1. 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. 2 Infrastructure Wireless Network Access Point

3. 3 Ad-hoc, multi-hop wireless networks

4. 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. 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(n 2 ) links –Pairwise interference  O(n 4 ) 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. 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. 7 Problem Formulation Two links, A->B and C->D –Throughputs X and Y when operating individually X // Y // –Throughputs X // and Y // when operating simultaneously X // Y //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: Estimate LIR for all link pairs without requiring O(n 4 ) experiments

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

9. 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 AB CD

10. 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 AB CD

11. 11 Impact of Interference on Unicast Transmissions: Other Possibilities 4.Data/ACK collision prevent reception of ACK at sender 5.ACK/ACK collision

12. 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(n 2 ) experiments instead of O(n 4 )

13. 13 Methodology Measure A’s receive rate @ B = M Measure C’s receive rate @ D = N Measure A’s receive rate @ B = M // Measure C’s receive rate @ D = N // Broadcast Interference Ratio (BIR) = (M // + N // ) / (M + N) = 1 no interference < 1 interference Pairwise Interference Individual Broadcasts Hypothesis: BIR is a good approximation of LIR BIR for all pairs can be calculated with O(n 2 ) experiments BIR Captures 1.Carrier sensing 2.Data/Data collisions BIR Ignores 1.Data/ACK collisions 2.ACK/ACK collsions 3.AutoRate

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

15. 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. 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. 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. 18 Backup Slides

19. 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. 20 What causes interference between two unicast transmissions? 1.Carrier sensing Senders can “hear” each other’s transmission  Only one sender sends at a time 2.Collisions Simultaneous data packet transmissions  One or both data packets lost Simultaneous data and ACK transmissions  Data and/or ACK packet lost