QoS Guarantee in Wirless Network

1
Fair Sharing of MAC under TCP in Wireless Ad Hoc
NetworksMobihoc 00
Mario Gerla Computer Science Department Universit
y of California, Los Angeles Los Angeles, CA
90095 http//www.cs.ucla.edu/NRL/wireless
2
Outline

• Overview of ad hoc networks.
• GloMoSim network simulator.
• Overview of CSMA, FAMA and IEEE 802.11.
• MAC performance with TCP.
• Variable Hop Length Experiments.
• Hidden Terminal Experiments.
• Ring Experiments.
• Grid Experiments.
• Static.
• Mobility.

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Ad-Hoc Network Characteristics

• Instantly deployable, reconfigurable
  infrastructure
• Node mobility
• Heterogeneous nodes (big/small fast/slow etc)
• Heterogeneous traffic (voice, image, video, data)
• Limited battery power
• Multihopping ( to save power, overcome obstacles,
  enhance spatial spectrum reuse, etc.)

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Simulation Using GloMoSim

• Detailed model of the protocol stack.
• Allows investigation of TCP and MAC layer
  interactions.
• Capability to simulate large number of nodes.
• GloMoSim web page.
• http//pcl.cs.ucla.edu/projects/domains/glomosim.h
  tml

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MAC Layer Protocols

• CSMA
• Requires carrier sensing before transmission.
• If the channel is free, the packet is transmitted
  immediately.
• Otherwise, it is rescheduled after a random
  timeout.
• FAMA
• Builds on CSMA.
• Uses the RTS (Request To Send) and CTS (Clear To
  Send) exchange to prepare the floor for data
  transmission.
• 802.11
• Uses carrier sensing and RTS/CTS, similar to
  FAMA.
• Utilizes link-level ACKs.
• Collision Avoidance scheme.

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Variable Hop Length Experiments Configuration

• Each node is 10 meters apart from its neighbors.
• Each node has a radio power range of 10 meters.
• 2Mbps channel bandwidth.
• FTP traffic.
• TCP window size varies from 1 to 16 packet size.
• Variable number of hops (single connection).
• i.e., FTP connection 0-1, 0-2, 0-3, 0-4, 0-5 (one
  at a time).

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Variable Hop Length Experiments Results
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Variable Hop Length Experiments Results (Contd)
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Variable Hop Length Experiments Results (Contd)

• CSMA and FAMA degrades with window size gt 1 pkt.
• Collisions between TCP data and ACKs.
• 802.11 performs the same no matter the window
  size.
• Link-level ACKs combat collisions.

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Hidden Terminal Experiments Configuration

• FTP traffic
• Connections from node 0 to node 1 and from node 2
  to node 1.
• Node 0 and node 2 cannot hear each other.

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Hidden Terminal Experiments Results

• CSMA suffers from hidden terminal.
• FAMA and 802.11 performs well due to RTS/CTS
  exchange.

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Ring Experiment Configuration

• FTP traffic
• Multiple single hop connections.
• i.e., 0-1, 1-2, 2-3, 3-4, 4-5, 5-6, 6-7, 7-0 (at
  the same time).

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Ring Experiment Results

• FAMA best in both fairness and aggregate
  throughput.
• 802.11 unfairness due to timers.

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802.11 Fairness

• Yield time.
• Time a node yields after transmitting a frame.

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Grid Experiment Configuration
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17
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26
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• Each node is 10 meters apart from its horizontal
  and vertical neighbors.
• Each node has a radio power range of 30 meters.
• FTP connections are established between node 18
  to node 26, node 36 to node 44, node 54 to node
  62, node 2 to node 74, node 4 to node 76 and node
  6 to node 78.

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Grid Experiment Configuration (Contd)

• 2Mbps channel bandwidth.
• Nodes move at a rate of 10 meters per second in a
  random direction with a probability of 0.5.
• When mobility is not considered, static routing
  is used.
• When mobility is introduced, Bellman-Ford routing
  is utilized with routing table updates occurring
  once every second.

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Grid Experiments Results (No Mobility)

• Without mobility
• CSMA performs poorly due to interference by
  neighboring streams and by intersecting streams.
• FAMA fair due to RTS/CTS and less aggressive
  yield time.
• 802.11 exhibits capture.

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Grid Experiments Results (With Mobility)

• CSMA and FAMA collapse with mobility due to lack
  of fast loss recovery facilities.
• 802.11 still operational.
• Link level ACKs help recover from loss caused by
  transient nodes.
• Capture exists.

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Conclusion

• RTS/CTS exchange improves fairness.
• Link-level ACKs important to combat packet loss
  in wireless ad-hoc environment.
• Impact of MAC layer timers needs further study.