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8/8/2019 IEEE Globecom

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Microelectronics and Multimedia Communications

Research Centre

Optimisation of RTS/CTS handshake in IEEE 802.11 Wireless

LANs for maximum performance

P. Chatzimisios1, A. C. Boucouvalas1 and V. Vitsas2

1Microelectronics and Multimedia Communications Research Centre,

School of Design, Engineering and Computing

Bournemouth University, UK

2Department of Information Technology,

Technological Educational Institution, Greece


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Contents

Brief description of the IEEE 802.11 protocol

Mathematical modelling including throughput and packet delay

analysis for both basic access and RTS/CTS schemes

Inefficiency of RTS/CTS scheme

Derivation of RTS threshold


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Analytical performance results

Conclusions

IEEE 802.11

MAC layer

DCF (Distributed Coordination Function)

o Asynchronous data transfer service (mandatory)

o Gives equal chance of accessing transmission medium

PCF (Point Coordination Function)


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o This optional service is designed for delay-sensitive traffic

o Access point polls stations according to a list

IEEE 802.11 DCF MAC access mechanisms

CSMA/CA Basic Access

o Collision avoidance via randomized backoff mechanism

o ACK packet for acknowledgement


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RTS/CTS

o Addresses the hidden terminal problem

o Shortens the collision duration

Basic Access mechanism

DIFS DIFS

SIFS

Bus medium

Contention Window

Slot time

Defer access

Backoff Window

Select slot and decrement backoff

as long as medium is idle


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RTS/CTS reservation mechanismDIFS

SourceTX

Destination(TX)

RTS

SIFS

CTS

SIFS

DATA

SIFS

ACK

Other

NAV (RTS)

NAV (CTS)

NAV (DATA)

DIFS

BackoffDefer access


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Mathematical modelling assumptions

Packets can encounter collisions only due to simultaneous transmissions (no

transmission errors)

There are no hidden stations (all stations can hear others transmissions).


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The network consists of a finite number of contending stations.

Saturated conditions, i.e. a station has always data ready for transmission.

The collision probability of a transmitted packet is constant and independent of

the number of retransmissions.

Analytical model

Utilizing a Markov chain model and after some algebra, theprobability that a station

transmits in a randomly chosen slot equal to:


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1

1 1

1

1 1 1

2(1 2 ) (1 ),

(1 (2 ) ) (1 ) (1 2 ) (1 )

2 (1 2 ) (1 )

(1 (2 ) ) (1 ) (1 2 ) (1 ) 2 (1 2 ) (1 )

m

m m

m

m m m m m m

p pm m

W p p p p

p p

W p p p p W p p p

+

+ +

+

+ + +

+

=

+ + , m m

>

where m is the retry limit, m'identifies the maximum number of backoff stages, Wis the

contention window (CW) size andp is the packet collision probability given by:

11 (1 )np =

Time interval durations

The values ofTsand Tcdepend on the medium access scheme and for the basic access are given by:bas bas

C S header ACK

lT T DIFS T SIFS T

C= = + + + +


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and for the RTS/CTS scheme:

RTSS RTS CTS header ACK

RTS

C RTS CTS

lT DIFS T SIFS T SIFS T SIFS T

C

T DIFS T SIFS T

= + + + + + + + +

= + + +

where lis the payload length, Cis the data rate, Ccontrol is the control rate (1 Mbit/s), Theader, TACK, TRTSand TCTSarethe time intervals required to transmit the packet payload header, the ACK, RTS andCTS control packets, respectively.

hdr hdr

header

control

MAC PHY T

C C= + ACKACK

control

lT

C= RTSRTS

control

lT

C=

CTSCTS

control

lT

C=

where lACK, lRTSand lCTS is the length of ACK, RTS and CTS control packets respectively, MAChdr isthe MAC header andPHYhdr is the physical header.


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Packet delay and throughput versus packet size

(n=5, C= 11 Mbit/s, Ccontrol= 2 Mbit/s)

0.004

0.005

0.006

0.007

0.008

delay(sec)

5

6

7

8

9

10

11

hput(Mbit/s)


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Packet delay and throughput versus packet size

(n=25, C= 11 Mbit/s, Ccontrol= 2 Mbit/s)

0.05

0.06

0.07

0.08

y(sec)

6

7

8

9

10

11

t(Mbit/s)


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Conclusions

An intuitive mathematical analysis and simple equations were presented for

throughput and packet delay performance of IEEE 802.11 DCF by utilizing a Markov

chain model.

The inefficiency of the RTS/CTS reservation scheme in reducing packet collisionduration was studied under certain scenarios; performance results have showed that the

lower rate RTS/CTS exchange reservation scheme has limited utility when it is

combined with higher transmission data rates.

Our work also carried out a simple analysis to derive an all-purpose expression for the

RTS threshold value, which determines when the RTS/CTS scheme should be

employed, aiming to minimize packet delay under IEEE 802.11 DCF.


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Conclusions (2)

Performance results demonstrated that the RTS threshold significantly depends on both

protocol parameters and network size. In fact, high data rates and a high packet retry

limit, bring about the considerable increase of RTS threshold values.

The use of a short physical packet overhead minimizes the main drawback of the extraoverhead for the RTS/CTS scheme and makes beneficial its employment for even

smaller data packets.

The derived analysis could be useful for simple performance improvements, through

the optimal use of the RTS/CTS scheme, however, it brings about the question of

effectiveness and necessity of the RTS/CTS reservation scheme in high-speed IEEE802.11 WLANs and in the absence of hidden stations.

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