Estimation of Link Interference in Static Multihop Wireless Networks

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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 Research University of Texas
Austin University of California, Berkeley
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Infrastructure Wireless Network
Access Point
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Ad-hoc, multi-hop wireless networks
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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-gtB and C-gtD
• Throughputs X and Y when operating individually
• Throughputs X// and Y// when operating
  simultaneously
• Link Interference Ratio (LIR) (X// Y// ) / (X
  Y)
• LIR 1 implies no interference
• LIR lt 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
A
B
C
D

• Carrier sensing
• A and C can hear each other.
• Only one transmits at a time.

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Impact of Interference on Unicast Transmissions
2
A
B
C
D

• Collision of data packets
• Transmissions from A and C collide at B
• Reception of data fails at B

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Impact of Interference on Unicast Transmissions
3
A
B
C
D

• Collision of data and ACK packets
• ACK from D collides with data from A
• Reception of data fails at B

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Impact of Interference on Unicast Transmissions
Other Possibilities

• Data/ACK collision prevent reception of ACK at
  sender
• 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
Individual Broadcasts
Pairwise Interference
Measure As receive rate _at_ B M
Broadcast Interference Ratio (BIR) (M// N//)
/ (M N)
Measure As receive rate _at_ B M//
1 no interference lt 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 Cs receive rate _at_ D N
Measure Cs receive rate _at_ D N//
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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
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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 (bahl_at_microsoft.com)

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Backup Slides
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Our Contribution

• An experimental methodology to estimate pair-wise
  link interference using O(n2) 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?

• Carrier sensing
• Senders can hear each others 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