A MeasurementBased Approach to Modelling Link Capacity in 802'11based Wireless Networks

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A Measurement-Based Approach to Modelling Link
Capacity in 802.11-based Wireless Networks

• Anab Kashyap, Samrat Ganguly, Samir R. Das
  _at_stonybrook univ.
• To be presented at Mobicom 07

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Outline

• Measurement-based capacity model
• Network modeling approach
• Modeling 802.11 behavior
• Analytical approach
• Experimentation
• Conclusion

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Measurement-based capacity model

• Objective
• Impact of interference by multiple surrounding
  nodes

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Measurement-based capacity model

• Impact of Interference
• Sender reducing maximum sending rate
• Receiver reducing the prob. Of packet reception
• - Random backoff, carrier-sense threshold, packet
  capture threshold

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Measurement-based capacity model

• Model based on actual measurement
• Eliminating unrealistic assumption
• Must be a reasonable measurement complexity to be
  practical
• Robust to potentially changing operating
  conditions
• To consider every possible combination of
  interfering transmitters

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Network modeling approach

• Problem formulation
• Capacity of a specific link in a 802.11 network
• Given the offered loads on a set of other links
• A set of n transmitters Zz1,,zn,
• A receiver x
• Throughput capacity

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Network modeling approach

• PHYlayer deferral, packet capture
• By one-time profiling experiments
• Profiling one time only, library for different
  NICs
• Link-wise measurement of RSS
• O(N) measurement, metrics for all the N(N-1)
  links

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Modeling 802.11 behavior

• Broadcast only no ACKs, exponential backoff
• Further extension for unicast
• Five states IDLE, DIFS, BACKOFF, DEFER, and
  XMIT
• Channel busy prob. p as prob. of deferral is
  constant

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Modeling 802.11 behavior
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Analytical approach

• Approximation
• The current state of the process does not depend
  on the previous state
• Sender the fraction of time a node is the XMIT
  state
• Receiver packet capture prob.
• Saturated condition with further extension

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Analytical approach
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Analytical approach
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Analytical approach

• Sender-side interference

P payload size H header size W channel bit
rate
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Analytical approach

• Empirical relationship for the prob. of deferral
  btw two nodes based on received signal strength

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Analytical approach

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Analytical approach

• Receiver-side interference

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Analytical approach

• Overall link capacity (bits/sec)

the fraction of time the sender zi transmits
along with some subset of the interferers
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Analytical approach-extensions

• Non-backlogged Interferers

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Analytical approach-extensions
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Analytical approach-extensions

• Modeling Unicast
• Idle time comp.

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Analytical approach-extensions

• Consideration of ACK
• Modeled by attributing a small prob. To a XMIT
• Simply ignore the effect of ACK

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Experimentation

• Profiling Experiments

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Experimentation
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Experimentation

• Capacity of senders

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Experimentation

• Capacity of links

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Experimentation
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Experimentation
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Conclusion

• Practical measurement-based methods
• Profile kept
• O(N) time
• Analytical solution
• Defects
• One time measure
• Profits of measurement