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Wireless Network Measurements

Wireless Network Measurements. Link-level measurements Loss rate between a pair of nodes: very low, very high or medium? Why? Interference between a pair of transmissions/links Do they interfere? Partially overlapping channels How to utilize them?

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Wireless Network Measurements

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  1. Wireless Network Measurements • Link-level measurements • Loss rate between a pair of nodes: very low, very high or medium? • Why? • Interference between a pair of transmissions/links • Do they interfere? • Partially overlapping channels • How to utilize them? • Three papers in multi-hop wireless networks

  2. Estimation of Link Interference in Static Multi-hop Wireless Networks Padhye, Agarwal, Padmanabhan, Lili Qiu, Ananth Rao, Brian Zill

  3. Outline • Introduction • Link Interference Ratio • Study and evaluation of simple heuristics • Broadcast Interference Ratio • Evaluation of proposed method • Related Work • Future Development

  4. Introduction • Interference is major cause of performance degradation • Which links in network interfere? • Given a set of wireless links • Their aggregate throughput when all links are active simultaneously lower compared to when they are active individually? • Empirical Testing not feasible: need to test O(n4) pairs.

  5. Link Interference Ratio • LAB : if packet loss rate in either direction <= a threshold • UAB : unicast throughput of link LAB • Link Interference Ratio (LIR): • LIR: [0,1] • LIR=1: links don’t interfere • LIR<1: links interfere.

  6. When do links interfere?

  7. Simple Heuristics • Heuristic 1 (pessimistic): • All links in a multi-hop path interfere with each other • Heuristic 2 (optimistic): • Two links interfere only if they share an endpoint • Heuristic 3: • dAB: distance between nodes A and B • LAB and LCD interfere if

  8. Evaluation of Heuristics • Experimental Setup • 22 nodes, each with two 802.11 wireless cards • RTS/CTS disabled • 75 randomly selected links • Each pair: 1000 byte UDP packets for 30 secs

  9. Heuristic 1: too pessimistic • Heuristic 2: too optimistic • Heuristic 3: pessimistic • Predict 56 links interfere (10 more than observed 46 links)

  10. Broadcast Interference Ratio • Ignore ACKs • SA : sending rate of node A • RAB : delivery rate of packets from A at B • For links LAB and LCD • O(n2) experiments required.

  11. Pros and Cons of proposed method • Pros • Impact of two senders considered • Impact of collision at receiver side considered • ACK packets are small in size and can be ignored • Cons • Does not consider retransmission of unicast packets • Data-ACK collision not considered

  12. Evaluation: Baseline Scenario Median LIR and BIR of 75 pairs CDF of |LIR-BIR|

  13. Evaluation: Other Scenarios BIR and LIR measured 5 days apart Three other scenarios

  14. Why do links interfere? • Define Carrier sense ratio (CSR) as • CSR=0.5, senders within carrier sense range of each other (34 links with LIR<0.9 had CSR=0.5) • Carrier Sensing major cause of interference?

  15. Related Work • Impact of interference on fairness [Nandogopal] • Impact of interference on overall capacity of multi-hop wireless networks [e.g., Gupta] • Heuristics to estimate link interference [Couto, Gupta, Kodialam, Xu] • Measuring various properties of wireless network [Aguago, Yarvis, Gupta]

  16. Future Work • Improve by considering factors such as unicast retransmission, ACK-DATA collision and RTS/CTS • Larger group of links? • Infer interference via passive measurements?

  17. Exploiting partially overlapped channels in wireless networks: Turning a peril into an advantage Arunesh Mishra, Eric Rozner, Suman Banerjee, William Arbaugh

  18. Outline • Introduction • Analytical basis behind spatial reuse • Applications to WLAN • Results • Applications to wireless mesh networks • Results • Future work

  19. Introduction • 802.11b characterized by 11 channels • Each channel represents a center frequency (channel 1 = 2.412 Ghz) • Channel centers separated by 5 Mhz • Channels have spread of ~30 Mhz around center • Implies 3 non-overlapping channels

  20. Introduction Continued • Interference Factor • Transmits on channel j • Pi: power received of a single at a location from channel i • I(i,j) = Pi / Pj • I(i,6):

  21. Applications to Mesh Networks • Neighboring nodes don’t have to be on same channel to communicate • Adds flexibility, decreases interference

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