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

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A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas S. J. De Couto MIT CSAIL (LCS) Daniel Aguayo, John Bicket, and Robert Morris – 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. De Couto
  • MIT CSAIL (LCS)
  • Daniel Aguayo, John Bicket, and Robert Morris
  • Presenter
  • Jiaji Huang and Zheng Zhai

2
Talk outline
  • Observations from experiments
  • Challenges
  • A new high-throughput metric (ETX)
  • Evaluation
  • Summary Criticism

3
Indoor wireless network
29 PCs with 802.11b radios (fixed transmit power)
in ad hoc mode
5th floor
3rd floor
2nd floor
6th floor
4th floor
4
Testbed UDP throughput
better
5
Hop-count penalty
6
Throughput differs between paths
Paths from 23 to 36
7
Talk outline
  • Observations from experiments
  • Challenges
  • A new high-throughput metric (ETX)
  • Evaluation
  • Summary Criticism

8
Challenge more hops, less throughput
  • Links in route share radio spectrum
  • Extra hops reduce throughput

Throughput 1
Throughput 1/2
Throughput 1/3
9
Challenge many links are asymmetric
Broadcast delivery ratios in both link directions.
Very asymmetric link.
Many links are good in one direction, but lossy
in the other.
10
A straw-man route metric
  • Maximize bottleneck throughput

B
50
Delivery ratio 100
C
A
51
51
D
A-B-C 50 A-D-C 51
Bottleneck throughput
A-B-C ABBABBABB 33 A-D-C AADDAADD
25
Actual throughput
11
Talk outline
  • Observations from experiments
  • Challenges
  • A new high-throughput metric (ETX)
  • Evaluation
  • Summary Criticism

12
New metric ETX
Minimize total transmissions per packet (ETX,
Expected Transmission Count)
Link throughput ? 1/ Link ETX
Delivery Ratio
Throughput
Link ETX
100
100
1
50
50
2
33
33
3
13
Calculating link ETX
Assuming 802.11 link-layer acknowledgments
(ACKs) and retransmissions P(TX success)
P(Data success) ? P(ACK success) Link ETX 1 /
P(TX success) 1 /
P(Data success) ? P(ACK success) Estimating
link ETX P(Data success) ? measured fwd
delivery ratio dfwd P(ACK success) ? measured
rev delivery ratio drev Link ETX ? 1 /
(dfwd ? drev)
14
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
  • drev ? pkts received / pkts sent(10)
  • Forward delivery ratios obtained from neighbors
    (piggybacked on probes)

15
Route ETX
Route ETX Sum of link ETXs
Route ETX
Throughput
1
100
2
50
2
50
3
33
5
20
16
ETX Properties
  • ETX predicts throughput for short routes
  • (1, 2, and 3 hops)
  • ETX quantifies loss
  • ETX quantifies asymmetry
  • ETX quantifies throughput reduction of longer
    routes

17
ETX caveats
  • ETX link probes are susceptible to MAC unfairness
    and hidden terminals
  • Route ETX measurements change under load
  • ETX estimates are based on measurements of a
    single link probe size (134 bytes)
  • Under-estimates data loss ratios, over-estimates
    ACK loss ratios
  • ETX assumes all links run at one bit-rate

18
Evaluation Setup
  • 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

19
ETX improves DSDV throughput
DSDVhop-count
better
DSDVETX
Best
20
DSR with ETX
DSRhop-count
DSRETX
Best
21
DSR with ETX (no TX feedback)
DSRhop-count
DSRETX
Best
22
Talk outline
  • Observations from experiments
  • Challenges
  • A new high-throughput metric (ETX)
  • Evaluation
  • Summary Criticism

23
Summary
  • ETX accounts for
  • Throughput reduction of extra hops
  • Lossy and asymmetric links
  • Link-layer acknowledgements
  • ETX finds better routes!

24
Criticism
  • For network with small amount of nodes
  • For network with frequent change, frequent probes
    results in high overhead
  • Difficulty in choosing an approximate probe size
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