# How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks Kaixin Xu,Mario Gerla, Sang Bae IEEE Globecom 2002.

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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 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) E RTS/CTS is equal to 1 (2)otherwise E RTS/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 A i 大於 A RTS/CST

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. – P r0.56  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

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

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 – 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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