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
Dec 19, 2015
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Estimation of Link Interference in Static Multi-hop Wireless Networks
Jitendra Padhye, Sharad Agarwal, Venkat Padmanabhan, Lili Qiu, Ananth Rao, Brian Zill
Microsoft ResearchUniversity of Texas Austin
University of California, Berkeley
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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, …] – …
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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.
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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
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Problem Formulation
• Two links, A->B and C->D– Throughputs X and Y when operating individually
– Throughputs XX// // and YY//// when operating simultaneously
• Link Interference Ratio (LIR) = (XX// // +YY// // ) / (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(n4) experiments
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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
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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
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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
CD
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Impact of Interference on Unicast Transmissions: Other Possibilities
4. Data/ACK collision prevent reception of ACK at sender
5. ACK/ACK collision
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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)
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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)
A
C
D
B
= 1 no interference< 1 interference
Pairwise InterferenceIndividual Broadcasts
Hypothesis: BIR is a good approximation of LIR
BIR for all pairs can be calculated with O(n2) experiments
BIR Captures1. Carrier sensing2. Data/Data collisions
BIR Ignores1. Data/ACK collisions2. ACK/ACK collsions3. AutoRate
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Sample Experimental Result
0
0.2
0.4
0.6
0.8
1
0 0.1 0.2 0.3 0.4 0.5
Error: Abs(LIR-BIR)
Fra
cti
on
802.11a, full power, 6Mbps, no RTS/CTS. 75 link pairs selected at random.
Average of 5 runs
Median error is zero!
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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.
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Future work
• More evaluation: – outdoor, differential power.
• Interference among larger groups of links (not just pairs)
• Predict interference by passively observing existing traffic?
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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 ([email protected])
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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.
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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