Application layer relays for wireless 802'11 mesh networks

1
Application layer relays for wireless 802.11 mesh
networks

• Yong Huang, Weibo Gong, Don Towsley
• University of Massachusetts, Amherst

2
Outline

• Motivation
• Experimental results
• Mathematical model
• Scheduling algorithm
• Conclusion and future work

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Motivation

• TCP likely to be predominant protocol in wireless
  mesh networks
• TCP relays proposed for improving performance in
  wired networks
• Do TCP relays improve performance in wireless
  mesh networks?

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TCP relays in wireline networks

• Break end-to-end TCP connection using relays
  Y. Liu 2004
• Thruput over links i,, j
• Padhye, Sigcomm 98
• Identical links
• without relays,
• with relays

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• Does this hold in a wireless mesh network?

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802.11a wireless mesh test-bed

• 5 laptop node linear chain topology, same
  channel, static routing, Dell 1470 dual band
  cards
• No interference with other wireless networks
• Mesh function supported by connectivity layer
  from MSR
• Performance metric TCP flow throughput, node 1
  to 5
• No CTS/RTS

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Experimental results

• Goodput in non-relay, relay cases

• Observation application layer relays do NOT
  provide gain as in wired networks
• Mathematical model needed to understand behavior
  in wireless mesh network

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Basic model

• For small p lt 0.01 and given Wmax, B(p)
  determined by RTT

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Comparison of RTT model
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Model verification

• Wmax 8, D 1240 bytes

• Error of B because TCP loss not considered in

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Discussion

• Observations
• Relay RTTs smaller than non-relay RTT
• However, maximum RTT of relay TCP close to
  non-relay RTT
• TCP Packet loss rate of first link in relay case
  larger than that of non-relay

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Model extension

• Assuming transmission of data and ACK packets
  follow Poisson process with attempt rate ? i and
  µi

Prob. data packet loss due to hidden node
problem

• With relay, attempt rates ? i larger or equal
  to those in non-relay case
• when attempt rate of source is same
• when link layer packet loss rate is same

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A simple scheduling algorithm

• Node 1 and 4 transmit parallel

• M appl. layer recv window
• S size of release signal

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Throughput with scheduling

• Nodes with same color transmit simultaneously
  with perfect scheduling

• Ignore S, for any hop distance larger than 3,
  steady state goodput is

• For homogeneous links,

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Experimental results with scheduling
M 1000, D 1240 bytes,S 20 bytes

• Practical issues
• Large M ? longer delay small M ? larger overhead
• With scheduling, link loss rate reduced network
  adaptor driver selects higher transmission rate

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Conclusion and future works

• Application layer relay may increase throughput
  also increases competition as relay nodes act
  more independently
• Simple scheduling algorithm can improve
  performance by coordinating relay nodes
• Future work
• More complex topology multiple flows
• Network coding

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• Thank you !