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A HighThroughput Path Metric for MultiHop Wireless Routing

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A High-Throughput Path Metric for Multi-Hop Wireless Routing. Douglas S. J. De Couto, Daniel ... 1 Mbps, 1 mW, small packets (134 bytes), RTS/CTS disabled ... – PowerPoint PPT presentation

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Title: A HighThroughput Path Metric for MultiHop Wireless Routing


1
A High-Throughput Path Metric for Multi-Hop
Wireless Routing
Douglas S. J. De Couto, Daniel Aguayo, John
Bicket, Robert Morris MIT Computer Science and
Artificial Intelligence Laborotory Presented by
Souvik Sen
2
Minimum Hop Count
  • Links in route share radio spectrum
  • Extra hops reduce throughput

Throughput 1
Throughput 1/2
Throughput 1/3
3
Problems
  • Min hop-count metric does not always choose
    highest-throughput path available!!!
  • Assumes links either work or dont work
  • Maximize the loss ratio of each hop
  • Arbitrarily chooses among same length paths
  • Retransmission reduces throughput increases
    interference
  • Thresholds to discard lossy links may disconnect
    network
  • Question Is there a better metric ?

4
Hop-count penalty
5
Challenge many links are asymmetric
Links with intermediate Loss Ratio
Very asymmetric link.
Many links are good in one direction, but lossy
in the other.
6
Effect of asymmetry
100
A
B
8
100
100
100
100
C
B successfully receives all of As route ads, and
installs a one-hop route to A.
But, throughput of B-A 0.08
B-C-A 0.5
7
Throughput differs between paths
Paths from 23 to 36
8
ETX metric Design goals
  • Find high-throughput paths
  • Account for lossy links
  • Account for asymmetric links
  • Account for inter-link interference
  • Independent of network load (dont incorporate
    congestion)?

9
A New Metric?
  • SINR threshold?
  • Nodes Unreachable
  • Multiply link ratios?

Throughput
1
1
1
1
.95
.95
2
  • End to End Delay? -Depends on interface queue
    lengths

10
ETX Minimize the Expected Transmission Count
Link throughput ? 1/ Link ETX
Delivery Ratio
Throughput
Link ETX
100
100
1
50
50
2
33
33
3
11
Calculating link ETX
  • Assuming 802.11 link-layer acknowledgements
    (ACKs) and retransmissions
  • P(TX success) P(Data success) ? P(ACK success)?
  • Estimating link ETX
  • P(Data success) ? measured fwd delivery ratio
    rfwd
  • P(ACK success) ? measured rev delivery ratio rrev
  • Link ETX ? 1 / (rfwd ? rrev)?
  • ETX (path) ? ETX(link)?

12
Measuring Delivery Ratios
  • Each node broadcasts small link probes (134
    bytes), once per second
  • Nodes remember probes received over past 10
    seconds
  • Reverse delivery ratios estimated as
  • rrev ? pkts received / pkts sent
  • Forward delivery ratios obtained from neighbors
    (piggybacked on probes)?

13
Test Specifics
  • Indoor network, 802.11b, ad hoc mode
  • 1 Mbps, 1 mW, small packets (134 bytes), RTS/CTS
    disabled
  • DSDV modifications to respect metrics
  • Packets are routed using route table snapshot to
    avoid route instability under load.
  • DSR modifications to respect metrics

14
ETX and DSDV
DSDVhop-count
DSDVETX
Best
15
Big packets
16
Comparison with min-hop (higher power)?
DSDV hop-count
DSDV ETX
17
DSR with ETX (no TX feedback)?
DSR hop-count
DSRETX
Best
18
DSR with ETX(With Tx Feedback)?
DSR hop-count
DSRETX
Best
19
ETX Performance
  • Broadcast has lower priority
  • Probe size ? Data/Ack size
  • Under-estimates data loss ratios,
  • over-estimates ACK loss ratios
  • For gt 4 hops, may choose a slower path with
    fewer hops
  • ETX assumes all links run at one bit-rate, at the
    same Power
  • Link-layer feedback already does a good job for
    DSR?

20
Conclusion / Thoughts
  • Proposed new metric to accommodate
    lossy/asymmetric links
  • Detailed experiments on real testbed
  • Favours shorter paths!
  • Inter-hop interference accounting problematic
  • Probe Packets may suffer from interference due to
    a hidden terminal
  • Is Traffic Independent
  • Congestion / Link loss separation ?
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