Microelectronics and Multimedia Communications
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Optimisation of RTS/CTS handshake in IEEE 802.11 Wireless
LANs for maximum performance
P. Chatzimisios1, A. C. Boucouvalas1 and V. Vitsas2
1
Microelectronics and Multimedia Communications Research Centre,
School of Design, Engineering and Computing
Bournemouth University, UK
2
Department of Information Technology,
Technological Educational Institution, Greece
Microelectronics and Multimedia Communications
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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
 Analytical performance results
 Conclusions
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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)
o This optional service is designed for delay-sensitive traffic
o Access point polls stations according to a list
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IEEE 802.11 DCF MAC access mechanisms
 CSMA/CA Basic Access
o Collision avoidance via randomized backoff mechanism
o ACK packet for acknowledgement
 RTS/CTS
o Addresses the hidden terminal problem
o Shortens the collision duration
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Basic Access mechanism
Contention Window
DIFS
DIFS
SIFS
Busy medium
Backoff Window
Slot time
Defer access
Select slot and decrement backoff
as long as medium is idle
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RTS/CTS reservation mechanism
DIFS
RTS
Source
(TX)
DATA
SIFS
SIFS
SIFS
CTS
ACK
Destination
(TX)
DIFS
NAV (RTS)
Other
NAV (CTS)
NAV (DATA)
Defer access
Backoff
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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 other’s transmissions).
 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.
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Analytical model
Utilizing a Markov chain model and after some algebra, the probability  that a station
transmits in a randomly chosen slot equal to:

2 (1  2 p) (1  p m 1 )

m 1
m 1
W (1  (2 p) ) (1  p)  (1  2 p) (1  p )

 

2 (1  2 p) (1  p m 1 )

W (1  (2 p) m 1 ) (1  p)  (1  2 p) (1  p m 1 )  W 2 m p m1 (1  2 p) (1  p m  m )
, m  m
, m  m
where m is the retry limit, m' identifies the maximum number of backoff stages, W is the
contention window (CW) size and p is the packet collision probability given by:
p  1  (1   )n1
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Time interval durations
The values of Ts and Tc depend on the medium access scheme and for the basic access are given by:
TCbas  TSbas  DIFS  Theader 
l
 SIFS  TACK
C
and for the RTS/CTS scheme:
TSRTS  DIFS  TRTS  SIFS  TCTS  SIFS  Theader 
l
 SIFS  TACK
C
TCRTS  DIFS  TRTS  SIFS  TCTS
where l is the payload length, C is the data rate, Ccontrol is the control rate (1 Mbit/s), Theader, TACK,
TRTS and TCTS are the time intervals required to transmit the packet payload header, the ACK, RTS
and CTS control packets, respectively.
Theader 
MAChdr PHYhdr

C
Ccontrol
TACK 
l ACK
Ccontrol
TRTS 
lRTS
Ccontrol
TCTS 
lCTS
Ccontrol
where lACK, lRTS and lCTS is the length of ACK, RTS and CTS control packets respectively, MAChdr
is the MAC header and PHYhdr 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)
0.008
11
10
0.007
0.006
8
0.005
7
6
0.004
5
Throughput
0.003
4
3
0.002
2
0.001
1
0
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Packet size (bits)
 Packet delay, Basic
 Throughput, Basic
 Packet delay, RTS/CTS
 Throughput, RTS/CTS
Throughput (Mbit/s)
Packet delay (sec)
9
Microelectronics and Multimedia
Communications Research Centre
Packet delay and throughput versus packet size
(n=25, C= 11 Mbit/s, Ccontrol= 2 Mbit/s)
0.08
11
10
0.07
0.06
8
0.05
7
6
0.04
5
0.03
Throughput
4
3
0.02
2
0.01
1
0
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Packet size (bits)
 Packet delay, Basic
 Throughput, Basic
 Packet delay, RTS/CTS
 Throughput, RTS/CTS
10000
Throughput (Mbit/s)
Packet delay (sec)
9
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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 collision
duration 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 extra
overhead 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 IEEE
802.11 WLANs and in the absence of hidden stations.