How Effective is the IEEE 802'11 RTSCTS Handshake in Ad Hoc Networks

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How Effective is the IEEE 802.11 RTS/CTS
Handshake in Ad Hoc Networks

• Kaixin Xu,Mario Gerla, Sang Bae
• IEEE Global Communications Conference
• (Globecom 2002)

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Outline

• INTRODUCTION
• EFFECTIVENESS OF RTS/CTS HANDSHAKE
• PROBLEM CAUSED BY LARGE INTERFERENCERANGE
• PROPOSED SCHEME AND SIMULATION EVALUATION
• CONCLUSION

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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

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Effectiveness of RTS/CTS handshake

• Three radio ranges
• Transmission Range
• Carrier Sensing Range (Rcs)
• Interference Range (Ri)

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Transmission Range (Rtx)

• The receiver can successfully decode the packet
• Almost 250 meters in simulation
• (Rt)

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Carrier-sensing Range (Rcs)

• The power from the transmitter can be sensed,
  indicating the busy state of the medium

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Interference Range (Ri )

• The range within which the receiving STA will be
  interfered by other STAs and thus suffer a packet
  loss.

collision
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Effectiveness of RTS/CTS handshake

• Two-ray ground reflection model

d distance between sender and receiver Pr
received power ht heights of the transmitter
antenna hr heights of the receiver antenna Gt
antenna gains of transmitter Gr antenna gains of
receiver L system loss
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Effectiveness of RTS/CTS handshake

• Signal to interference ratio (SIR)
• Neglecting ambient noise

Ps signal power Pi interference power ds
distance between sender and receiver di distance
between other node and receiver a signal
attenuation coefficient (4 in two-ray ground
reflection model) CPThresh Capture Threshold (10
in ns-2)

2009/11/23
Mu-Ying Lu
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Effectiveness of RTS/CTS handshake

• Investigation of the interference range
• Ri
• (SNR_THRESHOLD is usually set to 10)

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Effectiveness of RTS/CTS handshake

• Its relationship to the transmission range
• Notations

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Effectiveness of RTS/CTS handshake

• Analysis
• dlt 0.56 Rtx
• ERTS/CTS is equal to 1
• Otherwise
• ERTS/CTS is smaller than 1

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Effectiveness of RTS/CTS handshake

• effectiveness

many collisions may happen due to the large
interference range and hidden terminal problem
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Effectiveness of RTS/CTS handshake

• Influence of Physical Carrier Sensing

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Effectiveness of RTS/CTS handshake

• 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

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PROPOSED SCHEME--Conservative CTS Reply (CCR)

• main idea
• Replies a CTS packet for a RTS quest
• when receiving power of that RTS packet is larger
  than a certain threshold (CTS-REPLY-THRESHOLD)
• Pr0.56CTS-REPLY-THRESHOLD
• Only replies CTS packets to those nodes which are
  at most 0.56Rtx meters away

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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

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PROBLEM CAUSED BY LARGE INTERFERENCERANGE(1/7)

• NS2 simulator
• transmission range 250m
• interference range 550m

• Not considering the large interference range
• node 2 and node 3 can not transmit at the same
  time.
• capacity is reduced to 1/3

200m
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PROBLEM CAUSED BY LARGE INTERFERENCERANGE(2/7)

• considering the large interference range
• node 2, 3, 4 can not transmit at the same time.
• capacity is reduced to 1/4

• IEEE 802.11 MAC cannot achieve this bandwidth
  since a lot of bandwidth will be wasted due to
  collisions

200m
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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
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PROBLEM CAUSED BY LARGE INTERFERENCERANGE(4/7)
node 4 is out of the TX and in the Ri
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PROBLEM CAUSED BY LARGE INTERFERENCERANGE(5/7)
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PROBLEM CAUSED BY LARGE INTERFERENCERANGE(6/7)
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PROBLEM CAUSED BY LARGE INTERFERENCERANGE(7/7)
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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

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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

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SIMULATION EVALUATION(3/3)
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CONCLUSION

• First
• we analyze the interference range Ri
• The effectiveness of RTS/CTS handshake is also
  explored in theory
• Second
• frequent data packet corruptions due to
  interference range are verified through
  simulation
• Third
• 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

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Improving Spatial Reuse of IEEE 802.11 Based Ad
Hoc Networks

• Fengji Ye, Su Yi
• and Biplab Sikdar
• ECSE Department, Rensselaer Polytechnic Institute
• Troy, NY 12180

Globecom 03
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Outline

• Introduction
• Spatial reuse analysis
• The Signal to Interference Ratio Model
• Effectiveness of Virtual Carrier Sensing
• Proposed scheme
• Simulation
• Conclusion

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Introduction

• IEEE 802.11 ad hoc networks
• Distributed Coordination Function (DCF)
• Physical Carrier Sensing
• Backoff mechanism
• Virtual Carrier Sensing (VCS)

collision
A
B
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RTS
CTS
DATA
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Introduction

• To evaluate the effectiveness and efficiency of
  the RTS/CTS mechanism
• Spatial reuse can serve as an important benchmark.

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Analysis_ The Signal to Interference Ratio Model

• Two-ray ground reflection model

d distance between sender and receiver Pr
received power ht heights of the transmitter
antenna hr heights of the receiver antenna Gt
antenna gains of transmitter Gr antenna gains of
receiver L system loss
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Analysis_ The Signal to Interference Ratio Model

• Signal to interference ratio (SIR)
• Neglecting ambient noise

Ps signal power Pi interference power ds
distance between sender and receiver di distance
between other node and receiver a signal
attenuation coefficient (4 in two-ray ground
reflection model) CPThresh Capture Threshold (10
in ns-2)

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Analysis_ The Signal to Interference Ratio Model

• According to the SIR, the Ri is given by
• kSIR denote the multiplier, which depends on the
  SIR modle (kSIR 1.78 in ns-2)
• there is no fixed relation between Ri and Rt, Ri
  is proportional to the one-hop distance ds.

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Analysis_ Effectiveness of Virtual Carrier Sensing

• Sufficient condition
• Hear RTS/CTS -gt potential interfere
• Necessary condition
• Interfering -gt hear RTS/CTS

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Analysis_ Effectiveness of Virtual Carrier Sensing

• Underactive RTS/CTS Scenario
• Rt/kSIR lt d lt Rt
• 0.56Rt lt d lt Rt
• Sufficient

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Analysis_ Effectiveness of Virtual Carrier Sensing

• Moderate RTS/CTS Scenario
• Rt/(kSIR1) lt d lt Rt/kSIR
• 0.36Rt lt d lt 0.56Rt
• Sufficient
• Necessary

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Analysis_ Effectiveness of Virtual Carrier Sensing

• Overactive RTS/CTS Scenario
• d lt Rt/(kSIR1)
• d lt 0.36Rt
• Necessary

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Analysis_ Evaluation of 802.11 Spatial Reuse

• Spatial Reuse Index (SRI)

r d / Rt k kSIR
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Analysis_ Evaluation of 802.11 Spatial Reuse

• Spatial Reuse Index (SRI)

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Proposed scheme
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Proposed scheme_ Evaluation of AVCS

• The denominator shrinks to the intersection of
  the two transmission circles

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Proposed scheme_ Evaluation of AVCS
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Simulation

• CBR with a rate of 448Kbps
• Repeated 200 times

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Conclusion

• Three scenarios with different spatial reuse
  characteristics.
• Introduced SRI demonstrated its effectiveness in
  evaluating the spatial reuse.
• Proposed an improved virtual carrier sensing
  scheme.
• Increasing the spatial reuse
• Increasing network throughput

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Thank You
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Leveraging Spatial Reuse in 802.11 Mesh Networks
with Enhanced Physical Carrier Sensing

• ICC 2004
• Jing Zhu, Xingang Guo, L. Lily Yang, and W.
  Steven Conner
• Communication Technology Lab
• Intel Corporation

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Outline

• Introduction
• Enhanced physical carrier sensing
• Simulation model and results
• Conclusions

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Introduction

• Pathloss model
• The average signal strength at the receiver as a
  function of the T-R separation distance, d,

the pathloss exponent
the reference receiving signal strength as
measured at the reference distance
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Introduction

• A receiver can receive a packet with high
  probability of success only if the receiving
  strength

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Introduction

• With negligible noise (PN 0)

• D S-R separation distance
• I Interference range

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Introduction

• Physical carrier sensing range
• A transmitter will deem channel busy if it senses
  an energy level equivalent to a transmitter
  within that range

the carrier sensing threshold
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Enhanced physical carrier sensing

• Virtual carrier sensing was designed to avoid the
  well known hidden terminal problem
• The carrier sensing threshold should also be
  tuned to match network conditions

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Enhanced physical carrier sensing
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Enhanced physical carrier sensing
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Optimal PCS threshold

• Homogeneous networks with regular topology where
  neighboring nodes are separated by equal distance
  D

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Optimal PCS threshold
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Enhanced physical carrier sensing

• To consider the optimal tradeoff with exposed
  terminal

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Simulation model and results

• OPNET
• MAC data frame is 1024-byte long
• each node transmits at a fixed power of 0 dbm

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Simulation results - Point-to-point
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Simulation results - Large-scale chain network
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Simulation results - 2-D grid network
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• Thank you

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