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CMMI Appraisal Methods

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If the highest throughput path has three or fewer hops, ETX is likely to choose it ... Reduction of throughput due to interference among the successive hops of a route ... – PowerPoint PPT presentation

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Title: CMMI Appraisal Methods


1
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
2
Contents
  • Introduction
  • Performance of Minimum-Hop-Count Routing
  • ETX Metric Design
  • Implementation
  • Evaluation
  • Conclusion

3
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

4
Performance of Minimum Hop Count Routing
  • Data throughputs
  • Distribution of path throughputs

Best static route
DSDV hop count
5
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
df?dr
6
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

7
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

8
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

9
Evaluation (1/5)
  • Metric performance with DSDV

DSDV hop count
Best static route
DSDV ETX
10
Evaluation (2/4)
  • Metric performance with DSDV (contd)

DSDV hop count
DSDV ETX
11
Evaluation (3/4)
  • Metric performance with DSR

DSR hop count
DSR hop count
Best static route
Best static route
DSR ETX
DSR ETX
12
Evaluation (4/4)
  • Accuracy of link measurements

13
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
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