CMMI Appraisal Methods

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A High-Throughput Path Metric for Multi-Hop
wireless Routing
Douglas S. J. De Couto, Daniel Aguayo, John
Bicket, Robjert Morris ACM MOBICOM, 2003 2003.
11. 6 Presented by Park, Seunghun
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Contents

• Introduction
• Performance of Minimum-Hop-Count Routing
• ETX Metric Design
• Implementation
• Evaluation
• Conclusion

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Introduction

• Metric commonly used by existing ad hoc routing
  protocols is minimum hop-count
• In a dense network there may be many routes of
  the same minimum length, with widely varying
  qualities
• Arbitrary choice is not likely to select the best
• Proposed solution is ETX (Expected transmission
  count) metric
• Primary goal is to find paths with high
  throughput, despite losses
• Evaluation
• Demonstrate measurements taken from test-bed
  network

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Performance of Minimum Hop Count Routing

• Data throughputs
• Distribution of path throughputs

Best static route
DSDV hop count
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ETX Metric Design (1/2)

• Metric
• ETX of a link is the predicted number of data
  transmissions required to send a packet over that
  link, including retransmission
• ETX of a route is the sum of the ETX for each
  link in the route
• ETX of a link is calculated using the forward and
  reverse delivery ratios of the link
• Expected number of transmission
• ETX

1
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ETX Metric Design (2/2)

• Characteristics
• Based on delivery ratios, which directly affect
  throughput
• Detects and appropriately handles asymmetry by
  incorporating loss ratios in each direction
• Use precise link loss ratio measurements to make
  fine-grained decisions between routes
• Penalizes routes with more hops, which have lower
  throughput due to interference between different
  hops of the same path
• Choose the best path
• If the highest throughput path has three or fewer
  hops, ETX is likely to choose it
• If the best path has four or more hops, ETX may
  choose a slower path that has fewer hops

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Implementation (1/2)

• Background
• DSDV
• Every node has a routing table entry which
  contains four fields IP address, the next hop,
  the latest sequence number, the metric
• Every node broadcasts a route advertisement
  packet (full dump) containing its complete
  routing table
• Each route entry has an associated WST (Weighted
  Settling Time)
• WST is used together with triggered updates
• Triggered updates are not sent until at least
  2?WST has passed since first hearing the current
  sequence number
• DSR
• Node issues a route request when it has data to
  send
• Each node appending its own address to each
  request it receives, and then re-broadcasting
• Destination issues a route reply in response to
  every forwarded request it receives

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Implementation (2/2)

• Changes to DSDV
• Dose not advertise a route until 2?WST has passed
  since that particular route was heard
• Does not use link-level feedback to detect broken
  links and produce broken-route messages
• Full dumps are never sent on a triggered update
• Route is not used until it is allowed to be
  advertised
• Changes to DSR
• Node forwards a request, it appends not only its
  own address, but also the metric for the link
  over which it received the request
• When a node receives a request, it forwards it
  again if the accumulated route metric is better
  than the best which it has already forwarded with
  this request ID

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Evaluation (1/5)

• Metric performance with DSDV

DSDV hop count
Best static route
DSDV ETX
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Evaluation (2/4)

• Metric performance with DSDV (contd)

DSDV hop count
DSDV ETX
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Evaluation (3/4)

• Metric performance with DSR

DSR hop count
DSR hop count
Best static route
Best static route
DSR ETX
DSR ETX
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Evaluation (4/4)

• Accuracy of link measurements

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Conclusion

• Route selecting using ETX accounts for
• Link loss ratios
• Reduction of throughput due to interference among
  the successive hops of a route
• Contributions
• Explains why minimum hop count often finds routes
  with significantly less throughput than the best
  available
• Presents the design, implementation, and
  evaluation of the ETX metric
• Describes detailed design changes to the DSDV and
  DSR protocols, that enable them to more
  accurately choose routes with the best metric