pfc_analysis of ieee 802.15.4 in wbsn

Motivation and Objectives State-of-the-Art Analytical Model High Pathloss WBSN Results Conclusions

Performance Analysis of the Contention AccessPeriod in the slotted IEEE 802.15.4 for Wireless

Body Sensor Networks

Manuel AymerichTutor: Nadia Khaled

Dept. Teorıa de Senal y ComunicacionesUniversidad Carlos III de Madrid

Leganes, May 21, 2009

1 / 37


Outline

1 Motivation and ObjectivesMotivation and Objectives

2 State-of-the-ArtMAC design in WBSNOverview of the sloted IEEE 802.15.4 CAP

3 Analytical ModelDevelopmentAnalytical Formulation

4 High Pathloss WBSNAnalysisChanges in the Analytical Model

5 ResultsInitial ConsiderationsComparison ACK and non-ACK trafficPerformance Results for a high path loss WBSN

6 Conclusions2 / 37


Outline






6 Conclusions3 / 37


Motivation and Objectives

Motivation

WBSN ⇒ tremendous international

interest in recent years.

Advances in low power, lowcost, wireless MEMC systems.Significant progress in wearableand implantable biosensors.

WBSN Applications:

In-vivo monitoring: everydayhealthcare, sports.Video Games.

System requirements:

Single hop star topology.Low-power.Low-cost.Self-configuring.

ECG &Tilt Sensor

MotionSensors

SpO2 &Motion Sensor

Personal Server

Network CoordinatorTemperature &

Humidity Sensor

IEEE 802.15.4

4 / 37



Objectives

According to Dr. Leonard Fass, Director of GE Healthcare:

”One of the greatest barriers to the adoption of emerging BSNtechnologies is the whether or not they can be integrated withexisting systems, under common standards.”

The novel IEEE 802.15.4 standard is poised to become the globalstandard for low data rate, low energy consumption WSN.

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Objectives

Analyze the CAP of the slotted IEEE 802.15.4 standardworking under a WBSN application scheme.

1 Star topology.2 Acknowledged uplink traffic (nodes-to-coordinator).3 High pathloss human body channel.

How?

Extend an a state-of-the-art analytical model of the IEEE802.15.4 CAP for acknowledged traffic and under a WBSNchannel.Evaluate it in terms of energy consumption and throughput.Compare with ns-2 simulation results.

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Outline






6 Conclusions6 / 37


MAC design in WBSN

Energy Efficiency in WBSN MAC Protocols

The MAC layer directly controls energy operation.

Major causes of energy waste in WBSN:

1 Collisions2 Idle listening3 Overhearing4 Packet overhead

WBSN MAC design focuses on minimizing energyconsumption.

Contention based protocols: turning radio into sleep statewhen it is not needed.Scheduled based protocols: low duty cycling.

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Overview of the sloted IEEE 802.15.4 CAP

MAC Layer

Operational Modes:

IEEE 802.15.4 MAC

Beacon Enabled Non-Beacon Enabled

Superframe Unslotted CSMA/CA

Contention Access Period Contention Free Period

Slotted CSMA/CA GTS Allocation

Non-beacon-enabled mode:Distributed system without coordinator.Ad-hoc.

Beacon-enabled mode:CoordinatedSynchronization through beacon.Superframe time structure to organize communication.

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Beacon-Enabled Mode

Beacon frames are periodically sent by the coordinator every BI.Delimits the superframe structure and enables communication.

Superframe structure:

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CAP CSMA/CA Mechanism

NB = 0, CW = 2

Battery lifeextension?

BE = macMinBE

BE = lesser of(2, macMinBE)

Locate backoffperiod boundary

Delay forrandom(2BE - 1) unit

backoff periods

Perform CCA onbackoff period

boundary

Channel idle?

CW = 2, NB = NB+1,BE = min(BE+1, aMaxBE)

CW = CW - 1

CW = 0?NB>

macMaxCSMABackoffs?

Failure Success

Slotted CSMA

Y

Y Y

Y

N

N

N

N

Step 1. Init

Step 2. BackoffProcedure

Step 3. CCA

Step 4. ACK

Example...

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Outline






6 Conclusions11 / 37


Development

About the Analytical Model

Based on Ramachandran et al. model from University ofWashington.

Inspired on Bianchi’s analysis of IEEE 802.11.

Models sensors and channel using Markov chains.

Unacknowledged traffic.

No channel Model.

Choice:

Accuracy of the model with respect to ns-2 simulations.

Amenability for extension.

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Development

Model Assumptions

One-hop star topology

Fixed number of sensing devices (M)

Only CAP with no inactive period

No data packet retransmissions

Data packets of fixed N-backoff slots duration.

Packets arrive at the nodes according to a Poisson arrival rateλ.

No buffering at the nodes.

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Analytical Formulation

Markov Chain Model for a Sensing Node

1-p

IDLE

BO3

CS31

CS32

BO4

CS41

CS42

1-p3n 1-p4

n

p3n

pic

pi|ic

(1-p i

c )(1-p 4

n )

(1-p i|i

c )(1-p 4

n )

(1-p i|i

c )p 4n

(1-pic)p4

n

p4n

pic

pi|ic

p(1-p 1n )

pp1n

BO1

CS11

CS12

BO2

CS21

CS22

1-p1n 1-p2

n

p1n

pic

pi|ic

(1-p i

c )(1-p 2

n )

(1-p i|i

c )(1-p 3

n )

(1-p i|i

c )p 2n

(1-pic)p2

n

p2n

pic

pi|ic

(1-p i

c )(1-p 3

n )

(1-p i|i

c )(1-p 2

n )

(1-p i|i

c )p 3n

(1-pic)p3

n

BO5

CS51

CS52

(1-p i

c )(1-p 5

n )

(1-p i|i

c )(1-p 5

n )

(1-p i|ic )p 5

n

(1-pic)p5

n

p5n

pic

pi|ic

1-p5n

ACK

TX

(1-pic)

(1-pi|ic)

1

1

Backoff before channel sensing

Max number of backoffs/trials to re-access channel when sensed busy for one packet

Channel must be sensed idle during CW=2 consecutive

backoff slots

Channel Access failure

This Markov Chain is solved an equation relating pci and the probability that a

node accesses the channel pnt.

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Markov Chain Model For the Channel

IDLE,IDLE

SUCCESS

BUSY,IDLE

FAILURE

α

δ=1-α-β

β

11

1

NO node begins transmission

One and only one node begins transmission

More than one node begins transmission at

the same time

This Markov Chain is solved the second necessary equation relating pci and the probability

that a node accesses the channel pnt to characterize completely the whole system.

Consistent non linearequation system forpc

i/i , pci and pn

t .which can be solvedfollowing numericalapproximationtechniques.

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Time Spent in the ACK and (BUSY,IDLE) States

tack_min Lack

(a) Slot timing for the derivation of tsuccess

… idleACKdata …

… idle

tack_max

…

(a) Slot timing for the derivation of tfailure

collision

0.6 ≤ tack ≤ 1.6 (1)

The presence of acknowledgements makes the time spent in the (ACK) nodestate and (BUSY,IDLE) channel state non deterministic:

1 On the previous model, it was just one slot.

2 Determining these timings is an important aspect of our contributedmodel.

3 Probabilistic approach to determine the mean time spent on this states. 16 / 37



Performance MetricsAggregated throughput:

Relative time spent in the successful channel state.

S =Nπc

s

πcii + T c

B,Iπc

bi+ Nπc

s + Nπcf

=Nβ

1 + T cB,I

(1 − α) + N(β + δ)(2)

Average power consumption per node:

Relative time spent on transmitting, receiving and idle node states.

Yav = (pnidle − pn

beacon + pnbo − pn

ir )Yidle + (pncs + pn

ir + pnbeacon + pn

ack )Yrx + pntx Ytx (3)

Per node bytes-per-Joule capacity:

η =(S/M)(250 × 103/8)

Yav(4)

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Outline








Analysis

Path Loss Model for the Human Body

The human body is a very lossy medium.

Transmissions near the human body are not always possible.

Recently E. Reusens et al. and A. Fort et al. proposed the useof a lognormal model distribution+shadowing deviation todetermine the node’s communication range:

PL = PdB + Ps = P0,dB + 10nlog(d/d0) + tσ

The PL exponent n is varied empirically to match themeasured data.Ps = tσ is the shadowing component.t =√

2erfc−1[2(1− p)]

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Analysis

Parameter Values for the Shadowing Model

parameter value LOS value NLOS

d0 10 cm 10 cm

P0,dB 35.7 dB 48.8 dB

σ 6.2 dB 5.0 dB

n 3.38 5.9

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Changes in the Analytical Model

New Channel Markov Chain

IDLE,IDLE

SUCCESS

BUSY,IDLE

FAILURE

α

βPe+ δ

β(1-P e)

11

1

Inclusion of the packet loss rate Pe .

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Outline








Initial Considerations

Flow diagram to Obtain Results

ns-2

Config Script.tcl

Trace File .tr

gawkAnalyzerNAM

Topology.scn

Matlab

Output Data.txt

Analyzer script.awk

Seed Value

Simul Init

Performance Graphs

Nam File .nam

Topology Animator

Matlab

Analytical Init

Solution

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Parameters Used

aMinBE = 3 aMaxBE = 5CSMA/CA parameters macMaxCSMABackoffs = 5 CW = 2

BCO = 6 SFO = 6Analytical parameters pn

beacon = 1/3072Data Packet size N = Ldata = 10backoffslots nbeacon = 2backoffslots

Number of sensing Nodes M = 12

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CC2420 Energy State Values

Max [dBm] Min [dBm]Sensitivity S(R) -94 -90

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Comparison ACK and non-ACK traffic

Throughput

10−3

10−2

10−1

100

10−2

10−1

100

Per packet arrival rate λ [packet per packet duration]

Cha

nnel

thro

ughp

ut, S

Analytical ACK

Simulation ACK

Analytical NO ACK

Simulation NO ACK

10−3

10−2

10−1

100

0

2

4

6

8

10

12

14

Per packet arrival rate λ [packet per packet duration]%

cha

nge

in th

roug

hput

Excellent accuracy of our analytical model capturing throughput

performance.26 / 37



ns-2 Overhearing Bug

10−3

10−2

10−1

100

10−1

100

101

102


Per

−no

de p

ower

con

sum

ptio

n, Y

av [m

W]

Analytical NO ACKSimulation NO ACK

Figure: Per node power consumption27 / 37



ns-2 Overhearing Bug

10−3

10−2

10−1

100

10−2

10−1

100

101

102


Per

−no

de T

x po

wer

con

sum

ptio

n,Y

tx [m

W] Analytical NO ACK

Simulation NO ACK

10−3

10−2

10−1

100

10−2

10−1

100

101

102


,Per

−no

de R

x po

wer

con

sum

ptio

n,Y

rx [m

W]

Analytical NO ACKSimulation NO ACK

10−3

10−2

10−1

100

10−1

100

101

102


Per

−no

de Id

le p

ower

con

sum

ptio

n, Y

idle

[mW

] Analytical NO ACKSimulation NO ACK

Simulation Rx energy increases.

Simulation Idle energy decreases.

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Average per node power consumption

10−3

10−2

10−1

100

100

101


Per

−no

de p

ower

con

sum

ptio

n, Y

av [m

W]

Analytical ACKAnalytical NO ACK

10−3

10−2

10−1

100

0

1

2

3

4

5

6

7


% c

hang

e in

per

nod

e po

wer

con

sum

ptio

n

The inclusion of the ACK increases energy consumption.

28 / 37



Bytes per Joule capacity

10−3

10−2

10−1

100

102


Byt

es p

er J

oule

cap

acity

, η [K

B/J

]

Bytes per Joule capacity comparison

Analytical ACKAnalytical NO ACK

10−3

10−2

10−1

100

0

2

4

6

8

10

12

14

16

Per packet arrival rate λ [packet per packet duration]%

cha

nge

in b

ytes

−pe

r−Jo

ule

capa

city

The optimal energy-throughput trade off, archived for a datarate of

λ = 0.04 = 10kbps29 / 37


Performance Results for a high path loss WBSN

Throughput in the LOS channel

10−3

10−2

10−1

100

10−2

10−1

100


Cha

nnel

thro

ughp

ut, S

Throughput comparison WBSN channel with LOS

Analytical ACK Pe=0%Analytical ACK Pe=5%Simulation ACK Pt=1mWSimulation ACK Pt=0.1mW

Figure: Throughput comparison WBSN channel with LOS

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Average per node power consumption LOS channel

10−3

10−2

10−1

100

100

101


Per

−no

de p

ower

con

sum

ptio

n, Y

av [m

W]

LOS channel

Analytical ACK Pt=1mWAnalytical ACK Pt=0.1mW

Figure: Per-node power consumption in LOS channel

31 / 37



Throughput in NLOS channel

10−3

10−2

10−1

100

10−2

10−1

100


Cha

nnel

thro

ughp

ut, S

Throughput comparison BSN channel with NLOS

Simulation ACK Pt=1mWSimulation ACK Pt=0.32mWAnalytical ACK Pe=0%Analytical ACK Pe=5%

Figure: Throughput comparison WBSN channel with NLOS

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Hidden terminal problem

For high data rates, the hidden terminal problem becomesdominant, and collapses our model.

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Outline








Conclusions

Extension of an analytical model of the slotted CSMA/CAprocedure in the CAP of the IEEE 802.15.4 standard toacknowledged traffic.The validity of the analytical model has been demonstratedcomparing with simulation results.For the purpose of conducting near realistic simulations, theChipcon CC2420 IEEE 802.15.4 transceiver energy parametershave been used.The results of the analytical model resolution have been thenemployed to predict throughput and energy consumption.We have uncovered one of the main problems of using IEEE802.15.4 in a human body environment: hidden node problem⇒ multihop topology or the use of relays could be moresuited.

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Future Work

Solve the overhearing ns-2 simulation bug.

Include in the model, the possibility of hidden nodes.

Study the GTS implementation, particularly effective forWBSN applications that have timing constraints.

Use a multi-hop topology strategy to solve energy issues.

Study other sophisticated channel models available in theliterature to perform different evaluations and contrast studies.

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Questions?

Thank you for your attention!

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pfc_analysis of ieee 802.15.4 in wbsn

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