Unfairness in IEEE 802'11 Access Mechanism

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• Unfairness in IEEE 802.11 Access Mechanism
• By Li Zhifei

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Content Outline

• Introduction
• Motivation
• Fixed EIFS Problem in IEEE 802.11
• Enhanced VCS (EVCS)
• Conclusions
• Research Direction

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Medium Access Protocol (MAC)

• Basic objectives
• Maximize utilization of the limited bandwidth
• Fair access of the shared wireless medium

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Why do we need fairness in IEEE 802.11?

• The medium is shared by multiple contending nodes
• Every node operates in a distributed manner
• IEEE 802.11 is a random access protocol.

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Definition of Fairness

• There are many definitions about fairness
• Max-min fairness
• Proportional fairness and so on
• We use the simplest one
• To allocate the bandwidth equitably

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IEEE 802.11 Basics

• Designed for Wireless LAN
• Based on CSMA/CA
• CS including Physical CS and Virtual CS
• CA binary exponential back-off
• Four-way handshake RTS-CTS-Data-ACK
• For example Node A wants to send a packet to
  node B.

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Fairness of IEEE 802.11 in MANETs

• Unfairness may be caused by
• Imprecise CS
• Inappropriate CA
• Wireless error
• Mobility and so on.

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Motivation and Focus

• Most work focus on WLANs rather than MANETs
• Most work consider unfairness caused by CA
  algorithms
• Not much focus given to unfairness caused by CS
  in MANETs
• We focus on
• Study unfairness caused by CS
• Try to achieve fairness through enhanced CS
  mechanism

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One simple scenario
3-nodes with two single-hop flows
Flow 1 from node A to B Flow 2 from node B to
C Distance between neighbors 200 meters
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Simulation result
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Fixed EIFS problem

• Whenever a node detects a Sensing Range (SR)
  frame on the medium, it defers the transmission
  by a fixed duration, i.e. EIFS. This duration is
  sometimes smaller and sometimes larger than the
  desired period by which the transmission should
  be deferred, and it is the main cause of the
  unfairness.

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Enhanced Virtual CS (EVCS)

• A node should distinguish among the type of
  frames (RTS, CTS, Data and ACK) when detecting a
  SR frame.
• EIFS value should be directly linked to the type
  of SR frame detected.

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EVCS (contd)

• EIFS(collision or MAC error) Standard EIFS
• EIFS(RTS) SIFS TxTime(CTS)
• EIFS(CTS) SIFSTxTime(Max-Data-Length)
• EIFS(Data) SIFSTxTime(ACK)
• EIFS(ACK) 0

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Results under EVCS
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Another scenario

• 4-nodes with two single-hop flows

Flow 1 from node A to B Flow 2 from node D to
C Distance between neighbors 200 meters
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Simulation Results

• Under IEEE 802.11
• Flow from A to B gets 1.4 Mbps while the other
  flow completely starves.
• Under EVCS

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Conclusions

• IEEE 802.11 exhibits substantial unfairness
  caused by fixed value of EIFS
• EVCS solves the problems to a great extent and
  thus improve fairness substantially

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Future Research

• Consider other factors effects on fairness,such
  as mobility, hidden-terminal problem, capture
  and so on
• Study the short-term fairness
• Quality of Service (QoS) issues in MANETs

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• Thank you! Any question?