Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks?

1
Does the IEEE 802.11 MAC Protocol Work Well in
Multihop Wireless Ad Hoc Networks?

• Shugong Xu
• Tark Saadawi
• June, 2001
• IEEE Communications Magazine
• (Adapted from mnet.cs.nthu.edu.tw/paper/jbb/010704
  .pps)

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Overview

• IEEE 802.11 MAC Protocol
• The TCP Instability Problem and Analysis
• The Serious Unfairness and Analysis
• Conclusion

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IEEE 802.11 MAC Protocol (1)

• IEEE 802.11 MAC Protocol
• is the standard for wireless LANs
• combined network allocation vector (NAV) with
  clear channel assessment (CCA) to indicate the
  busy state of the medium.
• using RTS/CTS scheme to reduce the probability of
  two stations colliding due to not hearing each
  other.

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IEEE 802.11 MAC Protocol (2)

• Hidden terminal problem
• This will cause collision on data transmission.

A
C
B
5
IEEE 802.11 MAC Protocol (3)

• Exposed terminal problem
• If these terminals are not minimized, the
  available bandwidth is underutilized.

D
C
A
B
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IEEE 802.11 MAC Protocol (4)

• The IEEE 802.11 MAC protocol in wireless mobile
  ad hoc networks, multihop connectivity is one of
  the most prominent features.
• Can the IEEE 802.11 MAC Protocol Function Well in
  Multihop Networks?

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IEEE 802.11 MAC Protocol (5)

• The serious problems encountered in an IEEE
  802.11-based multihop ad hoc networks
• The TCP Instability Problem.
• The Serious Unfairness Problem.
• Authors concludes that the current version of
  this wireless LAN protocol does not function well
  in multihop ad hoc networks.

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TCP Instability Problem (1)

• The network topology

Interfering Range of node 4
250 m
250 m
100 m
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TCP Instability Problem (2)

• The TCP session is the only traffic in the
  network with four hops from source 1 to
  destination 5.

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TCP Instability Problem (3)

• If the network condition does not vary, the TCP
  throughput should stay stable within some range.

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TCP Instability Problem (4)
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TCP Instability Problem (5)
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TCP Instability Problem (6)

• Collision
• Collision will occur in node 2 when nodes 1 and 4
  is sending at the same time.
• Exposed Station
• Node 2 has to defer when node 4 is sending.

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TCP Instability Problem (7)
Collision
Collision
Exposed Terminal
Exposed Terminal
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TCP Instability Problem (8)

• Collision
• Node2 cannot receive RTS when node4 is sending
  (hidden terminal problem)
• Exposed Station Problem
• Node2 cannot send back CTS even if it receives
  the RTS from node1 correctly.
• After failing to receive CTS from node2 seven
  times, a route failure event occurs.

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TCP Instability Problem (9)

• By adjusting the Window size

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TCP Instability Problem (10)

• Discussion
• The maximum number for possible back-to-back
  sending is four, greatly reduces possible that
  other nodes might fail to access the channel in
  seven tries.

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Serious Unfairness (1)

• 2 TCP Connections
• First session starts at 10.0s ( 6 ? 4 )
• Second session starts 20.0s later ( 2 ? 3 )

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Serious Unfairness (2)
First session start
Second session start
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Serious Unfairness (3)

• The throughput of the first session is zero in
  most of its lifetime after the second session
  starts.
• There is not even a chance for it to restart.
• The loser session is completely shutdown even if
  it starts much earlier.

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Serious Unfairness (4)
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Serious Unfairness (5)
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Serious Unfairness (6)

• Discussion
• Node5 cannot reach node4 when
• Node2 is sending (collision)
• Node3 is sending ACK (defer)

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Serious Unfairness (7)

• Discussion
• TCP Instability / Unfairness Problem.
• These MAC layer problem appear when the traffic
  load becomes large enough, even if the traffic is
  not from TCP.

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2
3
4
5
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Conclusion

• The hidden terminal problem still exists in
  multihop networks.
• The exposed terminal problem will be more harmful
  in a multihop network and there is no scheme in
  IEEE 802.11 standard to deal with this problem.
• The binary exponential backoff scheme always
  favors the latest successful node. It will cause
  unfairness.