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A High-Throughput Path Metric for Multi-Hop Wireless Routing

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Minimizes signal strength - Maximizes the loss ratio. Uses a ... Uses precise link loss ratios measurements to make fine-grained decisions between routes ... – PowerPoint PPT presentation

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Title: A High-Throughput Path Metric for Multi-Hop Wireless Routing


1
A High-Throughput Path Metric for Multi-Hop
Wireless Routing
  • Douglas S.J. Couto
  • Daniel Aguayo
  • John Bicket
  • Robert Morris
  • Presented by Eric Rozner

2
Overview
  • Hop count metric
  • Describe ETX
  • Implementation
  • Experiments and Comparisons
  • Related Work
  • Conclusions
  • Discussion

3
Hop Count Metric
  • Paths decided upon by shortest route
  • Maximizes the distance traveled by each hop
  • Minimizes signal strength -gt Maximizes the loss
    ratio
  • Uses a higher TxPower -gt Interference
  • Possibly many shortest routes
  • Avoid lossy links?

4
Hop Count vs. Optimal
x axis throughput
y axis fraction of pairs with less throughput
5
Hop Count Route Selection
6
Motivation for a Better Routing Metric
Bidirectional loss rates
Fine-grained choices
Intermediate loss rates
7
Potential Ideas (and their cons)
  • Product of per-link delivery ratios
  • A perfect 2-hop route is viewed as better than a
    1-hop route with 10 loss ratio
  • Throughput of a paths bottleneck link
  • Same as above
  • End-to-End delay
  • Changes with network load as interference queue
    lengths vary can cause oscillations
  • Everyone convinced?

8
ETX
  • The predicted number of data transmissions
    required to send a packet over a link
  • The ETX of a path is the sum of the ETX values of
    the links over that path
  • Examples
  • ETX of a 3-hop route with perfect links is 3
  • ETX of a 1-hop route with 50 loss is 2

9
ETX continued
  • Expected probability that a transmission is
    successfully received and acknowledged is df x dr
  • df is forward delivery ratio
  • dr is reverse delivery ratio
  • Each attempt to transmit a packet is a Bernoulli
    trial, so

10
Hooray for ETX!
  • Based on delivery ratios, which affect throughput
  • Detects and handles asymmetry by incorporating
    loss ratios in each direction
  • Uses precise link loss ratios measurements to
    make fine-grained decisions between routes
  • Assumes you can measure these ratios precisely
  • Penalizes routes with more hops, which have lower
    throughput due to inter-hop interference
  • Assumes loss rates are generally equal over links
  • Tends to minimize spectrum use, which should
    maximize overall system capacity (reduce power
    too)
  • Each node spends less time retransmitting data

11
ETX always the best?
12
Acquiring ETX values
  • Measured by broadcasting dedicated link probe
    packets with an average period t (jittered by
    0.1t)
  • Delivery ratio
  • count(t-w,t) is the of probes received during
    window w
  • w/t is the of probes that should have been
    received
  • Each probe contains this information

13
Implementation and such
  • Authors modified DSDV and DSR
  • See paper for details
  • t 1 packet per second, w 10 sec
  • Multiple queues (different priorities)
  • Loss-ratio probes, protocol packets, data packets
  • Are these experiments unfair or unrealistic?
  • In DSDV w/ ETX, route table is a snapshot taken
    at end of 90 second warm-up period
  • In DSR w/ ETX, source waits additional 15 sec
    before initiating the route request

14
DSDV Performance
One hop
Asymmetric
ETX inaccurate
15
DSDV and High Transmit Power
16
Packet Size Problems
  • Less throughputadvantage than whendata
    packetsare smaller(134 bytes)

17
Packet sizes continued
ACKs smaller than probe packets
ETX underestimates ACK delivery ratios -gt
overestimates total number of transmissions per
packet
18
DSR Performance
Link-layer transmission feedback disabled
Link-layer transmission feedback enabled
19
Related Work
  • Yarvis et al. 33 propose a path metric which
    approximates the product of the per-link delivery
    ratios.
  • Link hand-shaking to avoid gray areas 8, 22
  • SNR ratio is also a possible path metric 14
  • SNR threshold value to filter links discovered by
    DSR Route Discovery.
  • See Related Works section of Comparison of
    Routing Metrics for Static Multi-Hop Wireless
    Networks

20
Related Work Continued
  • WCETT proposed in MR-LQSR
  • Weighted Cumulative Expected Transmission Time

21
Conclusions
  • Pros
  • ETX performs better or comparable to Hop Count
    Metric
  • Accounts for bi-directional loss rates
  • Can easily be incorporated into routing protocols
  • A lot of other work built off this paper
  • Cons
  • May not be best metric for all networks
  • Mobility, Power-limited, Adaptive Rate
    (multi-rate), Multi-radio
  • Predications of loss ratios not always accurate
  • Experiments (30 sec transfer of small packets)
    may not complement real-world scenarios

22
Questions?
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