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

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

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 AR

TS/CST

EFFECTIVENESS OF RTS/CTS HANDSHAKE (4/7)

Ai

EFFECTIVENESS OF RTS/CTS HANDSHAKE (5/7)

effectivenessmany collisions may happen due to the largeinterference 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 rec

eiving 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 d

iscovery– 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 t

hat 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

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

PROBLEM CAUSED BY LARGE INTERFERENCERANGE(2/7)

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 stand

ard 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– we analyze the interference range Ri– The effectiveness of RTS/CTS handshake is also explored in theor

y 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 interf

erence range. Main advantage

– our proposed scheme is that it is simple and only has a trivial modification to IEEE 802.11 standard