On the Performance Behavior of On the Performance Behavior of IEEE 802.11 Distributed IEEE 802.11 Distributed Coordination FunctionCoordination Function

M.K.Sidiropoulos, J.S.Vardakas and M.K.Sidiropoulos, J.S.Vardakas and M.D.LogothetisM.D.Logothetis

Wire Communications Laboratory,Wire Communications Laboratory,Department of Electrical & Computer Department of Electrical & Computer

Engineering,Engineering,University of Patras,University of Patras,

265 00 Patras, Greece265 00 Patras, GreeceE-mail: m-logo@wcl.ee.upatras.grE-mail: m-logo@wcl.ee.upatras.gr

Outline Outline

Purpose of the paperPurpose of the paper..

DCF-An Example.DCF-An Example.

Mathematical Analysis ( Assumptions ).Mathematical Analysis ( Assumptions ).

802.11 DCF: Markov Chain Model and Steady State 802.11 DCF: Markov Chain Model and Steady State AnalysisAnalysis

leading to a Saturation Throughput formula.leading to a Saturation Throughput formula.

Simulation results ( IEEE 802.11b network).Simulation results ( IEEE 802.11b network).

Conclusion.Conclusion.

Purpose of the paperPurpose of the paper

We propose a new Markov model for the DCF of We propose a new Markov model for the DCF of IEEE 802.11IEEE 802.11

based on Bianchi’s, Wu’s and Ziouva’s models.based on Bianchi’s, Wu’s and Ziouva’s models.

and derive an analytical formula for the Saturation and derive an analytical formula for the Saturation ThroughputThroughput

for both Basic and RTS/CTS access schemes.for both Basic and RTS/CTS access schemes.

Simulation StudySimulation Study: :

• Validation of our new Markov model based on Validation of our new Markov model based on throughput results by the NS-2.throughput results by the NS-2.

• Average end-to-end packet delay for both Average end-to-end packet delay for both access schemes.access schemes.

DCF-An ExampleDCF-An ExampleDCF employs 2 mechanisms:

Basic access scheme: A 2-way handshaking technique.

Note that: After a DIFS time interval each station defers for an additional random backoff time. The backoff counter is frozen if a transmission is detected on the channel BACKOFF SUSPENSION

DCF-An Example (cont.)DCF-An Example (cont.)RTS/CTS:

Request-To-Send / Clear-To- Send.

It is a 4-way handshaking technique.

Introduced to tackle the hidden terminal problem.

Improve throughput performance in case of long packets.

Mathematical Analysis Mathematical Analysis

AssumptionsAssumptions::

Ideal channel conditions ( error-free channel).Ideal channel conditions ( error-free channel).

Finite number of stations, each of which has always a Finite number of stations, each of which has always a packet packet

available for transmission. (saturation conditions)available for transmission. (saturation conditions)

Constant and independent collision probability p.Constant and independent collision probability p.

Probability Probability pb independent of the backoff procedure. independent of the backoff procedure.

802.11 DCF: Markov Chain Model802.11 DCF: Markov Chain Model

(1-p)/W0

(1-p)/W0

(1-p)/W0

1/W0

1-pb 1-pb 1-pb 1-pb

pb pb pb pb

1-pb 1-pb 1-pb 1-pb

pb pb pb pb

pb pb pb pb

pb pb pb pb

p/Wi

p/Wm

p/Wi+1

p/W1

1-pb 1-pb 1-pb 1-pb

1-pb 1-pb 1-pb 1-pb

0,0 0,1 0,2 0,W0-2 0,W0-1

m,0 m,2 m,1 m,Wm-2

i,0 i,Wi-1 i,Wi-2 i,2 i,1

i-1,0

i-1,1

i-1,2

i-1,wi-1-1 W0-1

i-1,wi-1-2

m,Wm-1

Saturation Throughput model.Saturation Throughput model.

Normalized Throughput: ( fraction of the channel time used for payload transmissions)

[ ]

[ ]

E payload transmitted in a slot timeS

E average length of a slot time

[ ]

(1 ) (1 )s tr

tr tr s S tr s c

PP E PS

P P PT P P T

Ts: average time of a successful transmission σ : duration of an empty slot time

Tc : average duration of a collision .

E[P]: average packet payload.

Ps : a successful transmission in a slot. Ptr : at least one transmission in a slot.

Steady State AnalysisSteady State Analysis

Stationary Distribution of the chain ( Steady State ):

, lim { ( ) , ( ) }i kt

b P s t i b t k

,0 0,0i

ib p b , [0, ]i m

1

,0 ,00

,

,0

1 0(1 )

(1)1

01

mi

j mji b

i k

ii

i b

W kp b b i

W pb

W kb i m

W p

From the chain we have:

Steady State Analysis (cont.)Steady State Analysis (cont.)

Normalization condition :

1

,0 0

1 (2)iWm

i ki k

b

2(3)

2

ii

mi

W W i m

W W i m

Contention Window :

'0,0(1), (2), (3) ( , , , , )bb f p p w m m

1

,0 0,00

1

1

mm

ii

pb b

p

Steady State Analysis (cont.)Steady State Analysis (cont.)

Collision probability :

Probability of channel being busy :

1

1

1 (1 )(5)

1 (1 )

n

nb

p

p

1 (1 )ntrP 1(1 )

1 (1 )

n

s n

nP

(4), (5) ,numerical techniques

p

Channel access probability : ( , ) (4)bf p p

Saturation Throughput model.Saturation Throughput model.

Normalized Throughput: ( fraction of the channel time used for payload transmissions)

[ ]

[ ]

E payload transmitted in a slot timeS

E average length of a slot time

[ ]

(1 ) (1 )s tr

tr tr s S tr s c

PP E PS

P P PT P P T

Ts: average time of a successful transmission σ : duration of an empty slot time

Tc : average duration of a collision .

E[P]: average packet payload.

Ps : a successful transmission in a slot. Ptr : at least one transmission in a slot.

Simulation StudySimulation Study

Performance metrics Performance metrics measuredmeasured

by simulation:by simulation:• Saturation throughput.Saturation throughput.• End-to-end average packet End-to-end average packet

delaydelay..

Simulations in NS-2Simulations in NS-2 IEEE 802.11b single-hopIEEE 802.11b single-hop networknetwork..

Network TopologyNetwork Topology: : • No hidden stations, all have No hidden stations, all have

LOSLOS..• CBR traffic over UDP links CBR traffic over UDP links

towards the AP.towards the AP.• No mobilityNo mobility..

Model Validation: Simulation vs. Model Validation: Simulation vs. Analysis:Analysis:

1Mbps. 1Mbps. Basic and RTS/CTSBasic and RTS/CTS Close match of analyticalClose match of analytical model and simulation model and simulation

resultsresults..

Our model is closer toOur model is closer to simulation than Wu’s.simulation than Wu’s.

The RTS/CTS gives higherThe RTS/CTS gives higher throughput than Basic throughput than Basic

due todue to the short RTS framesthe short RTS frames..

( Only exception for n ( Only exception for n =5).=5).

Basic 1Mbps

Number of Stations

0 10 20 30 40 50

Sa

tura

tio

n T

hro

ug

hp

ut

0.4

0.5

0.6

0.7

0.8

0.9

Basic, simulationBasic, new modelBasic, Wu's model

RTS/CTS, 1Mbps

Number of Stations

0 10 20 30 40 50

Sa

tura

tion T

hro

ughpu

t

0.5

0.6

0.7

0.8

0.9

RTS, simulationRTS, new modelRTS, Wu's model

Model Validation: Simulation vs. Model Validation: Simulation vs. Analysis: Analysis:

5.5 and 11 Mbps.5.5 and 11 Mbps. In both cases analysis andIn both cases analysis and simulation are in simulation are in

satisfactorysatisfactory agreementagreement..

Basic access scheme givesBasic access scheme gives higher throughput thanhigher throughput than RTS/CTS when channel bitRTS/CTS when channel bit rate rate ↑↑. ( RTS, CTS packets. ( RTS, CTS packets are transmitted at 1Mbps).are transmitted at 1Mbps).

Throughput Throughput ↓ ↓ as bit rateas bit rate↑↑ ( DIFS,SIFS, Backoff delay( DIFS,SIFS, Backoff delay remain unchanged)remain unchanged)

Number of Stations

0 10 20 30 40 50

Sa

tura

tio

n T

hro

ug

hp

ut

0.0

0.2

0.4

0.6

Basic, SimulationBasic, New ModelRTS, SimulationRTS, New Model

5.5 Mbps

Basic

RTS

Number of Stations

0 10 20 30 40 50

Sa

tura

tio

n T

hro

ug

hp

ut

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Basic, SimulationBasic, New ModelRTS, SimulationRTS, New Model

11 Mbps

Basic

RTS

Average Delay SimulationAverage Delay Simulation

As network size As network size ↑↑ delay delay ↑ for both access schemes.↑ for both access schemes. As channel bit rate ↑As channel bit rate ↑ delay ↓.delay ↓. RTS/CTS delay is lower than Basic delay only for RTS/CTS delay is lower than Basic delay only for

1Mbps1Mbps..

Not efficient to use RTS/CTS for high data ratesNot efficient to use RTS/CTS for high data rates

Number of stations

0 10 20 30 40 50

Satu

rati

on

Dela

y (

sec)

0.0

0.5

1.0

1.5

2.0

2.5

1 Mbps

5.5 Mbps

11 Mbps

RTS

Basic

ConclusionConclusion We have developed an analytical model to enhance We have developed an analytical model to enhance

Bianchi’s and Wu’s analytical model for the saturation Bianchi’s and Wu’s analytical model for the saturation throughput of the DCF of the IEEE 802.11 protocol.throughput of the DCF of the IEEE 802.11 protocol.

Our model gives greater throughput results than Wu’s Our model gives greater throughput results than Wu’s model for both access schemes, Basic and RTS/CTS. model for both access schemes, Basic and RTS/CTS.

Via numerous simulations with NS-2 we have shown that Via numerous simulations with NS-2 we have shown that our model is close to simulation, for all network sizes .our model is close to simulation, for all network sizes .

As channel bit rate increases:As channel bit rate increases: throughput decreasesthroughput decreasesAverage delay decreases.Average delay decreases.

Basic vs. RTS/CTSBasic vs. RTS/CTS::In low rates RTS is better than Basic.In low rates RTS is better than Basic.In higher rates Basic is preferable than RTS ( gives In higher rates Basic is preferable than RTS ( gives

greater throughput and lower delay).greater throughput and lower delay).