Distributed Fair Scheduling in a Wireless LAN

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Distributed Fair Scheduling in a Wireless LAN
Nitin Vaidya, Paramvir Bahl and Seema
Gupta (appeared in Mobicom 2000 Boston, MA)
Gautam Kulkarni EE206A (Spring 2001)

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Introduction

• Requirements of a scheduling discipline
• Ease of implementation
• Fairness and protection
• Performance bounds
• Ease of admission control (if needed)
• With fair scheduling bandwidth for a flow ?
  weight
• 802.11 MAC is not fair
• How to introduce fairness in wireless LANs ?

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Fair Queueing

• Ideal scheduling discipline Generalized
  Processor Sharing (GPS)
• All fair queueing disciplines try to emulate GPS
• Traditional GPS-like disciplines centralized in
  design
• Previous work on fairness in distributed MAC
  protocols
• Limited in scope provide equal bandwidth share
  (e.g. MACAW)
• Suffer in the presence of location-dependent
  errors

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Fair Scheduling

• Distributed Fair Scheduling (DFS) new protocol
  for fair scheduling
• A distributed algorithm derived from the
  Distributed Coordination Function (DCF) in 802.11
  
• Emulation of Self-Clocked Fair Queueing (SCFQ) in
  a distributed manner
• Scheduler maintains a virtual clock to keep
  track of packets to be serviced

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SCFQ

• Main idea
• Start tag of packet
• Finish tag of packet
• V(0) 0. Virtual time finish tag of packet in
  service
• Transmit packet with smallest finish tag
• Packets stamped on reaching the head of the queue

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802.11 Distributed Coordination Function

• CSMA/CA
• Node i chooses backoff interval Bi slots
• Bi uniformly distributed in 0, cw where cw
  size of contention window
• Decrement Bi
• Is Bi 0 ?
• Yes Send RTS
• Receive CTS
• No CTS ? Double cw, select new Bi and repeat from
  start
• Send data
• Receive ACK
• No Decrement Bi

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Distributed Fair Scheduling (DFS) Protocol

• Marriage of a distributed version of SCFQ with
  802.11 DCF
• Key idea select backoff interval proportional
  to the finish tag of the packet to be transmitted
• Each node maintains a local virtual clock vi(t)
• Backoff interval Scaling_Factor length /
  weight random number with mean 1

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DFS (contd.)

• Collision handling
• To reduce priority reversals, a small backoff
  interval is chosen after the first collision
• Backoff interval increased exponentially on
  further collisions
• Potential drawbacks
• Can exhibit short-term unfairness
• Impact of small weights of backlogged flows

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Impact of Small Weights

• Recall Backoff intervals are being used to
  compare length/weight
• Small weights can lead to high idle times
  throughput degradation
• Intuition Any non-decreasing function of
  length/weight may be used to obtain backoff
  intervals
• Need to explore alternate mappings

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Alternate Mappings
Chosen backoff interval
Scaling_factor length / weight random number
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Alternate Mappings (contd.)

• Advantage
• smaller backoff intervals
• less time wasted in counting down when weights of
  all backlogged flows are small
• Disadvantage
• backoff intervals that are different on a linear
  scale may become identical on the compressed
  scale
• possibility for greater number of collisions

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Performance Evaluation

• Using modified ns-2 simulator 2 Mbps channel
• Number of nodes N
• Number of flows N/2
• Odd-numbered nodes are destinations,
• even-numbered nodes are sources
• Unless otherwise specified
• flow weight 1 / number of flows
• backlogged flows with packet size 584 bytes
  (including UDP/IP headers)
• Scaling_Factor 0.02

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Fairness Index

• Fairness measured as a function of
• (throughput T / weight f) for each flow f over
  an interval of time
• Unless specified, the interval is 6 seconds

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Throughput/Weight Variation across Flows
Flatter curve is fairer DFS is fairer
Throughput / Weight
Flow destination identifier
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Throughput-Fairness Tradeoff
Fairness index
Number of flows
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Throughput-Fairness Tradeoff
Aggregate throughput (all flows combined)
Number of flows
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Scaled 802.11

• Fairness of 802.11 can be improved by using
  larger backoff intervals
• Is DFS fairer simply because it uses large
  backoff intervals ?
• Scaled 802.11 802.11 which uses backoff
• interval range comparable with DFS

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Short Term Fairness
Narrow distribution is fairer DFS is fairer
Frequency
Number of packets transmitted by a flow (over
0.04 second windows)
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Fairness Versus Sampling Interval Size
Fairness Index
Interval Size
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Scaling Factor

• How to select the scaling factor ?
• Small number May result in more collisions
• Large number Larger overhead

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Impact of Scaling Factor
Fairness Index
Scaling Factor
six flows with weights 1/2,1/4,1/8,1/16,1/32,1/32
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Impact of Scaling Factor
Aggregate Throughput
Scaling Factor
six flows with weights 1/2,1/4,1/8,1/16,1/32,1/32
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Conclusions

• DFS improves fairness compared to 802.11 and
  Scaled 802.11
• Alternative mappings somewhat beneficial
• No distributed fair scheduling protocol may
  accurately emulate work-conserving centralized
  protocols (unless clocks are synchronized)

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The Mandatory Critique!

• Need to evaluate the effect of collision
  resolution mechanisms to maintain priorities
• Selection of scaling factor could be adaptive
• Actually, a very good paper!

The End
Acknowledgements I have borrowed some slides
from Prof. Vaidyas webpage.