Contention Window Optimization for IEEE DCF Access Control

Contention Window Optimization for Contention Window Optimization for

IEEE 802.11 DCF Access Control IEEE 802.11 DCF Access Control

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

IEEE Transaction on Wireless Communication Speaker: Der-Jiunn Deng

Department 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: application transport

network physicallink

application transport

network physicallink network

physicallink

data data

Application &

Services OS &

Middleware Network

Data Link

Radio

Partitioning

Source Coding & DSP Context Adaptation Mobility Management Resource Management QoS Management

Rerouting

Impact on TCP Location Tracking Multiple Access Channel Allocation Link Error Control Modulation Schemes Channel Coding

RF Circuit

Destination Destination

DataData

ACKACK

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

Source Source

Other Other

DIFS

Busy Medium DIFS

Free access when medium is free longer than DIFS

CW

SIFS DIFS

Busy Medium

DIFS

Busy Medium CW DIFS

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

Destination Destination

DataData

ACKACK

RTS/CTS Mechanism RTS/CTS Mechanism

Source Source

Other Other

DIFS

Busy Medium DIFS

Free access when medium is free longer than DIFS

CW

SIFS DIFS

Busy Medium

DIFS

Busy Medium CW DIFS

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

Exponential

Exponential Backoff Backoff

DataData

DIFS

Busy Medium DIFS

CW

DIFS

Busy Medium SIFS

31

0

63 127

255

511

1023 initial attempt

first retransmission second retransmission third retransmission forth retransmission

CW min CW max

⎣

⎦ ^×

RTSRTS

Throughput Efficiency Throughput 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 inWireless Communications and Mobile Computing.

Throughput Efficiency Throughput 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 inWireless Communications and Mobile Computing.

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

ˆ 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.

ˆ 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)

ˆ 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 Scenario

Congested Scenario

ˆ In DCF access method, immediate positive

acknowledgement informs the sender of successful reception of each data frame

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

ˆ 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%.

10⁻4

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

Throughput

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

Noisy Environment

Noisy Environment

Geometric Distribution Geometric 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 ( )

( x p

p

f = −

⋅ 1 ≤ x ≤ ∞

∑ ∑

=

− −

⋅

=

⋅

=

1

) 1

1 ( )

( )

(

x

p x

p x x

f x X

E

∑

=

− −

=

1

) 1

1 _x (

p x

p x p

p p

p p

p 1

)) 1

( 1 (

1

1 ² =

−

−

⋅ −

= −

P P - - Persistent CSMA/CA Persistent CSMA/CA

p

W 1

2 + 1 =

2 − 1

=

⇒ W p

RTSRTS DataData

DIFS

Busy Medium DIFS

CW

DIFS

Busy Medium SIFS

DataData W

Runtime Estimate Channel Status Runtime Estimate Channel Status

attempts ion

transmiss of

Number

failures ion

transmiss of

Number

f = p

Average CW size

Average CW size

No. of Active Stations Estimation

No. of Active Stations Estimation

CW Optimization CW Optimization

M C p i

M p i

M p i p

M

M

i M

i opt

} 1 0 {

1 } {

} {

1 1

=

=

−

=

=

≥

=

⋅

=

⋅ ∑ ∑

=

=

BER Estimating BER Estimating

PSK Mode CCK Mode

BER vs. SNR for Intersil HFA3861B chipset

Using Block

Using Block - - Code Code

Priority Enforcement

Priority Enforcement

Theoretical Limit

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

slot SIFS

DATA ACK DATA

DIFS H

p

L L

DATA

W T R T

L T L

T T

BER

L ^{DATA ACK}

⋅ + + +

+ +

+ +

−

⋅ ⁺

2 2 2

2

) 1

( ^{( )}

τ

Theoretical Limit

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

slot SIFS

DATA

ACK DATA

CTS RTS

DIFS H

p

L L

L L DATA

W T R T

L L

L T L

T T

BER

L ^{RTS CTS DATA ACK}

⋅ + + +

+ + +

+ +

+

−

⋅ ^{+ + +}

3 2 4

4 4

) 1

( ^{( )}

τ

Simulation

Simulation – – Default Values Default Values

Achievable throughput versus channel BER

with different network sizes Blocking rate versus channel BER with different network sizes

Simulation

Simulation - - Congested Scenario Congested Scenario

Simulation

Simulation - - Noisy Environment Noisy Environment

Achievable throughput versus number of stations

under different channel BER Blocking rate versus number of stations under different channel BER

Simulation

Simulation – – Basic Access Method Basic Access Method

The performance of basic access method

Simulation

Simulation – – RTS/CTS Mechanism RTS/CTS Mechanism

The performance of RTS/CTS mechanism

Simulation

Simulation – – Proposed Scheme Proposed Scheme

The performance of proposed scheme

Simulation

Simulation – – Coexisted Environments Coexisted Environments

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

Simulation

Simulation – – Slow Slow - - Start Scheme Start Scheme

Simulation

Simulation – – Slow Slow - - Start Scheme Start Scheme

Simulation

Simulation – – Slow Slow - - Start Scheme Start Scheme

Conclusions Conclusions

ˆ The backoff parameters in IEEE 802.11 DCF access method are far from the optimal setting in heavy-load and error-prone

WLANs environment

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

ˆ 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

ˆ 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.