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Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks. Hongqiang Zhai and Yuguang Fang Dept of Electrical & Computer Engineering University of Florida Presented by Tae Hyun Kim. Contents. Problem Statement Analysis for optimum CS distance
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Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks Hongqiang Zhai and Yuguang Fang Dept of Electrical & Computer Engineering University of Florida Presented by Tae Hyun Kim
Contents • Problem Statement • Analysis for optimum CS distance • Interference models • … • End-to-end throughput • Simulation • Conclusion • Some comments
Problem Statement • To find optimum CS (Carrier Sense) distance that maximizes throughput • Considered factors • MAC overhead (frame headers and IFSs) • Bidirectional handshaking • Multirate – different TX ranges, RX sensitivities and required SINR • Multihop – forwarding, hidden/exposed nodes, and random topology
Analysis for Optimum CS distance • Notations • Two worst case interference models • Multiple CS distances for multiple rates? • Exposed/hidden node problems • Bidirectional handshaking intensifies interference • End-2-end throughput
Notations Payload size Frame header size Fixed-lengthoverhead
Two Interference Models • Worst case of 6 interferers
Two Interference Models (cont.) • Non-overlapping area of each TX • # of concurrent transmissions • Aggregate throughput • Thus, given SINR, we can find optimum CS distance while fixing transmission distance dt
Two Interference Models (cont.) • Optimum CS distance is, • SINR plays major role other than protocol overhead ≈ 10-10 ≈ 10-7
Two Interference Models (cont.) • 6 interferers scenario may be too conservative • Worst case of only ONE strong interferer • Compute dc’using same method • Optimum CS area reduces to 25~89% of 6 interferers • As shorter CS distance may greatly increase spatial reuse, we may be allowed to decrease dc
Multiple CS distances for Multiple Rates? • Optimum X in interference model varies much, given SINR requirement • Fortunately,RX sensitivities vary much, too • Recall
Multiple CS distances for Multiple Rates? • Optimum CS distances and thresholds by 6 interferers model • Single CS distance is sufficient to maximize throughput
Multiple CS distances for Multiple Rates? • We do have more reasons for single CS distance • High complexity to adapt multiple CS distances for multiple rates • Mobility, distance, channel fading, etc. • Multiple CS distances may introduce additional collisions
Exposed/Hidden Nodes • Exposed node problem • Nodes that are unnecessarily shut up • Let’s define interference range • (X-1)dt=dc-dtfrom receiver • Exposed-area ratio • E.g.) By using 6 interferers model,54 Mbps δ=0.24, 0.56 when X=10, 5, γ>3 Exposed area
Exposed/Hidden Nodes (cont.) • Shorter dc could • Alleviate exposed nodes problem • Achieve higher spatial reuse • Have potentially larger hidden nodes • Hidden node problem • A’s TX is not sensed by C • C may interfere TX from A to B • Increase collisions • Large CS distance can reduce hidden nodes
Exposed/Hidden Nodes (cont.) • Summary • Tradeoff between degrees of exposed nodes and hidden nodes
Bidirectional Handshaking • Bidirectional handshaking incurs • Packet collision by immediate ACK • Receiver blocking (permanent link failure) • Packet collision by immediate ACK • After successfully receiving DATA, ACK is transmitted without CS • RTS may mitigate this as following CTS is sent when channel is idle A B C D DATA DATA ACK
Bidirectional Handshaking (cont.) • Receiver blocking • Before transmitting either CTS or DATA, CS is performed • If there is nearby on-going transmission, receiver never replies to RTS • MAC decides that link has been broken A B C D DATA RTS A receiver does not replyas channel is busy
Bidirectional Handshaking (cont.) • Receivers of previous interferers become new interferers closer to yellow receiver • Modified 6 interferers model • Intuitively, larger dc required to prevent interferers from transmitting
Bidirectional Handshaking (cont.) • Compute optimum CS distance, again • For SINR > - 3dB, • Thus,
Bidirectional Handshaking (cont.) • This solution, • Sacrifices spatial reuse • Increases potential exposed nodes • Incurs MAC contention • But, this also reduces • Potential hidden terminals • Packet collision by immediate ACKs • Receiver blocking
Optimum CS distance • Summary of previous observations • Tradeoff between larger and smaller dc • For protocol stability, larger dc might be better • Optimum CS distance is determined by optimum X* • Simulation study will find μ
Multihop flow consideration • End-2-end throughput • Conditions for maximized spatial reuse along the path • Distance between TXs be less than dc • Not corrupting each other’s packet • N – # of hops between nearest concurrent TXs • 1/N – spatial reuse ratio of a multihop flow • Then, throughput upperbound is N=3 A B C D E F Hop distance
Multihop flow consideration (cont.) • Upperbound for one multihop flow throughput • Observations • Higher rate does not necessarily generate higher throughput IF MAC overhead is taken into account
Multihop flow consideration (cont.) • Consider interference from nearby concurrent transmissions in a regular chain topology • Achievable maximum E2E throughput • This is proportional to BDiP ( ) • May not be maximum in general topology (?!) dc' dc'-dt A B C D E F
Simulation • Modified Ns-2: cumulative interference • 150 nodes in 1000m x 1000 m area • To obtain “one hop” optimum CS distance • Observations • Maximum throughput when 60<CSth*<70 dBm • For some high rates, CSth* < RXse; starving flows exist • Max throughput can be sustained with some starving flows RX sensitivities for different rates
Simulation (cont.) • Optimum CS distance for multihop flows • Observations • CSth* for single hop does not work well • CSth* ~ 91 dBm (smaller CS distance) • Single CS distance could be optimal • Higher rates do not necessarily generate higher throughput Optimum CSth* CSth < RXse randomly selected 20 TCP connections with 500~600 E2E distance
Conclusion • This paper analyzes impact of CS distance to throughput from various perspectives • Found optimum CS distance • Single CS distance is sufficient • dc* may be less than dc due to conservativeness of 6 interferers model • dc * ~ dc + dt due to bidirectional handshaking • dc * = μ(dc / dt + 1) • dt = dh to get maximum E2E throughput • μ can be found by simulation according to network setup
Comments • Based on simplified interference models and extreme cases • Through analysis no relationships between any factors are drawn – only some intuitions • Ignorance on random access MAC overhead – much larger than frame header and IFSs overhead • Packet with higher rate has more overhead proportion, thus penalizing higher rates • Hard to compute BDiP • Does 801.11 do CS before sending CTS and DATA?? • It does not. Nevertheless, receiver blocking can happen due to virtual CS • If rate is adapted, then single CSth* may not be a good strategy
Multihop flow consideration (cont.) • Worst case model for condition (2) • Equivalent to bidirectional handshaking model • We have, • Bound for one multihop flow throughput
Multihop flow consideration (cont.) • Spatial reuse ratio = Ns-2 default: SINR=10 dB, γ=4 Spatial reuse ratio = 1/3