Eetd Wsn Basics 2 Dist

8/9/2019 Eetd Wsn Basics 2 Dist

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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


The fundamentals

Network lifetime

Wireless networking



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


The fundamentals

Network lifetime

Wireless networking



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


The fundamentals

Network lifetime

Wireless networking



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


The fundamentals

Network Lifetime

Wireless networking



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


The fundamentals

Network lifetime

Wireless networking



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


The fundamentals

Network lifetime

Wireless networking



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

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Eetd Wsn Basics 2 Dist