saturation throughput analysis and performance optimization of

Saturation Throughput Analysis and Saturation Throughput Analysis and Performance Optimization of BEB Performance Optimization of BEB

Algorithm Algorithm

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

National Changhua University of Education

Ad Hoc NetworksAd Hoc Networks

• Ad hoc network: mobile stations can dynamically form network without AP

• Applications:– “laptop” meeting in conference room, car– interconnection of “personal” devices– battlefield

• IETF MANET (Mobile Ad hoc Networks) working group

CSMA/CACSMA/CA

CSMA: sender- if sense channel idle for

DISF sec.then transmit entire frame (no collision detection)

-if sense channel busy then binary backoff

CSMA receiver:if received OK

return ACK after SIFS

CSMA: others• NAV: Network Allocation

Vector• 802.11 frame has

transmission time field• others (hearing sata) defer

access for NAV time units

CSMA/CACSMA/CA

DestinationDestination

DataData

ACKACK

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

SourceSource

OtherOther

DIFSBusy Medium

DIFS

Free access when medium is free longer than DIFS

CW

SIFSDIFSBusy Medium

DIFSBusy Medium CW

DIFSNAV

DIFSBusy Medium

DIFSBusy Medium

DIFSBusy Medium

Defer access

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

Binary Exponential Binary Exponential BackoffBackoff

DataDataDIFS

Busy Medium DIFS

CWDIFS

Busy Medium SIFS

31

0

63127

255511

1023

initial attempt

first retransmission

second retransmission

third retransmission

forth retransmission

CW min CW max

⎣ ⎦ifunctionrandom +× 52()_ time slot×

ACKACK

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 probabilityand 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

Thro

ughp

ut

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

Noisy EnvironmentNoisy Environment

Hidden Terminal effectHidden Terminal effect

• hidden terminals: A, C cannot hear each other– obstacles, signal attenuation– collisions at B

• goal: avoid collisions at B• CSMA/CA: CSMA with Collision Avoidance

Fairness Problem (hidden)Fairness Problem (hidden)

B. Bensaou , Y. Wang , and C. C. Ko, “Fair medium access in 802.11 based wireless ad-hoc networks,”in Proc. of the 1st ACM international symposium on Mobile ad hoc networking & computing, November 20, 2000, Boston, Massachusetts.

Collision Avoidance: Collision Avoidance: RTSRTS--CTS MechanismCTS Mechanism• CSMA/CA: explicit

channel reservation– sender: send short

RTS: request to send– receiver: reply with

short CTS: clear to send

• CTS reserves channel for sender, notifying (possibly hidden) stations

• avoid hidden station collisions

RTSRTS--CTS MechanismCTS Mechanism

• RTS and CTS short:– collisions less likely, of

shorter duration– end result similar to

collision detection• IEEE 802.11 alows:

– CSMA– CSMA/CA: reservations– polling from AP

RTSRTS

DestinationDestination

DataData

ACKACK

RTS/CTS MechanismRTS/CTS Mechanism

SourceSource

OtherOther

DIFSBusy Medium

DIFS

Free access when medium is free longer than DIFS

CW

SIFSDIFSBusy Medium

DIFSBusy Medium CW

DIFSNAV

DIFSBusy Medium

DIFSBusy Medium

DIFSBusy Medium

Defer access

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

CTSCTS

SIFS

SIFS

CBARTS

RTS/CTS (4RTS/CTS (4--way handshaking)way handshaking)

CBACTS

Lock


CBA

Keep silence

DATA


CBAACK


WCNC 2005WCNC 2005

Throughput Efficiency (multiThroughput Efficiency (multi--rate)rate)

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," Wireless Communications and Mobile Computing, Vol. 9, Issue 8, Aug. 2009, pp. 1102-1112.

Throughput EfficiencyThroughput Efficiency

multi-rate, 1, 2, 5.5, 11 Mbps 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," Wireless Communications and Mobile Computing, Vol. 9, Issue 8, Aug. 2009, pp. 1102-1112.

Exposed Terminal ProblemExposed Terminal Problem

In fact, B can simultaneously transmit with C

A B C DDATA

B is blocked by C’s transmission

DATA

Fairness Problem (exposed)Fairness Problem (exposed)

S. Xu and T. Saadawi, “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks?”IEEE Communications Magazine, vol. 39, issue 6, pp. 130-137, Jun. 2001.

Fairness Problem (Feb. 2010)Fairness Problem (Feb. 2010)

Fairness Problem (exposed)Fairness Problem (exposed)

Slow Start Slow Start BackoffBackoff Alg.Alg.

SlowSlow--Start Start BackoffBackoff Alg.Alg.

0,0 1,0

0,1 1,1

,0x

1,0x +

1,0m−

,0m

,1x

1,1x +

1,1m−

,1m , 1mm W −, 2mm W −, 2m

1,0 0 −W

11, 1mm W −− −

11, 1xx W ++ −

, 1xx W −

1,1 1 −W

2,0 0 −W

2,1 1 −W

, 2xx W −

11, 2xx W ++ −

11, 2mm W −− −1,2m−

1 111

111

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

0)1( Wp f−

1Wp f 0)1( Wp f−

1)1( Wp f−

f xp W

1f xp W +(1 )f xp W−

f xp W

1f mp W −

1(1 )f xp W +−

1f mp W −

f mp W 11 f mp W −−

f mp W

Saturation Throughput AnalysisSaturation Throughput Analysis

X X X XX1st 2nd 3rd xnd

1-p 1-p 1-p p

XX X

ppxf x ⋅−= −1)1()( ∞= ...2,1x

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

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

ppp

pp 1

))1(1(1

1 2 =−−−

⋅−

=

ACKACK

PP--Persistent CSMA/CAPersistent CSMA/CA

pW 1

21=

+12

−=⇒p

W

DataDataDIFS

Busy Medium DIFS

CWDIFS

Busy Medium SIFS

DataData

W

defer access decrement backoff as long as medium is idle

Saturation Throughput AnalysisSaturation Throughput Analysis

Optimal CW sizeOptimal CW size

IEEE TWC (Dec. 2008)IEEE TWC (Dec. 2008)

Runtime Estimate Channel StatusRuntime Estimate Channel Status

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

Simulation ResultsSimulation Results

saturation throughput analysis and performance optimization of

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