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