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(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 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 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 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 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 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 Impact of Interference on Unicast Transmissions: Other Possibilities 4.Data/ACK collision prevent reception of ACK at sender 5.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(n 2 ) experiments instead of O(n 4 )

13 Methodology Measure A’s receive B = M Measure C’s receive D = N Measure A’s receive B = M // Measure C’s receive 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 Sample Experimental Result a, full power, 6Mbps, no RTS/CTS. 75 link pairs selected at random. Average of 5 runs Median error is zero!

15 Summary of results BIR is a good approximation for LIR in various scenarios –Low power – 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 Support for academic researchers –Software (Mesh Academic Resource Toolkit) »Yes, includes source! –Hardware –$$$ Contact: Victor Bahl

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? 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