Wireless Sensor Networks Radio Realities Professor Jack Stankovic University of Virginia 2006.
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Transcript of Wireless Sensor Networks Radio Realities Professor Jack Stankovic University of Virginia 2006.
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Wireless Sensor Networks
Radio Realities
Professor Jack Stankovic
University of Virginia2006
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MotivationMotivation
– Significant Evidence of radio irregularity in physical environments• Theoretical• Practical (empirical evidence)
– Too many current solutions are via simulation with circular radio range assumed
– Need for simulation tools to model irregularity
– Need for better protocols to address irregularity• Many current protocols won’t work in
practice
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ExampleExample
AC
D
Bbeacon
data
beacon
data
beacon data
B, C, and D are the same distance from A.Note that this pattern changes over time.
Irregular Range of A
A and B areasymmetric
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OutlineOutline
• A radio energy model that considers irregularity and that can be used in simulators
• Study the impact of radio irregularity on – MAC layer – Routing layer– Other protocols (such as localization, topology
control)– Result: Common and non-negligible
• Solutions to deal with radio irregularity– Implicit– Explicit
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Antenna TypesAntenna Types
• Half-wave dipole (most efficient transmission)
• Quarter wave vertical
Half-wave dipole Quarter Wave Vertical
Radiation pattern Radiation pattern
Perfect IsotropicAntenna
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Line of Sight ImpairmentsLine of Sight Impairments
• Attenuation– Strength of the signal falls with distance – Attenuation is greater at higher
frequencies– Strength of signal must be detectable
by circuitry AND above noise
• Free Space Loss– Ratio of radiated power to the power
received by the antenna (antenna of certain area size)
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Line of Sight ImpairmentsLine of Sight Impairments
• Noise– Thermal– Crosstalk– Impulse (e.g., lightning)
• Atmosphere absorption– Vapor and oxygen contribute to
attenuation
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Line of Sight ImpairmentsLine of Sight Impairments
• Multipath– Reflection – bounce off objects are
arrive at destination late, together with original signal
– Diffraction – occurs at edge and looks like a new source (can have signal received even when no line of sight)
– Scattering – if size of obstacle is on order of size of wavelength
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Summary - Causes of Radio Irregularity
Summary - Causes of Radio Irregularity
• Devices– Antenna type (directional, omni-directional)– Sending power (non-linear)– Antenna gains– Receiver sensitivity (circuits)
• Propagation Media– Media type (air, water)– Background noise– Temperature, humidity– Obstacles– Rain
But how significant in WSN devices
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Real Measurements - Radio Signal
Real Measurements - Radio Signal
• Non-isotropic Path Loss: The radio signal from a transmitter has different path loss in different directions.
-65-64-63-62-61-60-59-58-57-56-55
0 25 50 75
Beacon SeqNo
South NorthWest East
Signal Strength over Time in Four Directions
(RSSI – Received Signal Strength Indicator)
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Non-isotropic Path LossNon-isotropic Path Loss
Signal Strength Values in Different Directions
-60
-58
-56
-54
-52
-50
1 48 95 142 189 236 283 330
Direction in Degree ( 10 feet)
-65
-60
-55
-50
-45
0 41 82 122 163 204 245 285 326
Direction in Degree (20 feet)
• Reasons:– Reflection, diffraction and scattering in environment– Hardware calibration (non-isotropic antenna gain)
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Radio Signal PropertyRadio Signal Property
• Continuous variation: The signal path loss varies continuously with incremental changes of the propagation direction from a transmitter.
Signal Strength Values in Different Directions
-60
-58
-56
-54
-52
-50
1 48 95 142 189 236 283 330
Direction in Degree ( 10 feet)
-65
-60
-55
-50
-45
0 41 82 122 163 204 245 285 326
Direction in Degree (20 feet)
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Radio Signal Property Radio Signal Property
• Heterogeneity: Different nodes have different signal sending power
-60
-59.5
-59
-58.5
-58
-57.5
-57
0 25 50 75
Beacon SeqNo
1.58V 1.4V1.32V 1.18V
(a) One mote with different battery status
-60-59.5
-59-58.5
-58-57.5
-57-56.5
-56-55.5
-55
0 25 50 75
Beacon SeqNo
Mote A Mote BMote C Mote D
(b) Different motes with the same battery status
• Reasons– Different hardware calibration and circuits
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RIM – Radio Irregularity Model
RIM – Radio Irregularity Model
• Degree of Irregularity (DOI): – Definition: the maximum received signal
strength percentage variation per unit degree change in the direction of radio propagation.
– Account for non-isotropic path loss
DOI = 0 DOI = 0.003 DOI = 0.01
Degree of Irregularity
Max range
Min range
Actual Range For this node
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RIM - VSPRIM - VSP
• Variance of Sending Power (VSP): – Definition: the maximum percentage variance
of the signal sending power among different devices.
– Account for heterogeneous sending power
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RIM – Propagation Formula
RIM – Propagation Formula
Signal receiving power = signal sending power - path loss + fading
Signal receiving power = signal sending power – DOI adjusted path loss + fading
DOI adjusted path loss = path loss* KD
Signal receiving power = VSP adjusted signal sending power – DOI adjusted path loss + fading
VSP adjusted signal sending power =
onDistributi Normal RandomNum Where
VSP)*RandomNum (1 *power sending signal
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Impact – MAC layerImpact – MAC layer• Impact on:
– Carrier Sense technique– Handshake technique – Used in CSMA, MACA,
MACAW, 802.11 DCF
B
C
A
(a) Carrier Sense Technique
B
C
A
RTS
X
CTS
DATA
(b) Handshake Technique
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Impact - RoutingImpact - Routing• Impact on:
– Path-Reversal technique
– Multi-Round technique – Used in AODV, DSR,
LAR
Source A
B Dest.RREQ
RREQ
RREP
RREP
Impact on Path-Reversal Technique
S DX
X
RREQ
RREP
Route Discovery Using Multi-Round Technique
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Impact - RoutingImpact - Routing• Impact on:
– Neighbor-Discovery technique
– Used in GF, GPSR, SPEED
AC
D
Bbeacon
Xdata
beacon
data
beacon data
Impact on Neighbor Discovery Technique
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Simulation TestSimulation Test
Components Setting
Simulator GloMoSim
Terrain (150m,150m)
Node Number 100
Node Placement Uniform
Payload Size 32 Bytes
Application 6 randomly chosen periodic multi-hop CBR streams
Routing Protocol AODV, DSR, GF
MAC Protocol CSMA, 802.11 (DCF)
Radio Model RIM
Radio Bandwidth 200Kb/s
Runs 140
Confidence Intervals The 95% confidence intervals are within 0~25% of the mean
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Quantify the ImpactQuantify the Impact
0%
10%
20%
30%
40%
50%
60%
70%
0 0.2 0.4 0.6 0.8 1
VSP-FACTOR
AODVDSRGF
0%
10%
20%
30%
40%
50%
60%
70%
0 0.002 0.004 0.006 0.008 0.01
DOI-FACTOR
AODVDSRGF
Increase DOI Increase VSP
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 0.002 0.004 0.006 0.008 0.01
DOI-FACTOR
AODVDSRGF
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 0.2 0.4 0.6 0.8 1
VSP-FACTOR
AODVDSRGF
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Quantify the ImpactQuantify the Impact
Increase DOI Increase VSP
0
200
400
600
800
1000
1200
0 0.002 0.004 0.006 0.008 0.01
DOI-FACTOR
AODVDSRGF
0
100
200
300
400
500
600
700
0 0.2 0.4 0.6 0.8 1
VSP-FACTOR
AODVDSRGF
0
1
2
3
4
5
6
7
8
9
0 0.002 0.004 0.006 0.008 0.01DOI-FACTOR
AODVDSRGF
0
1
2
3
4
5
6
7
8
0 0.2 0.4 0.6 0.8 1
VSP-FACTOR
AODVDSRGF
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Summary of the ImpactSummary of the Impact
• Radio irregularity has a greater impact on the routing layer than on the MAC layer.
• Routing protocols, such as AODV and DSR, that use multi-round discovery technique, can deal with radio irregularity, but with a high overhead.
• Routing protocols, such as geographic forwarding, which are based on neighbor discovery technique, are severely affected by radio irregularity.
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s d
Geographic ForwardingGeographic Forwarding
• GF always choose to node that is closest to the destination.
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Solution: Symmetric Geographic Forwarding
Solution: Symmetric Geographic Forwarding
• Beacon to discover neighbors• Exchange neighbor tables to detect
asymmetry• Delete asymmetric links from valid
neighbor table
34
11 2
3
4
14
31
Xx
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Symmetric Geographic
Forwarding (SGF)
Symmetric Geographic
Forwarding (SGF)Increase DOI Increase VSP
0%
10%
20%
30%
40%
50%
60%
70%
0 0.002 0.004 0.006 0.008 0.01
DOI-FACTOR
AODVDSRGFSGF
0%
10%
20%
30%
40%
50%
60%
70%
0 0.2 0.4 0.6 0.8 1
VSP-FACTOR
AODVDSRGFSGF
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 0.002 0.004 0.006 0.008 0.01
DOI-FACTOR
AODVDSRGFSGF
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 0.2 0.4 0.6 0.8 1
VSP-FACTOR
AODV DSRGF SGF
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Bounded Distance Forwarding
Bounded Distance Forwarding
• Bounded Distance Forwarding restricts the distance over which a node can forward a message in a single hop.
• Implemented in a surveillance/tracking system with 70 MICA2 motes
60%
65%
70%
75%
80%
85%
90%
95%
100%
8 16 24 32 40 48 100
Bounded Fowarding Distance(feet)
Percentage of Reporting Nodes
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Bounded Distance Forwarding
Bounded Distance Forwarding
• 8 ft – not enough nodes that close so some/many paths not possible
• 16 ft – best tradeoff• 24 ft and greater – too many
asymmetric links
Weaker signal
A
8 16
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Other Radio Realities?Other Radio Realities?
• Interference Range– Normally, interference range is greater
than communication range– Some protocols assume if more than 2
hops away then zero interference
– Not true: sum of energy from many distant communication nodes may cause interference (must deal with SNR and not hop count)
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Radio InterferenceRadio Interference
B
AC
Range 1 1Range
2
OKInterfere
s
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Other Radio RealitiesOther Radio Realities
• Logically, if two nodes are both transmitting and within 1 hop, then both messages are lost
– Not necessarily true – one packet may have enough signal strength to still be received correctly even if another node is transmitting at the same time (e.g., the second node may have a weak signal)
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Spread SpectrumSpread Spectrum
• Spread spectrum is a transmission technique in which a pseudo-noise (PN) code, independent of the information data, is employed as a modulation waveform to “spread” the signal energy over a bandwidth much greater than the signal information bandwidth.
• At the receiver the signal is “despread” using a synchronized replica of the pseudo-noise code.
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Two TypesTwo Types
• Frequency Hopping Spread Spectrum– Easier to explain
• Direct Sequenced Spread Spectrum– Used in MicaZ
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Basic Idea Basic Idea
0100100100 00 at freq A01 at freq B10 at freq C00 at freq D01 at freq E
Know the PN codeand reverse theencoding
Might have 16 freq channels to choose from
Sender Receiver
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AdvantagesAdvantages
• Jam resistant– If you jam on a freq you only knock out a
few bits (can be corrected)
• Eavesdroppers on a freq can only hear a few bits
• More resistant to noise and multi-path distortion
• Multiple users can transmit simultaneously with no (little) interference
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ExampleExample
• Use Spread Spectrum with a code
• User A has code that provides freq 3,7,2,8
• User B has code that provides disjoint set of freq, e.g., 5, 6, 14, 1, 4
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Example: Radio Chip CC 2420
Example: Radio Chip CC 2420• DSSS
• 250kbps effective data rate• Q-QPSK with half sine pulse shaping modulation• Low current consumption (RX: 19.7 mA, TX: 17.4
mA)• Programmable output power• 16 available frequency channels (IEEE 802.15.4
standard)– Fc = 2450 + 5 (k-11) MHz, k = 11, 12, …, 26
• Hardware MAC encryption
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More on Spread Spectrum
More on Spread Spectrum
• Tutorials on WEB
• Wireless Communications and Networks, W. Stallings, Prentice Hall, 2nd edition.
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SummarySummary
• Radio irregularities are commonplace• Many current protocols are susceptible to
poor performance because they ignore this problem (MAC, routing, localization, topology control)– They just don’t work in practice
• SGF, Bounded Distance, …solutions do exist for radio irregularities
• Radio interference realities are just being considered now
• Spread spectrum will likely become common