Wireless Sensor Networks
Radio Realities
Professor Jack Stankovic
University of Virginia2006
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
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
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
Antenna TypesAntenna Types
• Half-wave dipole (most efficient transmission)
• Quarter wave vertical
Half-wave dipole Quarter Wave Vertical
Radiation pattern Radiation pattern
Perfect IsotropicAntenna
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)
Line of Sight ImpairmentsLine of Sight Impairments
• Noise– Thermal– Crosstalk– Impulse (e.g., lightning)
• Atmosphere absorption– Vapor and oxygen contribute to
attenuation
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
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
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)
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)
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)
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
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
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
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
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
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
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
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
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
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
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.
s d
Geographic ForwardingGeographic Forwarding
• GF always choose to node that is closest to the destination.
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
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
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
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
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)
Radio InterferenceRadio Interference
B
AC
Range 1 1Range
2
OKInterfere
s
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)
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.
Two TypesTwo Types
• Frequency Hopping Spread Spectrum– Easier to explain
• Direct Sequenced Spread Spectrum– Used in MicaZ
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
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
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
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
More on Spread Spectrum
More on Spread Spectrum
• Tutorials on WEB
• Wireless Communications and Networks, W. Stallings, Prentice Hall, 2nd edition.
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
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