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
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
Research Centre
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
Microelectronics and Multimedia Communications
Research Centre
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
Microelectronics and Multimedia Communications
Research Centre
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
Microelectronics and Multimedia Communications
Research Centre
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
Microelectronics and Multimedia Communications
Research Centre
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
Microelectronics and Multimedia Communications
Research Centre
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.
Microelectronics and Multimedia Communications
Research Centre
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 m1 (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 )n1
Microelectronics and Multimedia Communications
Research Centre
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.
Microelectronics and Multimedia
Communications Research Centre
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
Microelectronics and Multimedia Communications
Research Centre
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.
Microelectronics and Multimedia Communications
Research Centre
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.
0
