
We can provide reliable networking today
Wireless communication is difficult
What you use today will not be interoperable
Not all networks created equal