How Effective is the IEEE 802.11
RTS/CTS Handshake in Ad Hoc
Networks
Kaixin Xu,Mario Gerla,
Sang Bae
IEEE Globecom 2002
Outline





INTRODUCTION
EFFECTIVENESS OF RTS/CTS
HANDSHAKE
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE
PROPOSED SCHEME AND SIMULATION
EVALUATION
CONCLUSION
INTRODUCTION



RTS/CTS handshake is mainly designed for
resolving hidden terminal problem
Such assumption may not hold when the
transmitter-receiver distance exceeds a
certain value
reveals that large interference range is a
serious problem
EFFECTIVENESS OF RTS/CTS
HANDSHAKE (1/7)

three radio ranges:
–
–
–
Transmission Range (Rtx) represents the range
within which a packet is successfully received
Carrier Sensing Range (Rcs) is the range within
which a transmitter triggers carrier sense
detection.
Interference Range (Ri) is the range within which
stations in receive mode will be “interfered with”
by an unrelated transmitter
EFFECTIVENESS OF RTS/CTS
HANDSHAKE (2/7)

Investigation of the interference range
–

Ri=
=
(SNR_THRESHOLD is usually set to 10)
Its relationship to the transmission range
–
Notations
當sender與
receiver的距離
大到一個程度
時，會造成Ri
大於Rtx，也就
是Ai大於
ARTS/CST
EFFECTIVENESS OF RTS/CTS
HANDSHAKE (3/7)
Analysis
(1)
ERTS/CTS is equal to 1
(2)otherwise
ERTS/CTS is smaller than 1

From equation (3)
Ri then exceeds the Rtx
EFFECTIVENESS OF RTS/CTS
HANDSHAKE (4/7)
Ai
EFFECTIVENESS OF RTS/CTS
HANDSHAKE (5/7)

effectiveness
many collisions may
happen due to the
large
interference range
and hidden terminal
problem
EFFECTIVENESS OF RTS/CTS
HANDSHAKE (6/7)

Influence of Physical Carrier Sensing
EFFECTIVENESS OF RTS/CTS
HANDSHAKE(7/7)

conclusions of Physical Carrier Sensing
–
–
–
The interference range at a node is not fixed as
the transmission range.
RTS/CTS handshake is not sufficient
effectiveness
Big carrier sensing range is not desired due to
hardware limitations and significant throughput
reduction
PROPOSED SCHEME--Conservative
CTS Reply (CCR)

main idea
–
–
a node only replies a CTS packet for a RTS quest when
receiving power of that RTS packet is larger than a certain
threshold (CTS-REPLY-THRESHOLD), even if the RTS
packet is received successfully and this node is idle.
Pr0.56


–
use it as the CTS-REPLY-THRESHOLD
Only replies CTS packets to those nodes which are at most
0.56*Rtx meters away
our scheme actually reduces the effective transmission
range to resolve the interference
PROPOSED SCHEME--Conservative
CTS Reply (CCR)

inconsistency between broadcasting and unicasting
–
–
–

broadcast packets are not protected by RTS/CTS
most routing protocols in MANETs use broadcast for route
discovery
routing protocols will discover a link which may be disabled
by our scheme
To solve this problem and maintain consistency
–
a node to drop broadcast packets if the receiving power of
that packet is below CTS-REPLY-THRESHOLD
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(1/7)

[4] J. Li, C. Blake, D. Couto, H. Lee, and R.
Morris, “Capacity of Ad Hoc Wireless
Networks,” ACM MobiCom 2001
–
–
–
NS2 simulator
transmission range: 250m
interference range: 550m
200m
 not considering the
large interference
range
 node 2 and node 3
can not transmit at the
same time.
 capacity is reduced to
1/3
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(2/7)

IEEE 802.11 MAC cannot
achieve this bandwidth since
a lot of bandwidth will be
wasted due to collisions
200m
 considering the
large interference
range
 node 2, 3, 4 can not
transmit at the same
time.
 capacity is reduced to
1/4
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(3/7)

To further demonstrate the performance
degradation due to large interference range
–
–
–
QualNet simulator
wireless radio is 367m
channel bandwidth: 2Mbps
300m
300m
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(4/7)
node 4 is out of the TX and in
the Ri
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(5/7)
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(6/7)
PROBLEM CAUSED BY LARGE
INTERFERENCERANGE(7/7)
SIMULATION EVALUATION(1/3)

Simulation Platform
–
–
–
–
–
QualNetTM simulator
incorporates a detailed and accurate model of the physical
channel and of the IEEE 802.11 MAC layer
parameters of QualNet are following the IEEE 802.11
standard and Lucent WaveLAN wireless card
transmission range: 367m
carrier sensing range : 670m
SIMULATION EVALUATION(2/3)

Simulation Evaluation
–
–
–
–
–
–
100 nodes
1500mX1500m
Channel bandwidth is 2Mbps
The CBR data packet size: 1024 byte
packet rate is 10pps
routing algorithm: DSDV
SIMULATION EVALUATION(3/3)
CONCLUSION

First
–
–

Second
–

frequent data packet corruptions due to interference range are
verified through simulation
Third
–

we analyze the interference range Ri
The effectiveness of RTS/CTS handshake is also explored in
theory
a simple MAC layer scheme is proposed to combat the large
interference range.
Main advantage
–
our proposed scheme is that it is simple and only has a trivial
modification to IEEE 802.11 standard