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Expected Data Rate (EDR): An Accurate High-Throughput Path Metric For Multi-Hop Wireless Routing

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Every link relies on supplying rate from previous link ... Supplying rate at link k 1 = TCD(k 1) ? TCD(k) ETX(k) ETX(k 1) ETX(k) TCD(k) TCD(k 1) = Min { 1, ... – PowerPoint PPT presentation

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Title: Expected Data Rate (EDR): An Accurate High-Throughput Path Metric For Multi-Hop Wireless Routing


1
Expected Data Rate (EDR) An Accurate
High-Throughput Path Metric For Multi-Hop
Wireless Routing
  • Jun Cheol Park (jcpark_at_cs.utah.edu)
  • Sneha Kumar Kasera (kasera_at_cs.utah.edu)
  • School of Computing
  • University of Utah

2
Multi-hop wireless networks
  • Flexible solution regardless of existence of
    fixed wired infrastructure
  • Efficient ad hoc routing necessary to achieve
    high throughput
  • Path metric crucial in selecting ad hoc paths

3
Related Work
  • ETX (Expected Transmission Count) MobiCom03
  • considers packet loss, but does not accurately
    model transmission interference
  • Existing transmission interference models do not
    consider packet loss
  • None of existing work has comprehensively
    addressed packet loss, transmission interference
    together

4
ETX
  • Average transmissions (including
    retransmissions) needed for successful packet
    delivery on wireless link with loss rate p
  • ETX sum of ad hoc path
  • sum of ETX of individual links
  • used as path metric for selecting best ad hoc path

Achievable Data Rate of a link Maximal data
rate / ETX ? Maximal data rate ? delivery ratio
1
ETX
1 - p
5
Limitations of ETX sum
  • UDP packet size 1500 bytes
  • Source node always backlogged (11 Mbps)
  • ETX sum cannot accurately differentiate ad hoc
    paths

6
Goal
  • Develop an accurate high-throughput path metric
    for multi-hop wireless networks

7
Outline
  • Problem Setting
  • EDR (Expected Data Rate)
  • Transmission Contention Degree
  • Back-off procedure
  • Performance Evaluation
  • Summary

8
Problem Setting
  • IEEE 802.11 networks
  • Distributed Coordination Function (DCF)
  • all links use single data rate
  • Load-insensitive path metric, routing
  • does not consider dynamic interference due to
    other flows
  • considers unavoidable transmission interference
    within single flow

1
2
3
4
9
Basic Ideas of EDR
  • Every link relies on supplying rate from previous
    link
  • EDR achievable data rate of whole ad hoc path
    achievable data rate of bottleneck link

B Bottleneck link
D Maximal Data rate on link B
ETX(B)
10
Basic Ideas of EDR
  • Every link relies on supplying rate from previous
    link
  • EDR achievable data rate of whole ad hoc path
    achievable data rate of bottleneck link

B Bottleneck link
D Maximal Data rate on link B
ETX(B)
I Total transmission interference factor
11
Total Transmission Interference Factor
  • Depends upon
  • TCD Transmission Contention Degree
  • RTCD Relatively Increased TCD
  • I Sum of all TCD and RTCD on links that
    interfere with bottleneck link B

12
Transmission Contention Degree for Link k
  • Represents how busy link k transmitting,
    retransmitting packets
  • range 0.0, 1.0, normalized value compared
    maximal data rate of link k
  • when node always backlogged, TCD 1.0
  • Considers load due to original transmission,
    retransmissions

13
How to calculate TCD?
  • Assume
  • ETX values of links are given
  • TCD(k1) in terms of TCD(k)?
  • TCD(1) 1.0

TCD(k1) ?
ETX(k)
14
Effect of 802.11 Back-off
  • No mechanism to differentiate packet loss due to
    collisions, channel noise
  • Upon packet loss exponential back-off used for
    occupying shared medium
  • Different loss rates between adjacent links
  • ? different average contention window sizes
  • ? different medium occupancy probabilities
  • ? relatively increased TCD (RTCD) on higher loss
    rate link

15
How to calculate RTCD?
  • Assume W(1) 5, W(2) 10
  • Node 1 twice more likely to occupy shared medium
    than Node 2
  • Thus, higher loss rate node (Node 2) experiences
    relative increase in TCD due to different window
    sizes
  • RTCD(k1) W(k1)/W(k) -1

Window size W(k)
10
5
1
2
3
16
EDR
  • D Maximum data rate on bottleneck link B
  • ETX(B) ETX of link B
  • I Sum of (TCD RTCD) over all links that
    interfere with link B

17
Performance Evaluation
  • NS-2 simulations
  • Independent, temporally correlated loss models
  • Randomly generate 270 ad hoc paths
  • hop lengths 2 - 5
  • link loss rates 0.0 - 0.5 (ETX 1.0 - 2.0)
  • Construct groups of 4 ad hoc paths between
    source, destination
  • for given group as input set, find how well each
    metric selects best ad hoc path
  • Use 1500-byte UDP packets, send rate at source
    node 11 Mbps

18
Independent loss
  • EDR performs much better than ETX sum
  • EDR for 90 of input cases, throughput more than
    90 of best

19
Temporally correlated loss
  • Packet burst loss modeled using two-state
    continuous time Markov chain
  • Burst length borrowed from experimental results
    Divert, MobiSys 04

20
Summary
  • Proposed a new metric, EDR
  • Showed that EDR can accurately determine
    achievable data rates of ad hoc paths
  • Future work
  • investigate TCP over EDR routing
  • apply EDR in multi-radio wireless networks

21
Backup
22
EDR for TCP on multi-rate paths
  • Bottleneck link B such that R Min D(k) /
    ETX(k)
  • I TCD(k)/TCDmax, k over interference range of
    link B,
  • Normalized total transmission contention degree
    in terms of B
  • For TCP flows, EDR does not include RTCD in I
    because TCP window mechanism is able to avoid
    unnecessary overhead of RTCD by adjusting send
    rate at source node

, TCD(1) 1.0
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