Cyclex

1
Cyclex

• An Efficient Cheating-Resistant MAC Protocol
• Jim Pugh
• Mentors Imad Aad, Mario Cagalj, Prof.
  Jean-Pierre Hubaux

2
Outline

• Problem with current protocol
• Cyclex protocol description
• How Cyclex solves current problem
• Pros and cons of Cyclex

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IEEE 802.11 Cheating Problem

• Nodes can modify their protocol to allow
  themselves to gain throughput without incurring
  any penalty (cheat)
• Instead of choosing a randomized backoff time,
  nodes will choose some low number
• Cheating nodes send packets more often than
  non-cheating nodes
• Difficult to detect, due to random nature of
  802.11

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Our Solution Cyclex

• Alternative to IEEE 802.11 for infrastructure
  mode
• Uses a turn-based approach to achieve time
  division multiple access (TDMA) cycle structure
• Cycle structure allows for easy cheating
  detection
• Operates on two different channels instead of one
• Uses Fast Collision Recovery to preserve order in
  cycle

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System Model

• Infrastructure mode with 1 access point, many
  nodes
• Reliable communication channels
• Nodes may join or leave network at any time

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Two Channel Approach

• Based on A Jamming-Based MAC Protocol to Improve
  the Performance of Wireless Multihop Ad Hoc
  Networks by Ye et al
• Upon hearing transmission from node on channel A,
  access point (AP) begins sending busy tone on
  channel B
• Eliminates hidden node problem

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Two Channel Approach
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Fast Collision Recovery

• Can detect collisions at nodes more quickly using
  two channel approach
• If error detected in packet header, AP will
  change from busy tone to error tone on channel
  B
• Transmitting nodes that hear error tone will know
  collision occurred immediately can attempt to
  transmit again sooner than nodes in IEEE 802.11

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Fast Collision Recovery
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Maintaining Order with Fast Collision Recovery

• In IEEE 802.11, colliding nodes may have long
  delay before next transmission
• With Fast Collision Recovery, colliding nodes can
  transmit soon after collision all other nodes
  wait longer
• If the maximum random backoff is short enough, we
  can guarantee that colliding nodes will transmit
  before any other nodes
• Preserves order, which is important for
  turn-based algorithm

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Full Protocol Description

• Nodes take turn transmitting specific order
• Transmissions form a cycle
• Acknowledgements from access point include node
  identity possible for all nodes to track who
  transmits

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Cyclex Operation
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Joining the Cycle

• To join, node must either wait X consecutive slot
  counts of channel idle time or hear a complete
  cycle
• If no nodes present, after X slot counts, node
  will start a cycle
• If nodes present, after hearing cycle (same node
  transmit twice), node will insert itself into
  cycle by preempting next node transmission

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Joining the Cycle
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Leaving the Cycle

• If node misses transmission, may have no data or
  may have left network
• If node misses transmission, next node waits one
  slot count, then transmits if two nodes in a row
  miss transmission, next node waits two slot
  counts etc
• If node misses Y transmissions in a row, it is
  removed from cycle

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Leaving the Cycle
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Collisions

• Nodes in cycle dont collide since ordered
• Joining nodes dont collide with nodes in cycle
  since preempting is faster
• Only collisions occur from nodes joining the
  cycle simultaneously
• Worst case analysis 0.15 loss of time
• In case of glitch, fast collision recovery allows
  order of network to be maintained

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Cyclex Cheating Detection

• Cheating difficult to detect in IEEE 802.11 due
  to random nature
• Cyclex is ordered easy cheating detection
• Active nodes listen to who transmits between
  their transmissions if some node transmits
  twice, it is cheating
• Once cheating detected, measures can be taken to
  punish node

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Cheating Detection
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Pros of Cyclex

• Solves hidden node problem less collisions
  occur, and time saved from not using RTS and CTS
• Cheating-resistant
• Very little lost transmission time in stable
  network operation

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Cons of Cyclex

• Two channels needed instead of one
• Busy tone means more power used by access point
• Current design only works in infrastructure mode

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Possible Modifications of Protocol

• Consecutive slot counts required before joining
  network
• Missed transmissions before node removal from
  cycle
• Adapt protocol for use in ad hoc networks

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References

• M. Raya, J. P. Hubaux, and I. Aad, DOMINO A
  System to Detect Greedy Behavior in IEEE 802.11
  Hotspots.
• M. Cagalj, S. Ganeriwal, I. Aad, and J. P.
  Hubaux, On Cheating in CSMA/CA Ad Hoc Networks.
• C. E. Koksal, H. Kassab, and H. Balakrishnan, An
  Analysis of Short-Term Fairness in Wireless Media
  Access Protocols, in Proc. ACM SIGMETRICS00,
  Santa Clara, CA, June 2000.
• J. L. Sobrinho and A. S. Krishnakumar,
  Quality-of-Service in Ad Hoc Carrier Sense
  Multiple Access Wireless Networks, IEEE Journal
  on Selected Areas in Communications, Vol. 17, No.
  8, August 1999.
• S. R. Ye, Y. C. Wang, and Y. C. Tseng, A
  Jamming-Based MAC Protocol to Improve the
  Performance of Wireless Multihop Ad Hoc Networks.