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Estimation of Link Interference in Static Multi-hop Wireless Networks

Estimation of Link Interference in Static Multi-hop Wireless Networks. Jitendra Padhye, Sharad Agarwal, Venkat Padmanabhan, Lili Qiu, Ananth Rao, Brian Zill. Microsoft Research University of Texas Austin University of California, Berkeley. Infrastructure Wireless Network. Access Point.

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Estimation of Link Interference in Static Multi-hop Wireless Networks

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  1. Estimation of Link Interference in Static Multi-hop Wireless Networks Jitendra Padhye, Sharad Agarwal, Venkat Padmanabhan, Lili Qiu, Ananth Rao, Brian Zill Microsoft Research University of Texas Austin University of California, Berkeley

  2. Infrastructure Wireless Network Access Point

  3. Ad-hoc, multi-hop wireless networks

  4. Motivation • Interference limits performance of (static) multi-hop wireless networks • Simultaneous transmissions on “nearby” links interact adversely • Knowledge of which links interfere with each other is useful for: • Capacity estimation [GK00, JPPQ03, …] • Routing [De Couto et. al. 03, DPZ04, …] • Channel assignment [RC05, …] • …

  5. Hard Problem … • Accurate, physical-level radio modeling is difficult • Environmental factors, hardware-specific details, … • Simple experimental measurements are not feasible: • Network with n nodes O(n2) links • Pairwise interference O(n4) experiments • Our testbed: • 22 nodes, over 100 “good” links over 10,000 link pairs • May have to repeat experiments periodically! • Our goal: Efficient experimental methodology to estimate pair-wise interference among all links.

  6. Previous Work • Punt on the problem … • Assume that interference information is “known” [JPPQ03, …] • Use simple heuristics • All links on a path interfere [De Couto et. al. 03, DPZ04, …] • Pessimistic • Only links that share endpoint interfere [KN03, …] • Optimistic • Interference range is twice the communication range [GK00, …] • Not valid in all environments

  7. Problem Formulation • Two links, A->B and C->D • Throughputs X and Ywhen operating individually • Throughputs X// and Y// when operating simultaneously • Link Interference Ratio (LIR) = (X// +Y// ) / (X + Y) • LIR = 1 implies no interference • LIR < 1 implies interference • Not just binary: full range of values between 0 and 1. • Goal:EstimateLIR for all link pairs without requiring O(n4) experiments

  8. Impact of Interference on Unicast Transmissions: #1 • Carrier sensing • A and C can hear each other. • Only one transmits at a time. A B C D

  9. Impact of Interference on Unicast Transmissions: #2 • Collision of data packets • Transmissions from A and C collide at B • Reception of data fails at B A B C D

  10. Impact of Interference on Unicast Transmissions: #3 • Collision of data and ACK packets • ACK from D collides with data from A • Reception of data fails at B A B D C

  11. Impact of Interference on Unicast Transmissions: Other Possibilities • Data/ACK collision prevent reception of ACK at sender • ACK/ACK collision

  12. Key Idea • Only consider carrier sensing (#1) and data packet collisions (#2) • Ignore ACKs • Broadcast packets are sufficient for measurements • Consider only sender pairs, instead of link pairs • O(n2) experiments instead of O(n4)

  13. Methodology Individual Broadcasts Pairwise Interference Measure A’s receive rate @ B = M Broadcast Interference Ratio (BIR) = (M//+ N//) / (M + N) Measure A’s receive rate @ B = M// • = 1 no interference • < 1 interference BIR for all pairs can be calculated with O(n2) experiments Hypothesis: BIR is a good approximation of LIR • BIR Captures • Carrier sensing • Data/Data collisions • BIR Ignores • Data/ACK collisions • ACK/ACK collsions • AutoRate Measure C’s receive rate @ D =N Measure C’s receive rate @ D = N//

  14. Sample Experimental Result Median error is zero! 802.11a, full power, 6Mbps, no RTS/CTS. 75 link pairs selected at random. Average of 5 runs

  15. Summary of results • BIR is a good approximation for LIR in various scenarios • Low power • 802.11 a/b/g • Autorate • BIR experiments need to be repeated regularly as link interference patterns change over time.

  16. Future work • More evaluation: • outdoor, differential power. • Interference among larger groups of links (not just pairs) • Predict interference by passively observing existing traffic?

  17. Microsoft Research Wireless Mesh Networking Project http://research.microsoft.com/mesh/ Support for academic researchers • Software (Mesh Academic Resource Toolkit) • Yes, includes source! • Hardware • $$$ Contact: Victor Bahl (bahl@microsoft.com)

  18. Backup Slides

  19. Our Contribution • An experimental methodology to estimate pair-wise link interference using O(n^2) experiments • Evaluation of this methodology in a variety of settings using an indoor, 22-node testbed.

  20. What causes interference between two unicast transmissions? • Carrier sensing • Senders can “hear” each other’s transmission • Only one sender sends at a time • Collisions • Simultaneous data packet transmissions • One or both data packets lost • Simultaneous data and ACK transmissions • Data and/or ACK packet lost

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