A Bidirectional Multi-channel MAC Protocol for Multihop Wireless Ad Hoc Networks

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A Bidirectional Multi-channel MAC Protocol for
Multihop Wireless Ad Hoc Networks

• Tianbo Kuang and Carey Williamson
• University of Calgary

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Multi-Hop Wireless Ad Hoc Networks
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Multi-Hop Wireless Ad Hoc Networks
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Multi-Hop Wireless Ad Hoc Networks
(Assume ideal world)
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Multi-Hop Wireless Ad Hoc Networks
(Reality check)
Problem 1 node A cant use both of these links
at the same time - shared wireless
channel - transmit or receive, but not
both
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Multi-Hop Wireless Ad Hoc Networks
Problem 2 cant use both of these links at same
time - range overlap at A -
hidden node problem - exposed node
problem
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Multi-Hop Wireless Ad Hoc Networks
Problem 3 LOTS of contention for the channel -
in steady state, all want to send - need RTS/CTS
to resolve contention
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RTS Request-To-Send CTS Clear-To-Send
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Multi-Hop Wireless Ad Hoc Networks
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RTS
CTS
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Multi-Hop Wireless Ad Hoc Networks
RTS
CTS
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Multi-Hop Wireless Ad Hoc Networks
Problem 4 TCP uses ACKS to indicate reliable
data delivery - bidirectional traffic (DATA,
ACKS) - even more contention!!!
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Multi-Hop Wireless Ad Hoc Networks
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TCP ACK
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A Bidirectional Multi-channel MAC Protocol for
Multihop Wireless Ad Hoc Networks

• Tianbo Kuang and Carey Williamson
• University of Calgary

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Agenda

• Problems of TCP over 802.11 MAC in
  multi-hop wireless ad hoc networks
• Proposed Solution Bi-MCMAC
• Performance Evaluation
• Fairness
• Throughput
• Response Time
• Conclusion

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Proposed Bi-MCMAC Protocol

• Idea 1 Use multiple channels (e.g., 4) on each
  wireless link, with 1 channel for control, and
  the others for data (suggested in literature
  previously, but not specifically for TCP
  problem)
• Idea 2 Extend the RTS/CTS to make bidirectional
  channel reservations (this handles DATA/ACK
  common case)

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Multi-Hop Wireless Ad Hoc Networks
Problem 2 cant use both of these links at same
time - range overlap at A -
hidden node problem - exposed node
problem
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Idea 1 Multi-Channel MAC Protocols
- Sender transmits RTS on control channel,
indicating candidate set of channels to use - CTS
reply indicates channel to use (if any) - Sender
and receiver tune to that channel for the
duration of that packet transmission
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A big win! - Confines contention to control
channel only - Reduces hidden node/exposed node
problems - Shortens the contention distance (3
to 2) - Significantly improves throughput
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Multi-Channel MAC Protocols
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Idea 2 Bidirectional Reservations
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Old way RTS CTS DATA 3 (ack) RTS CTS ACK 1
(ack)
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Our new way RTS CTS CRN DATA 3 ACK 1 (ack)
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25 less overhead in common case of TCP DATA/ACK
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Performance Evaluation

• Simulation using ns-2 network simulator
• Three protocols
• 802.11 MAC, MCMAC, Bi-MCMAC
• Static ad hoc networks
• Performance metrics
• Fairness
• Throughput (FTP-like bulk data transfer
  applications)
• Response Time (Web-like applications)

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Performance Evaluation
Channel Capacity 1 Mbps
Number of Channels 4
Propagation Model Two-ray ground
Transmission Range 250 m
Transport Protocol TCP NewReno
TCP Delayed ACK Enabled
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Fairness Example
MCMAC
802.11 MAC
Bi-MCMAC
Total 736 kbps
Total 1406 kbps
Total 1528 kbps
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Throughput
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Throughput Results
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Throughput (contd)
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Throughput (contd)
Random Topology
Grid Topology
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Response Time

Client
Server
Single Web object

Clients
Servers
Multiple Web objects
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Response Time (contd)
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Conclusions

• Bi-MCMAC protocol was proposed to improve TCP
  performance on multi-hop wireless ad hoc
  networks.
• Simulation results show 50 to 180 improvement
  in throughput.
• Significant improvements on fairness and transfer
  delay are also achieved.