Contention Window Optimization Contention Window Optimization for IEEE 802.11 DCF Access for IEEE 802.11 DCF Access

Control Control

D. J. Deng, C. H. Ke, H. H. Chen, and Y. M. Huang

IEEE Transaction on Wireless Communication

Speaker: Der-Jiunn DengDepartment of Computer Science and Information Engineering

National Changhua University of Education

Generic Mobile computing System Arch.Generic Mobile computing System Arch.

This talk will cover:applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

Application & Services

OS & Middleware

Network

Data Link

Radio

PartitioningSource Coding & DSPContext Adaptation

Mobility ManagementResource ManagementQoS ManagementReroutingImpact on TCPLocation Tracking

Multiple AccessChannel AllocationLink Error Control

Modulation SchemesChannel CodingRF Circuit

DestinationDestination

DataData

ACKACK

Basic Access Method (CSMA/CA)Basic Access Method (CSMA/CA)

SourceSource

OtherOther

DIFS

Busy Medium DIFS

Free access when medium is free longer than DIFS

CW

SIFSDIFS

Busy Medium

DIFS

Busy Medium CWDIFS

NAV

DIFS

Busy Medium

DIFS

Busy Medium

DIFS

Busy Medium

Defer access

Select a CW size and decrement backoff as long as medium is idle

RTSRTS

DestinationDestination

DataData

ACKACK

RTS/CTS MechanismRTS/CTS Mechanism

SourceSource

OtherOther

DIFS

Busy Medium DIFS

Free access when medium is free longer than DIFS

CW

SIFSDIFS

Busy Medium

DIFS

Busy Medium CWDIFS

NAV

DIFS

Busy Medium

DIFS

Busy Medium

DIFS

Busy Medium

Defer access

Select a CW size and decrement backoff as long as medium is idle

CTSCTS

SIFS

SIFS

Throughput EfficiencyThroughput Efficiency

single-rate, 1 Mbps single-rate, 11 Mbps

D. J. Deng, L. Bin, L. F. Huang, C. H. Ke, and Y. M. Huang, "Saturation Throughput Analysis of Multi-rate IEEE 802.11 Wireless Networks," accept for publications in Wireless Communications and Mobile Computing.

Throughput EfficiencyThroughput Efficiency

multi-rate, 1, 2, 5.5, 11 Mbps

D. J. Deng, L. Bin, L. F. Huang, C. H. Ke, and Y. M. Huang, "Saturation Throughput Analysis of Multi-rate IEEE 802.11 Wireless Networks," accept for publications in Wireless Communications and Mobile Computing.

multi-rate, 1, 2, 5.5, 11 Mbps

Exponential BackoffExponential Backoff

DataDataDIFS

Busy Medium DIFS

CWDIFS

Busy Medium SIFS

31

0

63

127

255

511

1023

initial attempt

first retransmission

second retransmission

third retransmission

forth retransmission

CW min CW max

ifunctionrandom 52()_ time slot

RTSRTS

The usage of backoff algorithm avoids long access delays when the load is light because it selects an initial (small) parameter value of contention window (CW) by assuming a low level of congestion in the system.

This strategy might allocate initial size of CW, only to find out later that it is not enough when the load increased, but each increase of the CW parameter value is obtained paying the cost of a collision (bandwidth wastage)

After a successful transmission, the size of CW is set again to the minimum value without maintaining any knowledge of the current channel status.

It incurs a high collision probability and channel utilization is degraded

in bursty arrival or congested scenarios

Congested ScenarioCongested Scenario

In DCF access method, immediate positive acknowledgement informs the sender of successful reception of each data frame

In case an acknowledgement is not received, the sender will presume that the data frame is lost due to collision, not by frame loss.

Unfortunately, wireless transmission links are noisy and highly unreliable, for example, for BER= , the probability of receiving a full data frame correctly is less than 30%.

410

0

0.2

0.4

0.6

0.8

1

1.2

1.00E-12 1.00E-11 1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 0.01

BER

Th

rou

gh

pu

t

The proper approach to dealing with lost framesis to send them again, and as auickly as possible

Noisy EnvironmentNoisy Environment

Geometric DistributionGeometric Distribution

Consider a sequence of Bernoulli trails with the probability of success on being P. Let r.v. X denote the number of trials up to and including the first success

,)1()( 1 ppxf x x1

1

1)1()()(x

xppxxfxXE

1

)1(1 x

xpxp

p

pp

p

p

p 1

))1(1(

1

1 2

ACKACK

PP-Persistent CSMA/CA-Persistent CSMA/CA

p

W 1

2

1

1

2p

W

DataDataDIFS

Busy Medium DIFS

CWDIFS

Busy Medium SIFS

DataData

W

defer access decrement backoff as long as medium is idle

Runtime Estimate Channel Runtime Estimate Channel StatusStatus

attemptsion transmissofNumber

failuresion transmissofNumber fp

Average CW sizeAverage CW size

No. of Active Stations EstimationNo. of Active Stations Estimation

BER EstimatingBER Estimating

PSK Mode CCK Mode

BER vs. SNR for Intersil HFA3861B chipset

CW OptimizationCW Optimization

CMpiMpiMpipM

M

i

M

iopt

1}0{1}{}{

11

Using Block-CodeUsing Block-Code

Priority EnforcementPriority Enforcement

Theoretical Limit Theoretical Limit (Basic Access Method)(Basic Access Method)

slotSIFSDATA

ACKDATADIFSHp

LLDATA

TW

TR

LLTTT

BERL ACKDATA

2222

)1( )(

Theoretical Limit Theoretical Limit (RTS/CTS Mechanism)(RTS/CTS Mechanism)

slotSIFSDATA

ACKDATACTSRTSDIFSHp

LLLLDATA

TW

TR

LLLLTTT

BERL ACKDATACTSRTS

23444

)1( )(

Simulation – Default ValuesSimulation – Default Values

Achievable throughput versus channel BER with different network sizes

Blocking rate versus channel BER with different network sizes

Simulation - Congested ScenarioSimulation - Congested Scenario

Simulation - Noisy EnvironmentSimulation - Noisy Environment

Achievable throughput versus number of stationsunder different channel BER

Blocking rate versus number of stationsunder different channel BER

Simulation – Basic Access Simulation – Basic Access MethodMethod

The performance of basic access method

Simulation – RTS/CTS Simulation – RTS/CTS MechanismMechanism

The performance of RTS/CTS mechanism

Simulation – Proposed SchemeSimulation – Proposed Scheme

The performance of proposed scheme

Simulation – Coexisted Simulation – Coexisted Environments Environments

The proposed scheme and legacy DCF access method coexist in a same BSS

ConclusionsConclusions The backoff parameters in IEEE 802.11 DCF access method are fa

r from the optimal setting in heavy-load and error-prone WLANs environment

In this paper, we attempt to identify the relationship between backoff parameters and channel BER and put forth a pragmatic problem-solving solution

The proposed scheme is performed at each station in a distributed manner, and it can be implemented in the present IEEE 802.11 standard with relatively minor modifications

There’s no such thing as a free lunch We believe that it is almost impossible to increase the probability of success of transmitting a frame excepting frames fragmentation or FEC (Forward Error Control) in an extremely noisy wireless environment.